JP2002369506A - Current drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a current drive circuit for current driving element, such as LEDs, etc., that can realize temperature compensation with a low loss, while the circuit drives a current with a constant current at high efficiency. SOLUTION: This current drive circuit is provided with a temperature detecting circuit 7, a current setting circuit 8 which sets the target current to be supplied to a current driving element 4, and a drive circuit 5 which changes its control output in accordance with the temperature detected by means of the circuit 7, the value of the target current set by means of the circuit 8, and a detected current. This circuit is also provided with a DC-DC converter section 10, which changes its output voltage in accordance with the control output of the driving circuit 5. This circuit supplies a required current to the current drive element 4, so as to impress the output voltage of the converter section 10 on the element 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current driving circuit, and more particularly, to a current driving circuit that reduces power consumption and improves reliability.

[0002]

2. Description of the Related Art When a current driving element such as a light emitting diode (hereinafter, referred to as an LED) or an organic electroluminescence element is driven at a constant voltage, there is a problem that the light emission luminance is changed by the influence of electrode resistance and temperature. To avoid this, for example, in Japanese Unexamined Patent Application Publication No. 11-338561, a circuit using a large number of current drive elements is used, and a current detection circuit detects a forward current for a predetermined voltage for each element and stores it in storage means. The constant current circuit is controlled according to the memory.

As shown in FIG. 8, the conventional constant current drive circuit for a current drive element such as an LED supplies a required maximum forward voltage Vfmax of the LED (a) using a fixed voltage + VB. The temperature compensating circuit is provided in the circuit to perform. In FIG. 8, reference symbol a denotes an LED, and reference symbol b.
Denotes a driving circuit, c denotes an operational amplifier, d denotes a temperature detection circuit, and e denotes a forward current setting circuit. In this circuit, the operational amplifier c compares the potential difference proportional to the current generated in the current detection resistor in the drive circuit b with the voltage set in the forward current setting circuit e, and determines the L level in the drive circuit b according to the result.
The current flowing through the ED (a) is controlled, and the current flowing through the LED (a) is controlled according to the temperature detected by the temperature detection circuit d.

The forward voltage Vf for causing a predetermined forward current IL to flow through the LED (a) is allowed to vary according to the standard. When an LED having a low forward voltage is used, the maximum forward voltage Vfmax is used. And the minimum forward voltage Vfmin, the difference voltage ΔVf, and the power due to the forward current IL (= ΔVf ×
IL) is a loss of the driving element Tr of the driving circuit b connected in series with the LED (a).

FIG. 9 shows a portion around the LED (a) in FIG. From this figure, assuming that the collector-emitter voltage of the transistor which is the driving element Tr is VCE, VB = Vf + VCE + R.IL Therefore, VCE1 is lower than VCE1 in the case of the forward voltage Vfmax of the LED (a) having a high forward voltage. LED
VB = Vfmax + VCE1 + R.IL = Vfmin + V between VCE2 in the case of the forward voltage Vfmin of (a).
CE2 + R · IL VCE2-VCE1 = Vfmax−Vfmin = △ Vf Therefore, when VB is fixed, when the LED (a) having a low forward voltage is used, the driving element T
At least ΔVf × IL between the collector and the emitter of r
Power loss occurs. As a result, the loss (for example, collector loss) of the driving element Tr of the driving circuit b connected in series with the LED (a) increases, so that the reliability of the driving element Tr deteriorates. For example, assuming that Vfmax = 4 V and Vfmin = 3 V when IL = 20 mA, the loss of the driving element Tr becomes a considerably high value of (4-3) V × 20 mA = 20 mW. Further, it is necessary to use a package having a large loss standard. Further, there is also a problem that the drive element Tr generates heat due to the large loss of the drive element Tr, and the reliability of the peripheral circuit is reduced.

[0006] In particular, there is a problem with processing corresponding to environmental temperature changes. In a conventional circuit for driving a constant current LED, from the viewpoint of the performance of the LED shown in FIG. In the case of driving at (for example, 85 ° C.), it is necessary to reduce the forward current to about 以下 or less, and if this is to be processed by the driving element Tr, the loss becomes extremely large.

[0007]

As described above, in the conventional current driving circuit, a countermeasure against temperature has been provided by providing a temperature compensation circuit in a circuit for supplying a maximum forward voltage using a fixed voltage. Therefore, the loss in the driving element at a high temperature is large, and this has been a major factor in lowering the reliability of the driving element and the reliability of peripheral circuits. The present invention solves this problem by a relatively simple method, and realizes temperature compensation with low loss while driving efficiently with a constant current.
It is an object to realize a current driving circuit for a current driving element such as an ED.

