JP2002203988A - Light emitting element driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform light control of an LED delicately with high efficiency using a DC/DC converter. SOLUTION: A power supply, an inductor and a switching element are connected in series, an output voltage boosted through the inductor is taken out by switching the switching element and a light emitting element is driven with that output voltage. In such a light emitting element driving circuit, light control of an LED can be performed delicately with high efficiency using a DC/DC converter by providing a first control system for turning the switching element off when a first detection resistor of low resistance detects a peak value of a current flowing through the switching element, and a second control system for detecting the average value of currents flowing through the LED detected by a second detection resistor of low resistance, comparing the average value with a predetermined reference level and controlling the switching element intermittently based on the difference thus obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting element driving circuit for lighting a light emitting element (LED) using a DC / DC converter.

[0002]

2. Description of the Related Art FIG. 7 is a circuit diagram showing a configuration example of a conventional light emitting element driving circuit. A switching transistor (FET) 2 is turned on and off by a switching control signal 100 output from the gate driver 1. As a result, while the transistor 2 is on, a current supplied from the battery power supply 3 flows through the transistor 2 via the coil 4,
While the transistor 2 is off, the current boosted from the coil 4 flows to the rectifier circuit composed of the diode 5 and the smoothing capacitor 6, and the DC voltage obtained here is applied to the current limiting resistor 7.
Is applied to a series circuit of a plurality of LEDs 8.

The output voltage of the rectifier circuit is divided by a voltage dividing circuit of a resistor R1, a variable resistor VR and a resistor R2, and the obtained divided voltage is inputted to a non-inverting input terminal (-) of an error amplifier circuit 9. Is done.

Since the reference voltage Vref is input to the inverting input terminal (+) of the error amplifier circuit 9, the divided voltage is compared with the reference voltage Vref, and the difference voltage is input to the PWM control circuit 10. The PWM control circuit 10 adjusts the pulse width of the switching control signal so that the difference voltage becomes 0, and outputs it to the gate driver 1. As a result, a predetermined voltage is applied to the series circuit of the plurality of LEDs 8 to light them. When the divided voltage is changed by changing the variable resistor VR, the voltage applied to the LED 8 can be changed.

[0005]

However, in the booster circuit using the conventional DC / DC converter as described above, when the boosted voltage is applied to the light emitting element and the necessary current value is caused to flow, the load of the load is reduced. Since the applied voltage must be finely adjusted due to the diode characteristics of the LED 8, the output voltage of the DC / DC converter must be accurately controlled in accordance with variations in the LED characteristics and temperature characteristics. In addition, when changing the applied voltage for dimming, it is difficult to set the current due to the constant voltage characteristic of the LED.

A general D which controls the output voltage as described above
In order to make it easy to set the LED current of the load in the C / DC converter, there is a method of inserting a current limiting resistor 7 in series with the load to reduce the sensitivity of the current change to the voltage change. In this case, the current flowing through the current limiting resistor 7 causes a resistance loss and a power loss.

Therefore, it is conceivable to apply a current feedback to the operation of the DC / DC converter to perform a constant current operation. However, in the conventional DC / DC converter, the reference voltage supplied to the error amplifier circuit 9 has a value of about 1 V. Therefore, when a current feedback circuit is assembled, the voltage generated by the load current detection resistor is also about 1 V. Voltage is required. In this case, the resistance loss for current detection becomes large, and it is not a convenient method when it is desired to increase the power efficiency.

Therefore, a circuit as shown in FIG. 8 has been known in which current feedback and voltage feedback are used in combination to reduce the amount of current feedback. In this circuit, the LED 8 is connected to the resistor R3
, And the output side of the rectifier circuit is connected to the connection point between the LED 8 and the resistor R3 via the resistors R4 and R5. As a result, the divided voltage at the connection point between the resistors R4 and R5 and the voltage at the connection point between the LED 8 and the resistor R3 are added by the adder 11, and the non-inverting input terminal (-) of the error amplifier circuit 9 is added.
Has been entered. Current feedback is LED8 and detection resistor R3
The voltage feedback is performed by feeding back the divided voltage at the connection point between the detection resistors R4 and R5 to the error amplifier circuit 9.

