WO2007074866A1

WO2007074866A1 - Light emitting device driving circuit

Info

Publication number: WO2007074866A1
Application number: PCT/JP2006/326052
Authority: WO
Inventors: Masashi Fukuda; Kengo Takahama
Original assignee: Sharp Kabushiki Kaisha
Priority date: 2005-12-28
Filing date: 2006-12-27
Publication date: 2007-07-05

Abstract

A constant current source (2) is connected in series to an LED array (7) to be driven. A booster circuit (3) boosts an input voltage (Vin) and supplies the obtained boost-up voltage (Vb) to the LED array (7) and the constant current source (2). A differential amplifier (4) detects the voltages at both ends of the constant current source (2), and a differential amplifier (5) compares the detected voltage with a reference voltage (E1). The booster circuit (3) changes the level of the boost-up voltage (Vb) according to the analog boost-up control signal (10) output from the differential amplifier (5). In a chopper-type booster circuit (3a), the switch that switches the flow of the current having passed through a coil is controlled by a control signal, the duty ratio of which varies according to the boost-up control signal (10). In a charge-pump-type booster circuit (3b), the switch that switches the connection states of a capacitor is also controlled by the control signal. This enables a light emitting device to be stably driven by a constant current at a low power consumption.

Description

Specification

Light emitting element drive circuit

Technical field

The present invention relates to a light emitting element driving circuit that drives a light emitting element at a constant current.

Background art

In order to cause a light emitting element to emit light, it is necessary to apply a predetermined current or voltage from the outside to the light emitting element. For example, a light emitting diode (hereinafter abbreviated as LED) emits light when a voltage higher than the forward voltage Vf is applied. The LED forward voltage Vf has the property that it increases with decreasing temperature and decreases with increasing temperature. Therefore, as a method of causing the LED to emit light correctly even when the forward voltage Vf changes with temperature, a constant current source has been conventionally connected in series with the LED to control the amount of current flowing through the LED. Methods are known (for example, Patent Documents 1 to 3).

FIG. 10 is a diagram showing a configuration of a conventional LED drive circuit. The LED drive circuit 81 shown in FIG. 10 drives four LEDs connected in series (hereinafter referred to as LED array 87) at a constant current. More specifically, the constant current source 82 is connected to the LED array 87 in series. The booster circuit 83 boosts the input voltage Vin and supplies the obtained boosted voltage Vb to the LED array 87 and the constant current source 82. The resistors 84a and 84b are connected in series and divide the boosted voltage Vb. The comparator 85 compares the divided voltage obtained by the resistors 84a and 84b with the reference voltage Vr provided by the reference power supply 86, and outputs a boost control signal 90 indicating the comparison result. The booster circuit 83 changes the level of the boost voltage Vb based on the boost control signal 90.

[0004] When the resistance values of the resistors 84a and 84b are Rl and R2, respectively, the boosted voltage Vb is given by the following equation (1). When the voltage across the constant current source 82 is Vi and the forward voltage of the LED is Vf, the following equation (2) is established.

Vb = VrX (Rl + R2) / R2… hi)

Vb = Vi + Vf X 4--(2)

[0005] In order to drive the LED array 87 using the LED drive circuit 81, the voltage Vi across the constant current source 82 needs to exceed the minimum operating voltage Vmin. Therefore, LED drive circuit 8 When designing 1, the boost voltage Vb is determined so as to satisfy the following equation (3) derived from the above equation (2) and Vi> Vmin.

Vb> Vmin + Vf X 4 '' (3)

[0006] As a technique related to the present invention, Patent Document 4 discloses a charge pump type booster circuit capable of arbitrarily setting an output voltage.

Patent Document 1: Japanese Patent Laid-Open No. 2002-359090

Patent Document 2: Japanese Unexamined Patent Publication No. 2003-152224

Patent Document 3: Japanese Unexamined Patent Publication No. 2005-11895

Patent Document 4: Japanese Unexamined Patent Publication No. 2000-166220

Disclosure of the invention

Problems to be solved by the invention

[0007] Within the operating temperature range of the LED drive circuit 81, the above equation (3) must always hold (in other words, even under the worst temperature condition). However, the above equation (

If the LED drive circuit 81 is designed so that 3) holds, there is a problem that the power consumption of the circuit increases.

