JP2006314168A - Power circuit for liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power circuit for liquid crystal drive which reduces the unevenness of brightness of a white LED illumination source in a liquid crystal display and besides operates stably regardless of lighting and lights out of the white LED illumination source. <P>SOLUTION: In a power circuit which has a DC power source, a constant voltage power source which is driven by the above DC power source, a constant current power source which is driven by the above DC power source, and a voltage rise controller which is shared by the above constant current power source, the above voltage rise controller performs control operation by the load state of the above constant current power source, and performs the control operation by the load state of the above constant voltage power source at operation stoppage of the above constant current power source. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a power supply circuit for a liquid crystal display device.

In a conventional general liquid crystal display device, for example, +15 V and −15 V, 5 V as a source driver voltage, and 3.3 V as a logic voltage of a control circuit are supplied from a constant voltage driving source as gate driver voltages necessary for driving the liquid crystal. is doing. As these constant voltage drive sources, there are many examples in which power supply blocks using switching regulators set to respective voltages are connected in parallel.
On the other hand, since the illumination brightness of white LEDs strongly depends on the supply current, it is common to supply power with a constant current power source in order to avoid fluctuations in the illumination brightness of the white LEDs.
In Patent Document 1 below, an LED backlight is used as a constant current circuit in order to keep the brightness of the LED backlight constant regardless of variations and fluctuations in the LED drive voltage and variations in forward voltage for each LED product. To drive.

  Hereinafter, a conventional technique will be described with reference to the drawings. FIG. 2 is a block diagram showing a configuration of an LED illumination source and a drive power supply circuit of a conventional liquid crystal display device. In the drawings, the same reference numerals indicate the same or corresponding parts.

In this figure, the current flowing through the white light emitting diode (white LED) 22 that is the first load that requires constant current drive is made constant by the regulation function of the switching regulator controller 12. In addition, the voltage applied to the liquid crystal display device 73, which is the second load that requires constant voltage driving, is made constant by the regulation function of the switching regulator controller 13.
JP-A-11-305198

In recent years, color liquid crystal (LCD) has been the mainstream for display of mobile devices (cell phones, PDAs, digital cameras, etc.), but since LCD does not emit light by itself, light is applied from the back (backlight), Or an illumination source is needed, such as shining light from the surface (front light). Recently, white light emitting diodes (white LEDs) are increasingly used as illumination sources for LCD display devices. However, the application of white LED illumination to LCD display devices has the following problems in terms of power supply devices. It was.
1. The liquid crystal display device includes the above-described various constant voltage power sources (drive power sources), but white LED lighting requires a constant current power source (illumination power source) for white LEDs separately from these constant voltage power sources.
2. Since the white LED switching regulator (illumination regulator) and the plurality of switching regulators (driving regulators) of the LCD drive power source are independent circuit components, the operating frequencies are not synchronized. For this reason, interference fringes may be displayed on the LCD screen. In addition, switching noise generated from a plurality of switching regulators may be superimposed on the LCD screen. Therefore, there is a possibility that the displayed image is deteriorated.
3. When the driving regulator for simplifying the circuit is connected in parallel with the white LED, the operating condition of the driving regulator varies when the white LED is turned off, and a stable voltage cannot be obtained.

  The present invention has been made to solve such problems, and the object thereof is to reduce unevenness in luminance of a white LED illumination source in an LCD display device, and to be stable regardless of whether the white LED illumination source is turned on or off. An object is to provide a power supply circuit for driving liquid crystal.

In order to achieve the above object, a power supply circuit according to the present invention includes a DC power supply, a constant voltage power supply unit driven by the DC power supply, a constant current power supply unit driven by the DC power supply, and the constant voltage power supply unit. In the power supply circuit having the constant voltage means and the boost control unit shared by the constant current power supply unit, the boost control unit performs a control operation according to a load state of the constant current power supply unit.
The power supply circuit according to the present invention is characterized in that the boost control unit performs a control operation according to a load state of the constant voltage power supply unit when the operation of the constant current power supply unit is stopped.
Further, in the power supply circuit of the present invention, the load of the constant voltage power source is a driver for driving liquid crystal of the liquid crystal display means, the load of the constant current power supply unit is a light emitting diode that illuminates the liquid crystal display means, and the constant voltage The means is a power supply circuit of a liquid crystal display device which is a series regulator connected to the constant current power supply unit. The present invention is characterized in that in the liquid crystal display device having the liquid crystal display means and the illumination means for illuminating the screen of the liquid crystal display, power is supplied by the power supply circuit.

  According to the present invention, a boost control circuit using a white light emitting diode as an LCD illumination light source as a constant current load is shared with a load that requires constant voltages of various constant voltage power sources for driving LCD liquid crystal, thereby simplifying the circuit. And switching noise of the power supply circuit can be reduced. In addition, by having the function of switching the control operation of the switching regulator controller between constant current control and constant voltage control, it is possible to supply stable power to various constant voltage loads for driving the liquid crystal even when the LED illumination source is turned off. It becomes possible.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a white LED illumination source and an LCD driving power supply circuit of an LCD display device according to the present invention. In the drawings, the same reference numerals indicate the same or corresponding parts.

  In this figure, 1 is a DC power source, 21 is a white LED that is a first load that requires constant current drive, and 71 is an LCD display device that is a second load that requires constant voltage drive.

  For the sake of explanation, a storage battery with a nominal voltage of 1.5 V is connected as a DC power source 1 and six white LEDs with a forward voltage drop of 3.5 V are connected in series as a first load white LED 21 to drive a constant current at a rated current of 20 mA. Assuming a module, further, + 15V is set as the power supply voltage V5 of the second load LCD display device 71, 1.2V is set as the reference voltage of the reference voltage source circuit voltage 161, and + 17V is set as the load terminal voltage V2 when the white LED 21 is extinguished. Assume that the collector-emitter voltage drop when the switching transistor 91 is on is 0.5V.