Also in this case, since current feedback is applied,
As described above, the reference voltage applied to the error amplifier circuit 9 is 1
V, and the voltage generated by the load current detection resistor is also 1
Since a voltage around V is required, the power consumption of the detection resistor increases. However, by successfully combining the voltage feedback and the current feedback, the power loss due to the detection resistor can be slightly reduced. Even so, the power efficiency cannot be increased so much, which is unsuitable when mounted on a portable device or the like.

Incidentally, the idea of lighting an LED by the principle of a DC / DC converter by applying current feedback has been proposed in the past in Japanese Unexamined Patent Publication No.
9-218586, which states that dimming is possible by changing the switching duty.

After all, when an LED mounted on a portable device or the like is driven by a conventional light emitting element driving circuit, the light amount is stabilized against a change in power supply voltage and the stability of the light amount against a change in the ambient temperature of the device is ensured. Therefore, it is essential that the light can be delicately adjusted according to a change in the operating environment. On the other hand, efficient use of the power supply is demanded in terms of the life of the battery power supply. Therefore, development of a light-emitting element drive circuit capable of adjusting light control with high efficiency and fine adjustment is demanded.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to make it possible to perform light control of a light emitting element with high efficiency and delicately using a DC / DC converter. An object of the present invention is to provide a light emitting element driving circuit.

[0013]

In order to achieve the above object, a feature of the present invention is that a power supply, an inductor and a switching element are connected in series, and the switching element is switched, thereby boosting the voltage by the inductor. A first current detection sensor for detecting a current flowing through the switching element, in a light emitting element driving circuit for lighting the light emitting element with the output voltage,
A peak current detection circuit for detecting a peak current flowing through the switching element from a current detected by the first current detection sensor; and a switching element when the peak current is detected by the peak current detection circuit. Turn off the first
Control circuit, a second current detection sensor that detects a current flowing through the light emitting element, and an average current detection that detects an average current of the current flowing through the light emitting element from the current detected by the second current detection sensor A circuit, an input circuit for inputting a reference voltage, a comparison circuit for comparing a current corresponding to the reference voltage input from the input circuit and an average current detected by the average current detection circuit to obtain a difference between the two, A second intermittently performing switching control of the switching element based on the difference obtained by the comparison circuit;
And a control circuit. The first current detection sensor and the second current detection sensor according to the second aspect of the invention are each a resistor having a low resistance value.

According to a third aspect of the present invention, the peak current detecting circuit utilizes a difference in base-emitter voltage of a pair of transistors having different emitter areas to detect a slight difference in detection voltage of the first current detection sensor. It is characterized by detecting a peak current.

According to a fourth aspect of the present invention, in the average current detecting circuit, the emitter voltages of a pair of transistors having the same emitter area are unbalanced by the detection voltage of the second current detection sensor, and the current flowing through both transistors is adjusted. The average current is detected by utilizing the flow of a current that compensates for the difference that occurs in the above.

According to a fifth aspect of the present invention, the switch element, the peak current detection circuit, the average current detection circuit, the comparison circuit, and the first and second control circuits are integrated on the same substrate. Is to be formed.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a light emitting element drive circuit according to a first embodiment of the present invention. The light emitting element drive circuit includes a switching transistor 21, a switch control circuit 22 for performing switching control of the transistor 21, a peak current detection circuit 23 for detecting a peak current of a current flowing through the transistor 21, and an LED 28.
Current detection circuit 2 for detecting the average current of the current flowing through
4. A comparison circuit 25 that compares the average current detected by the average current detection circuit 24 with the dimming reference, a battery power supply 26, a boosting coil 27, an LED 28 connected in series, and a current detection sensor that detects a current flowing through the LED 28. It has a (resistance) 29, a current detection sensor (resistance) 30 for detecting a current flowing through the transistor 21, and an input circuit 31 that inputs a dimming reference voltage and sets it as a corresponding reference current.

Next, the operation of this embodiment will be described. Switching control signal 1 output from switch control circuit 22
00 turns the transistor 21 on and off. When the transistor 21 is on, the current supplied from the battery power supply 26 flows through the transistor 21 via the coil 27. When the transistor 21 is off, the current boosted from the coil 27 flows to the series circuit of the LED 28, LED
28 is turned on. The current flowing through the transistor 21 is detected by the current detection sensor 30 and is input to the peak current detection circuit 23. When the detected current value reaches a predetermined current (peak current), the peak current detection circuit 23 notifies the switch control circuit 22 of this.

