KR20070015857A - Led driving device - Google Patents

Abstract

By driving the LED at a constant current and optimizing the voltage supplied to the LED, the heat generation of the group of circuit elements including the LED is reduced to provide an LED driving device capable of stably lighting the backlight unit even during dimming. The LED drive device of the present invention is the first negative feedback closed loop CL1 for supply voltage control to the DC / DC converter 8, the drive current control circuit 10, the voltage comparison circuit 12, and the output voltage control circuit 13. And a second negative feedback closed loop CL2 for constant current control with the drive current control circuit 10 and the constant current control circuit 11, and the frequency response characteristic of the first negative feedback closed loop CL1 for supply voltage control. Is set to 1/20 or less of the frequency response characteristic of the second negative feedback closed loop CL2 for constant current control so that the first negative feedback closed loop CL1 for supply voltage control can be stably operated without impairing the responsiveness of the constant current control. It is.

LED, current, voltage, output voltage, constant current, negative feedback closed loop, resistance, dimming rate

Description

LED driving device {LED DRIVING DEVICE}

1 is a block diagram of an LED driving circuit of a first embodiment of the present invention.

Fig. 2 is a circuit diagram of the LED driving circuit of the first embodiment of the present invention.

Fig. 3 is an oscillation waveform diagram of the voltage of the LED driving circuit of the first embodiment of the present invention.

Fig. 4 is a block diagram of the LED driving circuit of the second embodiment of the present invention.

Fig. 5 is a circuit diagram of the LED driving circuit of the second embodiment of the present invention.

Fig. 6 is an oscillation waveform diagram (No. 1) of the voltage of the LED driving circuit of the second embodiment of the present invention.

Fig. 7 is an oscillation waveform diagram (No. 2) of the voltage of the LED driving circuit of the second embodiment of the present invention.

Fig. 8 is a block diagram of the LED driving circuit of the third embodiment of the present invention.

Fig. 9 is a circuit diagram of the LED driving circuit of the third embodiment of the present invention.

10 is a circuit diagram showing an example of a control reference value creating circuit according to a third embodiment of the present invention.

Fig. 11 is an explanatory diagram showing the concept of setting V _REF2 in the third embodiment of the present invention.

12 illustrates a general backlight structure using LEDs.

13 is a block diagram of a conventional LED drive circuit.

14 is a circuit diagram of a conventional LED driving circuit.

Fig. 15 is a graph showing the characteristic between the dimming rate and the current in the third embodiment and the conventional example of the present invention.

<Description of the symbols for the main parts of the drawings>

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 condenser

18: transistor

19: negative feedback amplifier

20: voltage comparison amplifier

21: Comparator

22: negative side amplifier

23: plus side amplifier

24: control reference value creation circuit

25: mirror circuit

31: differential voltage output circuit

32: control reference value creation circuit

33: light control ratio determination circuit

[Patent Document 1] Japanese Unexamined Patent Publication No. 2003-152224

The present invention relates to an LED drive device.

Conventionally, a cold cathode fluorescent lamp containing mercury as a light source has been used as a backlight for liquid crystals such as navigation, but in order to reduce the use of mercury, which is a hazardous substance, a light emitting diode element such as white or RGB is used as a light source to replace. The used backlight unit attracts attention and is gradually commercialized.

12 is a diagram illustrating 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. On the LED mounting substrate 102, LEDs such as white and the like are arranged, and the light generated from these LEDs 101 is deflected by the light guide plate 103, passes through the optical sheet 105, and the like, and the display unit 107. Is irradiated to the liquid crystal. In addition, in the past, instead of the LED 101 and the LED mounting substrate 102, a cold cathode fluorescent lamp in which mercury is sealed as a light source has been used.

Hereinafter, a conventional technique of controlling the lighting of the LED will be described. The luminous brightness of the LED depends on the magnitude of the current flowing through it. For this reason, in order to make a LED light stably, it is necessary to drive LED with a constant current. Fig. 13 is a block diagram of a conventional lighting circuit configured to drive a constant current of an LED, and Fig. 14 is a circuit diagram of a specific circuit example.

In Fig. 13, two or more LEDs 201 are connected in series, and at one end thereof, a resistor R _CS for detecting a current flowing in the LED 201 is connected to ground (GND). In order to flow a constant current to the LED (201), by sending the comparison voltage by the voltage converting the current flowing through the resistor R to the _CS voltage comparing circuit 212, it is compared with a desired reference voltage. In addition, in accordance with the comparison result in the voltage comparison circuit 212, the output voltage control circuit 213 controls the DC / DC converter 8 such as a step-down or step-up switching method and smoothes the output voltage thereof. A negative feedback closed loop control CL3 is performed by supplying the LED 201 with a desired DC voltage value V _SW2 having good power efficiency at 219.

14 is a specific circuit example of this conventional LED drive circuit, and is configured as follows. Voltage conversion is performed by flowing a current flowing through the LED 201 through the detection resistor R _CS , and the voltage conversion value is used as a comparison value of the voltage comparison circuit 220 for error amplification composed of an operational amplifier or the like, and the reference voltage V _Ref. Compare with In addition, the output of the voltage comparison circuit 220 is compared with the triangular wave V _OSC2 by the output voltage control circuit 221 composed of a comparator, and generates a pulse-shaped square wave voltage V _G2 whose DUTY is changed by the comparison result. . Then, the gate of the switching element 214 formed of the P-channel FET is controlled by the pulse-shaped square wave voltage V _G2 of which the duty changes, and the DC voltage of the power supply V _IN is converted into a predetermined DC voltage and output. That is, it operates as a DC / DC converter 208 of the SW power supply system. After the drain terminal of the switching element 214 is connected to the cathode of the diode 215 and the inductor 216, the voltage output from the other end of the inductor 216 is converted into a direct current voltage V _SW2 by the smoothing capacitor 217. The high efficiency voltage is supplied to the LED 201 group.

