JP2011014945A - Led drive voltage supply circuit, and led device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To assure stable and normal operation of a constant current drive circuit by restraining and reducing power consumption generated in the constant current drive circuit in light emission drive of an LED.SOLUTION: The LED device includes: an LED array 12; LED driver ICs 14-14; a DC-DC converter 16; a first feedback circuit comprising voltage-dividing resistors 24, 26; and a headroom voltage-monitoring circuit including a controller 38 and a second feedback circuit 80. The second feedback circuit 80 feeds back headroom voltages HV0-HVobtained from current terminals OUT-OUTof the LED driver ICs 14-14, respectively, to the DC-DC converter 16.

Description

  The present invention relates to an LED device used for a backlight, illumination, a display, and the like, and an LED driving circuit for driving the LED to emit light.

  Nowadays, LEDs (light emitting diodes) that emit light with high luminance and those that emit various colors such as white are developed and mass-produced, and are widely used for backlights, illumination, displays, and the like.

FIG. 11 shows a circuit configuration of a conventional LCD device used for a backlight for LCD (liquid crystal display) -TV (television). As shown in the figure, this LCD device includes n × m (n and m are integers of 2 or more) LEDs [10 _(0,0) ... 10 _{(n−2,0) 1} , 10 _{(n−1 )} . _{, 0)} ] to [10 _{(0, m-1)} ... 10 _{(n-2, m-1)} , 10 _{(n-1, m-1)} ], for example, a 16-channel type. Or a plurality (N) of LED driver ICs (integrated circuits) 14 _{(0) to} 14 _(N−1) , a DC power source, for example, a DC-DC converter 16, and a controller 18.

In FIG. 11, LEDs 10 _{(0, y)} ... 10 _{(n−2, y)} , 10 _{(n−1, y)} (y = 0 to m−1) in each column are the DC-DC converters 16. The output terminal and each corresponding current terminal OUT _y of the LED driver IC 14 are electrically connected in series. For example, the LEDs 10 _{(0, 0)} ... 10 _(n−2 , ₀₎ , 10 _(n−1 , ₀₎ in the first column are the output terminal of the DC-DC converter 16 and the first LED driver IC 14 _{( 0) and} the first current terminal OUT ₀ are electrically connected in series. Further, the LEDs 10 _{(0, m−1)} ... 10 _{(n−2, m−1)} and 10 _{(n−1, m−1)} in the _m-th column are connected to the output terminal of the DC-DC converter 16 and N It is electrically connected in series with the last current terminal OUT _m−1 used in the second LED driver IC 14 _(N−1) .

This LED backlight is an area light system, and as shown in FIG. 12, the backlight region 22 is divided into m (m = i × j) blocks B ₀ , B ₁ ... B _m−1 in a matrix. divided, each corresponding row _LED10 in FIG. 11 in each block _{B y (0, y) ···} 10 (n-2, y), 10 (n-1, y) , for example as shown in FIG. 13 It is arranged two-dimensionally with a constant density distribution.

In FIG. 11, a DC-DC converter 16 is a switching power supply that operates as, for example, a chopper boost converter, and boosts a DC input voltage _VIN input at 24 volts, for example, to a constant level, for example, 50 volts DC voltage. Is output as the LED drive voltage V _LED .

The DC-DC converter 16 includes a feedback circuit including a reference voltage input terminal REF, a feedback voltage input terminal FB, and voltage dividing resistors 24 and 26 in order to perform constant voltage control on the output voltage, that is, the LED drive voltage V _LED. Have. More specifically, resistors 24 and 26 between the output terminal and the ground potential terminal of the DC-DC converter 16 are connected in series, the node N _A between the two resistors is connected to a feedback voltage input terminal FB . Assuming that the resistance values of the resistors ₂₄ and ₂₆ are R ₂₄ and R ₂₆ , respectively, a divided voltage V _A having a value obtained by multiplying the LED driving voltage V _LED by a coefficient R ₂₆ / (R ₂₄ + R ₂₆ ) is obtained at the node N _A. The divided voltage _VA is input to the feedback voltage input terminal FB as a feedback voltage. On the other hand, for example, a constant reference voltage V _REF from the controller 18 is input to the reference voltage input terminal REF. The DC-DC converter 16 operates the switching power supply so that the feedback voltage _VA from the resistance voltage dividing circuit [24, 26] becomes equal to the reference voltage _VREF .

Each LED driver IC 14 _(x) (x = 0 to N−1) has a 16-channel sink type constant current drive circuit, and the output terminal of each constant current drive circuit is connected to the current terminal OUT _y (y = 0 to m-1). The constant current drive circuit of each channel supplies a constant LED drive current I _y to the LEDs 10 _{(0, y)} ... 10 _{(n−2, y)} , 10 _{(n−1, y)} in each corresponding column. Work like so. However, in order to guarantee a stable constant current operation, a voltage exceeding a predetermined level must always be maintained as the headroom voltage HV _{y at} each current terminal OUT _y , so that this headroom voltage condition is satisfied. The output voltage of the DC-DC converter 16, that is, the LED drive voltage V _LED is set. Here, the headroom voltage HV _y at each current terminal OUT _y is represented by each corresponding LED series circuit [10 _{(0, y)} ... 10 _{(n−2, y)} , 10 _{(n−1, y)} ]. Then the total voltage drop _{V y} occurring _(0~n-1), represented by _{HV y = V LED -V y (} 0~n-1).

Each LED driver IC 14 _(x) receives data and a control signal for controlling the brightness of the LED backlight together with a required clock signal from the controller 18. In recent LCD-TVs, a local dimming method is used in which the brightness of an LED backlight is variably controlled in an area or block unit according to an image in one screen. To do this local Daimingu, serial transfer to the constant current driver grayscale data indicating the degree of intensity or brightness of each block B _y in gradation constant cycle (e.g., 120 Hz) each from the controller 18 to each corresponding fed in, so that the constant current driving circuit is variably controlled by the LED drive current I flowing time or the duty of the _y PWM (pulse width modulation) control scheme in one cycle based on the grayscale data.

