JP2011258797A - Drive control circuit of light-emitting diode and backlight system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a drive control circuit of a light-emitting diode which can drive the light-emitting diode with a simplified circuit, and to provide a backlight system.SOLUTION: The drive control circuit of a light-emitting diode 2 which drives the light-emitting diode by controlling a current being fed thereto comprises a first transistor Q1 to which a constant current is fed, a second transistor Q2 which is in current-mirror connection with the first transistor, a third transistor Q5 through which the same current as that of the second transistor flows, a drive transistor Q8 which is in current-mirror connection with the third transistor, and a switching element Q9 connected in series with the light-emitting diode and the drive transistor between a first power source line Vdd and a second power source line Vss.

Description

  The present invention relates to a drive control circuit for a light emitting diode and a backlight illumination device.

  In recent years, for example, light emitting diodes (LEDs) have been used as backlight illumination devices for liquid crystal display devices.

  An LED drive control circuit in such a backlight illumination device controls a switching transistor by, for example, an error amplifier (error amplifier), a PWM (Pulse Width Modulation) circuit, a logic control circuit, a driver circuit, and an overcurrent detection unit.

  Then, the charge stored in the output capacitor is controlled by the control of the switching transistor, and for example, a predetermined current is supplied to three or six LEDs connected in series to drive (emit) light stably. It is like that.

  By the way, conventionally, various LED drive control circuits or backlight illumination devices have been proposed.

JP 10-275691 A Japanese Patent Laid-Open No. 10-335089 JP 2001-215913 A JP 2003-058264 A

  As described above, as an LED drive control circuit, for example, a circuit that supplies a predetermined current to a plurality of LEDs connected in series to stably drive the LEDs is known.

  By the way, in recent years, for example, a backlight illuminating device for a liquid crystal display device in a portable terminal has been attracting attention.

  However, for example, a drive control circuit for supplying a predetermined current to three or six LEDs connected in series has a large circuit size and is redundant to drive one LED. It will also increase the price.

  Although a simplified drive control circuit for driving one LED can be considered, for example, the LED cannot be stably driven only by controlling the switching transistor with the output of the error amplifier.

  According to one embodiment, a drive control circuit for a light emitting diode is provided that controls and drives a current flowing through the light emitting diode.

  The light emitting diode drive control circuit includes: a first transistor through which a constant current flows; a second transistor that is current-mirror connected to the first transistor; and a third transistor through which the same current as the second transistor flows. Have.

  Further, the drive control circuit for the light emitting diode is connected in series with the light emitting diode and the drive transistor between the third transistor and the drive transistor connected in a current mirror and between the first power supply line and the second power supply line. Switching elements.

  The disclosed light emitting diode drive control circuit and backlight illumination device have an effect that the light emitting diode can be stably driven by a simplified circuit.

It is a block diagram which shows an example of the backlight illuminating device which uses a light emitting diode. It is a block diagram which shows an example of the backlight illuminating device as related technology. It is a block diagram which shows the backlight illuminating device of 1st Example. It is a block diagram which shows the backlight illuminating device of 2nd Example.

  First, before detailing the embodiments of the drive control circuit of the light emitting diode and the backlight illumination device, an example of the backlight illumination device using the light emitting diode, and an example of the backlight illumination device as a related technology, FIG. This will be described with reference to FIG. 1 and FIG.

  FIG. 1 is a block diagram illustrating an example of a backlight illumination device using a light emitting diode. In FIG. 1, reference numeral 101 is a drive control circuit, 102 is a step-up coil, 103 is a diode, 104 is an output capacitor, and 105 is a light emitting diode (LED).

  In FIG. 1, six LEDs 105 are connected in series, but for example, three LEDs may be connected in series.

  As shown in FIG. 1, the drive control circuit 101 includes an error amplifier (error amplifier) 111, a dimming logic (soft start) circuit 112, a capacitor 113, a PWM circuit 114, and a logic control circuit 115.