[0008] The above problem will be described in detail with reference to FIG. FIG. 11 is a graph showing the temperature characteristics of the boosted voltage Vb in the LED drive circuit 81. As shown in FIG. Figure 11 shows the boost voltage Vb when the operating temperature range is 20 ° C up to 85 ° C, and four times the LED forward voltage Vf (Vf X

4) is described. The difference between the two graphs shown in FIG. 11 is equal to the voltage Vi across the constant current source 82.

[0009] As shown in FIG. 11, the voltage Vi across the constant current source 82 is minimized at 20 ° C. Therefore, when designing the LED drive circuit 81, the boosted voltage Vb is determined so as to satisfy Vi (−20 ° C.)> Vmin. Here, when the current flowing through the constant current source 82 and the LED array 87 is Id, the power consumption Pi in the constant current source 82 is given by Pi = Vi X Id. Therefore, for example, the following equation (4) holds for power consumption Pi (25 ° C) at normal temperature.

Pi (25 ° C) = Vi (25 ° C) X ld

> Vi (-20 ° C) X ld > Vmin X Id '' (4)

[0010] The constant current source 82 operates when the minimum operating voltage Vmin is applied to both ends. Therefore, the power consumption in the constant current source 82 can be suppressed to (VminX Id) ideally. In fact, the above formula (4) is established for power consumption Pi (25 ° C) at room temperature, and the constant current source 82 consumes more power than necessary at room temperature. Since the above equation (4) holds even at room temperature, it can be said that the constant current source 82 always consumes more power than necessary within the operating temperature range.

Therefore, an object of the present invention is to provide a light emitting element driving circuit that drives a light emitting element at a constant current stably and with low power consumption.

Means for solving the problem

[0012] A first aspect of the present invention is a light emitting element driving circuit for driving a light emitting element at a constant current, a constant current source connected in series to the light emitting element to be driven,

A booster circuit that boosts an input voltage and applies the obtained boosted voltage to the light emitting element and the constant current source;

A detection circuit for detecting a voltage across the constant current source;

A comparison circuit that compares the voltage detected by the detection circuit with a reference voltage and outputs a boost control signal indicating a comparison result;

The booster circuit changes the level of the boosted voltage based on the boost control signal.

[0013] A second aspect of the present invention is the first aspect of the present invention,

The step-up circuit is a chitotsuba type step-up circuit including a coil and a switch that switches a flow of current that has passed through the coil.

The switch is controlled by a control signal whose duty ratio changes according to the boost control signal.

[0014] A third aspect of the present invention is the first aspect of the present invention,

The booster circuit is a charge pump booster circuit including a capacitor and a switch for switching a connection state of the capacitor,

The switch has a control signal whose duty ratio changes according to the boost control signal. It is controlled by.

[0015] A fourth aspect of the present invention provides, in the first aspect of the present invention,

The boost control signal is an analog signal indicating a difference between a voltage detected by the detection circuit and the reference voltage. The invention's effect

[0016] According to the first aspect of the present invention, the light emitting element driving circuit detects the voltage across the constant current source and compares the detected voltage with the reference voltage so that the voltage across the constant current source becomes the reference voltage. The boosted voltage is generated while performing feedback control so as to match the above. Therefore, even if the characteristics of the light emitting element (for example, the forward voltage Vf of the LED) fluctuate due to the operating temperature or the variation between elements, the voltage across the constant current source is kept constant, and the light emitting element is stably stabilized. Can be driven. Also, even if the operating temperature changes, keeping the voltage across the constant current source at a low level (for example, a level close to the minimum operating voltage) prevents the constant current source from consuming more power than necessary. can do. In this manner, the light emitting element can be driven with a constant current stably and with low power consumption.

[0017] According to the second aspect of the present invention, it is possible to obtain a light-emitting element driving circuit that drives a light-emitting element at a constant current stably and with low power consumption, using a chiyotsuba booster circuit.