In FIG. 11, between the LEDs 10 _{(0, y)} ... 10 _{(n−2, y)} , 10 _{(n−1, y)} in each column and the current terminals OUT _y of the corresponding channels. An NMOS transistor 28 is provided for protecting each constant current drive circuit from a high voltage when the LED is short-circuited. The NMOS transistor 28 is biased by a bias voltage V _K provided from a voltage dividing circuit including resistors 30 and 32, and limits the voltage of each current terminal OUT _y to a certain value (V _K + V _th ) or less. Here, V _th is the threshold voltage of the NMOS transistor 28.

In general, the forward voltage of an LED has a negative temperature characteristic. The lower the LED temperature, the larger the voltage drop that occurs in the light emitting LED, and the LED driver IC14 _(x) probably has a current drop at each current terminal OUT _y . The resulting headroom voltage HV _y is reduced. For this reason, the output voltage V _LED of the DC-DC converter 16 is set so that a headroom voltage HV _y of a certain value or more is guaranteed at each current terminal OUT _y even under the minimum operating temperature of the LCD-TV.

On the other hand, the higher the LED temperature is due to an increase in ambient temperature or the self-heating of the LED, the smaller the voltage drop that occurs in the LED in the light emitting state, and the LED driver IC 14 _(x) probably has a headroom voltage at each current terminal OUT _y . HV _y is high, which is a problem. That is, each constant current drive circuit operates so as to pass a constant LED drive current I _y , so that the power consumption of the constant current drive circuit increases as the headroom voltage HV _y increases. Further, if the power consumption (heat generation amount _{) of the} entire LED driver IC 14 _(x) exceeds the allowable loss of the IC package, the driver circuit is damaged or malfunctions and does not operate normally, resulting in a decrease in reliability.

  The present invention has been made in view of the problems of the prior art, and suppresses or reduces the power consumption generated in the constant current drive circuit during LED light emission drive, and stabilizes or normal operation of the constant current drive circuit. An object of the present invention is to provide an LED drive circuit and an LED device that guarantee the above.

  In order to achieve the above object, an LED driving circuit of the present invention is an LED driving circuit for driving one or a plurality of LEDs (light emitting diodes) electrically connected in series, A direct current power source that outputs an LED drive voltage, a constant current drive circuit connected in series with the LED to the direct current power source in order to inject a constant LED drive current into the LED, and a constant current drive circuit A headroom voltage monitoring circuit that variably controls the voltage level of the LED driving voltage by acting on the DC power supply so that the headroom voltage obtained at the current terminal is maintained near the first reference voltage; Have.

  In addition, the LED device of the present invention includes m DC (m is 2 or more) in which a DC power source that outputs a DC LED driving current and n (n is an integer of 2 or more) LEDs are electrically connected in series. An integer) LED series circuit electrically connected in parallel to the output terminal of the DC power source, and m for the DC power source to inject a constant LED drive current into the LED. M constant current drive circuits connected in series with the LED series circuits, and at least one of headroom voltages respectively obtained at current terminals of the m constant current drive circuits is a first reference voltage. A headroom voltage monitoring circuit that acts on the DC power supply and dynamically variably controls the voltage level of the LED drive voltage so as to be maintained in the vicinity.

  In the present invention, a constant LED drive current is injected into each LED by a DC power supply and a constant current drive circuit, and a headroom voltage obtained at a current terminal of the constant current drive circuit is monitored by a headroom voltage monitoring circuit. . The headroom voltage monitoring circuit acts on the DC power supply to dynamically variably control the output voltage, that is, the LED drive voltage, so that the headroom voltage is maintained near the first reference voltage. As a result, even if the voltage drop of the LED fluctuates due to the environmental temperature, LED self-heating, etc., even if it fluctuates in a particularly decreasing direction, the feedback loop via the headroom voltage monitoring circuit works and the headroom voltage Is stably maintained in the vicinity of the first reference voltage, so that the power consumption and the amount of heat generated in the constant current drive circuit can be suppressed within certain limits.

  According to a preferred aspect of the present invention, the DC power supply includes a first switching element that can be turned on / off at a high frequency, and the first switching element is turned on / off to generate a DC input voltage. A switching power supply unit that converts the LED drive voltage, a switching control unit that controls the on / off operation of the first switching element in the switching power supply unit, and a first feedback circuit that feeds back the LED drive voltage to the switching control unit; Have The headroom voltage monitoring circuit includes a second feedback circuit that feeds back the headroom voltage to the switching control unit of the DC power supply.

  In this case, preferably, the switching control unit has a reference voltage input terminal and a feedback voltage input terminal, and the voltage input to the feedback voltage input terminal is changed to the second reference voltage input to the reference voltage input terminal. The on / off operation of the first switching element is controlled so as to be equal. The first feedback circuit includes first and second resistors connected between the output terminal of the switching power supply unit and the reference potential terminal, and is between the first resistor and the second resistor. Are connected to the feedback voltage input terminal of the switching control unit. The second feedback circuit compares the first transistor connected between the feedback voltage input terminal and the reference potential terminal of the switching controller, the headroom voltage, and the first reference voltage, It has a comparator that outputs a comparison result signal indicating the level relationship between the two voltages, and a feedback control circuit that controls the first transistor in accordance with the comparison result signal output from the comparator.

  More preferably, in the second feedback circuit, a third resistor may be connected in series with the first transistor between the feedback voltage input terminal and the reference potential terminal of the switching control unit.