In the negative feedback closed loop CL3 configured in this manner, it is controlled to be I _LED × R _CS = V _Ref (constant), and the current I _LED flowing through the LED 201 is turned on at a constant current of approximately V _Ref / R _CS . do. That is, when the current flowing through the LED 201 is larger than the desired current, the pulsed square wave voltage V _G2 having a narrow on-period is supplied to the gate of the switching element 214 to smooth the applied to the LED 201. The voltage V _SW2 is lowered and functions in a direction of decreasing the current of the LED 201. On the contrary, when the current flowing in the LED 201 is smaller than the desired current, the pulse-shaped square wave voltage V _G2 having a wide on-period in the on period is supplied to the gate of the switching element 214 to be applied to the LED 201. The smoothed voltage V _SW2 becomes high and functions in the direction of increasing the current of the LED 201. By this negative feedback closed loop control, a desired constant current I _LED whose voltage value of the voltage detection resistor R _CS is equal to the reference voltage V _REF flows to the LED 201 to create a stable state.

The circuit example shown in FIG. 14 is a case where the voltage value of the power supply V _IN is greater than or equal to the maximum voltage value necessary to light up the LED 201 at a desired brightness, and constitutes a step-down type DC / DC converter 208. do. However, three types of step-down, step-up, and step-down types are used depending on the magnitude of the voltage value of the power supply V _IN and the maximum voltage value required to supply the LED 201 to the DC / DC converter 208. do.

In the structure of this conventional circuit, by controlling the output voltage of the DC / DC converter 208, the LED 201 can be driven with a constant current, and high efficiency driving can be realized regardless of the VF standard of the LED 201. Since the output of the DC / DC converter 208 is controlled with the required minimum driving voltage according to the desired constant current value and the temperature-dependent VF value, the loss due to the voltage margin considered from the VF specification does not occur. there was.

As an example of a patent similar to the conventional circuit example, Patent Document 1 discloses a technique in which the type of the DC / DC converter 208 is limited to a boost type and applied to a portable device.

However, in the conventional LED drive circuit as shown in Figs. 13 and 14, when the voltage variation of the power supply V _IN , the noise from the outside, the disturbance noise jumping into the current detection resistor portion, or the variation in the control loop occur, the constant current is generated. The control becomes unstable, and there is a technical limitation in that the response speed (frequency characteristic) and the gain of the control loop CL3 cannot be made very high. Therefore, the above-described conventional circuit is optimal to be employed to achieve a purpose as a backlight sufficiently by always passing a constant current to the LED 201 such as a mobile phone, but a product in which the desired constant current value changes frequently, for example For example, it was insufficient to be applied to products in fields requiring dimming functions. In particular, in the above-described conventional circuit, since the stable dimming performance is not obtained in the region where the dimming ratio is approximately 10% or less, and the current characteristic of the LED 201 with respect to the dimming ratio also increases, the error increases to 5%. Further improvement of technology has been required for the adoption of liquid crystal backlights for vehicle navigation, where linear characteristics are required up to the following extremely low dimming ratios.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the prior art, and drives light emitting diodes (LEDs) with a constant current and optimizes the voltage supplied to the LEDs to reduce heat generation of the group of circuit elements including the LEDs. Accordingly, an object of the present invention is to provide an LED driving device capable of stably lighting a backlight unit even during dimming.

The LED driving apparatus according to the first embodiment of the present invention includes a plurality of light emitting diodes connected in series and DC / converting the output voltage of a DC power supply to a predetermined voltage value to an anode side of the series connected light emitting diodes. A DC converter, a drive current control circuit having one end connected to the cathode side of the series-connected light emitting diode, a resistor for current detection having one end connected to the other end of the drive current control circuit and grounded at the other end thereof; And an output voltage control circuit for controlling the DC / DC converter to control output voltages supplied to the plurality of series-connected light emitting diodes, and to issue an output voltage command to the output voltage control circuit. The voltage obtained by converting the current flowing through the resistance of the circuit is regarded as the first comparison voltage, and the difference voltage is output to the output voltage control circuit in comparison with the desired reference voltage. A constant current control circuit for comparing the voltage comparison circuit and a second comparison voltage obtained by converting a current flowing through the current detecting resistor with a predetermined voltage for constant current control, and controlling the drive current control circuit so that the energized current becomes a constant current. And a closed loop composed of the DC / DC converter, the drive current control circuit, the voltage comparison circuit, and the output voltage control circuit as a first negative feedback closed loop for supply voltage control, and the drive current control circuit and the constant current control. The closed loop composed of the circuit is a second negative feedback closed loop for constant current control.