[0018] According to the third aspect of the present invention, it is possible to obtain a light emitting element driving circuit that drives a light emitting element at a constant current stably and with low power consumption using a charge pump booster circuit.

According to the fourth aspect of the present invention, the voltage across the constant current source can be matched with the reference voltage with high accuracy by controlling the booster circuit using the analog boost control signal.

Brief Description of Drawings

FIG. 1 is a diagram showing a configuration of an LED drive circuit according to first and second embodiments of the present invention.

2 is a circuit diagram of a constant current source of the LED drive circuit shown in FIG.

FIG. 3 is a circuit diagram of a booster circuit of the LED drive circuit according to the first embodiment of the present invention.

FIG. 4A is a signal waveform diagram showing changes in input / output signals when the voltage of the boost control signal is high in the booster circuit shown in FIG. FIG. 4B is a signal waveform diagram showing changes in input / output signals when the voltage of the boost control signal is low in the booster circuit shown in FIG.

5 is a graph showing the temperature characteristics of the boosted voltage of the LED drive circuit shown in FIG.

FIG. 6 is a circuit diagram of a booster circuit of an LED drive circuit according to a second embodiment of the present invention.

7 is a signal waveform diagram showing a change in the signal of the booster circuit shown in FIG.

8 is a diagram showing another usage pattern of the LED drive circuit shown in FIG.

9 is a graph showing the temperature characteristics of the boosted voltage when used in the form shown in FIG.

FIG. 10 is a diagram showing a configuration of a conventional LED drive circuit.

11 is a graph showing the temperature characteristics of the boosted voltage of the LED drive circuit shown in FIG.

Explanation of symbols

1—LED drive circuit

2, 31, 32 ... Constant current source

3 Booster circuit

3a ... Chiyotsuba type booster circuit

3b… Charge pump type booster circuit

4, 5 ... differential amplifier

6, 26, 36, 52

7, 8 · "LED array

Vin: Input voltage

Vb ... Boost voltage

10 ··· Boost control signal

21 ～ 23ΡΡMOS transistors

24, 25, 38 · "ΝCannnerole MOS transistor

27 ... resistance

28a-28c, 42a-42c, 56a-56c ... Terminal

33, SA1-SA8, SB1-SB5- "switch

34, 41, C1-C5- "capacitor

35, 51 Hysteresis 37 ... Comparator

39 ... Koinole

40 "diode

53 ... AND gate

54, 55

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] (First embodiment)

FIG. 1 is a diagram showing a configuration of an LED drive circuit according to the first embodiment of the present invention. The LED drive circuit 1 shown in FIG. 1 includes a constant current source 2, a booster circuit 3, a differential amplifier 4, 5, and a reference power supply 6, and includes four LEDs connected in series (LED array 7). Is driven at a constant current.

The input voltage Vin is supplied to the LED drive circuit 1. The booster circuit 3 boosts the input voltage Vin and outputs the obtained boosted voltage Vb. The booster circuit 3 changes the level of the boost voltage Vb based on the boost control signal 10. In the present embodiment, the booster circuit 3 increases the boost voltage Vb when the voltage of the boost control signal 10 is equal to or higher than a predetermined value, and decreases the boost voltage Vb otherwise (details will be described later).

The output terminal of the booster circuit 3 is connected to the current input terminal of the constant current source 2. The current output terminal of the constant current source 2 is connected to the anode side terminal of the LED array 7, and the power sword side terminal of the LED array 7 is grounded. Thereby, each LED included in the LED array 7 is driven by the constant current source 2 at a constant current.

The differential amplifier 4 functions as a detection circuit that detects the voltage across the constant current source 2. More specifically, the positive input terminal of the differential amplifier 4 is connected to the current input terminal of the constant current source 2, and the negative input terminal of the differential amplifier 4 is connected to the current output terminal of the constant current source 2. Yes. The differential amplifier 4 amplifies and outputs a potential difference (a voltage between both ends) between the current input terminal and the current output terminal of the constant current source 2.

