JP5428254B2 - LED drive device - Google Patents

Description

  The present invention relates to an LED driving device that drives an LED to emit light, and more particularly to an LED driving device that drives a WLED (white light emitting diode) used for a backlight of a liquid crystal monitor or the like, and more particularly, a charge pump that boosts and outputs an input voltage. The present invention relates to a technique suitable for use in a charge pump type LED drive device having

  In portable electronic devices such as cellular phones, WLEDs are used as backlights for display liquid crystal panels. Conventionally, a power supply device that generates a drive voltage for a WLED includes an LED drive device that uses a step-up type switching regulator, and switches the terminal voltage of a charged capacitor or transfers a charged charge to another capacitor. A charge pump type LED driving device (LED driver) that outputs a boosted voltage is known.

In any LED driver, constant current driving is performed in which a boosted voltage is applied to the LED and a constant current is passed through the LED. As an invention relating to a charge pump type LED driver, for example, there is one described in Patent Document 1.
JP 2006-254641 A

  In an LED driver that uses a battery as a power source and generates an LED drive voltage by boosting the battery voltage, a decrease in brightness of the LED due to a decrease in the battery voltage becomes a problem. In a switching regulator type LED driver, the drive current flowing through the LED is converted into voltage and fed back to the control circuit, and the switching element that intermittently flows current to the inductor (coil) is PWM driven, for example, to keep the drive current constant. Therefore, the LED brightness is prevented from decreasing due to a decrease in battery voltage.

  On the other hand, in the charge pump type LED driver, the boosting rate of the charge pump is switched in order to prevent a decrease in the brightness of the LED accompanying a decrease in the battery voltage. Conventionally, the switching of the boosting rate is generally performed by monitoring the input voltage (battery voltage) and increasing the boosting rate when the voltage falls below a predetermined level.

  However, in the method of monitoring the input voltage and switching, for example, the boost rate to three stages, the change in efficiency when switching the boost rate from 2 times to 1.5 times results in a boost rate of 1.5 times to 1 time. There is a problem that the total power efficiency is deteriorated because it is larger than the change in efficiency when switching to.

  Therefore, the LED driver disclosed in Patent Document 1 is configured to switch the step-up rate by monitoring the LED voltage in addition to the input voltage. However, voltage monitoring requires voltage comparators (comparators), and the number increases as the number of monitored voltages and the number of LEDs increase. Specifically, when the number of LEDs is six, in addition to the comparator 40 that compares the input voltage Vin and the reference voltage (3V), six that compare the cathode voltages of the six LEDs and the reference voltage Vref. Comparators 41 to 46 and five OR gates 51 to 55 for calculating the logical sum of the outputs of these comparators are required (see FIG. 5 of Patent Document 1). Therefore, there is a problem that the circuit scale becomes large and the chip size and thus the chip cost increase.

  When the number of LEDs is four, as shown in FIG. 5, the outputs of the comparators CMP1, CMP1, and CMP2 that directly compare the voltages of the LEDs 1 and 2 and the LEDs 3 and 4 are directly compared. Based on the output of comparators CMP3 and CMP3 that select and compare the higher voltage of LED1 and LED2 and the higher voltage of LED3 and LED4, the highest voltage and reference voltage Vref of LED1 to LED4 A method of providing a comparator CMP4 for comparing the above is also conceivable.

  However, even in the circuit system determined by the tournament system in FIG. 5, the same number of comparators as the number of LEDs are necessary, and the circuit scale becomes large. In FIG. 5, S1, S2; S3, S4; S5 and S6 are MOSFETs functioning as selectors. Moreover, in FIG. 5, the constant current source connected to LED1-LED4 is abbreviate | omitted.

  The present invention has been made paying attention to the above-described problems. The object of the present invention is to provide an LED driving device that monitors the voltage of the LED and detects the switching timing of the boosting rate of the boosting circuit. An object of the present invention is to make it possible to detect the switching timing of the step-up rate without significantly increasing it.