  As a preferred aspect, the feedback control circuit latches the comparison result signal output from the comparator at a predetermined timing every predetermined cycle, and the comparison result signal latched in the latch circuit is input as a control signal. When the comparison result signal indicates that the headroom voltage is higher than the first reference voltage, it is turned on to turn on the first transistor or increase the current flowing through the first transistor. When the comparison result signal indicates that the headroom voltage was lower than the first reference voltage, the headroom voltage is turned off to turn off the first transistor or reduce the current flowing through the first transistor. And a second transistor.

  As a preferable aspect, in the feedback control circuit, a time constant circuit may be connected between the output terminal of the second transistor and the control terminal of the first transistor. Further, a bias circuit that applies a constant bias voltage to the control terminal of the first transistor may be provided.

  As a preferred embodiment, the constant current drive circuit includes a constant current source for keeping the LED drive current constant, and a second switching element connected in series with the constant current source and capable of being turned on / off at a high frequency. And an LED brightness control circuit for turning on / off the second switching element at a constant cycle by a pulse width modulation method.

  In the LED device of the present invention, as a preferred embodiment, one surface light source is composed of m blocks, and m LED series circuits and m constant current drive circuits are respectively allocated to the m blocks. In each block, the n LEDs constituting the LED series circuit are two-dimensionally arranged with a constant density distribution. In this case, the duty by the pulse width modulation method may be individually controlled for each block.

  According to the LED device or the LED drive circuit of the present invention, the power consumption generated in the constant current drive circuit at the time of LED light emission drive is suppressed or reduced and the constant current drive circuit is stable or normal by the above configuration and operation. Operation can be guaranteed.

It is a circuit diagram which shows the circuit structure of the LED apparatus which has the LED drive circuit in one Embodiment of this invention. It is a circuit diagram which shows the structural example of the DC-DC converter used with the LED device of embodiment. It is a block diagram which shows the structural example in the LED driver IC used with the LED device of embodiment. In view showing an example of a DC relationship between the second control voltage _{V G} and the DC-DC converter at the node _{N C} in the feedback circuit of the output voltage (LED driving _{voltage) V LED} in LED device embodiments is there. It is a circuit diagram which shows the circuit structure at the time of making the LED array into the arrangement structure of n = 12 and m = 3 in the LED device of embodiment. It is a wave form diagram which shows the waveform of each part for demonstrating the effect | action of a certain condition of the LED apparatus (FIG. 5) of embodiment. It is a wave form diagram which shows the waveform of each part for demonstrating the effect | action of another condition of the LED apparatus (FIG. 5) of embodiment. It is a figure which shows the pattern of the duty control used in the experiment conducted in order to verify the effect of the LED device (FIG. 5) of embodiment in a local dimming function. It is a wave form diagram which shows the waveform of the headroom voltage and LED drive voltage which were obtained by the said experiment. As a comparative example, waveforms of a headroom voltage and an LED driving voltage obtained when the same experiment is performed by omitting the second feedback circuit in the LED device of the embodiment (FIG. 5) are shown. It is a circuit diagram which shows the circuit structure of the conventional LCD apparatus used for the backlight for LCD-TV. It is a figure which shows the structure which divides | segments LED backlight into many blocks in a matrix form. It is a figure which shows the arrangement configuration example of LED in each block of LED backlight.

  Hereinafter, a preferred embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 shows a circuit configuration of an LED device having an LED drive circuit according to an embodiment of the present invention. This LED device can be used for an LED backlight for LCD-TV, for example. In the figure, elements or circuits having the same configuration or function as those of the conventional apparatus of FIG. 11 are denoted by the same reference numerals.

This LED device has, as a main configuration in common with the conventional LED device (FIG. 11), n × m (n and m are integers of 2 or more) LEDs [10 _{(0, 0)} ... 10 _{(n -2,0)} , 10 _(m-1,0) ] to [10 _{(0, m-1)} ... 10 _{(n-2, m-1)} , 10 _{(n-1, m-1)} ] An LED array 12 comprising, for example, one or more (N) LED driver ICs 14 _{(0) to} 14 _(N-1) of a 16 channel type, a DC power source, for example, a DC-DC converter 16, and a feedback component. It has voltage resistors 24 and 26, a high-voltage protection transistor 28, and a bias circuit [30, 32]. In this embodiment, the feedback circuit constituted by the voltage dividing resistors 24 and 26 is the first feedback circuit.

Similarly to the conventional LED device (FIG. 11), in the LED array 12, the LEDs 10 _{(0, y)} ... 10 _{(n−2, y)} , 10 _{(n−1, y)} (y = 0 ₎ in each column. ˜m−1) is electrically in series between the output terminal of the DC-DC converter 16 and each corresponding current terminal OUT _{y of} any LED driver IC 14 _(x) (x = 0 to N−1). It is connected to the. In addition, this LED backlight is an area light system, and as shown in FIG. 12, m (m = i × j) blocks B ₀ , B ₁ ... B _m- divided into ₁ each corresponding row _LED10 in FIG. 1 in each block _{B y (0, y) ···} 10 (n-2, y), 10 (n-1, y) as shown in FIG. 13 It is arranged two-dimensionally with a constant density distribution.

  FIG. 2 shows a configuration example of the DC-DC converter 16. This DC-DC converter 16 includes a switching power supply unit 48 including an inductance coil 40, an NMOS transistor (switching element) 42, a diode 44, and a capacitor 46, and an on / off operation of the NMOS transistor 42 by a pulse control system, for example, a PWM control system. And a switching control circuit 50 for controlling. For PWM control, a clock signal CK having a constant frequency, for example, 150 kHz, is supplied to the switching control circuit 50 from the controller 38 or a clock circuit (not shown).

In the PWM control by the switching control circuit 50, during the period in which the NMOS transistor 42 is turned on within one cycle, the terminal of the ground potential passes through the inductance coil 40 and the NMOS transistor 42 from the voltage input terminal 52 for inputting the input voltage VIN. Current flows, and energy is stored in the inductance coil 40. When the NMOS transistor 42 is turned off within one cycle, the energy stored in the inductance coil 40 is released to the capacitor 46 side via the diode 44, and the capacitor 46 is charged to a voltage higher than the input voltage _VIN. The inter-terminal voltage 46 is output from the output terminal 54 as the LED drive voltage V _LED .

