JP2007042758A - Led driving device - Google Patents

Led driving device Download PDF

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JP2007042758A
JP2007042758A JP2005223304A JP2005223304A JP2007042758A JP 2007042758 A JP2007042758 A JP 2007042758A JP 2005223304 A JP2005223304 A JP 2005223304A JP 2005223304 A JP2005223304 A JP 2005223304A JP 2007042758 A JP2007042758 A JP 2007042758A
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voltage
circuit
output
constant current
led
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Toshiaki Shiba
俊明 司馬
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Toshiba Lighting and Technology Corp
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Harison Toshiba Lighting Corp
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Priority to TW095128188A priority patent/TW200717391A/en
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Abstract

<P>PROBLEM TO BE SOLVED: To provide an LED driving device which can drive an LED by constant current, reduce heat generation of a circuit element group including LEDs by optimizing voltage to be supplied to the LED, and stably light a backlight unit during light controlling. <P>SOLUTION: In the LED driving device, a first negative feedback closed loop CL1 for controlling supply voltage is comprised of a DC/DC convertor 8, an output driving element 10, a voltage comparison circuit 12, and an output voltage control circuit 13, and a second negative feedback closed loop CL2 for controlling constant-current is comprised of an output driving element 10 and a constant-current control circuit 11. The frequency response characteristic of the first negative feedback closed loop CL1 for controlling supply voltage is set to 1/20 or less of that of the second negative feedback closed loop CL2 for controlling constant current, so that the first negative feedback closed loop CL1 for controlling supply voltage can be stably operated without spoiling the response of controlling the constant current. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

本発明は、ＬＥＤ駆動装置に関する。   The present invention relates to an LED driving device.

従来、ナビゲーションなどの液晶用のバックライトには、光源として水銀が封入された冷陰極蛍光ランプが使用されてきたが、近年有害物質である水銀の使用を減らすため、代りの光源として白色やＲＧＢなどの発光ダイオード素子を使用したバックライトユニットが注目され徐々に商品化されつつある。   Conventionally, cold-cathode fluorescent lamps in which mercury is sealed as a light source have been used for backlights for liquid crystal such as navigation, but in recent years, white and RGB are used as alternative light sources in order to reduce the use of mercury, which is a harmful substance. Backlight units using light emitting diode elements such as these are attracting attention and are gradually being commercialized.

図１２は発光ダイオード素子（ＬＥＤ）を使った一般的なバックライトユニットを示す図である。バックライトユニットは導光板１０３、バックフレーム１０４、フロントフレーム１０６を備えている。バックフレーム１０４はＬＥＤ実装用基板１０２を内蔵している。ＬＥＤ実装用基板１０２には、白色などのＬＥＤ１０１が並べられ、これらのＬＥＤ１０１から発せられる光が導光板１０３で偏向され、光学シート１０５等を透過して表示部１０７から液晶へ照射される。尚、従来は、このＬＥＤ１０１とＬＥＤ実装用基板１０２の代りに、光源として水銀が封入された冷陰極蛍光ランプが使用されていたのである。   FIG. 12 is a diagram showing a general backlight unit using a light emitting diode element (LED). The backlight unit includes a light guide plate 103, a back frame 104, and a front frame 106. The back frame 104 incorporates an LED mounting substrate 102. White LEDs 101 are arranged on the LED mounting substrate 102, and light emitted from these LEDs 101 is deflected by the light guide plate 103, passes through the optical sheet 105 and the like, and is irradiated from the display unit 107 to the liquid crystal. Conventionally, instead of the LED 101 and the LED mounting substrate 102, a cold cathode fluorescent lamp in which mercury is sealed is used as a light source.

以下、ＬＥＤを点灯を制御する従来の技術を説明する。ＬＥＤの発光輝度は流れる電流の大きさに依存する。このため、ＬＥＤを安定して点灯させるためには、ＬＥＤを定電流にて駆動することが必要である。図１３はＬＥＤを定電流駆動するよう構成された従来の点灯回路のブロック図であり、図１４は具体的回路例の回路図である。   Hereinafter, a conventional technique for controlling lighting of the LED will be described. The light emission luminance of the LED depends on the magnitude of the flowing current. For this reason, in order to light an LED stably, it is necessary to drive the LED with a constant current. FIG. 13 is a block diagram of a conventional lighting circuit configured to drive an LED at a constant current, and FIG. 14 is a circuit diagram of a specific circuit example.

図１３では、２個以上のＬＥＤ２０１を直列に接続し、その一端にＬＥＤ２０１に流れる電流を検出するための抵抗ＲＣＳを接続し接地（ＧＮＤ）している。このＬＥＤ２０１に定電流を流すために、抵抗ＲＣＳに流れる電流を電圧変換した比較電圧を電圧比較回路２１２へ送り、所望の基準電圧と比較する。さらに、電圧比較回路２１２での比較結果に応じて、出力電圧制御回路２１３が降圧式又は昇圧式のスイッチング方式などのＤＣ／ＤＣコンバータ８を制御し、その出力電圧を平滑回路２１９で電力効率の良い所望のＤＣ電圧値（ＶＳＷ２）にしてＬＥＤ２０１へ供給することで、負帰還の閉ループ制御ＣＬ３を行う。 In Figure 13, connect two or more LED201 in series to connect the resistor R CS for detecting a current flowing through the LED201 to the one end ground (GND). To flow a constant current to the LED 201, sends the comparison voltage obtained by voltage conversion of the current flowing through the resistor R CS to the voltage comparator circuit 212, it is compared with a desired reference voltage. Furthermore, the output voltage control circuit 213 controls the step-down or step-up switching DC / DC converter 8 in accordance with the comparison result in the voltage comparison circuit 212, and the smoothing circuit 219 converts the output voltage to the power efficiency. A negative desired closed loop control CL3 is performed by supplying the LED 201 with a desired DC voltage value (V SW2 ).

図１４はこの従来のＬＥＤ駆動回路の具体的回路例であり、次のように構成されている。ＬＥＤ２０１に流れる電流を検出用抵抗ＲＣＳに流すことで電圧変換し、その電圧変換値をオペアンプ等で構成された誤差増幅用の電圧比較回路２２０の比較値として使い、基準電圧ＶＲｅｆと比較する。さらに電圧比較回路２２０の出力をコンパレータで構成される出力電圧制御回路２２１で三角波ＶＯＳＣ２と比較し、その比較結果によりＤＵＴＹが変化するパルス状の方形波電圧ＶＧ２を生成する。そして、このＤＵＴＹが変化するパルス状の方形波電圧ＶＧ２にてＰチャンネルＦＥＴで成るスイッチング素子２１４のゲートを制御し、電源ＶＩＮの直流電圧を所定の直流電圧に変換して出力する。つまり、ＳＷ電源方式のＤＣ／ＤＣコンバータ２０８として動作する。そして、このスイッチング素子２１４のドレイン端子をダイオード２１５のカソードとインダクタ２１６へ接続した後、インダクタ２１６の他端から出力される電圧を平滑用コンデンサ２１７で直流電圧ＶＳＷ２へ変換し、電力効率の良い電圧をＬＥＤ２０１群に供給する。 FIG. 14 shows a specific circuit example of this conventional LED driving circuit, which is configured as follows. And voltage conversion by passing a current flowing to the LED201 to the sensing resistor R CS, using the voltage conversion value as a comparison value of the voltage comparator circuit 220 for error amplification constituted by an operational amplifier or the like, is compared with the reference voltage V Ref . Further, the output of the voltage comparison circuit 220 is compared with the triangular wave V OSC2 by the output voltage control circuit 221 constituted by a comparator, and a pulsed square wave voltage V G2 in which DUTY changes is generated according to the comparison result. Then, the gate of the switching element 214 formed of a P-channel FET is controlled by the pulsed square wave voltage V G2 in which the DUTY changes, and the DC voltage of the power source VIN is converted into a predetermined DC voltage and output. In other words, it operates as a SW power source type DC / DC converter 208. Then, after connecting the drain terminal of the switching element 214 to the cathode of the diode 215 and the inductor 216, the voltage output from the other end of the inductor 216 is converted to the DC voltage V SW2 by the smoothing capacitor 217, and the power efficiency is high. A voltage is supplied to the LEDs 201 group.

このようにして構成される負帰還閉ループＣＬ３では、ＩＬＥＤ×ＲＣＳ＝ＶＲｅｆ（一定）になるように制御されることになり、ＬＥＤ２０１に流れる電流ＩＬＥＤは概ねＶＲｅｆ／ＲＣＳの定電流で点灯されることとなる。すなわち、ＬＥＤ２０１に流れる電流が所望の電流より大きい場合は、スイッチング素子２１４のゲートにオン期間の幅の狭いパルス状の方形波電圧ＶＧ２が供給されることでＬＥＤ２０１に印加される平滑化された電圧ＶＳＷ２は低くなりＬＥＤ２０１の電流を小さくする方向へ働く。逆に、ＬＥＤ２０１に流れる電流が所望の電流より小さくなった場合は、スイッチング素子２１４のゲートにオン期間の幅の広いパルス状の方形波電圧ＶＧ２が供給されることでＬＥＤ２０１に印加される平滑化された電圧ＶＳＷ２は高くなり、ＬＥＤ２０１の電流を大きくする方向へ働く。このような負帰還閉ループ制御により、電圧検出抵抗ＲＣＳの電圧値が基準電圧ＶＲＥＦと同じになるような所望の定電流ＩＬＥＤがＬＥＤ２０１に流れることで安定状態を作ることとなる。 In the negative feedback closed loop CL3 configured in this way, control is performed so that I LED × R CS = VRef (constant), and the current I LED flowing through the LED 201 is approximately a constant current of VRef / R CS. It will be lit. That is, when the current flowing through the LED 201 is larger than the desired current, the pulse-shaped square wave voltage V G2 having a narrow on-period is supplied to the gate of the switching element 214 and smoothed. The voltage V SW2 is lowered and works to reduce the current of the LED 201. On the contrary, when the current flowing through the LED 201 becomes smaller than the desired current, the smoothing applied to the LED 201 by supplying the pulse-shaped square wave voltage V G2 having a wide ON period to the gate of the switching element 214. The converted voltage V SW2 becomes higher and works to increase the current of the LED 201. By such negative feedback closed loop control, a desired constant current I LED whose voltage value of the voltage detection resistor R CS is the same as the reference voltage V REF flows to the LED 201 to create a stable state.