  Another object of the present invention is to provide an LED drive device that includes a charge pump type booster circuit and drives a plurality of light emitting diodes to light, and the voltage between the anode and cathode terminals of any one of the light emitting diodes is smaller than the maximum forward voltage. Therefore, it is possible to avoid stopping lighting of some of the light emitting diodes.

  In order to achieve the above object, the present invention generates a predetermined drive current that flows to each of a plurality of light emitting diodes according to a voltage from a booster circuit that can boost and output an input voltage and switch a boost rate stepwise. In the LED driving device for outputting, a monitoring circuit for monitoring each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes is provided, and the monitoring circuit has at least one of the anode-cathode terminal voltages of the plurality of light emitting diodes. When it is detected that the forward voltage of the diode having the largest forward voltage among the plurality of light emitting diodes has become smaller, the boosting rate of the booster circuit is switched to a higher one.

  More specifically, a plurality of external terminals to which each of the light emitting diodes can be connected, a booster circuit capable of outputting a boosted voltage by receiving the battery voltage as an input voltage, and capable of switching the boost rate stepwise, and the booster A plurality of constant current circuits that receive a voltage from the circuit and generate predetermined drive currents that flow through the plurality of light emitting diodes, respectively, in the LED driving device formed on one semiconductor chip, the plurality of light emitting diodes A monitoring circuit for monitoring each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes, wherein at least one of the anode-cathode terminal voltages of the plurality of light emitting diodes is the most forward voltage of the plurality of light emitting diodes. When it is detected that the voltage is lower than the forward voltage of the large diode, the boosting rate of the boosting circuit is cut to the higher side. It was configured to obtain.

  According to the configuration described above, when the input voltage decreases and the output voltage of the booster circuit decreases and the voltage between the anode and cathode terminals of any one of the light emitting diodes becomes smaller than the maximum forward voltage, the boosting ratio of the booster circuit is high. Therefore, it is possible to avoid stopping lighting of some of the plurality of light emitting diodes.

  Here, preferably, the monitoring circuit is connected to a plurality of transistors in parallel form receiving each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes at a control terminal, and a common drain terminal of the plurality of transistors. A node for supplying an output voltage of the booster circuit to a source terminal of the plurality of transistors, comprising: a constant current source; and potential determining means for determining a potential of a connection point between the common drain terminal and the constant current source. Is detected based on the fact that any one of the plurality of transistors is different from the other transistors in response to a decrease in the output voltage of the booster circuit, so that the potential at the connection point exceeds the determination level. It is configured to output a signal.

  As a result, the voltage between the anode and cathode terminals of any one of the light emitting diodes is the maximum forward voltage without providing a voltage comparison circuit for each of the plurality of light emitting diodes or providing the same number of voltage comparison circuits as the number of light emitting diodes to be connected. Therefore, it is possible to detect the switching timing of the boosting rate without significantly increasing the circuit scale.

  Preferably, the monitoring circuit is provided with an inverter circuit that determines the potential of the connection point based on a logical threshold value. Although the potential of the monitoring level can be determined by a voltage comparison circuit instead of the inverter circuit, the inverter circuit can be configured with a smaller number of elements than the voltage comparison circuit, so that the circuit scale can be further reduced. In addition, power consumption can be reduced.

  Further, preferably, a latch circuit for latching the output of the monitoring circuit is provided at the subsequent stage of the monitoring circuit. As a result, when the boosting rate of the booster circuit is increased, the output voltage of the booster circuit becomes higher, and when the output of the monitoring circuit is about to change to the non-detection state, the state once detected can be maintained, and the booster circuit becomes inoperative. A stable operating state can be prevented.