When the current value reaches a predetermined current, the switch control circuit 22 repeats the control of turning off the switching control signal 100 and then turning on the switching control signal 100. As a result, a current having a waveform as shown in FIG.
A load current having a waveform as shown in FIG.

The current detection sensor 29 detects a current flowing through a series circuit of the LEDs 28 and detects the detected current through the average current detection circuit 24.
Output to The average current detection circuit 24 is a current detection sensor 2
The average current of the current flowing through the series circuit of the LEDs 28 is detected from the detected current 9.

The comparison circuit 25 compares the average current flowing through the series circuit of the LEDs 28 with the dimming reference current, finds the difference, and outputs the difference to the switch control circuit 22. The switch control circuit 22 generates the switching control signal 100 intermittently as shown in FIG. 3A so that the difference becomes 0, and outputs this to the transistor 21. As a result, a current having a waveform as shown in FIG. 3B intermittently flows through the transistor 21, and a load current having a waveform as shown in FIG.

Here, when the dimming reference voltage is increased, the period during which the transistor 21 stops switching becomes shorter, the current flowing on the LED 28 side increases, and
If 8 is brighter and the dimming reference voltage is smaller,
The period during which the transistor 21 stops switching becomes longer, the current flowing through the LED 28 decreases, and L
The brightness of the ED 28 decreases.

By the above operation, the switch control circuit 22 detects a peak current flowing through the transistor 21 and turns off the transistor 21 when the switch control circuit 22 detects a peak current.
The average current flowing through the LED 28 is adjusted to the reference current.
Can be said to be composed of a second control circuit that performs the dimming operation.

FIG. 4 is a circuit diagram showing a detailed example of the circuit shown in FIG. The switch control circuit 22 mainly includes the oscillation circuit 2
21, a waveform shaping circuit 222, a flip-flop 223, and a gate driver A7.

The peak current detecting circuit 23 includes a transistor Q1
7 to Q21 and resistors R11 to R13. The average current detection circuit 24 includes transistors Q6 to Q10 and a resistor R
4 to R7 and a capacitor C3. The comparison circuit 25 includes transistors Q5 and Q6 and a resistor R3. Although not shown in FIG. 1, the input circuit 31 includes transistors Q1 to Q4 and resistors R0 to R2. The current detection sensor 29 is a small resistor R15, and the current detection sensor 30 is a small resistor R14. Although the bias current source 32 is not shown in FIG.
11 to Q16 and resistors R8 to R10.

Next, the operation of this embodiment will be described. The bias current source 32 generates a bias current from a circuit power supply input and distributes the bias current to the peak current detection circuit 23, the average current detection circuit 24, and the input circuit 31. The oscillation circuit 221 of the switch control circuit 22 oscillates, and the oscillation signal is
The waveform is shaped by the signal 22 and input to the flip-flop 223. The flip-flop 223 generates a switching control signal synchronized with the input signal and outputs the signal to the gate driver A7. The gate driver A7 inverts the input control signal and outputs the inverted signal to the gate of the transistor 21 to switch the transistor 21.

The switching current flowing through the transistor 21 becomes a resistance drop voltage of the resistor R14 and is applied to the emitter of the transistor Q20 of the peak current detection circuit 23.
A pair of transistors Q18 and Q of the peak current detection circuit 23
20 is biased by transistors Q17 and Q19 to operate with approximately equal collector currents. At this time, when the emitter area ratio of the transistors Q18 and Q20 is set to 1: 4, both transistors Q18 and Q2
A voltage ΔV having a VBE difference of 0 is ΔV = kT / q · ln (N
M) (1), for example, a stable detection voltage of 36 mV is obtained.

Here, k is Boltzmann's constant, T is absolute temperature, q is electron charge, N is the emitter area ratio of both transistors, M is the current ratio of both transistors, and k = 1.380.
× 10−23 (J / K), q = 1.602 × 10−19
(C).

That is, if the size of the transistor Q18 is made larger than the size of the transistor Q20,
Based on the difference ΔV between the two VBEs as shown in the equation (1), the current flowing through both transistors is balanced by the size ratio.

When the current flowing through the resistor R14 increases and the emitter voltage of the transistor Q20 becomes higher than the reference voltage, the current flowing through the transistor Q20 decreases. It flows through the transistor Q21. At this time, the collector of the transistor Q21 goes to a low level, and this low-level signal is input to the flip-flop 223 via the input buffers A10 and A11 of the switch control circuit 22, and cuts off the NAND gate A6. Output is high level. As a result, the output of the gate driver A7 goes low, turning off the transistor 21.