In addition, the LED driving apparatus according to the second embodiment of the present invention converts and supplies an output voltage of a DC power supply to a predetermined voltage value to a plurality of light emitting diodes connected in series and to an anode side of the light emitting diodes connected in series. A DC / DC converter, a drive current control circuit whose one end is connected to the cathode side of the series-connected light emitting diode, and one end thereof connected to the other end of the drive current control circuit, and the other end is used for current detection. The difference between the output voltage control circuit for controlling the DC / DC converter and the difference voltage between the both ends of the drive current control circuit in order to supply an optimum voltage efficiently to the plurality of series-connected light emitting diodes. In order to give an output voltage command to the voltage calculating circuit and the output voltage control circuit, the difference voltage calculated by the difference voltage calculating circuit is a first comparison voltage, A voltage comparison circuit for outputting a voltage difference to the output voltage control circuit in comparison with a reference voltage to be compared; and a second comparison voltage obtained by converting a current flowing through the resistance for current detection; A closed loop comprising a DC / DC converter, a drive current control circuit, a differential voltage calculation circuit, a voltage comparison circuit, and an output voltage control circuit, having a constant current control circuit for controlling the drive current control circuit so that the energizing current is a constant current. Is a first negative feedback closed loop for supply voltage control, and a closed loop composed of the drive current control circuit and the constant current control circuit is used as a second negative feedback closed loop for constant current control.

EMBODIMENT OF THE INVENTION Hereinafter, the Example of this invention is described in detail based on drawing.

(First embodiment)

1 is a block diagram of an LED driving device of a first embodiment of the present invention. In Fig. 1, reference numeral 1 denotes a light emitting diode (LED), reference numeral 8 denotes a DC / DC converter, reference numeral 9 a smoothing circuit, reference numeral 10 a driving current control circuit, reference numeral 11 a constant current control circuit, and reference numeral 12 Denotes a voltage comparison circuit, and reference numeral 13 denotes an output voltage control circuit.

Two or more LEDs 1 are connected in series, and on the anode side, step-down or step-up or step-down switching or chopper for DC / DC conversion of DC power from DC power V _IN such as a battery. A DC / DC converter 8 such as a system is connected via the smoothing circuit 9. On the cathode side of the LED 1, a drive current control circuit 10 such as a transistor or a FET, and a resistor R _CS for current detection are sequentially connected, and the other end of the resistor R _CS is grounded (GND). .

In order to supply the optimum optimal voltage to the LED 1, the DC / DC converter 8 is controlled by the output voltage control circuit 13. In addition, in order to control the drive current control circuit 10 to flow a constant current through the LED 1, the drive current control circuit 10 is controlled by the constant current control circuit 11.

The control of the DC / DC converter 8 is the same as the conventional one, and sends a comparison voltage (= R _CS × I _LED ) obtained by voltage-converting the current I _LED flowing through the resistor R _CS to the voltage comparison circuit 12 to provide a desired reference voltage. Compare to V _REF . In addition, according to the comparison result in the voltage comparison circuit 12, the output voltage control circuit 13 controls the DC / DC converter 8, and the output voltage is the desired DC having good power efficiency in the smoothing circuit 9. By supplying the LED 1 as the voltage value V _SW1 , the negative feedback closed loop control CL1 is performed.

The constant current control by the constant current control circuit 11 compares the comparison voltage of the said voltage detection resistor with a predetermined value, and controls the conduction current of the drive current control circuit 10 to become a constant current, and the closed loop control of negative feedback CL2 is performed.

The LED drive device of the present embodiment comprises a closed loop CL2 for control for flowing a constant current to the LED 1 in a circuit group constituting the closed loop CL1 for supply voltage control for supplying an optimal optimum voltage to the LED 1. The circuit group 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 flowing a constant current through the LED 1 is common. The common part of the closed loop CL1 and the closed loop CL2 is a part which converts the electric current which flows in LED1 into voltage information using the detection resistor _RCS , and the voltage of this detection resistor _RCS is a constant current control circuit 11 And voltage comparison circuit 12, respectively. 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 as comparison information for constant current control. using each of the closed loop control CL1, CL2 by the control unit will return to match the desired reference value _Ref V, I _o × V _R. Here, by setting the frequency response of the closed loop CL1 for supply voltage control to 1/20 or less of the frequency response of the closed loop CL2 for constant current control, the negative feedback closed loop CL1 for supply voltage control is not impaired. Can operate stably. In other words, since the response characteristics of the frequencies of the negative feedback closed loops CL1 and CL2 are 20 times or more different, they do not interfere with each other and become unstable, so that the gain and frequency response of the negative feedback closed loop CL2 for constant current control can be set high. In this respect, a very accurate constant current characteristic can be achieved.

2 is a specific circuit diagram of the LED driving device of the present embodiment. 2, reference numeral 14 denotes a MOS-FET, reference numeral 15 denotes a diode, reference numeral 16 denotes an inductor, reference numeral 17 denotes a capacitor, reference numeral 18 denotes an LED driving element (transistor), reference numeral 19 denotes a negative feedback amplifier circuit, Reference numeral 20 denotes a voltage comparison amplifier, and reference numeral 21 denotes a comparator (comparator). In addition, the code | symbol same as FIG. 1 has used the code | symbol "1" of LED.

It demonstrates, comparing FIG. 1 and FIG. Corresponding to the DC / DC converter 8 of FIG. 1 is the MOS-FET 14 and the diode 15 of the P-channel of FIG. 2, and the output voltage control circuit 13 is the comparator 21 and the voltage comparison. The circuit 12 is a voltage comparison amplifier 20, and the smoothing circuit 9 is 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. 1 corresponds to the drive current control circuit 10 and the transistor 18 corresponds to the constant current control circuit 11 and the negative feedback amplifier circuit 19 loaded with the transistor 18 as a load.