  Preferably, the booster circuit is configured such that its output voltage can be switched in two stages, and the output voltage of the booster circuit can be switched based only on the output of the monitoring circuit. Switching of the boosting rate of the booster circuit can be performed based on the voltage monitoring result of the anode terminal or the cathode terminal of the light emitting diode and the monitoring result of the input voltage, but the switching is performed based only on the output of the monitoring circuit. By doing so, the voltage comparison circuit for detecting the input voltage becomes unnecessary, and the circuit scale can be further reduced.

  According to the present invention, in the LED driving device that monitors the voltage of the LED and detects the switching timing of the boosting rate of the boosting circuit, the switching timing of the boosting rate can be detected without significantly increasing the circuit scale. In addition, in an LED drive device that drives a plurality of light emitting diodes to light, the voltage between the anode and cathode terminals of any one of the light emitting diodes is smaller than the maximum forward voltage, and the brightness of some of the light emitting diodes decreases or stops lighting There is an effect that can be avoided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 1 shows a first embodiment of an LED driving device to which the present invention is applied. Although not particularly limited, in this embodiment, a portion surrounded by a thick line in FIG. 1 is formed as a semiconductor integrated circuit (hereinafter referred to as a white LED driver IC) 10 on one semiconductor chip. The four white LEDs are connected to the white LED driver IC 10 as external elements, and the cathode terminals of the LEDs 1 to LED4 are respectively connected to the ground potential GND. Further, since the white LED driver IC 10 constitutes a charge pump type booster circuit together with the internal circuit of the IC, external terminals C1 +, C1-; C2 + C2 to which two external capacitance elements (capacitors) C1, C2 can be connected. -Is provided.

  The white LED driver IC 10 of the present embodiment has an input terminal VIN to which a battery voltage is applied as an input voltage Vin from a battery 20 such as a lithium ion battery, and an input voltage Vin applied to the input terminal VIN is increased by 1.5 times. It has a charge pump 11 constituting a booster circuit capable of boosting and outputting, control logic 12 for controlling the entire chip, and diode connection terminals LED1 to LED4 to which four white light emitting diodes can be connected. The control logic 12 generates a signal for controlling a circuit inside the IC, such as a charge pump, based on a control signal inputted from outside the IC and a signal inside the IC.

  The charge pump 11 includes the external capacitive elements C1 and C2, a switch element (not shown) for charging and discharging the capacitive elements C1 and C2, transferring charges between the capacitive elements, and transmitting voltage, and an oscillation. A clock generation circuit (not shown) that generates a clock signal for controlling the switch element based on an oscillation signal from the circuit OSC, and the like, is configured to appropriately select and control the switch elements that are turned on and off, thereby transferring charges and transmitting voltages. By switching these paths, 1-fold output or 1.5-fold boost output is possible.

  Specifically, the charge pump 11 transmits the input voltage Vin to the output terminal as it is in the 1 × output mode. In the case of the 1.5-fold boost mode, as shown in FIG. 3A, after charging one capacitor C1 to the input voltage Vin with reference to the ground potential, as shown in FIG. The two capacitors C1 and C2 are connected in parallel, and the charge of the capacitor C1 is distributed to C2 to be in the Vin / 2 charge state, and Vin is applied to the ground side terminals of C1 and C2. As a result, a voltage boosted to (Vin + Vin / 2) = 1.5 Vin is generated. By alternately repeating the states shown in FIGS. 3A and 3B, the voltage VOUT boosted 1.5 times by the charge transfer is output.

  Further, the white LED driver IC 10 of the present embodiment is required inside the IC, and constant current sources CS1 to CS4 that output a driving current to the diode connection terminals LED1 to LED4 using the voltage VOUT output from the charge pump 11 as a power supply voltage. A reference voltage generation circuit 13 for generating a reference voltage (constant voltage), a constant current circuit for receiving a reference voltage Vref generated by the reference voltage generation circuit 13 and generating a reference current, and the constant current sources CS1 to CS1 It has a constant current control circuit 14 comprising a transistor of CS4 and a transistor constituting a current mirror circuit.