The differential amplifier 5 functions as a comparison circuit that compares the voltage detected by the differential amplifier 4 with a reference voltage. More specifically, the negative input terminal of the differential amplifier 5 is connected to the output terminal of the differential amplifier 4, and the reference power supply 6 is connected to the positive input terminal of the differential amplifier 5. It is. The reference power supply 6 supplies a reference voltage E1. The differential amplifier 5 amplifies the difference between the output voltage of the differential amplifier 4 and the reference voltage E1, and outputs it as an analog boost control signal 10. The voltage of the boost control signal 10 is high when the reference voltage E1 is higher than the output voltage of the differential amplifier 4, and is low otherwise. Thus, the boost control signal 10 shows the result of comparing the output voltage of the differential amplifier 4 with the reference voltage E1.

[0027] As described above, the LED drive circuit 1 detects the voltage across the constant current source 2 and compares the detected voltage with the reference voltage E1, so that the voltage Vi across the constant current source 2 becomes the reference voltage E1. While performing feedback control to match, boost voltage Vb is generated.

FIG. 2 is a circuit diagram of the constant current source 2. The constant current source 2 shown in FIG. 2 includes P-channel MOS transistors 21 to 23, N-channel MOS transistors 24 and 25, a reference power supply 26, and a resistor 27. The constant current source 2 has terminals 28a to 28c. The terminal 28a is supplied with a boosted voltage Vb, the terminal 28b is connected to the negative input terminal of the differential amplifier 4, and the terminal 28c is connected to the anode side terminal of the LED array 7.

[0029] As shown in FIG. 2, the sources of the P-channel MOS transistors 21 to 23, 25 are connected to the terminal 28a, and the drains of the P-channel MOS transistors 21, 22 are connected to both the terminals 28b, 28c. . The gates of P-channel MOS transistors 21 to 23 and the drain of P-channel MOS transistor 23 are connected to the drain of N-channel MOS transistor 24. The source of the N-channel MOS transistor 24 is connected to the reference power supply 26, and the source of the N-channel MOS transistor 25 and the gates of the N-channel MOS transistors 24 and 25 are connected to the resistor 27.

N-channel MOS transistors 24 and 25 constitute a current mirror circuit. When the reference voltage supplied by the reference power supply 26 is E and the resistance value of the resistor 27 is R, the current flowing through the drain of the N-channel MOS transistor 24 and the current flowing through the drain of the N-channel MOS transistor 25 are either Is also given by Ic = EZR.

[0031] P-channel MOS transistors 21 and 23 form a current mirror circuit, and similarly, P-channel MOS transistors 22 and 23 also form a current mirror circuit. The current Ic flows through the source of the P-channel MOS transistor 23. When P-channel MOS transistors 21 to 23 have the same characteristics, the source of P-channel MOS transistors 21 and 22 , The same current Ic flows. Therefore, the constant current source 2 outputs a current (I c X 2) twice the current Ic to the LED array 7 via the terminal 28c. The potential difference between terminals 28a and 28b matches the voltage drop across P-channel MOS transistors 21 and 22.

FIG. 3 is a circuit diagram of the booster circuit 3. The booster circuit 3a shown in FIG. 3 includes constant current sources 31, 32, a switch 33, a capacitor 34, a hysteresis comparator 35, a reference power supply 36, a comparator 37, an N-channel MOS transistor 38, a coil 39, a diode 40, and a capacitor 41. Is a Chietsuba type booster circuit. Further, the booster circuit 3a has terminals 42a to 42c. An input voltage Vin is applied to the terminal 42a, a boost control signal 10 is applied to the terminal 42b, and a boost voltage Vb is output from the terminal 42c.

As shown in FIG. 3, the input voltage Vin is applied to the current input terminal of the constant current source 31 via the terminal 42a. The current output terminal of the constant current source 31 is connected to the “a” contact of the switch 33. The current input terminal of the constant current source 32 is connected to the b contact of the switch 33, and the current output terminal of the constant current source 32 is grounded. The output contact of the switch 33 is connected to one electrode of the capacitor 34, the plus side input terminal of the hysteresis comparator 35, and the minus side input terminal of the comparator 37. The other terminal of the capacitor 34 is grounded. The negative input terminal of the hysteresis comparator 35 is connected to a reference power supply 36 that supplies a reference voltage E2.