FIG. 3 shows a circuit configuration example in the LED driver IC 14 ₍₀₎ . The other LED driver ICs 14 _{(1) to} 14 _(N-1) also have the same circuit configuration.

As shown in FIG. 3, 16-channel constant current drive circuits 60 _{(0) to} 60 ₍₁₅₎ are provided in the LED driver IC 14 ₍₀₎ . Each constant current drive circuit 60 _(y) (y = 0 to 15) has LEDs 10 _{(0, y)} ... 10 _{(n−2, y)} , 10 _{(n−1, y)} ( Figure 1) and the switching element 62 connected in series between the terminal of the ground potential and _(y) and a constant current source 64 _(y), with gradation degree of luminance or brightness of each corresponding block B _y The main component is a gray scale PWM control circuit 66 _(y) that controls the on / off operation of the switching element 62 _(y) by the PWM control method based on the gray scale data GS _y to be instructed.

In order to perform local dimming, grayscale data GS _y sent by serial transfer from the controller 38 (FIG. 1) at a constant cycle (for example, 120 Hz) is transferred to each GS register via the input shift registers 68 and 70. 72 _(y) . Each PWM control circuit 66 _(y) determines the time during which the switching element 62 _(y) is turned on within one cycle, that is, the LED driving current, based on the grayscale data GS _y loaded in each GS register 72 _(y). The time (pulse width) in which I _y flows is variably controlled by the PWM control method. If gray scale data GS _y is 12 bits, for example, is capable of pulse-width control 4096 ^{(2 12)} comprises the LED drive current _{I y} of each channel, thereby brightness control 4096 tones in each block _{B y} Is possible.

In the LED driver IC 14 ₍₀₎ , as another incidental function, constant current sources 64 ₍₀₎ to 64 ₍₁₅₎ are individually provided so as to eliminate variations in the LED driving currents I _{0 to} I ₁₅ between channels. Dot correction circuits 74 ₍₀₎ to 74 ₍₁₅₎ are provided for control. Dot correction data DC _y of each channels controllers 38 at initialization from (FIG. 1) transmitted by serial transfer is loaded through the input shift register 68, 70 to the respective DC register _{78 (y).} Each dot correction circuit 74 _(y) corrects the current value that each constant current source 64 _(y) flows, that is, the LED drive current I _y , based on the dot correction data DC _y loaded in each DC register 78 _(y). It is supposed to be. If dot correction data is 6 bits, for example, it is possible to perform fine adjustment of 64 steps for LED driving current I _y of each channel. Furthermore, in the LED driver IC 14 ₍₀₎ , an LED open detection circuit 76 ₍₀₎ to detect an open circuit caused by LED breakage or the like in the constant current drive circuits 60 _{(0) to} 60 _(15). 76 ₍₁₅₎ etc. are also provided.

Referring again to FIG. 1, the LED device of this embodiment is most different from the conventional LED device (FIG. 11) in that each of _m current terminals OUT _{0 to} OUT _m−1 connected to the LED array 12 is provided. The m headroom voltages HV _{(0) to} HV _{( m−1)} obtained are fed back to the DC-DC converter 16 via a second feedback circuit 80 described later. The controller 38 of this LED device performs predetermined control not only on the LED driver ICs 14 _{(0) to} 14 _(N-1) and the DC-DC converter 16, but also on the second feedback circuit 80. In this embodiment, the controller 38 and the feedback circuit 80 constitute a headroom voltage monitoring circuit in the present invention.

The second feedback circuit 80 includes a resistor 82 and an NMOS transistor 84 connected in series between the feedback voltage input terminal FB of the DC-DC converter 16 and the ground potential terminal, and m current terminals OUT _{0 to} OUT _m. the m comparators ₈₆ for comparing each obtained in _-1 the m headroom voltage _{HV (0) ~HV} the _(m-1) with a predetermined reference voltage _{V S} and _{(0) ~86 (m-1} ), A feedback control circuit 88 that controls the NMOS transistor 84 in accordance with _m comparison result signals CO _{0 to} CO _m−1 respectively output from the comparators 86 _{(0) to} 86 _{( m−1).} Yes.

Each comparator _{86 (y) (y = 0~m} -1) inputs the headroom voltage HV _y of each current terminal OUT _y to one input terminal (+), the predetermined from the reference voltage generating circuit 95 The reference voltage V _S is input to the other input terminal (−), and when the head room voltage HV _y is higher than the reference voltage V _{S, the} H level comparison result signal CO _y is output, and the head room voltage HV _y is the reference. When the voltage is lower than the voltage V _{S, the} L level comparison result signal CO _y is output.

The feedback control circuit 88 includes a logic circuit 90 connected to the output terminals of the m number of comparators 86 _{(0) to} 86 _{( m−1)} , and a D-type flip-flop connected to the output terminal of the logic circuit 90. A latch circuit 92, a PMOS transistor 94 connected to the output terminal of the latch circuit 92, and a time constant circuit 96 connected between the output terminal of the PMOS transistor 94 and the gate terminal of the NMOS transistor 84. is doing.

In the logic circuit 90, each cathode terminal is connected to the output terminals of the comparators 86 _{(0) to} 86 _{( m−1)} , and each anode terminal is commonly connected to the data input terminal (D) of the latch circuit 92. a number of diodes _{98 (0) ~98 (m-} 1), is connected between the anode terminal or node _{N B} and the terminal of the power supply voltage _{V CC} of the diodes _{98 (0) ~98 (m-} 1) And a pull-up resistor 100. Comparator _{86 (0) ~86 (m-} 1) comparison result signals output from the _CO 0 _{~CO m-1} of all of H level judgment signal SA is obtained at the node _{N B} is at the H level, comparison results At least one of the signals _CO 0 _{to CO m-1} but is adapted to the node _{N B} in the L-level judgment signal SA is at the L level is obtained. Thus, the logic circuit 90 in this embodiment functions as an AND circuit.