[0008]

In order to achieve the above object, the present invention comprises a current detecting means and a current supply power supply means for controlling a current supply in accordance with a detection result of the current detecting means. In a current drive circuit for supplying a current to a drive element, the current supply power supply means includes a temperature detection means, a current setting means for setting a target current to be supplied to the current drive element, and a detection means for detecting the temperature detection means. A driving means for changing a control output in accordance with a temperature, a target current value set by the current setting means, and a detection current of the current detection means, and a DC-controller for varying an output voltage in accordance with the control output of the driving means. And a DC converter for supplying a required current to the current drive element by applying an output voltage of the DC-DC converter of the current supply power supply. This makes it possible to vary the output voltage applied to the current drive element according to the situation, and realize a current drive circuit for a current drive element such as an LED that can realize temperature compensation with low loss while driving efficiently with a constant current. Can be realized.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a current driving circuit according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of an embodiment of a current driving circuit according to the present invention, and FIG. 2 is a detailed circuit diagram of the embodiment of FIG. 1 and 2, reference numeral 1 is a switching unit, reference numeral 2 is a filter unit, reference numeral 3 is a current detection unit, reference numeral 4 is a semiconductor light emitting element unit (LED), reference numeral 5 is a drive circuit, reference numeral 6 is a current adjustment circuit, Reference numeral 7 denotes a temperature detection circuit, reference numeral 8 denotes a current setting circuit, reference numeral 9 denotes a DC-DC converter IC, reference numeral 91 denotes an operational amplifier circuit, reference numeral 92 denotes a triangular wave generator, reference numeral 93 denotes a comparator circuit, and a switching unit 1 and a filter unit. 2 and the DC-DC converter IC 9 constitute a DC-DC converter unit 10.

In the block configuration shown in FIG. 1, the LED 4 is supplied with a forward voltage Vf from the non-rectified power supply voltage UNREG via the switching unit 1 and the filter unit 2 so as to obtain a desired forward current If. The main parts of the drive circuit are a forward current detection unit 3, a drive circuit unit 5, and a DC-D
It comprises a C converter unit 10. The current setting circuit 8 and the current adjustment circuit 6 set an initial current value of the LED 4. Further, as shown in the temperature-allowable forward current characteristic of FIG. 4, the LED 4 needs to reduce the current at a high temperature (for example, 85 ° C.) to about １ ／ or less of the allowable current at a normal temperature (for example, 25 ° C.). Therefore, the forward current value is reduced by the temperature detection circuit 7 from the initial setting so as to satisfy this standard.

As shown in FIG. 2, this circuit has a DC-DC
Converter IC 9 and DC-DC converter unit 10 including switching unit 1 and filter unit 2
, The voltage F fed back from the drive circuit 5
B operates to output a desired voltage Vdd and make the current IL flowing through the LED unit 4 constant. Normally, the current value of the LED 4 is equal to the detection resistance R of the current detection unit 3.
1. The transistors Q1 and Q2 of the drive circuit 5 and the resistor R
The output current Vdd of the DC-DC converter unit 10 is controlled in accordance with the variation of the forward voltage Vf of the LED 4 (for example, 3 V to 4 V) by controlling the constant current at 2, R3, and R4.

The DC-DC converter 10 compares the reference voltage Vref of the operational amplifier with the feedback voltage FB, and operates such that a current of IL '(= Vref / R4) flows through the resistor R2. At this time, assuming that the base voltage of the transistor Q1 is Vbg1, a voltage of Vdd-Vbg1 is generated at both ends of the detection resistor R1, and the forward current IL of the LED = (Vd-Vbg1) / R1} is determined.

On the other hand, transistors Q3 and Q4 and a resistor R
5, R6, a current adjustment circuit 6 composed of an operational amplifier IC2, a current setting circuit 8 composed of resistors R7 and R8, and a temperature detection circuit 6 composed of a temperature sensing element (for example, a thermistor) TH. The voltage VLEV is connected to the resistor R7,
The voltage divided by R8 is applied to the resistor R5 as the voltage Vr5, and the current IL ″ (= Vr5 / R5) flows through the resistor R5. The current IL ″ changes the current value of IL ′, and as a result, the feedback voltage FB Changes, and the output voltage V
As dd changes, the current IL flowing through the LED section 4 changes. If the current adjustment terminal voltage VLEV increases, the current IL "increases, Vdd increases, and the current IL increases. As a result, the current IL flowing through the LED section 4 can be set.

FIG. 3 shows the relationship between the temperature and the resistance of the thermistor. In the temperature detection circuit 7, at normal temperature (for example, 25 ° C.), the resistance R6 and the resistance value of the thermistor (TH25 in FIG. 3)
Is set to be higher than the voltage Vr5, and operates at the current value set by the current adjustment terminal voltage VLEV. The voltage VTH divided by the resistance value (TH85 in FIG. 3) is set to be smaller than the voltage Vr5. At this time, assuming that the base-emitter voltage of the transistor Q3 is VBE in the resistor R5, the current IL ″ ｛(VTH−VB
E) / R5} flows. That is, when the temperature becomes high, the current IL "becomes small, and as a result, DC-DC
As the output voltage Vdd of the converter unit 10 becomes smaller,
The current IL flowing through the ED section 4 decreases. In this way, the drive current of the current drive element can be set by optimizing the forward voltage, thereby improving the efficiency of the current supply power supply circuit, reducing power consumption, and realizing a highly reliable circuit. can do.