Here, the emitter output current of the transistor 18 partially returns through the resistor, and is fed back to an input terminal of one of the negative feedback amplifier circuits 19 constituted by the differential amplifier. The output of this negative feedback amplifier circuit 19 is supplied to the base electrode of the transistor 18. The feedback circuit constituted by the transistor 18 and the negative feedback amplifier circuit 19 constitutes a pulse transmission circuit, and the duty ratio of the transmission pulse is connected to the other input terminal of the negative feedback amplifier circuit 19. It can be adjusted by adjusting the supplied reference voltage (I ₀ × voltage determined by the resistance value of the variable resistor V _R) level.

Next, operation | movement of the LED drive device comprised in this way is demonstrated. First, the negative feedback closed loop CL1 for supply voltage control for supplying the LED 1 with the required lowest optimum voltage with good power efficiency operates as follows. Since the driving current control circuit 10 that controls the current flowing through the LED 1 does not operate in a saturation region that cannot follow constant current operation and responsiveness, most of the required minimum voltage values are previously set as a control target value. 20) is set as the first reference value V _Ref . In the voltage comparison amplifier 20, the current flowing through the LED 1 is converted into the voltage value V _E (= R _CS × I _LED ) using the detection resistor R _cs , and the voltage value V _E is used as comparison information. The reference voltage value V _Ref is compared with the comparison voltage V _E. The comparator 21 compares the output of the voltage comparison amplifier 20 with the triangular wave V _OSC1, and generates a pulse-shaped square wave voltage V _G1 whose DUTY changes as a result of the comparison. Then, the gate of the P-channel FET 14 is controlled by the pulse-shaped square wave voltage V _G1 in which the duty changes, and the DC voltage V _IN of the DC power supply is converted into the predetermined DC voltage V _SW1 and output. The direct current output of the FET 14 is smoothed by the smoothing inductor 16 and the capacitor 17, and is supplied to the LED 1 as a voltage having good power efficiency.

On the other hand, the operation of the closed loop CL2 for the constant current control for flowing the desired constant current to the LED 1 uses the negative feedback voltage value V _E for the negative feedback loop CL1 for the previous supply voltage control in common. as compared to a second reference voltage value (I ₀ × resistance value of the variable resistor V _R) corresponding to a desired current value, the feedback loop control unit a drive current control circuit 10 according to a variation.

Here, as described above, the frequency response of the closed loop CL1 for supply voltage control is set to 1/20 or less of the frequency response of the closed loop CL2 for constant current control. This setting is mainly possible by setting the time constants of the smoothing inductor 16 and the capacitor 17 included in the closed loop CL1 for supply voltage control to a predetermined value. That is, by setting this time constant large, it is possible to always supply a constant supply voltage to the LED 1 for the on / off operation of the transistor 18 in the closed loop CL2 for constant current control.

3 is an oscillation waveform diagram of the voltage of the LED driving apparatus of the present embodiment. It can be seen that the voltage value V _SW1 having a good power conversion efficiency required for the desired constant current to flow to the LED 1 is supplied to the input voltage V _IN , and a linear and stable constant current I _LED is provided. Here, the voltage V _SW1 having a good power conversion efficiency constitutes the voltage plus the driving current control circuit 10 plus each V _F (forward voltage) required for the LED 1 to flow a desired current under ambient temperature conditions. The voltage of the collector-emitter electrode and the voltage of the detection resistor R _CS in the state in which the transistor 18 operates without saturation (saturation) becomes the total voltage value.

According to the LED driving apparatus of the present embodiment, in addition to the first negative feedback closed loop for controlling the voltage supplied to the plurality of light emitting diodes connected in series, the second part for controlling the current supplied to the plurality of light emitting diodes connected in series Since the feedback closed loop is set, the light emitting diode is always driven at a constant current and the voltage supplied to the light emitting diode is optimized, thereby eliminating heat generation in the group of circuit elements including the light emitting diode, thereby stabilizing the backlight unit even during dimming. Can be turned on.

That is, according to the LED drive device of the present embodiment, extremely low dimming performance is required, for example, a liquid crystal backlight used for in-vehicle navigation, and as a result, even in a device in which the current value supplied to the light emitting diode changes frequently, By making it variable according to a dimming ratio, it can fully respond and can obtain stable dimming performance even in the extremely low dimming region like 5% or less.

Further, according to the present invention, the response of the constant current control is set by setting the frequency response characteristic of the first negative feedback closed loop for supply voltage control 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 damaging. That is, since the response characteristics of the frequencies of the negative feedback closed loops are 20 times or more different, they do not interfere with each other and become unstable, so that the gain and the frequency response of the second negative feedback closed loop for constant current control can be set high, so that the accuracy is quite high. Good constant current characteristics can be achieved.

Therefore, by varying the voltage reference according to the dimming rate, the product can be sufficiently adapted to a product in a field where a desired constant current value changes frequently or a field in which a very low dimming performance is required, in particular 5% or less required for a liquid crystal backlight for vehicle navigation. Linear current characteristics in the extremely low dimming region can be achieved.

(2nd Example)

4 is a block diagram of the LED driving device of the second embodiment of the present invention. This embodiment is characterized in that a difference voltage calculating circuit 31 is added to the configuration of the LED drive device of the first embodiment shown in FIG. The difference voltage calculating circuit 31 calculates the voltage between the drain and source electrodes or the voltage between the collector and emitter electrodes of the drive current control circuit 10, and calculates the calculated difference voltage with respect to the voltage comparison circuit 12. To print. In addition, in FIG. 4, the same code | symbol is shown for the same element as FIG.