  The drive control circuit 101 further includes an overcurrent detection unit 116, a driver circuit 117, a switching transistor 118 made up of an n-channel MOS transistor, and a resistor 119. Here, the overcurrent detection unit 116 includes an overcurrent protection circuit 61 and a current detection circuit 62.

  The current detection circuit 62 detects the current flowing through the switching transistor 118 via a voltage signal IS obtained by voltage conversion by the resistor 119, and supplies a signal indicating the detected current value to the overcurrent protection circuit 61 and the PWM circuit 114. To do.

  When the current value of the switching transistor 118 detected by the current detection circuit 62 exceeds a predetermined level, the overcurrent protection circuit 61 controls the logic control circuit 115 to control the signal DVR from the driver circuit 117 for switching. The transistor 118 is protected (turned off).

  The error amplifier 111 receives the magnitude of the current flowing through the LEDs 105 connected in series as a voltage signal FB converted into a voltage by the resistor 106, and outputs an output so that the voltage signal FB matches the voltage level from the dimming logic circuit 112. Change.

  Here, the dimming logic circuit 112 receives an external dimming control signal CS, and controls the current flowing through the LED 105 to adjust its brightness. The dimming logic circuit 112 also has a soft start function that alleviates stress and inrush current when the power is turned on, for example, and gradually increases the current flowing through the LED 105.

  The output of the error amplifier 111 is supplied to the input of the PWM circuit 114 together with the output of the current detection circuit 62. The PWM circuit 114 controls the switching (on / off) period of the switching transistor 118 by the signal DVR via the logic control circuit 115 and the driver circuit 117.

  One end of the booster coil 102 is connected to, for example, a 5 V power supply line, and the other end is connected to the drain of the switching transistor 118 and the cathode of the diode 103. The anode of the diode 103 is connected to the other end of the output capacitor 104 whose one end is grounded and the first stage cathode of the LED 105 connected in series.

  Note that the anode of the final stage of the LEDs 105 connected in series is grounded via a resistor 106. Further, as described above, the resistor 106 converts the current flowing through the LED 105 into the voltage signal FB, and the voltage signal FB is fed back to the error amplifier 111.

  The backlight illumination device of FIG. 1 uses a back electromotive voltage generated in the booster coil 102 by turning on / off the switching transistor 118 that is PWM-controlled, and the diode 103 and the output capacitor 103 cause, for example, about 20V to the node N0. Generate voltage.

  As described above, the backlight illumination device of FIG. 1 is configured to feedback-control the current flowing through the LEDs 105 connected in series to cause the LEDs 105 to emit light.

  However, for example, the backlight illumination device (drive control circuit) shown in FIG. 1 has a large circuit size and redundancy for driving one LED, which may increase the price of the backlight illumination device. Become.

  FIG. 2 is a block diagram showing an example of a backlight illuminating device as a related technique, and shows one using one LED 207.

  In FIG. 2, reference numeral 200 denotes a drive control circuit, 201 denotes a reference voltage generation circuit, 202 denotes a dimming control switch, 203 denotes a capacitor, 204 denotes an error amplifier, 205 denotes a drive transistor, and 206 denotes a resistor.

  As shown in FIG. 2, in the related-art backlight illumination device, for example, an LED 207, a drive transistor 205, and a resistor 206 are connected in series between a high-potential power line of 3.3 V or 5 V and the ground. .

  The resistor 206 converts the magnitude of the current flowing through the LED 207 (driving transistor 205) into a voltage signal, and the voltage signal FB is fed back to the input of the error amplifier 204.

  The error amplifier 204 changes its output so that the voltage signal FB from the resistor 206 matches the voltage level due to the charge stored in the capacitor 203 when the output voltage of the reference voltage generation circuit 201 is applied by the dimming control switch 202. Let

  That is, the drive transistor 205 is feedback-controlled so that the voltage from the reference voltage generation circuit 201, the dimming control switch 202, and the capacitor 203 is equal to the voltage signal FB. As a result, the LED 207 emits light when a predetermined current flows.