Some other embodiments will be described as modified examples of the present invention. FIG. 5 shows an example in which the DC-DC converter unit 10 is connected to the LED 4 with a negative power supply. As shown in FIG. 6, the DC-DC converter unit 10
When there are a plurality of channels of -1, 10-2, different initial settings are made for each of the LEDs 4-1 and 4-2,
The temperature correction can be performed by the common temperature correction control circuit 11 for each of the drive circuits 5-1 and 5-2. Of course, a temperature correction control circuit can be provided for each LED having a different allowable current curve. Further, as shown in FIG. 7, a plurality of temperature correction control circuits 11-1 and 11-2 may be set, and a correction curve may be selected and used according to conditions. In the above description, LE
D4 is shown to be used in series, but individual LEDs
Can be used in parallel connection if the current adjustment can be performed.

As described above, according to the current driving circuit of the present invention, when a constant current is set to a current driving element such as an LED, a voltage can be supplied at an optimum value of the forward voltage, so that the efficiency of the power supply circuit is improved. Is improved, which can contribute to power reduction. At the same time, the loss of the drive circuit is reduced, the reliability can be prevented from lowering due to heat generation, and the reliability is improved. Even if the number of current driving elements such as LEDs changes, the circuit can operate as a constant current driving circuit, so that a common design can be performed regardless of the number of current driving elements, standardization of the circuit can be achieved, and cost reduction can be achieved. As the temperature rises, the forward current is reduced in accordance with the permissible current standard of the current drive element such as an LED, so that the reliability at a high temperature can be improved while driving efficiently with a constant current. Advantages such as are born.

[0018]

As described above, according to the first aspect of the present invention, in a current driving circuit for supplying a current to a current driving element such as an LED, a temperature detecting means and a target current to be supplied to the current driving element are provided. Current setting means, a driving means for changing a control output according to a detected temperature, a target current value and a detected current, and a DC-DC conversion means for varying an output voltage according to a control output of the driving means. Provided, DC-DC
The output voltage of the conversion means is applied to the current driving element to supply a required current. As a result, the voltage of the optimum value of the forward voltage can be supplied to the current driving element, so that the efficiency of the power supply circuit can be improved, the power can be reduced, the heat generation and the like can be prevented, and the reliability of the element and the circuit can be improved. be able to.

According to a second aspect of the present invention, the target current value set by the current setting means is changed according to the detected temperature detected by the temperature detecting means so as to be suitable for the allowable forward current characteristic of the current driving element. As a result, it is possible to increase the reliability at high temperatures while driving with high efficiency at a constant current.

[Brief description of the drawings]

FIG. 1 is a block diagram of a current drive circuit according to the present invention.

FIG. 2 is a detailed circuit diagram of the current drive circuit of FIG.

FIG. 3 is an explanatory diagram showing a relationship between a temperature and a resistance value of a thermistor.

FIG. 4 is an explanatory diagram showing temperature-allowable forward current characteristics of an LED.

FIG. 5 is a circuit diagram showing a modified example of the current drive circuit of the present invention.

FIG. 6 is a block diagram showing another modification of the current drive circuit of the present invention.

FIG. 7 is a block diagram showing another modified example of the current drive circuit of the present invention.

FIG. 8 is a block diagram of a conventional current drive circuit.

FIG. 9 is an explanatory diagram extracting and showing a part of the block diagram of FIG. 8;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Switching part, 2 ... Filter part, 3 ... Current detection part, 4 ... Semiconductor light emitting element part (LED), 5 ... Drive circuit,
6 current adjustment circuit, 7 temperature detection circuit, 8 current setting circuit, 9 DC-DC converter IC, 10 DC-DC
Converter unit, 11: temperature correction control circuit, 91: operational amplifier circuit, 92: triangular wave generator, 93: comparator circuit, a: LED, b: drive circuit, c: operational amplifier, d ...
Temperature detection circuit, e: forward current setting circuit.

Continued on the front page (72) Inventor Hideaki Abe 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (72) Inventor Toshiaki Isagawa 6-35-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Sony Stock Company F term (reference) 5H730 AA20 BB13 DD02 DD26 DD34 FD31 FG05 FV07 XX19 XX38

Claims (2)

[Claims] 1. A current drive circuit comprising: a current detection means; and a current supply power supply means for controlling a current supply in accordance with a detection result of the current detection means, and supplying a current to a current drive element. Power supply means, temperature detection means, current setting means for setting a target current to be supplied to the current drive element, detected temperature detected by the temperature detection means, target current value set by the current setting means, Driving means for changing a control output in accordance with the detection current of the current detection means; and D for varying an output voltage in accordance with the control output of the driving means.
A current drive circuit comprising: a C-DC converter; and supplying a required current to the current driver by applying an output voltage of the DC-DC converter of the current supply power supply. 2. The apparatus according to claim 1, wherein a target current value set by said current setting means is changed in accordance with a detected temperature detected by said temperature detecting means so as to be suitable for an allowable forward current characteristic of said current driving element. Item 2. The current drive circuit according to item 1.