The clock terminal (C) of the latch circuit 92 is connected to the clock terminal (C) at a predetermined timing (that is, the LED driving current I _LED in each LED driver IC 14 _(x) ) at a predetermined timing (that is, the LED The sampling clock SCK is supplied from the controller 38 immediately after the start of the current duration in which the drive current I _LED flows. The latch circuit 92 latches the determination signal SA in response to the sampling clock SCK, and provides an output (Q) having the same logic level as the latched determination signal SA to the gate terminal of the PMOS transistor 94.

The PMOS transistor 94 has a source terminal connected to the terminal of the power supply voltage _VCC , a drain terminal (output terminal) connected to the ground potential terminal via the resistor 102, and the NMOS transistor 84 via the time constant circuit 96. Connected to the gate terminal. The time constant circuit 96 includes a resistor 104 and a capacitor 106.

When the output signal (Q) of the latch circuit 92 is at H level, that is, when the headroom voltages HV _{0 to} HV ₁₅ of all the channels are higher than the reference voltage V _S at the timing of the immediately preceding sampling clock SCK, the PMOS transistor 94 is turned off. When PMOS transistor 94 is turned off, when the capacitor 106 of the time constant circuit 96 is discharged through the resistor 104, 102, lowers the gate voltage _{V G} of the node _{N C} potential clogging NMOS transistor 84. Thus, the first bypass current i decreases flowing from the node _{N A} of the voltage dividing resistors 24 and 26 constituting the feedback circuit through the resistor 82 and the NMOS transistor 84, or the current i does not flow, DC-DC converter The feedback voltage V _FB input to the 16 feedback voltage input terminals FB rises.

When the output signal (Q) of the latch circuit 92 is at L level, that is, when at least one of the headroom voltages HV _{0 to} HV ₁₅ is lower than the reference voltage V _S at the timing of the immediately preceding sampling clock SCK, the PMOS transistor 94 is Turns on. When PMOS transistor 94 is turned on, the capacitor 106 of the time constant circuit 96 is charged through the PMOS transistor 94 and the resistor 104, the gate voltage _{V G} of the node _{N C} potential clogging NMOS transistor 84 is increased . Thereby, the bypass current i increases flowing from the node _{N A} of the voltage dividing resistors 24 and 26 through the resistor 82 and the NMOS transistor 84, a feedback voltage _{V FB} is reduced.

As described above, in the second feedback circuit 80 in this embodiment, the headroom voltages HV _{0 to} HV ₁₅ of all the channels are higher than the reference voltage V _S at the timing of the sampling clock SCK given from the controller 38 at a constant period. When this occurs, the feedback voltage V _FB to the DC-DC converter 16 is raised, and when at least one of the headroom voltages HV _{0 to} HV ₁₅ is lower than the reference voltage V _S , the feedback voltage V _FB is lowered.

In the DC-DC converter 16, when the feedback voltage V _FB is lower than the reference voltage V _REF , the switching control circuit 50 (see FIG. 5) so as to make this error zero, that is, increase the voltage level of the output voltage V _LED . 2) increases the duty of the on / off operation of the switching element 42. On the contrary, when the feedback voltage V _FB is higher than the reference voltage V _REF , the switching control circuit 50 turns on the switching element 42 so as to make this error zero, that is, to lower the voltage level of the output voltage V _LED.・ Reduce the duty of OFF operation.

The transmission characteristic of the second feedback circuit 80 can be arbitrarily adjusted, and the time constant of the time constant circuit 96, the resistance values of the resistors 24, 26, and 82, the reference voltage V _{REF and the} like are appropriately selected. Good.

4, when the DC relationship _{(V G -V LED} characteristics between the output voltage (LED driving _{voltage) V LED} node _N control obtained _C voltage _{V G} and the DC-DC converter 16 of the time constant circuit 96 ) Is an example. In this example, the control voltage V _G is set to change within the range of 1.0 to 1.6 volts so that the fluctuation of the LED drive voltage V _LED falls within the range of 39 to 42.5 volts. Is done. The allowable fluctuation range of the LED drive voltage V _LED is determined depending on the configuration of the LED array, the forward voltage characteristics of the LED, the ambient temperature, and the like.

  Next, the operation of the LED device in this embodiment will be described with reference to FIGS. Here, for the sake of simplicity of explanation and convenience of understanding, a case where n = 12, m = 3 in the LED array 12 as shown in FIG. 5 is taken as an example. As shown in FIG. 5, in the second feedback circuit 80, it is also possible to provide a resistance bias circuit 108, 110 for applying a constant bias voltage to the gate terminal of the NMOS transistor 84 or the node NB. In this configuration, the bypass current i can be varied by turning on the NMOS transistor 84 at all times.

In FIG. 6, an example of the waveform of each part at the time of steady state in this LED device is shown. FIG. 6A shows a horizontal blanking signal BLANK given from the controller 38 to each LED driver IC 14 _(x) at a constant cycle (for example, 120 Hz).

FIG. 6B shows LED drive currents I ₀ , I ₁ , and I ₂ for all channels of the LED array 12. Here, all LED drive currents I ₀ , I ₁ and I ₂ are controlled to have the same pulse width by PWM control.

FIG. 6C shows the sampling clock SCK supplied from the controller 38 to the latch circuit 92 of the second feedback circuit 80. As shown in the figure, the timing of the sampling clock SCK is set immediately after the start of the variable pulse time of all LED drive currents I ₀ , I ₁ , I ₂ by PWM control.

FIG. 6D shows the headroom voltages HV ₀ , HV ₁ and HV ₂ for all channels. Here, it is assumed that all headroom voltages HV ₀ , HV ₁ , HV ₂ change with the same waveform.