この図１４に示した回路例は、電源ＶＩＮの電圧値がＬＥＤ２０１を所望の明るさに点灯させるために必要な最大電圧値以上の場合であり、降圧式タイプのＤＣ／ＤＣコンバータ２０８を構成していることになる。しかし、ＤＣ／ＤＣコンバータ２０８が、電源ＶＩＮの電圧値とＬＥＤ２０１に供給するのに必要な最大電圧値の大小比較に応じて降圧式、昇圧式、昇降圧式の３種類のタイプが考えられ、ＤＣ／ＤＣコンバータ２０８の駆動方式は限定されていない。 The circuit example shown in FIG. 14 is a case where the voltage value of the power source VIN is equal to or higher than the maximum voltage value necessary for lighting the LED 201 to a desired brightness, and constitutes a step-down type DC / DC converter 208. Will be. However, depending on the magnitude comparison between the voltage value of the power source VIN and the maximum voltage value required for the LED 201 to be supplied by the DC / DC converter 208, there are three types, a step-down type, a step-up type, and a step-up / down type. The driving method of the DC / DC converter 208 is not limited.

この従来回路の構成では、ＤＣ／ＤＣコンバータ２０８の出力電圧を制御することによってＬＥＤ２０１を定電流で駆動できるとともに、ＬＥＤ２０１のＶＦ規格によらず高効率駆動が実現でき、所望の定電流値と温度に依存したＶＦ値に応じた必要最小限の駆動電圧にＤＣ／ＤＣコンバータ２０８の出力が制御されるため、ＶＦ規格から考慮される電圧マージンによる損失は発生しないという特徴があった。   In this conventional circuit configuration, the LED 201 can be driven with a constant current by controlling the output voltage of the DC / DC converter 208, and a high-efficiency drive can be realized regardless of the VF standard of the LED 201, and a desired constant current value and temperature can be achieved. Since the output of the DC / DC converter 208 is controlled to the minimum necessary drive voltage corresponding to the VF value depending on the VF value, there is a feature that no loss due to the voltage margin considered from the VF standard occurs.

上記従来回路例に類似した特許の一例として、ＤＣ／ＤＣコンバータ２０８の形式を昇圧式に限定し、携帯機器に応用した技術が特開２００３−１５２２２４号公報（特許文献１）に開示されている。   As an example of a patent similar to the above conventional circuit example, Japanese Patent Laid-Open No. 2003-152224 (Patent Document 1) discloses a technique in which the DC / DC converter 208 is limited to a step-up type and applied to a portable device. .

しかしながら、従来の図１３、図１４に示したようなＬＥＤ駆動回路では、電源ＶＩＮの電圧変動や外部からのノイズ、さらに電流検出抵抗部分へ飛び込む外乱ノイズ、制御ループ内の変動が発生すると定電流制御が不安定となり、そのため制御ループＣＬ３の応答速度（周波数特性）及びゲインをあまり高くすることができない技術的な限界がある。そのため、上記の従来回路は、携帯電話などのように常時決まった定電流をＬＥＤ２０１に流すことで十分バックライトとしての目的を達成できるものには採用するには最適であったが、頻繁に所望の定電流値が変わるような商品、例えば調光機能が必要な分野の商品に採用するには不十分であった。特に、上記の従来回路では、調光率が概ね１０％以下の領域で安定した調光性能が得られない上に、調光率に対するＬＥＤ２０１の電流特性も誤差が増大し非線形となってしまうため、５％以下の極めて低い調光率まで線形な特性が要求される車載用のナビゲーション用の液晶バックライトヘの採用のためにはさらに技術改良が求められていた。
特開２００３−１５２２２４号公報 However, in the conventional LED drive circuit as shown in FIGS. 13 and 14, it is determined that voltage fluctuation of the power source VIN , noise from the outside, disturbance noise jumping into the current detection resistor portion, and fluctuation in the control loop occur. Current control becomes unstable, so there is a technical limit that the response speed (frequency characteristic) and gain of the control loop CL3 cannot be made so high. For this reason, the conventional circuit described above was optimal for use in a device such as a mobile phone that can achieve the purpose of a backlight sufficiently by flowing a constant current that is always determined to the LED 201. However, it is not sufficient for use in a product that changes the constant current value of the light source, for example, a product in a field that requires a dimming function. In particular, in the above-described conventional circuit, a stable dimming performance cannot be obtained in a region where the dimming rate is approximately 10% or less, and the current characteristic of the LED 201 with respect to the dimming rate also increases in error and becomes nonlinear. In order to adopt a liquid crystal backlight for in-vehicle navigation that requires linear characteristics up to an extremely low dimming rate of 5% or less, further technical improvement has been demanded.
JP 2003-152224 A

本発明は、上述したような従来技術の問題点に鑑みてなされたものであり、発光ダイオード（ＬＥＤ）を定電流で駆動すると共に、ＬＥＤに供給する電圧を最適化することでＬＥＤを含む回路素子群の発熱を低減し、調光時でもバックライトユニットを安定して点灯させることができるＬＥＤ駆動装置を提供することを目的とする。   The present invention has been made in view of the above-described problems of the prior art, and a circuit including an LED by driving a light emitting diode (LED) with a constant current and optimizing a voltage supplied to the LED. An object of the present invention is to provide an LED drive device that can reduce the heat generation of the element group and can stably light a backlight unit even during dimming.

請求項１の発明のＬＥＤ駆動装置は、直列接続された複数の発光ダイオードと、前記直列接続された発光ダイオードのアノード側に直流電源から電力効率の良い電圧値を供給するＤＣ／ＤＣコンバータと、前記直列接続された発光ダイオードのカソード側に一端が接続された出力駆動素子と、前記出力駆動素子の他端に、その一端が接続され、その他端が接地された電流検出用の抵抗と、前記複数の直列接続された発光ダイオードに効率の良い最適値の電圧を供給するために、前記ＤＣ／ＤＣコンバータを制御する出力電圧制御回路と、前記出力電圧制御回路に出力電圧指令を与えるために、前記電流検出用の抵抗に流れる電流を変換した電圧を第１の比較電圧とし、所望の基準電圧と比較して差電圧を前記出力電圧制御回路に出力する電圧比較回路と、前記電流検出用の抵抗に流れる電流を変換した第２の比較電圧を定電流制御のための所定電圧と比較し、前記出力駆動素子をその通電電流が定電流になるように制御する定電流制御回路とを備え、前記ＤＣ／ＤＣコンバータ、出力駆動素子、電圧比較回路及び出力電圧制御回路にて構成される閉ループを供給電圧制御用の第１の負帰還閉ループとし、前記出力駆動素子及び定電流制御回路にて構成される閉ループを定電流制御用の第２の負帰還閉ループとしたものである。   An LED driving device according to a first aspect of the present invention includes a plurality of light emitting diodes connected in series, a DC / DC converter that supplies a power-efficient voltage value from a direct current power source to the anode side of the light emitting diodes connected in series, An output driving element having one end connected to the cathode side of the light emitting diodes connected in series; the other end of the output driving element; a current detecting resistor having one end connected to the other end and grounded; In order to supply an efficient optimum voltage to a plurality of light emitting diodes connected in series, an output voltage control circuit for controlling the DC / DC converter, and an output voltage command to the output voltage control circuit, A voltage obtained by converting a current flowing through the current detection resistor is set as a first comparison voltage, and a voltage for outputting a differential voltage to the output voltage control circuit in comparison with a desired reference voltage A comparison circuit and a second comparison voltage obtained by converting a current flowing through the current detection resistor are compared with a predetermined voltage for constant current control, and the output drive element is controlled so that the energization current becomes a constant current. A closed loop composed of the DC / DC converter, the output drive element, the voltage comparison circuit, and the output voltage control circuit is defined as a first negative feedback closed loop for supply voltage control, and the output drive The closed loop constituted by the element and the constant current control circuit is a second negative feedback closed loop for constant current control.

請求項２の発明のＬＥＤ駆動装置は、直列接続された複数の発光ダイオードと、前記直列接続された発光ダイオードのアノード側に直流電源から電力効率の良い電圧値を供給するＤＣ／ＤＣコンバータと、前記直列接続された発光ダイオードのカソード側に一端が接続された出力駆動素子と、前記出力駆動素子の他端に、その一端が接続され、その他端が接地された電流検出用の抵抗と、前記複数の直列接続された発光ダイオードに効率の良い最適値の電圧を供給するために、前記ＤＣ／ＤＣコンバータを制御する出力電圧制御回路と、前記出力駆動素子の両端の差電圧を算出する差電圧算出回路と、前記出力電圧制御回路に出力電圧指令を与えるために、前記差電圧算出回路の算出した差電圧を第１の比較電圧とし、所望の基準電圧と比較して電圧差を前記出力電圧制御回路に出力する電圧比較回路と、前記電流検出用の抵抗に流れる電流を変換した第２の比較電圧を定電流制御のための所定電圧と比較し、前記出力駆動素子をその通電電流が定電流になるように制御する定電流制御回路とを備え、前記ＤＣ／ＤＣコンバータ、出力駆動素子、差電圧算出回路、電圧比較回路及び出力電圧制御回路にて構成される閉ループを供給電圧制御用の第１の負帰還閉ループとし、前記出力駆動素子及び定電流制御回路にて構成される閉ループを定電流制御用の第２の負帰還閉ループとしたものである。   An LED driving device according to a second aspect of the present invention includes a plurality of light emitting diodes connected in series, a DC / DC converter that supplies a power-efficient voltage value from a direct current power source to the anode side of the light emitting diodes connected in series, An output driving element having one end connected to the cathode side of the light emitting diodes connected in series; the other end of the output driving element; a current detecting resistor having one end connected to the other end and grounded; An output voltage control circuit for controlling the DC / DC converter and a differential voltage for calculating a differential voltage between both ends of the output drive element in order to supply an efficient optimum voltage to a plurality of light emitting diodes connected in series. In order to give an output voltage command to the calculation circuit and the output voltage control circuit, the difference voltage calculated by the difference voltage calculation circuit is used as a first comparison voltage and compared with a desired reference voltage. A voltage comparison circuit that outputs a voltage difference to the output voltage control circuit; and a second comparison voltage obtained by converting a current flowing through the current detection resistor is compared with a predetermined voltage for constant current control, and the output drive A constant current control circuit that controls the element so that the energization current becomes a constant current, and is configured by the DC / DC converter, the output drive element, the differential voltage calculation circuit, the voltage comparison circuit, and the output voltage control circuit. The closed loop is a first negative feedback closed loop for supply voltage control, and the closed loop composed of the output drive element and the constant current control circuit is a second negative feedback closed loop for constant current control.