[0034] The hysteresis comparator 35 is a comparison circuit having hysteresis characteristics with a hysteresis width Vh. The hysteresis comparator 35 outputs a high level signal when the voltage applied to the positive input terminal (hereinafter referred to as voltage Vp) exceeds (E2 + VhZ2), and when the voltage Vp falls below (E2−VhZ2), the hysteresis comparator 35 outputs a low level signal. A level signal is output.

[0035] When the contact a of the switch 33 and the output contact are in conduction, a current flows from the constant current source 31 to the capacitor 34 via the switch 33, the electric charge is accumulated in the capacitor 34, and the voltage Vp increases. To do. When the voltage Vp becomes (E2 + VhZ2) or higher, the output of the hysteresis comparator 35 goes high.

[0036] When the output of the hysteresis comparator 35 becomes high level, the b contact and the output contact of the switch 33 are brought into conduction. In this state, the electric charge accumulated in the capacitor 34 is discharged toward the constant current source 32 via the switch 33, and the voltage Vp drops. Voltage Vp is (E2—Vh When Z2) or less, the output of hysteresis comparator 35 goes low.

[0037] When the output of the hysteresis comparator 35 becomes low level, the contact a and the output contact of the switch 33 are brought into conduction again. As a result, a current flows from the constant current source 31 to the capacitor 34 via the switch 33, electric charges are accumulated in the capacitor 34, and the voltage Vp rises again. As the booster circuit 3a repeats the above operation, the voltage Vp changes in a triangular waveform (see FIGS. 4A and 4B).

[0038] The boost control signal 10 is given to the positive side input terminal of the comparator 37 via the terminal 42b. The comparator 37 compares the voltage Vp applied to the negative input terminal with the voltage of the boost control signal 10 applied to the positive input terminal. The output terminal of the comparator 37 is connected to the gate of the N-channel MOS transistor 38. The source of the N-channel MOS transistor 38 is grounded, and the drain of the N-channel MOS transistor 38 is connected to one end of the coil 39 and the anode of the diode 40. The input voltage Vin is applied to the other end of the coil 39 via the terminal 42a. The power sword of the diode 40 is connected to one electrode of the capacitor 41 and the terminal 42c. The other electrode of the capacitor 41 is grounded.

4A and 4B are signal waveform diagrams showing changes in input / output signals in the booster circuit 3a. 4A shows a signal change when the voltage of the boost control signal 10 is high, and FIG. 4B shows a signal change when the voltage of the boost control signal 10 is low. In either case, the negative input terminal of the comparator 37 is supplied with a voltage Vp that changes in a triangular waveform in a range from (E2−VhZ2) to (E2 + VhZ2). On the other hand, the boost control signal 10 at a certain level is applied to the positive input terminal of the comparator 37. The comparator 37 outputs a high level signal when the voltage of the boost control signal 10 is equal to or higher than the voltage Vp, and outputs a low level signal otherwise. Therefore, the output signal of the comparator 37 is a signal having a certain duty ratio that changes between a high level and a low level.

[0040] While the output signal of the comparator 37 is at a high level, the N-channel MOS transistor 38 is turned on. During this time, the current passing through the coil 39 flows to the ground via the N-channel MOS transistor 38. When the output signal of the comparator 37 changes to low level, the N-channel MOS transistor 38 is turned off, and the current passing through the coil 39 is Flows into capacitor 41 via node 40. Thereby, the capacitor 41 is charged. In this way, the booster circuit 3a boosts the input voltage Vin by switching the current flow that has passed through the coil 39 using the N-channel MOS transistor 38.