FIG. 6E shows the output signal (Q) of the latch circuit 92. (F) in FIG. 6 illustrates a control voltage _{V G} obtained in the node _{N C} in the second feedback circuit 80. FIG. 6G shows the bypass current i that flows through the NMOS transistor 84 of the second feedback circuit 80. FIG. _6H shows the feedback voltage V _FB input to the feedback voltage input terminal FB of the DC-DC converter 16. (I) of FIG. 6 shows the LED drive voltage V _LED output from the DC-DC converter 16.

As shown in FIG. 6, at the timing of the sampling clock SCK ₍₁₎ , all the headroom voltages HV ₀ , HV ₁ , HV ₂ are higher than the reference voltage V _S. Then, in the second feedback circuit 80, the output signal (Q) of the latch circuit 92 changes from the previous L level to the H level, and the control voltage V _G changes from the linear increase until then to the linear decrease. The bypass current i changes from a linear increase until then to a linear decrease. As a result, in the DC-DC converter 16, the feedback voltage V _FB changes from a linear decrease until then to a linear increase, and the output voltage, that is, the LED drive voltage V _LED changes from a linear increase until then to a linear decrease. When the LED drive voltage V _LED decreases linearly, the headroom voltages HV ₀ , HV ₁ , HV _{2 become} linear during the period in which the LED drive currents I ₀ , I ₁ , I ₂ are flowing in each cycle of PWM control. The headroom voltage HV ₀ , HV ₁ , HV ₂ decreases linearly even during the period when the LED drive currents I ₀ , I ₁ , I ₂ do not flow.

At the timing of the next sampling clock SCK ₍₂₎ , all the headroom voltages HV ₀ , HV ₁ , HV ₂ become lower than the reference voltage V _S. Then, in the second feedback circuit 80, the output signal (Q) of the latch circuit 92 changes from the previous H level to the L level, and the control voltage V _G changes from a linear decrease until then to a linear increase. The bypass current i changes from a linear decrease until then to a linear increase. Thus, the DC-DC converter 16, turned the feedback voltage V _FB from linear rise to a linear decrease, the output voltage, that LED driving voltage V _LED starts to linear increase from the previous linear decrease to. When the LED drive voltage V _LED rises linearly, the headroom voltages HV ₀ , HV ₁ , and HV _{2 become} linear during the period in which the LED drive currents I ₀ , I ₁ , and I ₂ flow in each cycle of PWM control. The headroom voltages HV ₀ , HV ₁ , HV ₂ rise linearly even during the period when the LED drive currents I ₀ , I ₁ , I ₂ do not flow.

Thereafter, as shown in FIG. 6, the same operation as described above is repeated. Thus, in the LED device of this embodiment, as headroom voltage _{HV 0,} HV 1, HV ₂ is maintained near the reference voltage _{V S} with or exceeded by dividing the reference voltage _{V S,} the second The feedback circuit 80 acts on the DC-DC converter 16, and the LED drive voltage V _LED is dynamically variably controlled.

In the example of FIG. 6, when the LED drive currents I ₀ , I ₁ , I ₂ of all channels are controlled to the same pulse width by PWM control, the headroom voltages HV ₀ , HV ₁ , HV _{2 respectively.} However, the waveforms may differ between the headroom voltages HV ₀ , HV ₁ , and HV ₂ . FIG. 7 shows the waveforms of the respective parts when the waveforms of the headroom voltages HV ₀ and HV ₁ are the same and the waveforms of the headroom voltage HV ₂ are different.

In the case of the example of FIG. 7, the process is almost the same as that of the example of FIG. 6 until immediately before the third sampling clock SCK ₍₃₎ . However, at the timing of the sampling clock _{SCK (3),} but headroom voltage HV ₂ is higher than the reference voltage _{V S,} since the headroom voltage _HV 0, HV ₁ becomes lower than the reference voltage _{V S,} the second In the feedback circuit 80, the output signal (Q) of the latch circuit 92 continues to maintain the L level until then. Thus, the control voltage V _G maintains a linear increase, the bypass current i maintains a linear increase. As a result, in the DC-DC converter 16, the feedback voltage V _FB maintains a linear decrease, and the output voltage pressure V _LED maintains a linear increase.

However, at the timing of the next fourth sampling clock SCK ₍₄₎ , all the headroom voltages HV ₀ , HV ₁ , HV ₂ become higher than the reference voltage V _S. Then, in the second feedback circuit 80, the output signal (Q) of the latch circuit 92 changes from the previous L level to the H level, and the control voltage V _G changes from the linear increase until then to the linear decrease. The bypass current i changes from a linear increase until then to a linear decrease. As a result, in the DC-DC converter 16, the feedback voltage V _FB changes from a linear decrease until then to a linear increase, and the output voltage V _LED changes from a linear increase until then to a linear decrease.

Thus, also in this case, the headroom voltages HV ₀ and HV ₁ and the headroom voltage HV ₂ are maintained in the vicinity of the reference voltage V _S while dividing or exceeding the reference voltage V _S while having a different period. The second feedback circuit 80 acts on the DC-DC converter 16, and the LED drive voltage V _LED is dynamically variably controlled.

Although not shown, the headroom voltage HV is different when the waveforms of the headroom voltages HV ₀ , HV ₁ , HV ₂ are all different, and even when the pulse widths of the LED drive currents I ₀ , I ₁ , I ₂ are varied. Through the second feedback circuit 80, all or part of ₀ , HV ₁ , HV ₂ is maintained in the vicinity of the reference voltage V _S while dividing or exceeding the reference voltage V _S while changing the period. The output voltage V _LED of the DC-DC converter 16 is dynamically variably controlled.