A plurality of LEDs 1 are connected in series, and on the anode side thereof, a step-down or step-up or step-down switching or chopper method for supplying a power-efficient voltage from a DC power supply V _IN such as a battery. The DC / DC converter 8 is connected via the smoothing circuit 9. The cathode side of the LED 1 connects a resistor R _CS for current detection via a drive current control circuit 10 such as a transistor or a FET, and grounds the other end of the resistor R _CS to ground (GND).

The LED drive device of the present embodiment is a circuit group constituting a closed loop CL1 for supply voltage control for supplying an optimal voltage with high power efficiency to the LED 1, and for constant current control for flowing a constant current through the LED 1. The circuit group constituting the closed loop CL2 is a completely separate configuration.

Next, the operation of the LED driving device configured as described above will be described. First, in order to supply the LED group with the lowest power required and the most efficient and efficient, the drive current control circuit 10 that controls the current flowing through the LED 1 cannot be followed in constant current operation and responsiveness. Since most operations do not operate, most of the minimum voltage values required are set as control target values in advance and set as the first reference values of the voltage comparison circuit 12. In the voltage comparison circuit 12, the reference voltage value and the voltage comparison value calculated by the difference voltage calculating circuit 31, that is, the voltage between the drain-source electrode or the collector-emitter electrode of the drive current control circuit 10 Compare with voltage. In response to the output of the voltage comparison circuit 12, the output voltage control circuit 13, the DC / DC converter 8, and the smoothing circuit 9 cause the drive current control circuit 10 to become a voltage that does not saturate. The negative feedback closed loop CL1 for supply voltage control is operated.

On the other hand, the operation of the closed loop CL2 for constant current control for flowing a desired constant current to the LED 1 converts the current flowing through the LED 1 into voltage information I _LED × R _cs using the detection resistor R _cs . The voltage value and the reference voltage value corresponding to the desired current value are compared, and the driving current control circuit 10 performs negative feedback loop control according to the error.

Fig. 5 is a specific circuit diagram of the LED driving device of the second embodiment of the present invention. The circuit configuration of this second embodiment is characterized by a configuration in which a negative side amplifier 22 and a plus side amplifier 23 are additionally added to the circuit configuration of the first embodiment shown in FIG. In addition, in FIG. 5, the same code | symbol is shown for the same element as FIG.

It demonstrates, comparing FIG. 5 with FIG. Corresponding to the DC / DC converter 8 in FIG. 4 is the MOS-FET 14 and the diode 15 of the P channel in FIG. 5, and the output voltage control circuit 13 is the comparator 21 and the voltage comparison. The circuit 12 is a voltage comparison amplifier 20, and the smoothing circuit 9 is an inductor 16 and a capacitor 17. The circuit of this embodiment has a configuration of a step-down switching power supply DC / DC converter similarly to the first embodiment. In FIG. 4, the transistor 18 corresponds to the drive current control circuit 10, and the negative feedback amplifier circuit 19 loaded with the transistor 18 corresponds to the constant current control circuit 11.

First, in the closed loop CL1 for supply voltage control, the transistor 18 which controls the current which flows through the LED 1 in order to supply the LED 1 with the required lowest optimal voltage V _SW1 with good power efficiency is subjected to constant current operation and response. The voltage comparison amplifier 20 is previously assigned a control target value V _Ref1 (I ₁ × R ₂ ) as the control target value V _Ref1 (I ₁ × R ₂ ), in order to not operate in the saturation region that cannot be followed. The voltage difference between the drain-source and the voltage difference between the collector and the emitter of the driving current control circuit 10 is measured by the amplifier 22, the amplifier 23, the resistor R 31, the mirror circuit 25, and the resistor R ( It calculates by the difference voltage calculation circuit 31 comprised by 32, and outputs the difference voltage to the voltage comparison amplifier 20 as a comparison value V _COMP . Similar to the first embodiment, the voltage comparison amplifier 20, the voltage comparison circuit 12, and the comparator 21, the output voltage control circuit 13, supply the supply voltage such that the comparison value V _COMP coincides with the target value V _Ref1. The negative feedback closed loop CL1 for control is operated. The reference value V _Ref1 (I ₁ × R ₂ ) of the voltage amplifier 20 for controlling the supply voltage is not limited to a constant value, and can be selected according to the current or dimming rate that flows to the LED 1 from two or more kinds of set values. You may do so.

On the other hand, in the closed loop CL2 for constant current control, the current I _LED flowing through the LED 1 is current-voltage-converted by the current detecting resistor R _CS , and the voltage value V _E (= I _LED × R _CS ) is used for the constant current. The base current of the transistor 18 is controlled to match the reference voltage value V _Ref2 (= I ₀ × R ₁ ) corresponding to the current of the target LED 1 as the comparison information of the negative feedback amplifier 19 of N s. To operate the negative feedback amplifier circuit. As a result, the transistor 18 operates to draw the desired constant current from the LED 1.

Here, as in the first embodiment described above, the frequency response of the closed loop CL1 for supply voltage control is set to 1/20 or less of the frequency response of the closed loop CL2 for constant current control. As in the first embodiment, this setting is mainly possible by setting time constants of the smoothing inductor 16 and the capacitor 17 included in the closed loop CL1 for supply voltage control to a predetermined value. That is, by setting this time constant large, it is possible to always supply a constant supply voltage to the LED 1 for the on / off operation of the transistor 18 in the closed loop CL2 for constant current control.