請求項３の発明は、請求項２のＬＥＤ駆動装置において、前記差電圧算出回路に対して、外部から与えられる調光率に対応して可変な基準電圧を与える制御基準作成回路を備えたことを特徴とするものである。   According to a third aspect of the present invention, in the LED driving device according to the second aspect, a control reference creation circuit is provided that gives a variable reference voltage to the difference voltage calculation circuit in accordance with a dimming rate given from the outside. It is characterized by.

本発明によれば、供給電圧制御用の第１の負帰還閉ループの周波数応答特性を定電流制御用の第２の負帰還閉ループの周波数応答特性の１／２０以下に設定することで、定電流制御の応答性を損なわずに供給電圧制御が安定して行える。すなわち、各負帰還閉ループの周波数の応答特性が２０倍以上も違うことからお互いが干渉して不安定になることがないので、定電流制御用の第２の負帰還閉ループの利得及び周波数応答を高く設定でき、非常に精度の良い定電流特性が達成できる。   According to the present invention, the frequency response characteristic of the first negative feedback closed loop for supply voltage control is set to 1/20 or less of the frequency response characteristic of the second negative feedback closed loop for constant current control. Supply voltage control can be performed stably without impairing control responsiveness. That is, since the frequency response characteristics of the negative feedback closed loops are different from each other by 20 times or more, they do not become unstable due to interference with each other. Therefore, the gain and frequency response of the second negative feedback closed loop for constant current control are High constant current characteristics can be achieved with high accuracy.

また、本発明によれば、電圧基準を調光率に応じて可変とすることで、頻繁に所望の定電流値が変わるような分野の商品や極めて低い調光性能が必要な分野の商品にも十分対応可能であり、特に車載用のナビゲーションの液晶バックライトに必要な５％以下の極めて低い調光率域での線形な電流特性を達成できる。   In addition, according to the present invention, by making the voltage reference variable according to the dimming rate, it is possible to produce products in a field in which a desired constant current value frequently changes or products in a field requiring extremely low dimming performance. In particular, it is possible to achieve a linear current characteristic in a very low dimming rate region of 5% or less necessary for a liquid crystal backlight for a vehicle-mounted navigation.

以下、本発明の実施の形態を図に基づいて詳説する。   Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

（第１の実施の形態）図１は本発明の第１の実施の形態のＬＥＤ駆動装置のブロック図である。図１において、１は発光ダイオード（ＬＥＤ）、８はＤＣ／ＤＣコンバータ、９は平滑回路、１０は出力駆動素子、１１は定電流制御回路、１２は電圧比較回路、１３は出力電圧制御回路を表している。   (First Embodiment) FIG. 1 is a block diagram of an LED driving apparatus according to a first embodiment of the present invention. In FIG. 1, 1 is a light emitting diode (LED), 8 is a DC / DC converter, 9 is a smoothing circuit, 10 is an output drive element, 11 is a constant current control circuit, 12 is a voltage comparison circuit, and 13 is an output voltage control circuit. Represents.

２個以上のＬＥＤ１を直列に接続し、そのアノード側には、バッテリー等のＤＣ電源ＶＩＮからの直流電力をＤＣ／ＤＣ変換するための降圧式又は昇圧式、あるいは昇降圧式のスイッチング又はチョッパ方式などのＤＣ／ＤＣコンバータ８が平滑回路９を介して接続してある。また、ＬＥＤ１のカソード側には、トランジスタやＦＥＴなどの出力駆動素子１０、電流検出用の抵抗ＲＣＳが順に接続してあり、この抵抗ＲＣＳの他端は接地（ＧＮＤ）してある。 Two or more LEDs 1 are connected in series, and the anode side has a step-down or step-up or step-up / step-down switching or chopper method for DC / DC conversion of DC power from a DC power source VIN such as a battery. A DC / DC converter 8 is connected via a smoothing circuit 9. Further, the cathode side of the LED1, the output drive element 10 such as a transistor or FET, the resistance R CS for current detection Yes connected in this order, the other end of the resistor R CS is are grounded (GND).

ＬＥＤ１に効率の良い最適値の電圧を供給するために、出力電圧制御回路１３にてＤＣ／ＤＣコンバータ８を制御する。また、出力駆動素子１０がＬＥＤ１に定電流を流すように制御するために、この出力駆動素子１０は定電流制御回路１１にて制御する。   The DC / DC converter 8 is controlled by the output voltage control circuit 13 in order to supply the LED 1 with an efficient optimum voltage. Further, the output drive element 10 is controlled by a constant current control circuit 11 in order to control the output drive element 10 to flow a constant current through the LED 1.

ＤＣ／ＤＣコンバータ８の制御は従来と同様であり、抵抗ＲＣＳに流れる電流ＩＬＥＤを電圧変換した比較電圧（＝ＲＣＳ×ＩＬＥＤ）を電圧比較回路１２へ送り、所望の基準電圧ＶＲＥＦと比較する。さらに、電圧比較回路１２での比較結果に応じて、出力電圧制御回路１３がＤＣ／ＤＣコンバータ８を制御し、その出力電圧を平滑回路９で電力効率の良い所望のＤＣ電圧値（ＶＳＷ１）にしてＬＥＤ１へ供給することで、負帰還の閉ループ制御ＣＬ１を行う。 The control of the DC / DC converter 8 is the same as in the prior art, and a comparison voltage (= R CS × I LED ) obtained by converting the current I LED flowing in the resistor R CS into a voltage is sent to the voltage comparison circuit 12, and a desired reference voltage V REF Compare with Further, the output voltage control circuit 13 controls the DC / DC converter 8 in accordance with the comparison result in the voltage comparison circuit 12, and the smoothing circuit 9 converts the output voltage to a desired DC voltage value (V SW1 ) with good power efficiency. Thus, the negative feedback closed-loop control CL1 is performed by supplying to the LED1.

定電流制御回路１１による定電流制御は、上記の電圧検出抵抗の比較電圧を所定値と比較し、出力駆動素子１０の通電電流が定電流になるように制御することで、負帰還の閉ループ制御ＣＬ２を行う。   The constant current control by the constant current control circuit 11 compares the comparison voltage of the voltage detection resistor with a predetermined value and performs control so that the energization current of the output drive element 10 becomes a constant current, thereby closing the negative feedback. CL2 is performed.

本実施の形態のＬＥＤ駆動装置は、ＬＥＤ１に効率の良い最適値の電圧を供給するための供給電圧制御用閉ループＣＬ１を構成する回路群の中に、ＬＥＤ１に定電流を流すための制御用の閉ループＣＬ２を構成した回路群が存在する構成であり、供給電圧制御用の閉ループＣＬ１とＬＥＤ１に定電流を流すための定電流制御用の閉ループＣＬ２との経路の一部が共通する構成となっている。閉ループＣＬ１と閉ループＣＬ２の共通部分は、ＬＥＤ１に流れる電流を、検出抵抗ＲＣＳを使って電圧情報に変換する部分であり、この検出抵抗ＲＣＳの電圧は定電流制御回路１１及び電圧比較回路１２それぞれの比較値としている。すなわち、ＬＥＤ１に流れる電流を電流検出用抵抗ＲＣＳによって電流−電圧変換した電圧値（ＩＬＥＤ×ＲＣＳ）を供給電圧制御用の比較情報とすると同時に、定電流制御用の比較情報としても利用することで、それぞれの制御用の閉ループＣＬ１，ＣＬ２は所望の基準値ＶＲｅｆ，Ｉ０×ＶＲに一致するように負帰還制御する。ここで、供給電圧制御用の閉ループＣＬｌの周波数応答が定電流制御用の閉ループＣＬ２の周波数応答の１／２０以下に設定することで、定電流制御の応答性を損なわずに供給電圧制御用の負帰還閉ループＣＬ１を安定に動作させることができる。すなわち、各負帰還閉ループＣＬ１，ＣＬ２の周波数の応特性が２０倍以上も違うことからお互いが干渉して不安定になることがないので、定電流制御用の負帰還閉ループＣＬ２の利得及び周波数応答を高く設定できることから、非常に精度の良い定電流特性を達成できる。 The LED driving device according to the present embodiment is used for control for supplying a constant current to the LED 1 in the circuit group constituting the supply voltage control closed loop CL1 for supplying the LED 1 with an efficient optimum voltage. A circuit group constituting the closed loop CL2 exists, and a part of the path between the closed loop CL1 for supply voltage control and the closed loop CL2 for constant current control for causing a constant current to flow through the LED 1 is common. Yes. Intersection of closed loop CL1 and loop CL2 is the current flowing through the LED1, the detection resistor with the R CS is a portion for converting the voltage information, the voltage of the detection resistor R CS is constant current control circuit 11 and the voltage comparator circuit 12 Each comparison value is used. That is, the voltage value (I LED × R CS ) obtained by current-voltage conversion of the current flowing through the LED 1 by the current detection resistor R CS is used as comparison information for supply voltage control and also used as comparison information for constant current control. doing, a closed loop of each control CL1, CL2 is desired reference value VRef, negative feedback control so as to match the I 0 × V R. Here, by setting the frequency response of the closed loop CLl for supply voltage control to 1/20 or less of the frequency response of the closed loop CL2 for constant current control, it is possible to control the supply voltage without impairing the responsiveness of the constant current control. The negative feedback closed loop CL1 can be stably operated. That is, since the frequency response characteristics of the negative feedback closed loops CL1 and CL2 are different by 20 times or more, they do not become unstable due to interference with each other. Therefore, the gain and frequency response of the negative feedback closed loop CL2 for constant current control Can be set high, so that a highly accurate constant current characteristic can be achieved.