  Here, the current control (constant current control) of the LED 207 is controlled by turning on / off the dimming control switch 202, and the dimming control of the LED 207 is controlled by an on / off interval (duty ratio) of the dimming control switch 202. Is controlled by changing (PWM).

  Further, for example, soft start can also be realized by changing the duty ratio of the dimming control switch 202. In the drive control circuit 200 shown in FIG. 2, there are two external terminals.

  However, in the related-art backlight illumination device (drive control circuit) shown in FIG. 2, for example, the voltage signal FB from the external resistor 206 is fed back to the error amplifier 204, so the feedback constant changes. May oscillate.

  Note that the oscillation in the feedback control appears as flickering of the LED 207, for example. That is, the drive control circuit 200 of FIG. 2 has a problem that the circuit itself can be simplified, but the LED 207 cannot be driven stably.

  Embodiments of a drive control circuit for a light emitting diode and a backlight illumination device will be described in detail below with reference to the accompanying drawings. FIG. 3 is a block diagram showing the backlight illumination device of the first embodiment.

  In FIG. 3, reference numeral 1 is a drive control circuit, 2 is an LED, 3 is a resistor (external resistor), 10 is a bias control circuit, Q1 and Q2 are p-channel MOS transistors, and Q3 to Q9 are n-channel types. A MOS transistor is shown.

  As shown in FIG. 3, the backlight illumination device of the present embodiment is, for example, between a high potential power line (first power line) Vdd and a low potential power line (second power line) Vss of 3 to 5 V. The LED 2 and the transistors Q6, Q8, and Q9 are connected in series. Here, the transistor Q8 is a drive transistor, and the transistor Q9 is a switching transistor (switching element).

  The bias control circuit 10 includes a transistor Q1, a transistor Q4, and an error amplifier 11. The transistors Q1 and Q4 are connected in series with the external resistor 3 between the high potential power supply line Vdd and the low potential power supply line Vss.

  As a result, the current I1 flowing through the transistor Q4 (Q1) is controlled by the output of the error amplifier 11 so that the voltage signal obtained by converting the current I1 by the external resistor 3 and the reference voltage VREF coincide.

  That is, the current I1 is controlled so as to be a constant current in which the voltage signal from the external resistor 3 matches the reference voltage VREF. Here, the transistor Q1 is current-mirror connected to the transistor Q2, and the transistor Q3 is current-mirror connected to the driving transistor Q8.

  Transistors Q2, Q7, Q3 and Q5 are connected in series between high potential power line Vdd and low potential power line Vss, and transistors Q6, Q8 and Q9 are connected to high potential power line Vdd and low potential power line Vss. Are connected in series via the LED 2.

  Here, as an example only, the gate widths of the transistors Q1 and Q2 are 1: 1, the gate widths of the transistors Q3 and Q8 are 1:20, and the current flowing through the transistor Q1 (Q4) is 1 mA. .

  At this time, since the transistor Q1 is current-mirror connected to the transistor Q2, the current I2 flowing through the transistor Q2 (Q3) is 1 mA, which is the same as the current I1. The transistor Q3 is current-mirror connected to the driving transistor Q8 and has a gate width of 1:20, so that the current Id flowing through the driving transistor Q8 (LED2) is 20 mA.

  In this way, the LED 2 can be driven (emitted) by setting the current Id flowing through the LED 2 to a predetermined current value (for example, 20 mA).

  Here, the transistor Q6 provided between the LED 2 and the driving transistor Q8 and having a gate connected to the high potential power supply line Vdd is a transistor for countermeasures against 5V tolerance. The transistor Q7 whose gate is connected to the high potential power line Vdd is provided corresponding to the transistor Q6, and the transistor Q5 whose gate is connected to the high potential power line Vdd is connected to the switching transistor Q9. Correspondingly provided.

  The switching transistor Q9 is controlled to be switched (on / off) by a control signal SCS.