6 is an oscillation waveform diagram of the collector voltage V _C , the emitter voltage V _E of the transistor 18, which is the driving current control circuit 10, and FIG. 7 is a DC power supply voltage V _{IN , an} LED driving voltage V _SW1 , and an LED current. Oscilloscope waveform of I _LED . It can be seen that the voltage value V _SW having a good power conversion efficiency required for the desired constant current to flow to the LED 1 is supplied to the input voltage V _IN , and a linear and stable constant current I _LED is provided. Here, the voltage V _SW having high power conversion efficiency is a voltage obtained by adding each V _F necessary for the LED 1 to flow a desired current under ambient temperature conditions and in an operating state in which the transistor 18 is not saturated. collector-emitter voltage electrode chiyida the total voltage of the sense resistor R _CS liver.

According to this 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 supply voltage is not impaired without compromising the responsiveness of the constant current control. The negative feedback closed loop CL1 for control can be operated stably. That is, since the frequency response of each of the negative feedback closed loops CL1 and CL2 is 20 times or more different, they do not interfere with each other and become unstable, so that the gain and frequency response of the negative feedback closed loop CL2 for constant current control can be set high. As a result, a very accurate constant current characteristic can be achieved. Therefore, by varying the voltage reference according to the dimming rate, it is possible to sufficiently cope with a product in a field where a desired constant current value changes frequently or a product in a field where a very low dimming performance is required, and is particularly required for a liquid crystal backlight for vehicle navigation. Linear current characteristics can be achieved in the following extremely low dimming region.

(Third Embodiment)

Fig. 8 is a block diagram of the LED driving device of the third embodiment of the present invention. This embodiment is characterized by the configuration in which the control reference value creating circuit 32 and the dimming rate determination circuit 33 are added to the second embodiment shown in FIG. 8, the same code | symbol is shown for the same element as FIG. 1, FIG. 4, and detailed description is abbreviate | omitted.

Step-down or step-up or step-down switching or chopper methods for connecting a plurality of LEDs 1 in series and supplying a power-efficient voltage from a DC power supply V _IN such as a battery to the anode side. DC / DC converter 8 is connected via smoothing circuit 9. In addition, a driving current control circuit 10 such as a transistor or a FET, and a resistor R _CS for current detection are sequentially connected to the cathode side of the LED 1, and the other end of the resistor R _CS is grounded.

Also in this embodiment, the circuit group constituting the supply voltage control closed loop CL1 for supplying the optimal value of power efficiency to the LED 1 and the closed loop CL2 for constant current control for flowing a constant current to the LED 1 are provided. The circuit group constituting the circuit is completely separate. First, the operation of the negative feedback closed loop CL1 for supply voltage control is as follows. In order to supply the LED group with the lowest power required and the most efficient, the drive current control circuit 10 for controlling the current flowing through the LED 1 operates in a constant current, and in a saturation region incapable of following responsiveness. The minimum voltage value necessary for not operating is set in advance as a control target value. This control target value is created by the control reference value creation circuit 32. The control reference value creating circuit 32 creates a control reference value corresponding to the dimming ratio determination value from the dimming ratio determination circuit 33 and outputs it as a reference value to the voltage comparison circuit 12.

Here, the dimming control signal DIM is supplied to the dimming rate determination circuit 33. This dimming control signal DIM is a control signal for changing the light emission luminance of the LED 1 and is a signal in which a dimming ratio, which is a ratio of luminance to the maximum luminance of the LED 1, is associated with, for example, a current level. The dimming rate determining circuit 33 determines the dimming rate from the current level of the input dimming control signal DIM, and outputs a signal corresponding to the dimming rate.

The voltage comparison circuit 12 compares the voltage between the drain-source electrodes or the voltage between the collector-emitter electrodes of the drive current control circuit 10 with the reference value calculated by the difference voltage calculating circuit 31 and the reference value, and compares them. The result is output to the output voltage control circuit 13. The output voltage control circuit 13 operates in the same manner as the conventional example, controls the output voltage of the DC / DC converter 8 and flows from the smoothing circuit 9 to the drive current control circuit 10 through the LED 1. The direct current is controlled so as to be a voltage which does not saturate the drive current control circuit 10.

On the other hand, the operation of the closed loop CL2 for constant current control for flowing a desired constant current to the LED 1 converts the current flowing through the LED 1 into voltage information using the detection resistor R _cs , and the voltage value and the desired value. The reference voltage value corresponding to the current value is compared, and the driving current control circuit 10 performs negative feedback loop control according to the error.

The dimming control signal DIM is also supplied to the constant current control circuit 11 to control the duty ratio of the output pulses of the transmitting circuit including the transistor 18 in accordance with the dimming control signal DIM.

9 is a specific circuit diagram of the LED driving circuit of this embodiment. In Fig. 9, reference numeral 24 denotes a control reference value creating circuit. 9, the same code | symbol is shown for the same element as FIG. 2, FIG.