図２は本実施の形態のＬＥＤ駆動装置の具体回路図である。図２において、１４はＭＯＳ−ＦＥＴ、１５はダイオード、１６はインダクタ、１７はコンデンサ、１８はＬＥＤ駆動素子（トランジスタ）、１９は負帰還増幅回路、２０は電圧比較増幅器、２１は比較器（コンパレータ）を表している。尚、ＬＥＤの符号「１」は図１と同様の符号を用いている。   FIG. 2 is a specific circuit diagram of the LED driving device of the present embodiment. In FIG. 2, 14 is a MOS-FET, 15 is a diode, 16 is an inductor, 17 is a capacitor, 18 is an LED driving element (transistor), 19 is a negative feedback amplifier circuit, 20 is a voltage comparison amplifier, and 21 is a comparator (comparator). ). In addition, the code | symbol "1" of LED uses the code | symbol similar to FIG.

図１と図２を照合しながら説明する。図１のＤＣ／ＤＣコンバータ８に相当するのが、図２のＰチャンネルのＭＯＳ−ＦＥＴ１４とダイオード１５であり、出力電圧制御回路１３がコンパレータ２１、電圧比較回路１２が電圧比較増幅器２０、平滑回路９がインダクタ１６と平滑コンデンサ１７である。本実施の形態の回路は降圧型のスイッチング電源方式のＤＣ／ＤＣコンバータ構成になっている。さらに、図１中の出力駆動素子１０に相当するのが、トランジスタ１８であり、定電流制御回路１１に相当するのがトランジスタ１８を負荷にした負帰還増幅回路１９である。   Description will be made with reference to FIG. 1 and FIG. The DC / DC converter 8 in FIG. 1 corresponds to the P-channel MOS-FET 14 and the diode 15 in FIG. 2, the output voltage control circuit 13 is the comparator 21, the voltage comparison circuit 12 is the voltage comparison amplifier 20, and the smoothing circuit. Reference numeral 9 denotes an inductor 16 and a smoothing capacitor 17. The circuit of this embodiment has a DC / DC converter configuration of a step-down switching power supply system. Further, the transistor 18 corresponds to the output drive element 10 in FIG. 1, and the negative feedback amplifier circuit 19 using the transistor 18 as a load corresponds to the constant current control circuit 11.

先ず、ＬＥＤ１に所望の一定の電流を流すための定電流制御用の閉ループＣＬ２の動作は、ＬＥＤ１に流れる電流を検出抵抗ＲＣＳを使って電圧情報ＶＥ（＝ＲＣＳ×ＩＬＥＤ）に変換し、その電圧値ＶＥと所望の電流値に対応した基準電圧値ＶＲｅｆ１（＝Ｉ０×可変抵抗ＶＲの抵抗値）とを比較し、その誤差に応じて出力駆動素子１０を負帰還ループ制御する。 First, the operation of the closed loop CL2 for constant current control for causing a desired constant current to flow through the LED 1 converts the current flowing through the LED 1 into voltage information V E (= R CS × I LED ) using the detection resistor R CS. Then, the voltage value V E is compared with a reference voltage value V Ref1 (= I0 × resistance value of the variable resistor VR) corresponding to a desired current value, and the output drive element 10 is controlled in a negative feedback loop according to the error. To do.

他方、ＬＥＤ１に電力効率の良い必要最低な最適電圧を供給するための供給電圧制御用の負帰還閉ループＣＬ１は、次のように動作する。ＬＥＤ１に流れる電流を制御する出力駆動素子１８が定電流動作及び応答性に追従できなくなる飽和領域で動作しないために必要な概ね最低電圧値を予め制御目標値として電圧比較増幅器２０の基準値ＶＲｅｆ２として設定している。電圧比較増幅器２０では、先の定電流制御用の負帰還ループＣＬ２に対する負帰還電圧値ＶＥを比較情報として共通に用い、基準電圧値ＶＲＥＦ２とこの比較電圧ＶＥとを比較する。さらに電圧比較増幅器２０の出力をコンパレータ２１で三角波ＶＯＳＣ１と比較し、その比較結果によりＤＵＴＹが変化するパルス状の方形波電圧ＶＧ１を生成する。そして、このＤＵＴＹが変化するパルス状の方形波電圧ＶＧ１にてＰチャンネルＦＥＴ１４のゲートを制御し、直流電源の直流電圧ＶＩＮを所定の直流電圧ＶＳＷ１に変換して出力する。このＦＥＴ１４の直流出力は、平滑用のインダクタ１６とコンデンサ１７にて平滑化し、電力効率の良い電圧としてＬＥＤ１に供給する。 On the other hand, the negative feedback closed loop CL1 for controlling the supply voltage for supplying the LED 1 with the lowest necessary optimum voltage with high power efficiency operates as follows. The reference value V Ref2 of the voltage comparison amplifier 20 is preliminarily set to a control target value that is approximately the minimum voltage value necessary for the output drive element 18 that controls the current flowing through the LED 1 not to operate in a saturation region where it cannot follow constant current operation and responsiveness. It is set as. In the voltage comparison amplifier 20, the negative feedback voltage value V E for the negative feedback loop CL2 for constant current control is commonly used as comparison information, and the reference voltage value V REF2 is compared with the comparison voltage V E. Further, the output of the voltage comparison amplifier 20 is compared with the triangular wave V OSC1 by the comparator 21, and a pulsed square wave voltage V G1 in which DUTY changes is generated based on the comparison result. Then, by controlling the gate of the P-channel FET14 at pulsed square-wave voltage V G1 of the DUTY is changed, it converts the DC voltage V IN of the DC power supply to a predetermined DC voltage V SW1. The DC output of the FET 14 is smoothed by the smoothing inductor 16 and the capacitor 17 and supplied to the LED 1 as a voltage with good power efficiency.

図３は本実施の形態のＬＥＤ駆動装置の電圧のオシロ波形図である。入力電圧ＶＩＮに対してＬＥＤ１に所望の定電流が流れるのに必要な電力変換効率の良い電圧値ＶＳＷ１が供給され、しかも線形で安定な定電流ＩＬＥＤとなっていることがわかる。ここで、電力変換効率の良い電圧ＶＳＷ１とは、ＬＥＤ１が周囲の温度条件下で所望の電流を流すのに必要な各ＶＦを足した電圧と出力駆動素子１０がサチレーションしない電圧と検出抵抗ＲＣＳの電圧を総和した電圧値となる。 FIG. 3 is an oscilloscope waveform diagram of the voltage of the LED driving device of the present embodiment. It can be seen that the voltage value V SW1 with good power conversion efficiency necessary for a desired constant current to flow through the LED 1 with respect to the input voltage VIN is supplied, and that the linear and stable constant current I LED is obtained. Here, a good voltage V SW1 power conversion efficiency, the detection resistor and voltage LED1 voltage and the output drive element 10 plus the V F required to pass a desired current at a temperature of ambient not Sachireshon a voltage value obtained by summing the voltage of the R CS.

本実施の形態のＬＥＤ駆動装置によれば、各負帰還閉ループの周波数の応答特性が２０倍以上も違うことからお互いが干渉して不安定動作になることがないため、定電流制御用の負帰還閉ループＣＬ２の利得及び周波数応答を高く設定でき、これによって非常に精度良く高速に応答する定電流回路が達成できる。そのため、電圧基準を調光率に応じて可変とすることで、頻繁に所望の定電流値が変わるような分野の商品や極めて低い調光性能が必要な分野の商品にも十分対応可能であり、特に車載用のナビゲーションの液晶バックライトに必要な５％以下の極めて低い調光率域での線形な電流特性を達成できる。   According to the LED driving device of the present embodiment, since the frequency response characteristics of the respective negative feedback closed loops are different by 20 times or more, they do not interfere with each other to cause unstable operation. The gain and frequency response of the feedback closed loop CL2 can be set high, thereby achieving a constant current circuit that responds very accurately and at high speed. Therefore, by making the voltage reference variable according to the dimming rate, it is possible to sufficiently handle products in fields where the desired constant current value frequently changes and products that require extremely low dimming performance. In particular, it is possible to achieve a linear current characteristic in an extremely low dimming rate range of 5% or less, which is necessary for a liquid crystal backlight for a vehicle-mounted navigation.

（第２の実施の形態）図４は本発明の第２の実施の形態のＬＥＤ駆動装置のブロック図である。本実施の形態は、図１に示した第１の実施の形態のＬＥＤ駆動装置の構成に対して、差電圧算出回路３１を付加したことを特徴とする。この差電圧算出回路３１は、出力駆動素子１０のドレイン−ソース間電圧又はコレクターエミッタ間電圧を算出するものであり、算出した差電圧を電圧比較回路１２に対して出力するものである。尚、図４において、図１と同一の要素には同一の符号を用いて表している。   (Second Embodiment) FIG. 4 is a block diagram of an LED driving apparatus according to a second embodiment of the present invention. The present embodiment is characterized in that a differential voltage calculation circuit 31 is added to the configuration of the LED driving device of the first embodiment shown in FIG. The difference voltage calculation circuit 31 calculates a drain-source voltage or a collector-emitter voltage of the output drive element 10, and outputs the calculated difference voltage to the voltage comparison circuit 12. In FIG. 4, the same elements as those in FIG. 1 are denoted by the same reference numerals.