  In the constant current control circuit 14, the constant current sources CS1 to CS4 convert a current generated based on the voltage from the reference voltage generation circuit into a voltage, and generate a constant current by performing voltage-current conversion based on the voltage. Thus, even if the voltage VOUT as the power supply voltage changes, a current proportional to the constant current of the constant current control circuit 14 is generated and output to the terminals LED1 to LED4 as the LED drive current, thereby generating white light emission. The diode can be driven at a constant current.

  Furthermore, the white LED driver IC 10 of this embodiment monitors the voltage of the terminals LED1 to LED4, that is, the anode voltage of each white light emitting diode connected to the terminals LED1 to LED4, and detects the switching timing of the boosting rate. And a latch circuit 16 for latching the output of the monitoring circuit 15. In response to the output signal BMC of the latch circuit 16, the charge pump 11 performs 1-fold output or 1.5-fold boost output.

  By providing the latch circuit 16 that latches the output of the monitoring circuit 15, the output voltage of the charge pump is increased by increasing the step-up rate, the output of the monitoring circuit is changed to the non-detection state, and returns to the original state. It is possible to prevent the oscillation state 11 repeats the 1 × output mode and the 1.5 × boost mode. In addition, even if the potential of the monitoring node fluctuates due to noise or the like, the state once detected can be maintained, and the charge pump 11 can be prevented from oscillating.

  The monitoring circuit 15 includes four P-channel MOSFETs (insulated gate field effect transistors: hereinafter referred to as MOS transistors), each having a source terminal connected to a node to which a voltage VOUT is supplied and a gate terminal connected to the terminals LED1 to LED4. Q1-Q4, an N-channel MOS transistor Q5 connected between a common drain terminal of Q1-Q4 and a ground point, and a MOS transistor connected in series with a constant current source CS0 and commonly connected to the gate of Q5 Q0, an inverter INV1 as potential determining means for receiving the potential of the connection node N1 between the common drain of Q1 to Q4 and Q5, and an inverter INV2 for inverting the output thereof.

  The MOS transistor Q0 has a so-called diode coupling in which the gate and drain are coupled, and converts the constant current I0 from the constant current source CS0 into a voltage. By connecting the gate terminals of the transistors Q0 and Q5, Q0 and Q5 constitute a current mirror circuit, and a current proportional to the size ratio of Q0 and Q5 flows through Q5. Thereby, Q5 functions as a constant current element.

  Although it is possible to use a resistor instead of Q5, the resistance on the semiconductor chip has a larger variation than the variation of the MOS transistor. Therefore, if the resistor is used, the relative judgment level shift by the inverter INV1 is large. Therefore, the constant current element is preferable. When the power supply voltage does not change, the inverter INV1 as the potential determination means can be easily designed by using a CMOS inverter. However, when the voltage that changes such as the output voltage VOUT of the charge pump is used as the power supply voltage, Therefore, it is necessary to design in consideration that the logic threshold value also changes when a CMOS inverter is used. Alternatively, the inverter INV1 may have a configuration in which a MOS transistor and a constant current source are connected in series, and the logic threshold value may be determined based on a power supply voltage reference or a ground potential reference.

  In this embodiment, although not particularly limited, the constant current I0 is designed to be about 1 μA, and the size ratio N between Q0 and Q5 is, for example, “1”. Further, in the monitoring circuit 15 of this embodiment, when any one of the transistors Q1 to Q4 is turned off, the potential of the node N1 changes across the logical threshold value of the inverter INV1, and the potentials of the external terminals LED1 to LED4. Are constants of the transistors constituting the monitoring circuit 15 so that the corresponding transistors of Q1 to Q4 are turned off when the forward voltage of the light-emitting diodes connected to these terminals has the highest forward voltage. Designed.