Hereinafter, it demonstrates, comparing FIG. 9 with FIG. Equivalent to the DC / DC converter 8 in FIG. 8 is the MOS-FEF 14 and the diode 15 of the P channel in FIG. 9, and the output voltage control circuit 13 is the comparator 21 and the voltage comparison. The circuit 12 is a voltage comparison amplifier 20, and the smoothing circuit 9 is an inductor 16 and a capacitor 17. The inductor 16 and the capacitor 17 constitute a time constant circuit similarly to the first and second embodiments. In the circuit of this embodiment, the DC / DC converter structure of the step-down switching power supply system is constructed. 8 corresponds to the drive current control circuit 10 is the transistor 18, and the equivalent to the constant current control circuit 11 is the negative feedback amplifier circuit 19 loaded with the transistor 18. In FIG. 9, the amplifier 22, the amplifier 23, the resistor R 31, the mirror circuit 25, and the resistor R 32 correspond to the difference voltage calculating circuit 31 in FIG. 8. In addition, the control reference value creation circuit 24 in FIG. 9 corresponds to the control reference value creation circuit 32 in FIG. 8. The control reference value generation circuit 24, a constant current I ₁ and the variable current I ₂ sum current I ₁ + by flowing the I ₂ to the resistor R _2, the voltage comparing amplifier 20 value of the variable voltage as a first voltage threshold value V _Ref1 in To give. The sum current I ₁ + I ₂ is determined in accordance with the output signal of the dimming rate determination circuit 33.

Next, in the negative feedback closed loop CL2 for constant current control, the voltage value V _E (= I _LED × R _CS ) obtained by current-to-voltage conversion of the current I _LED flowing through the LED 1 by the current detection resistor R _CS is used for the constant current. Negative feedback is performed as comparison information of the negative feedback amplifier 19. And the negative feedback amplifier (19), of the comparison voltage V _E The LED (1) a second reference voltage value V _Ref2 transistor 18 to match (= I ₀ × V _R1) corresponding to the current of the aiming The base current is controlled to cause the transistor 18 to draw the desired constant current from the LED 1.

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 through the LED 1 in order to supply the LED 1 with the required lowest optimal voltage V _SW1 having good power efficiency. ) Is set as a control reference value V _Ref2 (= I ₁ × R ₂ ) in advance, and is set as a control reference value V _Ref2 (= I ₁ × R ₂ ) in order to operate in a constant current and not operate in a saturation region which cannot follow the response. The voltage difference between the drain-source electrode or the collector-emitter electrode, V _COMP , is a difference voltage calculation circuit consisting of an amplifier 22, an amplifier 23, an R 31, a mirror circuit 25, and a resistor R 32. Calculated by (31), the voltage comparison amplifier 20 compares this difference voltage _VCOM with control reference value _VRef2 . Similar to the first embodiment, the voltage comparison circuit 20, the voltage comparison amplifier 20, and the comparator 21, the output voltage control circuit 13, are supplied so that the comparison value V _COMP coincides with the control reference value V _Ref2. The negative feedback closed loop CL1 for voltage control is operated.

In addition, in the present embodiment, the reference value V _Ref1 of the voltage comparison amplifier 20 for supply voltage control is not limited to a constant value, but is variably set in accordance with the current or dimming rate that flows to the LED 1 from two or more kinds of set values. . Therefore, although the control reference value creation circuit 24 creates the reference value V _Ref2 by (I ₁ + I ₂ ) × R ₂ , I ₁ is a constant value and I ₂ is variable in proportion to the current value flowing to the LED 1. In this case, when the set current value of the LED 1 increases, I ₂ also increases. On the contrary, when the set current value of the LED 1 decreases, I ₂ also decreases.

FIG. 10 shows a specific circuit example of the control reference value creating circuit 24 that changes the reference value V _Ref1 in accordance with the dimming ratio. FIG. 11 shows the setting of the reference value V _Ref2 created by the control reference value creating circuit 24. It is explanatory drawing which shows a concept. That is, the dimming signal DIM1 from the outside is judged by the dimming rate judging circuit 33, and when the dimming rate is relatively high, for example, 100%, only the signal SW1 is turned on is input to the control reference value creating circuit 24. When the dimming ratio is relatively low, for example, at 50%, signals of both SW1 and SW2 are input. In this way, the reference voltage values can be set precisely by turning SW1 and SW2 on and off in accordance with the dimming ratio.

In FIG. 10 and FIG. 11, since the response speed of the drive current control circuit 10 depends on the terminal voltage, the terminal voltage of the drive current control circuit 10 (drain-source in the case of FET when the dimming ratio is low). In the case of a terminal voltage between electrodes and a transistor, it is indicated that the set value of the collector-emitter electrode) must be set higher than when the dimming ratio is high. In general, when the dimming ratio is not determined, assuming that the dimming ratio is extremely low (for example, 0.5%), setting of a high terminal voltage is necessary.

According to this 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, without impairing the responsiveness of the constant current control. The negative feedback closed loop CL1 for supply voltage control can be stably operated. That is, since the response characteristics of the frequencies of the negative feedback closed loops CL1 and CL2 are 20 times or more different, they do not interfere with each other and become unstable, so that the gain and frequency response of the negative feedback closed loop CL2 for constant current control can be set high. Therefore, a very accurate constant current characteristic can be achieved. Therefore, it is possible to meet the product of the field where the desired constant current value changes frequently and the product of the field requiring very low dimming performance, and in particular, the linearity in the extremely low dimming range of 5% or less required for the liquid crystal backlight of vehicle navigation Current characteristics can be achieved.

Fig. 15 is a graph showing the characteristic A of the dimming rate and the LED current by the LED driving apparatus of the present embodiment in comparison with the characteristic B of the conventional example and the characteristic C of the backlight for a mobile phone. In the LED driving apparatus of the present embodiment, It can be seen that a linear current characteristic in an extremely low dimming region of 5% or less is obtained.