The booster circuit 3a changes the level of the boost voltage Vb based on the boost control signal 10 by the following method. When the voltage of the boost control signal 10 is high (FIG. 4A), since the period during which the voltage of the boost control signal 10 is equal to or higher than the voltage Vp is long, the output signal from the comparator 37 has a long and low period. In contrast, when the voltage of the boost control signal 10 is low (FIG. 4B), the period during which the voltage of the boost control signal 10 is equal to or lower than the voltage Vp is long, so the low level period is long in the output signal of the comparator 37. . As described above, the duty ratio of the output signal of the comparator 37 changes according to the boost control signal 10.

The boosted voltage Vb increases when the charge accumulation amount of the capacitor 41 is large, and decreases when the charge accumulation amount of the capacitor 41 is small. The amount of charge stored in the capacitor 41 increases as the period during which the N-channel MOS transistor 38 is on is longer (that is, as the period during which the output signal of the comparator 37 is at a high level is longer). Therefore, the boost voltage Vb increases when the voltage of the boost control signal 10 is high, because the charge storage amount of the capacitor 41 is large. When the voltage of the boost control signal 10 is low, the charge storage amount of the capacitor 41 is small! / , Descend for.

As described above, in the chopper type booster circuit 3 a, the boost control signal 10 is controlled by controlling the N-channel MOS transistor 38 using the switch control signal whose duty ratio changes according to the boost control signal 10. Based on this, the level of the boost voltage Vb can be changed.

Hereinafter, the effect of the LED drive circuit 1 will be described with reference to FIG. FIG. 5 is a diagram showing the temperature characteristics of the boosted voltage Vb in the LED drive circuit 1. Figure 5 shows the step-up voltage Vb when the operating temperature range is −20 ° C to 85 ° C, and four times the LED forward voltage Vf (V f X 4). The difference between the two graphs shown in FIG. 5 is equal to the voltage Vi across the constant current source 2.

[0045] In general, the forward voltage Vf of an LED increases as the temperature decreases and decreases as the temperature increases. The LED drive circuit 1 is designed so that the voltage Vi across the constant current source 2 matches the reference voltage E1. Perform feedback control. Therefore, even if the operating temperature changes, the voltage Vi across the constant current source 2 does not change and always matches the reference voltage E1. In FIG. 5, the graph of the boosted voltage Vb is a graph obtained by translating the voltage (Vf X 4) graph vertically by the reference voltage E1.

If the current flowing through the constant current source 2 and the LED array 7 is Id, the power consumption Qi in the constant current source 2 is given by Qi = ViX Id. However, even if the temperature changes, both the voltage Vi and the current Id of the constant current source 2 do not change, so the power consumption Qi in the constant current source 2 is constant regardless of the temperature. Therefore, the constant current source 2 does not consume more power than necessary.

[0047] In addition, the booster circuit 3 is controlled by using an analog boost control signal 10 indicating the difference between the voltage Vi across the constant current source 2 and the reference voltage E1, so the voltage Vi across the constant current source 2 is used as a reference. The voltage E1 can be matched with high accuracy.

[0048] As described above, the LED drive circuit 1 according to the present embodiment detects the voltage Vi between both ends of the constant current source 2, and compares the detected voltage with the reference voltage E1, whereby both ends of the constant current source 2 are detected. While performing feedback control so that the voltage V i matches the reference voltage E1, the boost voltage Vb is generated. Therefore, even if the forward voltage Vf of the LED fluctuates due to the operating temperature and variations between elements, the voltage Vi across the constant current source 2 can always be kept constant, and the LED can be driven stably at a constant current. Even if the operating temperature changes, the constant current source 2 consumes more power than necessary by keeping the voltage Vi across the constant current source 2 at a low level (for example, a level close to the minimum operating voltage Vmin). Can be prevented. As described above, according to the LED driving circuit 1 according to the present embodiment, the LED can be driven with a constant current stably and with low power consumption.

[0049] (Second Embodiment)

The LED drive circuit according to the second embodiment of the present invention has the configuration shown in FIG. 1, and includes a booster circuit 3b shown in FIG. 6 instead of the booster circuit 3a shown in FIG. Hereinafter, details of the booster circuit 3 b will be described, and description of points that are common to the first embodiment will be omitted.