Thereafter, when the current flowing through the resistor R14 decreases, the transistor Q21 turns off and its collector voltage goes to a high level. This high-level signal is supplied to the flip-flop 2 via the input buffers A10 and A11.
23 to make the NAND gate A6 conductive. For this reason, the flip-flop 223 is connected to the waveform shaping circuit 222.
In synchronization with the next rise of the oscillation signal input from the gate driver A7, the output is set to the low level, and the high-level switching control signal 100 is supplied from the gate driver A7 to the transistor 21.
And the transistor 21 is turned on. Or later,
The above operation is repeated.

On the other hand, the current flowing through the LED 28 is the resistance R1
5, and is applied to the emitter of the transistor Q10 of the average current detection circuit 24 via the resistor R7. The transistors Q10 and Q8 are transistors of the same size, and if the current flowing through the LED 28 is 0,
The currents flowing through the transistors Q10 and Q8 are the same and are balanced.

In this state, when a current flows through the LED 28, a resistance drop voltage of the resistor R15 is generated, and the emitter voltage of the transistor Q10 is increased.
The current flowing through 0 decreases. The reduced current component is
The transistor Q6 turns on and flows to the resistor R5 through the transistor Q6. Therefore, when the emitter voltage of the transistor Q8 rises and rises by the resistance drop voltage of the resistor R15, the operations of Q8 and Q10 are balanced,
The current flowing through the transistor Q6 is determined.

Here, the load current detection resistor R15 is a minute resistance of approximately 0.5Ω. By comparing the voltage generated by the minute resistance with the emitter current flowing through the resistor R5 of the transistor Q6, the collector current of the transistor Q6 is determined. For example, if R5 = 500Ω, 1/10
It works as a 00 current level conversion circuit. At this time, the collector voltage of the transistor Q6 is held by the capacitor C3, and the voltage of the capacitor C3 corresponds to the average current of the current flowing through the LED 28.

On the other hand, when a dimming reference voltage is applied to the base of the transistor Q1 of the input circuit 31, a constant current corresponding to this reference voltage flows through the transistors Q3 and Q4,
Further, the current flows to the transistor Q5. Here, LED 28
When the current flowing through the transistor Q6 increases, the current flowing through the transistor Q6 increases, so that the voltage at the connection point between the transistors Q5 and Q6 constituting the comparison circuit 25 decreases, and a low-level signal is input to the input buffer A8 of the switch control circuit 22. , The NAND gate A1 of the oscillation circuit 221 is cut off.
Accordingly, the oscillation of the oscillation circuit 221 is stopped, and the switching control signal is not output from the flip-flop 223, so that the switching of the transistor 21 is stopped.

Thereafter, when the current flowing through the LED 28 decreases, the current flowing through the transistor Q6 decreases.
The voltage at the connection point between the transistors Q5 and Q6 rises, and a high-level signal is
8 to turn on the NAND gate A1 of the oscillation circuit 221. Accordingly, the oscillation of the oscillation circuit 221 is started, a switching control signal is output from the flip-flop 223, and the switching of the transistor 21 is restarted. Thereafter, by repeating the above operation, the transistor 21 is intermittently switched, and a current corresponding to the dimming reference voltage is supplied to the LED 28 to turn on the LED at a predetermined brightness.

According to the present embodiment, LE is obtained by current feedback.
Since dimming is performed by controlling the current flowing through D28, delicate dimming can be performed, and stabilization of the amount of light with respect to changes in battery power supply 26 and changes in ambient temperature can be ensured.
Moreover, since there is no rectifier circuit, there is no power loss here. In addition, since current detection is performed using the base-emitter voltages of the transistors Q18 and Q20 or the transistors Q8 and Q10, a small change in voltage can be detected.
/ 200 to 1/20) can be used, the power loss due to these detection resistors can be minimized, and when mounted on a portable device, the battery life can be extended. it can.

By making the dimming reference voltage dependent on temperature, the luminance of the LED can be changed at the same temperature.

FIG. 5 shows a second embodiment of the light emitting device driving circuit according to the present invention.
FIG. 3 is a block diagram showing a configuration according to the embodiment. The configuration of this example is the same as that of the first embodiment shown in FIG.
The difference is that the diode 51 and the capacitor 5 are connected between the output side of the switching transistor 21 and the LED 28.
2 has been inserted.