複数個のＬＥＤ１を直列に接続し、そのアノード側には、バッテリー等のＤＣ電源ＶＩＮから電力効率の良い電圧を供給するための降圧式又は昇圧式、あるいは昇降圧式のスィッチング又はチョッパ方式などのＤＣ／ＤＣコンバータ８を平滑回路９を介して接続している。また、ＬＥＤ１のカソード側はトランジスタやＦＥＴなどの出力駆動素子１０を介して電流検出用の抵抗ＲＣＳを接続し、この抵抗ＲＣＳの他端を接地（ＧＮＤ）している。 A plurality of LEDs 1 are connected in series, and on the anode side, a step-down or step-up or step-up / step-down switching or chopper method for supplying a power-efficient voltage from a DC power source VIN such as a battery is used. A DC / DC converter 8 is connected via a smoothing circuit 9. The cathode side of LED1 is connected to resistor R CS for current detection through the output drive element 10 such as transistors and FET, grounded and the other end of the resistor R CS (GND).

本実施の形態のＬＥＤ駆動装置は、ＬＥＤ１に電力効率の良い最適値の電圧を供給するための供給電圧制御用閉ループＣＬ１を構成する回路群と、ＬＥＤ１に一定の電流を流すための定電流制御用の閉ループＣＬ２を構成する回路群とを完全に別に存在する構成にしている。   The LED driving apparatus of the present embodiment includes a circuit group constituting a supply voltage control closed loop CL1 for supplying a power-efficient optimum voltage to the LED 1, and a constant current control for causing a constant current to flow through the LED 1. The circuit group constituting the closed loop CL2 is completely separate.

先ず、ＬＥＤ１に所望の一定の電流を流すための定電流制御用の閉ループＣＬ２の動作は、ＬＥＤ１に流れる電流を、検出抵抗ＲＣＳを使って電圧情報ＩＬＥＤ×ＲＣＳに変換し、その電圧値と所望の電流値に対応した基準電圧値とを比較し、その誤差に応じて出力駆動素子１０を負帰還ループ制御する。他方、ＬＥＤ群に電力効率の良い必要最低な最適電圧を供給するため、ＬＥＤ１に流れる電流を制御する出力駆動素子が定電流動作及び応答性に追従できなくなる飽和領域で動作しないために必要な概ね最低電圧値を予め制御目標値として電圧比較回路１２の基準値として設定している。電圧比較回路１２ではこの基準電圧値と、差電圧算出回路３１で算出した電圧比較値、つまり、出力駆動素子１０のドレイン−ソース間電圧又はコレクターエミッタ間電圧と比較する。そしてこの電圧比較回路１２の出力に応じて、出力電圧制御回路１３、ＤＣ／ＤＣコンバータ８、平滑回路９により出力駆動素子１０が飽和しないような電圧になるように供給電圧制御用の負帰還閉ループＣＬ１を動作させる。 First, the operation of the closed loop CL2 for constant current control for causing a desired constant current to flow through the LED 1 is to convert the current flowing through the LED 1 into voltage information I LED × R CS using the detection resistor R CS , and the voltage The value is compared with a reference voltage value corresponding to a desired current value, and the output drive element 10 is subjected to negative feedback loop control according to the error. On the other hand, in order to supply the LED group with a power-efficient minimum necessary optimum voltage, the output drive element that controls the current flowing through the LED 1 does not operate in a saturation region where it cannot follow constant current operation and responsiveness. The minimum voltage value is set in advance as a reference value for the voltage comparison circuit 12 as a control target value. The voltage comparison circuit 12 compares this reference voltage value with the voltage comparison value calculated by the differential voltage calculation circuit 31, that is, the drain-source voltage or the collector-emitter voltage of the output drive element 10. Then, according to the output of the voltage comparison circuit 12, a negative feedback closed loop for supply voltage control so that the output drive element 10 is not saturated by the output voltage control circuit 13, the DC / DC converter 8, and the smoothing circuit 9. Operate CL1.

図５は本発明の第２の実施の形態のＬＥＤ駆動装置の具体回路図である。この第２の実施の形態の回路構成は、図２に示した第１の実施の形態の回路構成に対して、負側増幅器２２、正側増幅器２３を追加的に付加した構成を特徴としている。尚、図５において、図２と同一の要素には同一の符号を用いて表している。   FIG. 5 is a specific circuit diagram of the LED driving apparatus according to the second embodiment of the present invention. The circuit configuration of the second embodiment is characterized in that a negative side amplifier 22 and a positive side amplifier 23 are additionally added to the circuit configuration of the first embodiment shown in FIG. . In FIG. 5, the same elements as those in FIG. 2 are denoted by the same reference numerals.

図５について図４と照合しながら説明する。図４中のＤＣ／ＤＣコンバータ８に相当するのが、図５中のＰチャンネルのＭＯＳ−ＦＥＴ１４とダイオード１５であり、出力電圧制御回路１３がコンパレータ２１、電圧比較回路１２が電圧比較増幅器２０、平滑回路９がインダクタ１６とコンデンサ１７である。本実施の形態の回路は、第１の実施の形態と同様に降圧型のスイッチング電源方式のＤＣ／ＤＣコンバータ構成になっている。さらに、図４中の出力駆動素子１０に相当するのがトランジスタ１８であり、定電流制御回路１１に相当するのが、トランジスタ１８を負荷にした負帰還増幅回路１９である。   5 will be described with reference to FIG. 4 corresponds to the P-channel MOS-FET 14 and the diode 15 in FIG. 5, the output voltage control circuit 13 is a comparator 21, the voltage comparison circuit 12 is a voltage comparison amplifier 20, The smoothing circuit 9 is an inductor 16 and a capacitor 17. The circuit of this embodiment has a step-down switching power supply type DC / DC converter configuration as in the first embodiment. Further, the transistor 18 corresponds to the output drive element 10 in FIG. 4, and the negative feedback amplifier circuit 19 using the transistor 18 as a load corresponds to the constant current control circuit 11.

まず、定電流制御用閉ループＣＬ２では、ＬＥＤ１に流れる電流ＩＬＥＤを電流検出用抵抗ＲＣＳによって電流−電圧変換し、その電圧値ＶＥ（＝ＩＬＥＤ×ＲＣＳ）を定電流用の負帰還増幅器１９の比較情報とし、目標とするＬＥＤ１の電流に対応した基準電圧値ＶＲｅｆ１（＝Ｉ０×Ｒ１）と一致するようにトランジスタ１８のベース電流を制御し、負帰還増幅回路動作させる。これにより、トランジスタ１８は目的の定電流をＬＥＤ１から引っ張ろうと動作する。 First, in the closed loop CL2 for constant current control, the current I LED flowing through the LED 1 is subjected to current-voltage conversion by the current detection resistor R CS , and the voltage value V E (= I LED × R CS ) is negative feedback for constant current. As the comparison information of the amplifier 19, the base current of the transistor 18 is controlled so as to coincide with the reference voltage value V Ref1 (= I 0 × R 1 ) corresponding to the target current of the LED 1 , and the negative feedback amplifier circuit is operated. As a result, the transistor 18 operates to pull a target constant current from the LED 1.

他方、供給電圧制御用閉ループＣＬ１では、ＬＥＤ１に電力効率の良い必要最低な最適電圧ＶＳＷ１を供給するため、ＬＥＤ１に流れる電流を制御するトランジスタ１８が定電流動作及び応答性に追従できなくなる飽和領域で動作しないために必要な最低電圧以上の値を予め制御目標値ＶＲｅｆ２（Ｉ１×Ｒ２）として電圧比較増幅器２０に与える。そして、出力駆動素子１０のドレイン−ソース間の差電圧又はコレクターエミッタ間の差電圧を増幅器２２、増幅器２３、抵抗Ｒ３１、ミラー回路２５、抵抗Ｒ３２で構成されている差電圧算出回路３１で算出し、その差電圧を比較値ＶＣＯＭＰとして電圧比較増幅器２０に対して出力する。第１の実施の形態と同様に、電圧比較回路１２である電圧比較増幅器２０、出力電圧制御回路１３であるコンパレータ２１は、この比較値ＶＣＯＭＰが目標値ＶＲｅｆ２と一致するように供給電圧制御用の負帰還閉ループＣＬ１を動作させる。尚、この供給電圧制御用の電圧増幅器２０の基準値ＶＲｅｆ２（Ｉ１×Ｒ２）は一定値に限定せず、２種類以上の設定値からＬＥＤ１に流す電流や調光率に応じて選択できるようにしてもよい。 On the other hand, in the closed loop CL1 for supply voltage control, in order to supply the LED 1 with the lowest necessary optimum voltage V SW1 with good power efficiency, the transistor 18 that controls the current flowing through the LED 1 cannot follow constant current operation and responsiveness. A value equal to or higher than the minimum voltage necessary for not operating at is supplied to the voltage comparison amplifier 20 as a control target value V Ref2 (I 1 × R 2 ) in advance. Then, the difference voltage between the drain and the source of the output drive element 10 or the difference voltage between the collector and the emitter is calculated by the difference voltage calculation circuit 31 including the amplifier 22, the amplifier 23, the resistor R31, the mirror circuit 25, and the resistor R32. The difference voltage is output as a comparison value V COMP to the voltage comparison amplifier 20. Similar to the first embodiment, the voltage comparison amplifier 20 that is the voltage comparison circuit 12 and the comparator 21 that is the output voltage control circuit 13 control the supply voltage so that the comparison value V COMP matches the target value V Ref2. The negative feedback closed loop CL1 is operated. Note that the reference value V Ref2 (I 1 × R 2 ) of the voltage amplifier 20 for controlling the supply voltage is not limited to a constant value, but is selected from two or more set values according to the current flowing through the LED 1 and the dimming rate. You may be able to do it.