  Here, the design concept of the monitoring circuit 15 will be described. The white light emitting diodes currently on the market have a variation in the forward voltage Vf, and the diode does not emit light unless a voltage higher than the forward voltage Vf is applied to the anode terminal. Therefore, as in the above embodiment, when a plurality of white light emitting diodes are connected to the cathode common and a drive current is supplied from each anode terminal by the constant current circuit to emit light, the power supply voltage of the constant current circuit decreases and the diode When the anode voltage is lowered, light is emitted from the highest forward voltage. As a result, in a liquid crystal monitor using a white light emitting diode as a backlight, display unevenness occurs.

  Thus, although set makers strive to use diodes with uniform forward voltages as much as possible, variations are still inevitable. Generally, an allowable range of the forward voltage of the diode to be used is set, and a diode that is larger than the minimum forward voltage and smaller than the maximum forward voltage is selected and used. Therefore, if the potential of the diode connection terminal of the LED driver IC is smaller than the maximum forward voltage and the control is performed so as to increase the anode voltage, the light emission stop of the diode due to the voltage drop can be avoided. Can do. In the LED driver IC of the above embodiment, the monitoring circuit 15 is designed under such a concept.

  Next, the operation of the monitoring circuit 15 will be described in detail. In the LED driver IC of the above-described embodiment, when the battery voltage that supplies the input voltage Vin is sufficiently high, the charge pump 11 uses the input voltage Vin as it is as the output voltage VOUT as it is to the constant current sources CS1 to CS4 and the monitoring circuit 15. Supply. At this time, since the output voltage VOUT is sufficiently high and the voltage between the VOUT and LED terminals is also large, all the P channel MOS transistors Q1 to Q4 of the monitoring circuit 15 are turned on. Therefore, the potential of the node N1 becomes lower than the logic threshold value of the inverter INV1, and the output of the inverter INV2 becomes low level.

  Thereafter, when the battery voltage decreases, the voltage between the VOUT and LED terminals also decreases. At this time, the voltage between the VOUT and LED terminals has the smallest potential difference between the potential of the terminal to which the diode having the largest forward voltage is connected and VOUT. Become. When the LED terminal voltage to which the one having the largest forward voltage is connected becomes smaller than the forward voltage, the transistor corresponding to that terminal among Q1 to Q4 is turned off. Then, the current flowing through the entire Q1 to Q4 decreases and the potential of the node N1 becomes low. When the logic threshold value of the inverter INV1 is exceeded, the output of the inverter INV2 changes to a low level.

  As a result, the latch circuit 16 performs a latch operation, the output becomes a high level, and the charge pump 11 is switched from the 1 × output mode to the 1.5 × boost mode. As a result, the output voltage VOUT of the charge pump 11 becomes high, and the light emission stop of the diode due to the voltage drop is avoided.

  When the output voltage VOUT of the charge pump 11 increases, all the transistors Q1 to Q4 in the monitoring circuit 15 are turned on again and the output of the inverter INV2 is inverted, but the latch circuit 16 maintains the previous state. Therefore, the original 1 × output mode is not returned from the 1.5 × boost mode. The latch circuit 16 can be configured such that the latch is released by an enable signal input from the outside of the chip.

  FIG. 2 shows a second embodiment of a white LED driver IC to which the present invention is applied. The white LED driver IC of the first embodiment is a current output type driver, whereas the white LED driver IC of the second embodiment is a current drawing type driver.

  In this embodiment, a terminal OUT that outputs the output voltage VOUT of the charge pump 11 is provided in the chip, and the anode terminals of the four white light emitting diodes are commonly connected to the output terminal OUT, and the cathode terminals of the respective diodes are connected. It is connected to LED connection terminals LED1 to LED4, respectively. In the chip, constant current sources CS1 to CS4 are provided for drawing drive currents of the diodes from the LED connection terminals LED1 to LED4, respectively.