FIG. 6 is a circuit diagram of the booster circuit 3 included in the LED drive circuit according to the second embodiment. The booster circuit 3b shown in FIG. 6 is a charge pump type booster including a hysteresis comparator 51, a reference power supply 52, an AND gate 53, noters 54 and 55, switches SA1 to SA8, SB1 to SB5, and capacitors C1 to C5. Circuit. In addition, the booster circuit 3b includes terminals 56a to 5 6c. An input voltage Vin is applied to the terminal 56a, a boost control signal 10 is applied to the terminal 56b, and a boost voltage Vb is output from the terminal 56c. The boost voltage Vb is 5 times the input voltage Vin.

As shown in FIG. 6, one electrode of the capacitor C1 is grounded via the switch SA1, and the other electrode is connected to the terminal 56a via the switch SA2. The same applies to capacitors C2 to C4. A switch SB1 force is provided between the terminal 56a and one electrode of the capacitor C1 (the electrode on the side where the switch SA1 is connected). A switch SB2 is provided between the other electrode of the capacitor C1 and one electrode of the capacitor C2 (the electrode on the side to which the switch SA3 is connected). Switches SB3 and SB4 are provided in the same manner. A switch SB5 force is provided between the other electrode of the capacitor C4 (the electrode on the side where the switch SA8 is connected) and the terminal 56c. Capacitor C5 is provided between terminal 56c and ground.

The AND gate 53 is supplied with a clock signal CLK having a predetermined frequency. The AND gate 53 outputs a logical product of the clock signal CLK and the output signal of the hysteresis comparator 51. The nofer 54 outputs the output signal of the AND gate 53 in a non-inverted manner. The output signal of the notch 54 is given to the control terminals of the switches SB1 to SB5. On the other hand, the notifier 55 inverts the output signal of the AND gate 53. The output signal of the buffer 55 is applied to the control terminals of the switches SA1 to SA8. Therefore, the switches SA1 to SA8 are turned on at the same time, and the switches SB1 to SB5 are turned on at the same time when the switches SA1 to SA8 are turned off (see FIG. 7).

[0053] When the switches SA1 to SA8 are in the on state and the switches SB1 to SB5 are in the off state, the ground voltage and the input voltage Vin are applied to each electrode of the capacitor C1 via the switches SA1 and SA2, respectively. . As a result, the capacitor C1 is charged. At this time, capacitors C2 to C4 are also charged.

[0054] On the other hand, when the switches SA1 to SA8 are in the off state and the switches SB1 to SB5 are in the on state, the capacitor C5 is the sum of the input voltage Vin and the voltage charged in the capacitors C1 to C4, that is, the input Charged by a voltage 5 times the voltage Vin. The potential difference between both electrodes of capacitor C5 is output as boosted voltage Vb. In this way, the booster circuit 3b The input voltage Vin is boosted by controlling the switches SA1 to SA8 and the switches SB1 to SB5 alternately on and off, and switching the connection state of the capacitors C1 to C5.

The booster circuit 3b changes the level of the boost voltage Vb based on the boost control signal 10 by the following method. As shown in FIG. 6, the boost control signal 10 is given to the positive side input terminal of the hysteresis comparator 51 via the terminal 56b. The negative input terminal of the hysteresis comparator 51 is connected to the reference power supply 52 that supplies the reference voltage E3.

The hysteresis comparator 51 is a comparison circuit having hysteresis characteristics with a hysteresis width VH. The hysteresis comparator 51 outputs a high level signal when the voltage of the boost control signal 10 becomes (E3 + VHZ 2) or more, and outputs a low level signal when the voltage of the boost control signal 10 becomes (E3−VHZ 2) or less. Output.

Therefore, after the voltage of the boost control signal 10 becomes equal to or higher than (E3 + VHZ2), the output signal of the NAND gate 53 changes at a predetermined frequency similarly to the clock signal CLK. As a result, the charge pump circuit composed of the switches SA1 to SA8, SB1 to SB5 and the capacitors C1 to C5 performs a boosting operation, and the boosted voltage Vb rises. At this time, the duty ratio of the output signal of the AND gate 53 is a value other than zero (for example, 50%).