図６は出力駆動素子１０であるトランジスタ１８のコレクタ電圧ＶＣ、エミッタ電圧ＶＥのオシロ波形図、図７は直流電源電圧ＶＩＮ、ＬＥＤ駆動電圧ＶＳＷ、そしてＬＥＤ電流ＩＬＥＤのオシロ波形図である。入力電圧ＶＩＮに対してＬＥＤ１に所望の定電流が流れるのに必要な電力変換効率の良い電圧値ＶＳＷが供給され、しかも線形で安定な定電流ＩＬＥＤとなっていることがわかる。ここで、電力変換効率の良い電圧ＶＳＷとは、ＬＥＤ１が周囲の温度条件下で所望の電流を流すのに必要な各ＶＦを足した電圧とトランジスタ１８がサチレーションしない電圧と検出抵抗ＲＣＳの電圧を総和した電圧値である。 6 is an oscilloscope waveform diagram of the collector voltage V C and the emitter voltage V E of the transistor 18 as the output drive element 10, and FIG. 7 is an oscilloscope waveform diagram of the DC power supply voltage V IN , the LED drive voltage V SW , and the LED current I LED . It is. It can be seen that the voltage value V SW with good power conversion efficiency necessary for a desired constant current to flow through the LED 1 with respect to the input voltage VIN is supplied, and that the linear and stable constant current I LED is obtained. Here, a good voltage V SW power conversion efficiency, detection and voltage voltage and transistor 18 LED1 is plus each V F required to pass a desired current at a temperature of ambient not Sachireshon resistor R CS Is the sum of the voltages of

本実施の形態によれば、供給電圧制御用の負帰還閉ループＣＬ１の周波数応答が定電流制御用の負帰還閉ループＣＬ２の周波数応答の１／２０以下に設定することで、定電流制御の応答性を損なわずに供給電圧制御用の負帰還閉ループＣＬ１を安定に動作させることができる。すなわち、各負帰還閉ループＣＬ１，ＣＬ２の周波数の応特性が２０倍以上も違うことからお互いが干渉して不安定になることがないので、定電流制御用の負帰還閉ループＣＬ２の利得及び周波数応答を高く設定でき、これによって非常に精度の良い定電流特性が達成できる。そのため、電圧基準を調光率に応じて可変とすることで、頻繁に所望の定電流値が変わるような分野の商品や極めて低い調光性能が必要な分野の商品にも十分対応可能であり、特に車載用のナビゲーションの液晶バックライトに必要な５％以下の極めて低い調光率域での線形な電流特性を達成できる。   According to the present embodiment, the frequency response of the negative feedback closed loop CL1 for supply voltage control is set to 1/20 or less of the frequency response of the negative feedback closed loop CL2 for constant current control, so that the response of constant current control is achieved. The negative feedback closed loop CL1 for supply voltage control can be stably operated without impairing the above. That is, since the frequency response characteristics of the negative feedback closed loops CL1 and CL2 are different by 20 times or more, they do not become unstable due to interference with each other. Therefore, the gain and frequency response of the negative feedback closed loop CL2 for constant current control Can be set high, thereby achieving a highly accurate constant current characteristic. Therefore, by making the voltage reference variable according to the dimming rate, it is possible to sufficiently handle products in fields where the desired constant current value frequently changes and products that require extremely low dimming performance. In particular, it is possible to achieve a linear current characteristic in an extremely low dimming rate range of 5% or less, which is necessary for a liquid crystal backlight for a vehicle-mounted navigation.

（第３の実施の形態）図８は本発明の第３の実施の形態のＬＥＤ駆動装置のブロック図である。本実施の形態は、図４に示した第２の実施の形態に対して、制御基準値作成回路３２と、調光率判定回路３３を付加した構成を特徴としている。尚、図８において、図１、図４と同一の要素には同一の符号を用いて表している。   (Third Embodiment) FIG. 8 is a block diagram of an LED driving apparatus according to a third embodiment of the present invention. The present embodiment is characterized in that a control reference value creating circuit 32 and a dimming rate determining circuit 33 are added to the second embodiment shown in FIG. In FIG. 8, the same elements as those in FIGS. 1 and 4 are denoted by the same reference numerals.

複数個のＬＥＤ１を直列に接続し、そのアノード側に、バッテリー等のＤＣ電源ＶＩＮから電力効率の良い電圧を供給するための降圧式又は昇圧式、あるいは昇降圧式のスイッチング又はチョッパ方式などのＤＣ／ＤＣコンバータ８を、平滑回路９を介して接続している。また、ＬＥＤ１のカソード側にはトランジスタやＦＥＴなどの出力駆動素子１０、電流検出用の抵抗ＲＣＳを順に接続し、この抵抗ＲＣＳの他端は接地させている。 A plurality of LEDs 1 are connected in series, and a DC of a step-down or step-up or step-up / step-down switching or chopper method for supplying a power-efficient voltage from a DC power source VIN such as a battery to the anode side thereof. / DC converter 8 is connected via smoothing circuit 9. Further, the cathode side of the LED1 connected output drive element 10 such as transistors and FET, a resistor R CS for current detection in this order, the other end of the resistor R CS is then grounded.

本実施の形態でも、ＬＥＤ１に電力効率の良い最適値の電圧を供給するための供給電圧制御用閉ループＣＬ１を構成する回路群と、ＬＥＤ１に一定の電流を流すための定電流制御用の閉ループＣＬ２を構成する回路群とが完全に別に存在する構成にしている。先ず、ＬＥＤ１に所望の一定の電流を流すための定電流制御用の閉ループＣＬ２の動作は、ＬＥＤ１に流れる電流を検出抵抗ＲＣＳを使って電圧情報に変換し、その電圧値と所望の電流値に対応した基準電圧値とを比較しその誤差に応じて出力駆動素子１０を負帰還ループ制御している。 Also in the present embodiment, a circuit group constituting a supply voltage control closed loop CL1 for supplying the LED 1 with a power efficient optimum value voltage, and a constant current control closed loop CL2 for supplying a constant current to the LED 1 The circuit group that constitutes is configured to exist completely separately. First, the operation of the closed loop CL2 for constant current control for supplying the desired constant current to LED1 is converted into voltage information by using a detection resistor R CS the current flowing through the LED1, the voltage value and the desired current value that And the output drive element 10 is subjected to negative feedback loop control according to the error.

他方、供給電圧制御用の負帰還閉ループＣＬ１の動作は次の通りである。ＬＥＤ群に電力効率の良い必要最低な最適電圧を供給するため、ＬＥＤ１に流れる電流を制御する出力駆動素子１０が定電流動作し、かつ応答性に追従できなくなる飽和領域で動作しないようにするために必要な最低電圧値を予め制御目標値とする。この制御目標値は、制御基準値作成回路３２にて作成する。この制御基準値作成回路３２は調光率判定回路３３からの調光率判定値に応じた制御基準値を作成し、電圧比較回路１２に対して基準値として出力する。電圧比較回路１２は、出力駆動素子１０のドレイン−ソース間電圧又はコレクタ−エミッタ間電圧を差電圧算出回路３１で算出した比較値と基準値とを比較し、比較結果を出力電圧制御回路１３に出力する。出力電圧制御回路１３は従来例と同様に動作し、ＤＣ／ＤＣコンンバータ８の出力電圧を制御し、平滑回路９よりＬＥＤ１を介して出力駆動素子１０に流れる直流が出力駆動素子１０を飽和させない電圧になるように制御する。   On the other hand, the operation of the negative feedback closed loop CL1 for supply voltage control is as follows. In order to supply the LED group with a power-efficient minimum and optimum voltage, the output drive element 10 that controls the current flowing in the LED 1 operates at a constant current and does not operate in a saturation region where the response cannot be followed. The minimum voltage value necessary for the above is set as a control target value in advance. This control target value is created by the control reference value creation circuit 32. The control reference value creation circuit 32 creates a control reference value corresponding to the dimming rate determination value from the dimming rate determination circuit 33 and outputs it to the voltage comparison circuit 12 as a reference value. The voltage comparison circuit 12 compares the comparison value calculated by the difference voltage calculation circuit 31 with the drain-source voltage or the collector-emitter voltage of the output drive element 10 and the reference value, and the comparison result is output to the output voltage control circuit 13. Output. The output voltage control circuit 13 operates in the same manner as in the conventional example, controls the output voltage of the DC / DC converter 8, and the direct current flowing from the smoothing circuit 9 to the output drive element 10 via the LED 1 does not saturate the output drive element 10. Control to become.

図９は本実施の形態のＬＥＤ駆動回路の具体回路図である。図９において、２４は制御基準値作成回路である。尚、図９において、図２、図５と同一の要素には同一の符号を用いて表している。   FIG. 9 is a specific circuit diagram of the LED drive circuit of the present embodiment. In FIG. 9, reference numeral 24 denotes a control reference value creation circuit. In FIG. 9, the same elements as those in FIGS. 2 and 5 are denoted by the same reference numerals.

以下、図９を図８と照合しながら説明する。図８中のＤＣ／ＤＣコンバータ８に相当するのが、図９中のＰチャンネルのＭＯＳ−ＦＥＴ１４とダイオード１５であり、出力電圧制御回路１３がコンパレータ２１、電圧比較回路１２が電圧比較増幅器２０、平滑回路９がインダクタ１６とコンデンサ１７である。本実施の形態の回路は降圧型のスイッチング電源方式のＤＣ／ＤＣコンバータ構成になっている。また、図８中の出力駆動素子１０に相当するのが、トランジスタ１８であり、定電流制御回路１１に相当するのがトランジスタ１８を負荷にした負帰還増幅回路１９である。図９中の増幅器２２、増幅器２３、抵抗Ｒ３１、ミラー回路２５、抵抗Ｒ３２は、図８中の差電圧算出回路３１に相当する。制御基準値作成回路２４は、図８における制御基準値作成回路３２に対応している。この制御基準値作成回路２４は、定電流Ｉ１と可変電流Ｉ２との和電流Ｉ１＋Ｉ２を抵抗Ｒ２に流すことで、電圧基準値ＶＲｅｆ２として可変電圧値を電圧比較増幅器２０に与える。 Hereinafter, FIG. 9 will be described with reference to FIG. 8 corresponds to the P-channel MOS-FET 14 and the diode 15 in FIG. 9. The output voltage control circuit 13 is a comparator 21, the voltage comparison circuit 12 is a voltage comparison amplifier 20, and the like. The smoothing circuit 9 is an inductor 16 and a capacitor 17. The circuit of this embodiment has a DC / DC converter configuration of a step-down switching power supply system. Further, the transistor 18 corresponds to the output drive element 10 in FIG. 8, and the negative feedback amplifier circuit 19 using the transistor 18 as a load corresponds to the constant current control circuit 11. The amplifier 22, the amplifier 23, the resistor R31, the mirror circuit 25, and the resistor R32 in FIG. 9 correspond to the differential voltage calculation circuit 31 in FIG. The control reference value creation circuit 24 corresponds to the control reference value creation circuit 32 in FIG. The control reference value creation circuit 24 supplies the variable voltage value to the voltage comparison amplifier 20 as the voltage reference value V Ref2 by flowing the sum current I 1 + I 2 of the constant current I 1 and the variable current I 2 through the resistor R2. .