Next, the effect of this embodiment in the local dimming function will be described. In FIG. 8, the duty of PWM control in the device configuration of FIG. 5 is switched alternately between 5% and 95% every fixed period (for example, 500 sec), and the three blocks B ₁ , B ₂ , B ₃ of the LED array 12 are switched. The pattern of the experiment example which changed the brightness | luminance uniformly is shown. FIG. 9 shows waveforms of the headroom voltage (HV ₀ , HV ₁ , HV ₂ ) and the LED drive voltage V _LED obtained in this experimental example.

As shown in FIG. 9, in the LED device of this embodiment, the LED drive voltage V _LED is about 41.0 volts when the duty cycle is 5% and about 40.0 volts when the duty cycle is 95%. Two values were taken, which kept the headroom voltages (HV ₀ , HV ₁ , HV ₂ ) around 1.5 volts throughout the entire cycle. When the LED drive current (I ₀ , I ₁ , I ₂ ) is 100 mA, the LED array 12, LED driver ICs 14 ₍₁₎ , 14 ₍₂₎ , 14 ₍₃₎ and the DC-DC converter 16 generate The total power consumption was 6719 mW at an ambient temperature of 25 ° C. and 6499 mW at an ambient temperature of 60 ° C.

FIG. 10 shows, as a comparative example, the headroom voltages (HV ₀ , HV ₁ , HV ₂ ) obtained when the second feedback circuit 80 is omitted in the apparatus configuration of FIG. and an LED driving voltage _{V LED} of the waveform. In this case, the LED drive voltage V _LED changes only slightly to about 41.1 volts when the duty cycle is 5% and about 41.2 volts when the duty cycle is 95%, while the headroom voltage (HV ₀ , HV ₁ , HV ₂ ) fluctuated greatly to about 1.7 volts in a cycle with a duty of 5% and about 2.6 volts in a cycle with a duty of 95%. In this comparative example, when the LED drive current (I ₀ , I ₁ , I ₂ ) is 100 mA, the LED array 12, the LED driver ICs 14 ₍₁₎ , 14 ₍₂₎ , 14 _(3), and the DC-DC The total power consumption generated by the converter 16 was 6863 mW at an ambient temperature of 25 ° C. and 6894 mW at an ambient temperature of 60 ° C.

  As described above, it has been experimentally confirmed that the LED device of the present embodiment can improve the stability of the headroom voltage and the reduction of the power consumption even in the local dimming function.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the technical idea.

For example, in the above-described embodiment, the headroom voltage monitoring circuit [38, 80] monitors the headroom voltages HV _{0 to} HV _m−1 of all channels, but it is also possible to monitor only a part of the headroom voltages. It is. In particular, when the variation in characteristics of the LEDs 10 constituting the LED array 12 is small, only the headroom voltage of one or several channels selected as a representative is fed back to the DC-DC converter 16 via the second feedback circuit 80. May be.

In the LED driver ICs 14 _{(0) to} 14 _(N-1) , although not shown, the LED open detection circuits 76 _{(0) to} 76 _(m-1) may be configured by a comparator, a logic circuit, and a latch circuit. it can. In this case, the voltage of the current terminal OUT _y of each channel is input to one input terminal of each comparator, and a predetermined reference voltage V _OP is input to the other input terminal from a dedicated reference voltage generation circuit. Therefore, the reference voltage V _S for headroom voltage monitoring and the reference voltage V _OP for LED open detection are switched in a time division manner, and the same comparator, logic circuit and latch circuit are switched to the first feedback circuit 80 and the LED open. A configuration can also be used for the detection circuits 76 _{(0) to} 76 _{( m−1)} .

Other configurations in each LED driver IC 14 _(x) , in particular, configurations such as the constant current drive circuit 60 _(y) and the PWM control circuit 66 _(y) can be variously modified. Further, the DC-DC converter 16 is not limited to the chopper boost type, and other types such as an insulation type using a transformer are also possible.

  The LED device of the present invention is not limited to a backlight, but can be applied to other LED applications such as lighting and a display.

10 LED
12 LED array 14 _{(0) to} 14 _(N-1) LED driver IC
16 DC-DC converter 24, 26 Voltage dividing resistor (first feedback circuit)
38 controller 48 switching power supply unit 50 switching control circuit FB feedback voltage input terminal REF reference voltage input terminal 60 _{(0) to} 60 ₍₁₅₎ constant current drive circuit 62 _{(0) to} 62 ₍₁₅₎ switching element 64 ₍₀₎ to 64 ₍₁₅₎ Constant current source 66 _{(0) to} 66 _{( 15)} Gray scale PWM control circuit 80 Second feedback circuit 82 Resistance 84 NMOS transistor 86 _{(0) to} 86 _(m-1) Comparator 88 Feedback control circuit 90 Logic circuit 92 Latch circuit 94 PMOS transistor 95 Reference voltage generation circuit 96 Time constant circuit 98 _{(0) to} 98 _(m-1) Diode

Claims (18)