On the other hand, after the voltage of the boost control signal 10 becomes equal to or lower than (E3−VHZ2), the output signal of the AND gate 53 is fixed at the low level. Therefore, after the voltage of the boost control signal 10 becomes (E3−VHZ2) or less, the charge pump circuit does not perform the boost operation and the boost voltage Vb drops. At this time, the duty ratio of the output signal of the AND gate 53 is 0%. As described above, the duty ratio of the output signal of the AND gate 53 changes according to the boost control signal 10.

[0059] As described above, in the charge pump type booster circuit 3b, the switches SA1 to SA8 and SB1 to SB5 are controlled by using the switch control signal whose duty ratio changes according to the boost control signal 10, thereby boosting the voltage. Based on the control signal 10, the level of the boost voltage Vb can be changed.

[0060] According to the LED drive circuit of this embodiment including the booster circuit 3b shown in FIG. As in the LED driving circuit according to the first embodiment provided with the booster circuit 3a shown in FIG. 2, the LED can be driven with a constant current stably and with low power consumption.

It should be noted that the 1S LED drive circuit that has been described so far for the case where the LED drive circuit drives four LEDs may drive one or more arbitrary LEDs. For example, the LED driving circuit 1 may drive two LEDs connected in series (hereinafter referred to as LED array 8) as shown in FIG. Even in this case, the LED drive circuit 1 generates the boost voltage Vb while performing feedback control so that the voltage across the constant current source 2 matches the reference voltage E1.

FIG. 9 is a diagram showing temperature characteristics of the boosted voltage Vb when the LED array 8 is driven.

In this case, the following equation (5) is established among the boosted voltage Vb, the voltage Vi across the constant current source 2, and the forward voltage Vf of each LED. In FIG. 9, the graph of the boosted voltage Vb is a graph obtained by translating the voltage (Vf X 2) graph by the reference voltage E1 in the vertical direction.

Vb = Vi + Vf X 2--(5)

[0063] When driving the LED array 8, even if the temperature changes, the voltage Vi across the constant current source 2 and the current flowing through the constant current source 2 and the LED array 8 do not change. The power consumption is constant regardless of the temperature. Therefore, the constant current source 2 does not consume more power than necessary. The same applies to driving one or more arbitrary LEDs.

[0064] The same LED driving circuit 1 can be used when driving four LEDs and when driving two LEDs. In general, the same LED driving circuit 1 can be used to drive n or less LEDs as long as the boosting circuit 3 can generate a boosted voltage Vb that satisfies the following equation (6).

Vb = Vi + Vf X n · '· (6)

[0065] Although the LED driving circuit for driving the LED has been described as an example of the light emitting element driving circuit so far, the electoric luminescence element (EL element) is driven by the same method in the same way. A light emitting element driving circuit can be obtained. These EL element drive circuit and filament ball drive circuit have the same effect as the LED drive circuit.

Industrial applicability

[0066] The light-emitting element driving circuit according to the present invention provides a constant current drive for a light-emitting element stably and with low power consumption. Since it has the effect of being able to move, it can be used for light emitting element drive circuits that drive LEDs, EL elements, and filament balls.

Claims

The scope of the claims

[1] A light emitting element driving circuit for driving a light emitting element at a constant current,

A constant current source connected in series to the light emitting element to be driven;

A detection circuit for detecting a voltage across the constant current source;

The light-emitting element driving circuit, wherein the booster circuit changes a level of the boosted voltage based on the boost control signal.

[2] The booster circuit is a chitotsuba booster circuit including a coil and a switch that switches a flow of current that has passed through the coil.

2. The light emitting element driving circuit according to claim 1, wherein the switch is controlled by a control signal whose duty ratio changes according to the boost control signal.

[3] The booster circuit is a charge pump booster circuit including a capacitor and a switch for switching a connection state of the capacitor.

4. The light emitting element drive circuit according to claim 1, wherein the boost control signal is an analog signal indicating a difference between a voltage detected by the detection circuit and the reference voltage.