先ず、定電流制御用の負帰還閉ループＣＬ２では、ＬＥＤ１に流れる電流ＩＬＥＤを電流検出用抵抗ＲＣＳによって電流−電圧変換した電圧値ＶＥ（＝ＩＬＥＤ×ＲＣＳ）を定電流用の負帰還増幅器１９の比較情報として負帰還させる。そして負帰還増幅器１９は、この比較電圧ＶＥが目標とするＬＥＤ１の電流に対応した基準電圧値ＶＲｅｆ１（＝Ｉ０×ＶＲ１）と一致するようにトランジスタ１８のベース電流を制御し、トランジスタ１８に目的の定電流をＬＥＤ１から引っ張らせる。 First, in the negative feedback closed loop CL2 for constant current control, the voltage value V E (= I LED × R CS ) obtained by current-voltage conversion of the current I LED flowing through the LED 1 by the current detection resistor R CS is negative for constant current. Negative feedback is provided as comparison information for the feedback amplifier 19. The negative feedback amplifier 19 controls the base current of the transistor 18 so that the comparison voltage V E matches the reference voltage value V Ref1 (= I 0 × V R1 ) corresponding to the target current of the LED 1. 18 makes the target constant current be pulled from the LED 1.

他方、供給電圧制御用の負帰還閉ループＣＬ１では、ＬＥＤ１に電力効率の良い必要最低な最適電圧ＶＳＷ１を供給するため、ＬＥＤ１に流れる電流ＩＬＥＤを制御するトランジスタ１８が定電流動作し、かつ、応答性に追従できなくなる飽和領域で動作しないために必要な最低電圧以上の値を予め制御基準値ＶＲｅｆ２（＝Ｉ１×Ｒ２）として設定し、トランジスタ１８のドレイン−ソース間電圧又はコレクタ−エミッタ間の差電圧ＶＣＯＭＰを増幅器２２、増幅器２３、Ｒ３１、ミラー回路２５、抵抗Ｒ３２で構成された差電圧算出回路３１で算出し、電圧比較増幅器２０でこの差電圧ＶＣＯＭＰと制御基準値ＶＲｅｆ２とを比較させる。第１の実施の形態と同様に、電圧比較回路１２である電圧比較増幅器２０、出力電圧制御回路１３であるコンパレータ２１は、この比較値ＶＣＯＭＰが制御基準値ＶＲｅｆ２と一致するように供給電圧制御用の負帰還閉ループＣＬ１を動作させる。 On the other hand, in the negative feedback closed loop CL1 for supply voltage control, the transistor 18 for controlling the current I LED flowing in the LED 1 operates at a constant current in order to supply the LED 1 with the lowest necessary optimum voltage V SW1 with good power efficiency, and A value equal to or higher than the minimum voltage necessary for not operating in the saturation region where the response cannot be followed is set in advance as a control reference value V Ref2 (= I 1 × R 2 ), and the drain-source voltage of the transistor 18 or the collector- A difference voltage V COMP between the emitters is calculated by a difference voltage calculation circuit 31 including an amplifier 22, amplifiers 23, R31, a mirror circuit 25, and a resistor R32. The voltage comparison amplifier 20 calculates the difference voltage V COMP and a control reference value V. Ref2 is compared. As in the first embodiment, the voltage comparison amplifier 20 that is the voltage comparison circuit 12 and the comparator 21 that is the output voltage control circuit 13 are supplied with the supply voltage so that the comparison value V COMP matches the control reference value V Ref2. The negative feedback closed loop CL1 for control is operated.

また、本実施の形態の場合、供給電圧制御用の電圧比較増幅器２０の基準値ＶＲｅｆ２は一定値に限定せず、２種類以上の設定値からＬＥＤ１に流す電流や調光率に応じて可変設定する。そのために、制御基準値作成回路２４が（Ｉ１＋Ｉ２）×Ｒ２により基準値ＶＲｅｆ２を作成するが、Ｉ１は一定値、Ｉ２はＬＥＤ１に流す電流値に比例した可変値とし、ＬＥＤ１の設定電流値が増えるとＩ２も増え、逆にＬＥＤ１の設定電流値が減るとＩ２も減るようにしている。 In the case of the present embodiment, the reference value V Ref2 of the voltage comparison amplifier 20 for controlling the supply voltage is not limited to a constant value, but can be varied from two or more kinds of setting values according to the current flowing through the LED 1 and the dimming rate. Set. For this purpose, the control reference value creation circuit 24 creates the reference value V Ref2 by (I 1 + I 2 ) × R 2 , where I 1 is a constant value, I 2 is a variable value proportional to the current value flowing through the LED 1 , LED1 set current value is increased when the I 2 also increases, so that I 2 is also reduced when LED1 set current value conversely decreases.

図１０は、調光率に応じて基準値ＶＲｅｆ２を変化させる制御基準値作成回路２４の具体的な回路例を示しており、図１１はこの制御基準値作成回路２４が作成する基準値ＶＲｅｆ２の設定の概念を示す説明図である。すなわち、外部からの調光信号ＤＩＭ１を調光率判定回路３３で判定し、調光率が比較的高いとき、例えば１００％のときにはＳＷ１のみがオンする信号をこの制御基準値作成回路２４に入力し、調光率が比較的低いとき、例えば５０％のときにはＳＷ１、ＳＷ２の両方がオンする信号を入力する。このように、調光率に応じてＳＷ１、ＳＷ２をオン、オフさせることで基準電圧値の設定がきめ細かく行える。 FIG. 10 shows a specific circuit example of the control reference value creation circuit 24 that changes the reference value V Ref2 according to the dimming rate. FIG. 11 shows the reference value V created by the control reference value creation circuit 24. It is explanatory drawing which shows the concept of the setting of Ref2 . That is, the dimming signal DIM1 from the outside is determined by the dimming rate determination circuit 33, and when the dimming rate is relatively high, for example, 100%, a signal that turns on only SW1 is input to the control reference value generation circuit 24. When the dimming rate is relatively low, for example 50%, a signal for turning on both SW1 and SW2 is input. Thus, the reference voltage value can be set finely by turning on and off SW1 and SW2 in accordance with the dimming rate.

図１０、図１１では、出力駆動素子１０の応答速度が端子電圧に依存することから、調光率が低い場合は出力駆動素子１０の端子電圧（ＦＥＴの場合はドレイン−ソース間の端子電圧、トランジスタの場合はコレクタ−エミッタ間の電圧）の設定値を、調光率が高い場合よりも高くする必要があることを示している。一般に、調光率判定をしない場合は、調光率が極めて低い時（例えば０．５％）を想定して、高い端子電圧の設定が必要となる。   10 and 11, since the response speed of the output drive element 10 depends on the terminal voltage, the terminal voltage of the output drive element 10 (the terminal voltage between the drain and source in the case of FET, In the case of a transistor, it is indicated that the set value of the collector-emitter voltage needs to be higher than when the dimming rate is high. In general, when the dimming rate is not determined, it is necessary to set a high terminal voltage assuming that the dimming rate is extremely low (for example, 0.5%).

本実施の形態によれば、供給電圧制御用の負帰還閉ループＣＬ１の周波数応答特性を定電流制御用の負帰還閉ループＣＬ２の周波数応答特性の１／２０以下に設定することで、定電流制御の応答性を損なわずに供給電圧制御用の負帰還閉ループＣＬ１を安定に動作させることができる。すなわち、各負帰還閉ループＣＬ１，ＣＬ２の周波数の応答特性が２０倍以上も違うことからお互いが干渉して不安定になることがないので、定電流制御用の負帰還閉ループＣＬ２の利得及び周波数応答を高く設定でき、非常に精度の良い定電流特性が達成できる。そのため、頻繁に所望の定電流値が変わるような分野の商品や極めて低い調光性能が必要な分野の商品にも十分対応可能であり、特に車載用のナビゲーションの液晶バックライトに必要な５％以下の極めて低い調光率域での線形な電流特性を達成できる。   According to the present embodiment, the frequency response characteristic of the negative feedback closed loop CL1 for supply voltage control is set to 1/20 or less of the frequency response characteristic of the negative feedback closed loop CL2 for constant current control. It is possible to stably operate the negative feedback closed loop CL1 for controlling the supply voltage without impairing the responsiveness. That is, since the frequency response characteristics of the negative feedback closed loops CL1 and CL2 are different by 20 times or more, they do not interfere with each other and become unstable. Therefore, the gain and frequency response of the negative feedback closed loop CL2 for constant current control Can be set high, and a highly accurate constant current characteristic can be achieved. Therefore, it is fully compatible with products in fields where the desired constant current value frequently changes and products in fields where extremely low dimming performance is required, especially 5% required for in-vehicle navigation LCD backlights. Linear current characteristics in the following extremely low dimming ratio regions can be achieved.