  Further, a current mirror comprising a constant current circuit 14a that generates a reference current upon receipt of the reference voltage Vref generated by the reference voltage generation circuit 13, and MOS transistors Q11 and Q12 that fold back the constant current generated by the constant current circuit. A circuit 14b is provided, and is configured to generate a diode driving current by supplying the drain current of Q12 to the constant current sources CS1 to CS4. A combination of the constant current circuit 14a and the current mirror circuit 14b in FIG. 2 corresponds to the constant current control circuit 14 in FIG.

  Furthermore, in the present embodiment, a step-up rate switching circuit 17 that receives the potentials of the LED connection terminals LED1 to LED4 is provided. The step-up rate switching circuit 17 corresponds to a circuit combining the monitoring circuit 15 and the latch circuit 16 in FIG. However, the monitoring circuit in the present embodiment is vertically symmetrical with the monitoring circuit 15 in FIG. 1, that is, an N-channel MOS transistor is substituted for the P-channel MOS transistors Q1 to Q4, and a P-channel MOS is substituted for the N-channel MOS transistors Q0 and Q5. A transistor is used, the source terminals of Q1 to Q4 are connected to the ground point, and the source terminals of Q0 and Q5 are connected to the output terminal.

  The operation of the circuit is the same as that in FIG. 1, and when the potential difference between the output voltage VOUT and the voltages of the terminals LED1 to LED4 becomes smaller than the one with the highest forward voltage among the light emitting diodes connected to them. It operates so that the corresponding transistor of Q1 to Q4 is turned off. As a result, when the battery voltage drops to some extent, the charge pump 11 is switched from the 1 × output mode to the 1.5 × boost mode, the output voltage VOUT of the charge pump 11 is increased, and the light emission stop of the diode due to the voltage drop is avoided. .

  FIG. 4 shows a third embodiment of a white LED driver IC to which the present invention is applied.

  In the above-described embodiment, the charge pump can be switched between the 1 × output and the 1.5 × boost output, whereas in the third embodiment, the charge pump 11 has the 1 × output and the 1.5 × boost. It is possible to switch between three levels of output and double boost output. Further, a voltage comparison circuit CMP that detects the level of the input voltage Vin by comparing with the reference voltage Vref is provided, and based on the output of the voltage comparison circuit CMP and the output of the monitoring circuit 15 described in the first embodiment, The boosting rate of the charge pump 11 is switched.

  For example, when the output of the monitoring circuit 15 and the output of the voltage comparison circuit CMP are both at low level, the boosting rate is switched to 1 × output, and either the output of the monitoring circuit 15 or the output of the voltage comparison circuit CMP is changed to high level. When the output of the monitoring circuit 15 and the output of the voltage comparison circuit CMP are both at the high level, it may be controlled to switch to the double boost. Note that the double boost output of the charge pump is obtained by, for example, charging two capacitors together to the input voltage Vin based on the ground potential, and then boosting (boosting) the terminal to which the ground potential is applied by switching to the input voltage Vin. Can be obtained at

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the embodiment. For example, in the above-described embodiment, the driver IC in which the number of light-emitting diodes that can be driven is four is shown, but the present invention can also be applied to the case where the number of light-emitting diodes is five or more.

  In the above embodiment, the monitoring circuit 15 is formed of a MOS transistor. However, the monitoring circuit 15 may be formed of a bipolar transistor. Furthermore, in the above-described embodiment, it has been described that the latch circuit 16 at the subsequent stage of the monitoring circuit 15 is configured to be unlatched by an enable signal input from the outside of the chip. However, the control logic 12 is connected to an input voltage detection circuit (not shown) If it is detected by a signal or the like that the input voltage has become sufficiently high, the latch circuit 16 may be reset to release the latch.

  In the above description, the example in which the present invention is applied to an LED driver IC for lighting a white light emitting diode used as a backlight of a liquid crystal monitor has been described. However, the present invention is not limited to the present invention, and a boosted voltage is applied. It can be widely used for ICs that generate and want to control the output current.