Next, the operation of this embodiment will be described.
In this example as well, the switching transistor 21 switches intermittently, and a pulse current is supplied from the boosting coil 27 to the series circuit of the plurality of LEDs 28 through the diode 51 to light these LEDs 28. At this time, LED
Since the high-frequency component of the pulse current supplied to the LED 28 is short-circuited to the ground side through the capacitor 52, the high-frequency component is removed from the current flowing through the LED 28, and the plurality of LEDs 28
However, for example, unnecessary radiation can be reduced even if they are arranged so as to form a loop as shown in FIG.
The diode 51 prevents the charge held in the capacitor 52 from flowing back to the switching transistor 21 side.

This embodiment has the same effects as the first embodiment, but can reduce unnecessary radiation.

The present invention is not limited to the above-described embodiment, and may be embodied in various other forms with specific configurations, functions, operations, and effects without departing from the gist thereof. .

[0043]

As described in detail above, according to the present invention, dimming of an LED can be performed with high efficiency and subtlety using a DC / DC converter.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a light emitting element drive circuit according to a first embodiment of the present invention.

FIG. 2 is a signal waveform diagram for explaining the operation of the circuit shown in FIG.

FIG. 3 is another signal waveform diagram for explaining the operation of the circuit shown in FIG. 1;

FIG. 4 is a circuit diagram showing a detailed example of the circuit shown in FIG. 1;

FIG. 5 is a block diagram showing a configuration of a light emitting element drive circuit according to a second embodiment of the present invention.

FIG. 6 is a diagram illustrating an example of arrangement of light emitting elements.

FIG. 7 is a circuit diagram showing a configuration example of a conventional light emitting element drive circuit.

FIG. 8 is a circuit diagram showing another configuration example of a conventional light emitting element drive circuit.

[Explanation of symbols]

 Reference Signs List 21 switching transistor 22 switch control circuit 23 peak current detection circuit 24 average current detection circuit 25 comparison circuit 26 battery power supply 27 coil 28 LED 29, 30 current detection sensor 31 input circuit 32 bias current source 51 diode 52 capacitor 221 oscillation circuit 222 Waveform shaping circuit 223 Flip-flop R14, R15 Resistance

Continued on the front page F term (reference) 5F041 AA09 BB03 BB10 BB24 BB25 5H730 AA14 BB14 DD04 FD31 FG05 5J050 AA01 AA44 CC08 DD06 EE21 EE31 EE34 EE35 EE37 FF26

Claims (5)

[Claims] 1. A light emitting element driving circuit for connecting a power supply, an inductor, and a switching element in series, switching the switching element to extract an output voltage boosted by the inductor, and lighting the light emitting element with the output voltage. A first current detection sensor that detects a current flowing through the switching element; a peak current detection circuit that detects a peak current flowing through the switching element from a current detected by the first current detection sensor; When the peak current is detected by the peak current detection circuit,
A first control circuit for turning off the switching element; a second current detection sensor for detecting a current flowing through the light emitting element; and a current flowing through the light emitting element based on the current detected by the second current detection sensor. An average current detection circuit that detects an average current of the input current; an input circuit that inputs a reference voltage; and a current corresponding to the reference voltage input from the input circuit and an average current detected by the average current detection circuit. A light emitting element drive, comprising: a comparison circuit for obtaining a difference between the two; and a second control circuit for intermittently performing switching control of the switching element based on the difference obtained by the comparison circuit. circuit. 2. The first current detection sensor and the second current detection sensor.
2. The light emitting element drive circuit according to claim 1, wherein each of the current detection sensors includes a resistor having a low resistance value. 3. The peak current detection circuit detects a peak current by using a difference in base-emitter voltage of a pair of transistors having different emitter areas and a slight detection voltage difference of the first current detection sensor. The light-emitting element drive circuit according to claim 1, wherein the light-emitting element drive circuit detects. 4. The average current detection circuit detects a difference in a current flowing through both transistors when emitter voltages of a pair of transistors having the same emitter area are unbalanced by a detection voltage of the second current detection sensor. 4. The light-emitting element driving circuit according to claim 1, wherein the average current is detected by utilizing a flow of a current to be compensated. 5. The switch element, the peak current detection circuit, the average current detection circuit, the comparison circuit, and the first,
5. The light emitting element driving circuit according to claim 1, wherein the second control circuit is integrated and formed on the same substrate.