According to the present invention, in addition to the first negative feedback closed loop for controlling the voltage supplied to the plurality of light emitting diodes connected in series, a second negative feedback closed loop for controlling the current supplied to the plurality of light emitting diodes connected in series is provided. Since the light emitting diode is always driven at a constant current and the voltage supplied to the light emitting diode is optimized, heat generation of a circuit element including the light emitting diode can be reduced, and the backlight unit can be stabilized and turned on even during dimming. .

That is, according to the present invention, even in an apparatus in which extremely low dimming performance is required, for example, a liquid crystal backlight used for vehicle-mounted navigation, and as a result, the current value supplied to the light emitting diode is frequently changed, the voltage reference is set to the dimming ratio. By varying accordingly, it is possible to cope sufficiently, and stable dimming performance can be obtained even in an extremely low dimming region such as 5% or less.

Further, according to the present invention, the response of the constant current control is set by setting the frequency response characteristic of the first negative feedback closed loop for supply voltage control 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 damaging. That is, since the response characteristics of the frequencies of the negative feedback closed loops are 20 times or more different, they do not interfere with each other and become unstable, so that the gain and the frequency response of the second negative feedback closed loop for constant current control can be set high, so that the accuracy is quite high. Good constant current characteristics can be achieved.

Claims (9)

A plurality of light emitting diodes connected in series, A DC / DC converter for converting and supplying an output voltage of a DC power supply to a predetermined voltage value on an anode side of the series-connected light emitting diodes; A drive current control circuit having one end connected to a cathode side of the series-connected light emitting diode, A resistor for current detection having one end connected to the other end of the drive current control circuit and the other end grounded; An output voltage control circuit for controlling the DC / DC converter to control output voltages supplied to the plurality of series connected light emitting diodes; 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 detecting resistor is used as a comparison voltage, and the difference voltage is compared to the first reference voltage. Output voltage comparison circuit, A constant current control circuit for comparing the comparison voltage obtained by converting the current flowing through the current detecting resistor with a second reference voltage for constant current control, and controlling the drive current control circuit so that the energized current becomes a constant current; A closed loop composed of the DC / DC converter, a drive current control circuit, a voltage comparison circuit, and an output voltage control circuit is a first negative feedback closed loop for supply voltage control, and is composed of the drive current control circuit and the constant current control circuit. An LED drive device comprising a closed loop as a second negative feedback closed loop for constant current control. The method of claim 1, The frequency response of the first negative feedback closed loop is set to 1/20 or less of the frequency response of the second negative feedback closed loop. The method of claim 2, The output voltage control circuit is a circuit for outputting a square wave voltage having a variable duty ratio, and the DC / DC converter is a circuit including a transistor switching element that is opened and closed by an output square wave voltage of the output voltage control circuit. The driving current control circuit is a transistor connected in series to the plurality of light emitting diodes connected in series. The method of claim 3, The first reference voltage of the voltage comparison circuit is variable in response to the dimming ratio. A plurality of light emitting diodes connected in series, A DC / DC converter for converting and supplying an output voltage of a DC power supply to a predetermined voltage value on an anode side of the series-connected light emitting diodes; A drive current control circuit having one end connected to a cathode side of the series-connected light emitting diode, A resistor for current detection having one end connected to the other end of the drive current control circuit and the other end grounded; An output voltage control circuit for controlling the DC / DC converter to control output voltages supplied to the plurality of series connected light emitting diodes; A differential voltage calculating circuit for calculating differential voltages at both ends of the driving current control circuit; A voltage comparing circuit for comparing the difference voltage calculated by the difference voltage calculating circuit with a first reference voltage to output an output voltage command to the output voltage control circuit, for giving an output voltage command to the output voltage control circuit; A constant current control circuit for controlling the drive current control circuit so that the energized current becomes a constant current, by comparing a comparison voltage obtained by converting a current flowing through the current detecting resistor with a second reference voltage, A closed loop composed of the DC / DC converter, a drive current control circuit, a differential voltage calculation circuit, a voltage comparison circuit, and an output voltage control circuit is used as a first negative feedback closed loop for supply voltage control, and the drive current control circuit and the constant current. An LED drive device comprising a closed loop composed of a control circuit as a second negative feedback closed loop for constant current control. The method of claim 5, The frequency response of the first negative feedback closed loop is set to 1/20 or less of the frequency response of the second negative feedback closed loop. The method of claim 6, The output voltage control circuit is a circuit for outputting a square wave voltage having a variable duty ratio, and the DC / DC converter is a circuit including a transistor switching element that is opened and closed by an output square wave voltage of the output voltage control circuit, The driving current control circuit is a transistor connected in series to the plurality of light emitting diodes connected in series. The method of claim 7, wherein The transistor constituting the drive current control circuit is supplied with an output current of a negative feedback amplifier circuit composed of a differential amplifier to its base electrode, and part of the emitter current is fed back to one input terminal of the differential amplifier. And a pulse oscillator supplied with a reference voltage to the other input terminal of the differential amplifier. The method of claim 8, An input terminal of a differential amplifier constituting a first reference voltage supplied as one input signal of a voltage comparison circuit included in the first negative feedback closed loop and a negative feedback amplification circuit included in the second negative feedback closed loop The second reference voltage supplied to the LED drive device, characterized in that the change corresponding to the dimming rate supplied from the outside.

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