It is a block block diagram which shows 1st Embodiment of the LED drive device (LED driver IC) to which this invention is applied. It is a block block diagram which shows 2nd Embodiment of the LED drive device (LED driver IC) to which this invention is applied. It is a circuit explanatory drawing which shows the operation principle of 1.5 time boosting of the charge pump in the LED driver IC of the embodiment. It is a circuit block diagram which shows the specific example of the step-up rate switching circuit in 3rd Embodiment of the LED drive device (LED driver IC) to which this invention is applied. It is a circuit block diagram which shows an example of the LED terminal voltage monitoring circuit in the LED driver examined prior to this invention.

Explanation of symbols

10 LED driver IC
DESCRIPTION OF SYMBOLS 11 Charge pump 12 Control logic 13 Reference voltage generation circuit 14 Constant current control circuit 15 Monitoring circuit 16 Latch circuit 17 Boosting ratio switching circuit OSC Oscillation circuit CS0-CS4 Constant current source LED1-LED4 Light emitting diode connection terminal

Claims (5)

An LED driving device that generates and outputs a predetermined driving current that flows to each of a plurality of light emitting diodes by a voltage from a boosting circuit that can boost and output an input voltage and can stepwise switch a boosting rate,
A monitoring circuit for monitoring each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes;
When the monitoring circuit detects that at least one of the anode-cathode terminal voltages of the plurality of light emitting diodes is smaller than the forward voltage of the diode having the largest forward voltage among the plurality of light emitting diodes. , Configured to switch the boosting rate of the booster circuit to a higher one,
The monitoring circuit is
A plurality of transistors in parallel form that receive each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes at a control terminal;
A constant current source connected to a common drain terminal of the plurality of transistors;
A potential determining means for determining a potential at a connection point between the common drain terminal and the constant current source;
The source terminals of the plurality of transistors are connected to a node to which the output voltage of the booster circuit is supplied ,
As one of the plurality of transistors becomes different from the other transistors in response to a decrease in the output voltage of the booster circuit, a detection signal is output when the potential at the connection point exceeds the determination level. An LED driving device characterized by being configured as described above . A plurality of external terminals to which each light-emitting diode can be connected, a booster circuit that can output a boosted voltage by receiving the battery voltage as an input voltage, and that can switch the boost rate stepwise, and a voltage from the booster circuit A plurality of constant current circuits that generate predetermined drive currents respectively flowing through the plurality of light emitting diodes, and an LED drive device formed on one semiconductor chip,
A monitoring circuit for monitoring each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes;
When the monitoring circuit detects that at least one of the anode-cathode terminal voltages of the plurality of light emitting diodes is smaller than the forward voltage of the diode having the largest forward voltage among the plurality of light emitting diodes. , Configured to switch the boosting rate of the booster circuit to a higher one,
The monitoring circuit is
A plurality of transistors in parallel form that receive each anode terminal voltage or cathode terminal voltage of the plurality of light emitting diodes at a control terminal;
A constant current source connected to a common drain terminal of the plurality of transistors;
A potential determining means for determining a potential at a connection point between the common drain terminal and the constant current source;
The source terminals of the plurality of transistors are connected to a node to which the output voltage of the booster circuit is supplied ,
As one of the plurality of transistors becomes different from the other transistors in response to a decrease in the output voltage of the booster circuit, a detection signal is output when the potential at the connection point exceeds the determination level. An LED driving device characterized by being configured as described above . The LED driving device according to claim 2 , wherein the monitoring circuit includes an inverter circuit that determines the potential of the connection point based on a logical threshold value as the potential determination unit. The LED driving device according to claim 2, wherein a latch circuit that latches an output of the monitoring circuit is provided at a subsequent stage of the monitoring circuit. The booster circuit is configured to be switchable its output voltage in two stages, according to claim, characterized in that it is configured so that the output voltage of the boosting circuit based on only the output of the monitoring circuit is switched 2 The LED drive device in any one of -4 .

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