図１５は本実施の形態のＬＥＤ駆動装置による調光率とＬＥＤ電流との特性Ａを、従来例の特性Ｂ、また携帯電話用バックライトの特性Ｃと比較して示したグラフであり、本実施の形態のＬＥＤ駆動装置では、５％以下の極めて低い調光率域での線形な電流特性が得られていることがわかる。   FIG. 15 is a graph showing the characteristics A between the dimming rate and the LED current by the LED driving device of the present embodiment in comparison with the characteristics B of the conventional example and the characteristics C of the backlight for the mobile phone. In the LED driving device of the embodiment, it can be seen that a linear current characteristic in an extremely low dimming rate region of 5% or less is obtained.

本発明の第１の実施の形態のＬＥＤ駆動回路のブロック図。The block diagram of the LED drive circuit of the 1st Embodiment of this invention. 本発明の第１の実施の形態のＬＥＤ駆動回路の回路図。The circuit diagram of the LED drive circuit of the 1st Embodiment of this invention. 本発明の第１の実施の形態のＬＥＤ駆動回路の電圧のオシロ波形図。The oscilloscope waveform figure of the voltage of the LED drive circuit of the 1st Embodiment of this invention. 本発明の第２の実施の形態のＬＥＤ駆動回路のブロック図。The block diagram of the LED drive circuit of the 2nd Embodiment of this invention. 本発明の第２の実施の形態のＬＥＤ駆動回路の回路図。The circuit diagram of the LED drive circuit of the 2nd Embodiment of this invention. 本発明の第２の実施の形態のＬＥＤ駆動回路の電圧のオシロ波形図（その１）。FIG. 6 is an oscilloscope waveform diagram of voltage of an LED drive circuit according to a second embodiment of the present invention (part 1). 本発明の第２の実施の形態のＬＥＤ駆動回路の電圧のオシロ波形図（その２）。FIG. 7 is an oscilloscope waveform diagram of voltage of the LED drive circuit according to the second embodiment of the present invention (part 2). 本発明の第３の実施の形態のＬＥＤ駆動回路のブロック図。The block diagram of the LED drive circuit of the 3rd Embodiment of this invention. 本発明の第３の実施の形態のＬＥＤ駆動回路の回路図。The circuit diagram of the LED drive circuit of the 3rd Embodiment of this invention. 本発明の第３の実施の形態の制御基準値作成回路の一例を示す回路図。The circuit diagram which shows an example of the control reference value preparation circuit of the 3rd Embodiment of this invention. 本発明の第３の実施の形態のＶＲＥＦ２の設定の概念を示す説明図。Explanatory drawing which shows the concept of the setting of VREF2 of the 3rd Embodiment of this invention. ＬＥＤを用いた一般的なバックライト構造示す図。The figure which shows the general backlight structure using LED. 従来のＬＥＤ駆動回路のブロック図。The block diagram of the conventional LED drive circuit. 従来のＬＥＤ駆動回路の回路図。The circuit diagram of the conventional LED drive circuit. 本発明の第３の実施の形態と従来例との調光率−電流間の特性を示すグラフ。The graph which shows the characteristic between the light control rate-electric current of the 3rd Embodiment of this invention and a prior art example.

符号の説明Explanation of symbols

１ ＬＥＤ（発光ダイオード）
８ ＤＣ／ＤＣコンバータ
９ 平滑回路
１０ 出力駆動素子
１１ 定電流制御回路
１２ 電圧比較回路
１３ 出力電圧制御回路
１４ スイッチング素子
１５ ダイオード
１６ インダクタ
１７ 平滑コンデンサ
１８ トランジスタ
１９ 負帰還増幅器
２０ 電圧比較増幅器
２１ 比較器
２２ 負側増幅器
２３ 正側増幅器
２４ 制御基準値作成回路
２５ ミラー回路
３１ 差電圧算出回路
３２ 制御基準値作成回路
３３ 調光率判定回路 1 LED (light emitting diode)
8 DC / DC converter 9 Smoothing circuit 10 Output drive element 11 Constant current control circuit 12 Voltage comparison circuit 13 Output voltage control circuit 14 Switching element 15 Diode 16 Inductor 17 Smoothing capacitor 18 Transistor 19 Negative feedback amplifier 20 Voltage comparison amplifier 21 Comparator 22 Negative side amplifier 23 Positive side amplifier 24 Control reference value creation circuit 25 Mirror circuit 31 Difference voltage calculation circuit 32 Control reference value creation circuit 33 Dimming rate determination circuit

Claims (3)

直列接続された複数の発光ダイオードと、
前記直列接続された発光ダイオードのアノード側に直流電源から電力効率の良い電圧値を供給するＤＣ／ＤＣコンバータと、
前記直列接続された発光ダイオードのカソード側に一端が接続された出力駆動素子と、
前記出力駆動素子の他端に、その一端が接続され、その他端が接地された電流検出用の抵抗と、
前記複数の直列接続された発光ダイオードに効率の良い最適値の電圧を供給するために、前記ＤＣ／ＤＣコンバータを制御する出力電圧制御回路と、
前記出力電圧制御回路に出力電圧指令を与えるために、前記電流検出用の抵抗に流れる電流を変換した電圧を第１の比較電圧とし、所望の基準電圧と比較して差電圧を前記出力電圧制御回路に出力する電圧比較回路と、
前記電流検出用の抵抗に流れる電流を変換した第２の比較電圧を定電流制御のための所定電圧と比較し、前記出力駆動素子をその通電電流が定電流になるように制御する定電流制御回路とを備え、
前記ＤＣ／ＤＣコンバータ、出力駆動素子、電圧比較回路及び出力電圧制御回路にて構成される閉ループを供給電圧制御用の第１の負帰還閉ループとし、前記出力駆動素子及び定電流制御回路にて構成される閉ループを定電流制御用の第２の負帰還閉ループとしたことを特徴とするＬＥＤ駆動装置。 A plurality of light emitting diodes connected in series;
A DC / DC converter that supplies a power-efficient voltage value from a direct current power source to the anode side of the light emitting diodes connected in series;
An output driving element having one end connected to the cathode side of the light emitting diodes connected in series;
A current detection resistor having one end connected to the other end of the output drive element and the other end grounded;
An output voltage control circuit for controlling the DC / DC converter in order to supply an efficient optimum voltage to the plurality of light emitting diodes connected in series;
In order to give an output voltage command to the output voltage control circuit, a voltage obtained by converting a current flowing through the current detection resistor is set as a first comparison voltage, and a difference voltage is compared with a desired reference voltage. A voltage comparison circuit that outputs to the circuit;
Constant current control for comparing the second comparison voltage obtained by converting the current flowing through the current detection resistor with a predetermined voltage for constant current control and controlling the output drive element so that the energization current becomes a constant current. With circuit,
A closed loop constituted by the DC / DC converter, output drive element, voltage comparison circuit and output voltage control circuit is defined as a first negative feedback closed loop for supply voltage control, and is constituted by the output drive element and constant current control circuit. The LED driving device is characterized in that the closed loop is a second negative feedback closed loop for constant current control. 直列接続された複数の発光ダイオードと、
前記直列接続された発光ダイオードのアノード側に直流電源から電力効率の良い電圧値を供給するＤＣ／ＤＣコンバータと、
前記直列接続された発光ダイオードのカソード側に一端が接続された出力駆動素子と、
前記出力駆動素子の他端に、その一端が接続され、その他端が接地された電流検出用の抵抗と、
前記複数の直列接続された発光ダイオードに効率の良い最適値の電圧を供給するために、前記ＤＣ／ＤＣコンバータを制御する出力電圧制御回路と、
前記出力駆動素子の両端の差電圧を算出する差電圧算出回路と、
前記出力電圧制御回路に出力電圧指令を与えるために、前記差電圧算出回路の算出した差電圧を第１の比較電圧とし、所望の基準電圧と比較して電圧差を前記出力電圧制御回路に出力する電圧比較回路と、
前記電流検出用の抵抗に流れる電流を変換した第２の比較電圧を定電流制御のための所定電圧と比較し、前記出力駆動素子をその通電電流が定電流になるように制御する定電流制御回路とを備え、
前記ＤＣ／ＤＣコンバータ、出力駆動素子、差電圧算出回路、電圧比較回路及び出力電圧制御回路にて構成される閉ループを供給電圧制御用の第１の負帰還閉ループとし、前記出力駆動素子及び定電流制御回路にて構成される閉ループを定電流制御用の第２の負帰還閉ループとしたことを特徴とするＬＥＤ駆動装置。 A plurality of light emitting diodes connected in series;
A DC / DC converter that supplies a power-efficient voltage value from a direct current power source to the anode side of the light emitting diodes connected in series;
An output driving element having one end connected to the cathode side of the light emitting diodes connected in series;
A current detection resistor having one end connected to the other end of the output drive element and the other end grounded;
An output voltage control circuit for controlling the DC / DC converter in order to supply an efficient optimum voltage to the plurality of light emitting diodes connected in series;
A differential voltage calculation circuit for calculating a differential voltage between both ends of the output drive element;
In order to give an output voltage command to the output voltage control circuit, the difference voltage calculated by the difference voltage calculation circuit is used as a first comparison voltage, and a voltage difference is output to the output voltage control circuit by comparison with a desired reference voltage. Voltage comparison circuit to
Constant current control for comparing the second comparison voltage obtained by converting the current flowing through the current detection resistor with a predetermined voltage for constant current control and controlling the output drive element so that the energization current becomes a constant current. With circuit,
A closed loop composed of the DC / DC converter, the output drive element, the differential voltage calculation circuit, the voltage comparison circuit, and the output voltage control circuit is defined as a first negative feedback closed loop for supply voltage control, and the output drive element and the constant current An LED driving device characterized in that a closed loop constituted by a control circuit is a second negative feedback closed loop for constant current control. 前記差電圧算出回路に対して、外部から与えられる調光率に対応して可変な基準電圧を与える制御基準作成回路を備えたことを特徴とする請求項２に記載のＬＥＤ駆動装置。

3. The LED drive device according to claim 2, further comprising a control reference creation circuit that gives a variable reference voltage to the difference voltage calculation circuit in accordance with a dimming rate given from the outside.

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