An LED driving circuit for driving light emission of one or a plurality of LEDs (light emitting diodes) electrically connected in series,
A DC power supply that outputs a DC LED drive voltage;
A constant current drive circuit connected in series with the LED to the DC power supply to inject a constant LED drive current into the LED;
A head that dynamically variably controls the voltage level of the LED drive voltage by acting on the DC power supply so that the headroom voltage obtained at the current terminal of the constant current drive circuit is maintained near the first reference voltage. An LED driving circuit having a room voltage monitoring circuit; The DC power supply is
A switching power supply unit that has a first switching element that can be turned on and off at a high frequency, and that turns on and off the first switching element to convert a DC input voltage into the LED driving voltage;
A switching control unit for controlling on / off operation of the first switching element in the switching power supply unit;
A first feedback circuit that feeds back the LED drive voltage to the switching controller;
The headroom voltage monitoring circuit includes a second feedback circuit that feeds back the headroom voltage to the switching control unit of the DC power supply.
The LED drive circuit according to claim 1. The switching control unit has a reference voltage input terminal and a feedback voltage input terminal, and a voltage input to the feedback voltage input terminal is equal to a second reference voltage input to the reference voltage input terminal. , Controlling the on / off operation of the first switching element,
The first feedback circuit has first and second resistors connected between an output terminal of the switching power supply unit and a terminal of a reference potential, and the first resistor and the second resistor A node between is connected to the feedback voltage input terminal of the switching controller,
The second feedback circuit comprises:
A first transistor connected between the feedback voltage input terminal and the reference potential terminal of the switching control unit;
A comparator that compares the headroom voltage with the first reference voltage and outputs a comparison result signal indicating a level relationship between the two voltages;
A feedback control circuit that controls the first transistor in accordance with the comparison result signal output from the comparator;
The LED drive circuit according to claim 2.   4. The LED drive circuit according to claim 3, further comprising a third resistor connected in series with the first transistor between the feedback voltage input terminal and the reference potential terminal of the switching control unit. The feedback control circuit comprises:
A latch circuit that latches the comparison result signal output from the comparator at a predetermined timing every predetermined cycle;
When the comparison result signal latched in the latch circuit is input as a control signal and the comparison result signal indicates that the headroom voltage is higher than the first reference voltage, the comparison result signal is turned on. When the comparison result signal indicates that the first transistor is turned on or the current flowing through the first transistor is increased and the headroom voltage is lower than the first reference voltage. A second transistor that is in an off state to turn off the first transistor or reduce current flowing through the first transistor;
The LED drive circuit of Claim 3 or Claim 4.   The LED drive circuit according to claim 5, wherein the feedback control circuit includes a time constant circuit connected between an output terminal of the second transistor and a control terminal of the first transistor.   The LED drive circuit according to claim 3, further comprising a bias circuit that applies a constant bias voltage to a control terminal of the first transistor. The constant current drive circuit is
A constant current source for keeping the LED driving current constant;
A second switching element connected in series with the constant current source and capable of being turned on and off at a high frequency;
The LED drive circuit according to any one of claims 1 to 7, further comprising: an LED brightness control circuit that turns on and off the second switching element at regular intervals by a pulse width modulation method. A direct current power source for outputting direct current LED drive current;
m (m is an integer of 2 or more) LED series circuits formed by electrically connecting n (n is an integer of 2 or more) LEDs in series are electrically connected in parallel to the output terminal of the DC power supply. A connected LED array;
M constant current drive circuits respectively connected in series with the m LED series circuits to the DC power source to inject a constant LED drive current into the LEDs;
The voltage level of the LED driving voltage is adjusted by acting on the DC power supply so that at least one of the headroom voltages respectively obtained at the current terminals of the m constant current driving circuits is maintained near the first reference voltage. A headroom voltage monitoring circuit that dynamically and variably controls the LED device. The DC power supply is
A switching power supply unit that has a first switching element that can be turned on and off at a high frequency, and that turns on and off the first switching element to convert a DC input voltage into the LED driving voltage;
A switching control unit for controlling on / off operation of the first switching element in the switching power supply unit;
A first feedback circuit that feeds back the LED drive voltage to the switching controller;
The headroom voltage monitoring circuit has a second feedback circuit that feeds back at least one headroom voltage to the switching control unit of the DC power supply.
The LED device according to claim 9. The switching control unit has a reference voltage input terminal and a feedback voltage input terminal, and a voltage input to the feedback voltage input terminal is equal to a second reference voltage input to the reference voltage input terminal. , Controlling the on / off operation of the first switching element,
The first feedback circuit has first and second resistors connected between an output terminal of the switching power supply unit and a terminal of a reference potential, and the first resistor and the second resistor A node between and to the feedback signal input terminal of the switching controller,
The second feedback circuit comprises:
A first transistor connected in series between the feedback voltage input terminal and the reference potential terminal of the switching control unit;
One or more comparators that compare at least one of the headroom voltage and the first reference voltage and output a binary level comparison result signal indicating a level relationship between the two voltages;
A feedback control circuit that controls the first transistor in accordance with one or more of the comparison result signals output from one or more of the comparators, respectively.
The LED device according to claim 10.   The LED device according to claim 11, further comprising a third resistor connected in series with the first transistor between the feedback voltage input terminal and the reference potential terminal of the switching control unit. The feedback control circuit comprises:
A latch circuit for latching a binary level determination signal representing a logical product or logical sum of one or a plurality of the comparison result signals respectively output from one or a plurality of the comparators at a predetermined timing every predetermined cycle When,
The determination signal latched in the latch circuit is input as a control signal, and the determination signal indicates that all of the headroom voltages input to all the comparators are higher than the first reference voltage. The first transistor is turned on, or the current flowing through the first transistor is increased, and at least one of the headroom voltages is lower than the first reference voltage. A second transistor that is turned off when the determination signal indicates that the first transistor is turned off or the current flowing through the first transistor is reduced,
The LED device according to claim 11 or 12.   The LED device according to claim 13, wherein the feedback control circuit includes a time constant circuit connected between an output terminal of the second transistor and a control terminal of the first transistor.   The LED device according to claim 11, further comprising a bias circuit that applies a constant bias voltage to a control terminal of the first transistor. Each of the constant current drive circuits is
A constant current source for keeping the LED driving current constant;
A second switching element connected in series with the constant current source and capable of being turned on and off at a high frequency;
The LED device according to claim 15, further comprising: an LED brightness control circuit that turns on and off the second switching element at a predetermined cycle by a pulse width modulation method. One surface light source consists of m blocks,
m number of the LED series circuits and m number of constant current driving circuits are allocated to the m number of blocks, respectively.
The LED device according to any one of claims 9 to 16, wherein n LEDs constituting the LED series circuit in each of the blocks are two-dimensionally arranged with a constant density distribution.   The LED device according to claim 17, wherein the duty of the pulse width modulation method is individually controlled for each block.

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