  That is, the current control (dimming control) of the LED 2 is controlled by varying (PWM) the on / off interval (duty ratio) of the switching transistor Q9.

  Furthermore, for example, soft start can also be realized by changing the duty ratio of the switching transistor Q9. In the drive control circuit 1 shown in FIG. 3, the external terminals are two terminals for connecting the external resistor 3 and the LED 2.

  As described above, according to the backlight illumination device (drive control circuit 1) of the first embodiment, the feedback circuit such as the drive control circuit of FIG. 2 described above is unnecessary, and the LED 2 can be driven stably. it can. Further, the circuit can be simplified as compared with the drive control circuit of FIG. 1 described above.

  FIG. 4 is a block diagram showing the backlight illumination device of the second embodiment. As is clear from the comparison between FIG. 4 and FIG. 3 described above, the backlight illumination device (drive control circuit 1a) of the second embodiment is configured to drive the two LEDs 2a and 2b.

  That is, as shown in FIG. 4, two sets (2a, Q6a, Q8a, Q9a, LED2 and transistors Q6, Q8, Q9, connected in series between the high potential power supply line Vdd and the low potential power supply line Vss, And 2b, Q6b, Q8b, Q9b).

  Here, each pair of drive transistors Q8a and Q8b is current-mirror connected to the transistor Q3. In FIG. 4, different control signal control signals SCSa and SCSb are supplied to the gates of the switching transistors Q9a and Q9b, and the currents Ida and Idb flowing through the LEDs 2a and 2b are independently controlled. You may control by a signal. Further, the LED2 and the transistors Q6, Q8, and Q9 are not limited to two sets.

  In the above description of each embodiment, the transistor has been described as a MOS transistor. However, a bipolar transistor may be used, and it is needless to say that the conductivity of the transistor is not limited to that of the embodiment.

1,200 Drive control circuit 2, 2a, 2b, 105, 207 Light emitting diode (LED)
3 Resistance (external resistance)
10 Bias control circuit 11, 111, 204 Error amplifier (error amplifier)
Q1, Q2 p-channel MOS transistors Q3-Q7, Q6a, Q6b n-channel MOS transistors Q8, Q8a, Q8b drive transistors (n-channel MOS transistors)
Q9, Q9a, Q9b Switching transistor (n-channel MOS transistor)

Claims (6)

A drive control circuit for a light emitting diode that controls and drives a current passed through the light emitting diode,
A first transistor through which a constant current is passed;
A second transistor in current mirror connection with the first transistor;
A third transistor through which the same current as the second transistor flows;
A drive transistor connected in current mirror with the third transistor;
A drive control circuit for a light emitting diode, comprising a switching element connected in series with the light emitting diode and the drive transistor between a first power supply line and a second power supply line. The drive control circuit for a light emitting diode according to claim 1, further comprising:
A fourth transistor connected in series with a resistor and the first transistor between the first power line and the second power line;
An error amplifier that varies an output of the resistor to match a reference voltage, and an output of the error amplifier is supplied to a control terminal of the fourth transistor. Drive control circuit. In the drive control circuit of the light emitting diode according to claim 1,
The second transistor and the third transistor are connected in series between the first power supply line and the second power supply line via a fifth transistor corresponding to the switching transistor. Diode drive control circuit. The light emitting diode drive control circuit according to claim 3, further comprising:
A sixth transistor having a control terminal connected to the first power line between the driving transistor and the light emitting diode;
A drive control circuit for a light emitting diode, comprising: a seventh transistor connected between the second transistor and the third transistor and having a control terminal connected to the first power supply line. A drive control circuit for a light emitting diode according to any one of claims 2 to 4,
A backlight illumination device comprising: a light emitting diode and a resistor provided externally to the drive control circuit. The backlight illumination device according to claim 5,
A backlight illumination device comprising a plurality of sets of the light emitting diode, the driving transistor, and the switching element connected in series between the first power supply line and the second power supply line.

Images