  The white LED 21 is turned on and off by the control voltage V3. When the control voltage V3 is at a high level, the control signal is converted to a low level by the logic inverting element 92. Therefore, when the transistor switch 91 is turned on, a current is supplied to the white LED 21 and a lighting operation is performed. At this time, since the transistor switch 93 is in an OFF state, no current flows through the resistor 81.

  On the other hand, when the control voltage V3 is at the low level, the control signal is converted to the high level by the logic inverting element 92. Therefore, when the transistor switch 91 is turned off, the current supply to the white LED 21 is cut off and the light is turned off. At this time, since the transistor switch 93 is in the ON state, the load current I1 flows to the resistor 61 through the resistor 81.

  During the lighting control of the white LED 21, the current flowing through the white LED 21 as the first load can be set by connecting a resistor 61 between the inverting input terminal of the error detection comparator 141 and the GND terminal. The value of the current setting resistor 61 can be obtained by dividing the reference voltage of 1.2 V by the set current of 20 mA, and in this example is 60Ω.

  Further, during the lighting control of the white LED 21, the load current I1 is made constant by the regulation function of the switching regulator controller 11, and the operation is as follows.

  When the inverting input terminal voltage V4 of the error detection comparator 141 is equal to or higher than the reference voltage 1.2V applied to the non-inverting input terminal of the error detection comparator 141, the load current I1 is supplied from the output side capacitor 101. At the same time, the current flowing through the load resistor 71 through the three-terminal regulator 10 that is a series regulator is also supplied from the output-side capacitor 101. When the V4 voltage becomes equal to or lower than the reference voltage 1.2V, the built-in MOS transistor switch 121 is turned on and a current starts to flow through the coil 31.

  This current is monitored by the boost control circuit 131 as a voltage drop of the resistor 171. When, for example, the set value reaches 350 mA, a reset signal is output from the boost control circuit 131, and the built-in MOS transistor switch 121 is turned off. As a result, a counter electromotive force is generated in the coil 31, and current is supplied to the white LED 21 through the diode 41. At the same time, a current flowing through the load resistor 71 is also supplied via the three-terminal regulator 10. By repeating this cycle until I1 reaches the set value of 20 mA, I1 is maintained constant, so that the white LED lighting operation without unevenness in brightness is performed.

  The constant voltage diode 51 is in an open state when the white LED 21 is disconnected and when the transistor switch 91 and the transistor switch 93 are simultaneously in the off state, the boost output is increased, and a voltage exceeding the withstand voltage of the switching regulator controller 11 is applied. In order to prevent this, a product that operates at a constant voltage of 25 V, which is larger than about 21 V of the normal boost output V2 as an open protection circuit, is connected.

On the other hand, when the white LED 21 is turned off, the constant voltage drive voltage V2 can be set by the resistance voltage value of the resistor 61 and the resistor 81 connected between the inverting input terminal of the error detection comparator 143 and the GND terminal. The values of the resistor 61 that also serves as a constant current drive and the constant voltage drive setting resistor 81 can be obtained by a 1: 1.75 proportional combination in which the resistance voltage dividing value of the load setting voltage 17V is the reference voltage 1.2V. In this example, the resistance 61 is 60Ω and the resistance 81 is 765Ω.
During the turn-off control of the white LED 21, the load voltage V2 is made constant by the regulation function of the switching regulator controller 11, and the operation is as follows.

  When the inverting input terminal voltage V4 of the error detection comparator 141 is equal to or higher than the reference voltage 1.2V applied to the non-inverting input terminal of the error detection comparator 141, the current flowing through the load resistor 71 via the three-terminal regulator 10 is Supplied from the output side capacitor 101. When the V4 voltage becomes equal to or lower than the reference voltage 1.2V, the built-in MOS transistor switch 121 is turned on and a current starts to flow through the coil 31.

This current is monitored by the boost control circuit 131 as a voltage drop of the resistor 171. When, for example, the set value reaches 350 mA, a reset signal is output from the boost control circuit 131, and the built-in MOS transistor switch 121 is controlled to be turned off. . As a result, a counter electromotive force is generated in the coil 31, and a current is supplied to the load resistor 71 through the diode 41. By repeating this cycle until the load terminal voltage V2 reaches the set value of 17V, the load voltage V2 is kept constant.
The operating voltage of the switching regulator controller 11 is supplied from the bias circuit 111.

  On the other hand, the connection voltage V2 of the white LED 21 which is the first constant current load is about 21.5V when the white LED 21 is lit and about 17V when the white LED 21 is lit, so by connecting the three-terminal regulator 10 to the V2 point, The resistor 71, which is the second load, is always supplied with a stable constant voltage of 15V.

  As described above, regardless of whether the white LED 21 is turned on or off, the DC input voltage V1 is supplied to the control voltage of the shutdown circuit 151 and is always set in an active state. And a stable constant voltage driving voltage can be provided for the liquid crystal display device as the second load.

  In order to simplify the description, the embodiment disclosed this time generates only 15V as the power source for driving the liquid crystal as the second load, but the V2 voltage point is the same as in the 3-terminal regulator 10 for 15V. By connecting three-terminal regulators for generating 3V, 5V, and -15V, it is possible to generate various voltages necessary for driving the liquid crystal by one switching regulator controller.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows typically the structure of the power supply circuit which is embodiment of this invention. It is a figure which shows typically the structure of the liquid crystal panel power supply circuit centering on the conventional LED illumination source drive circuit.

Explanation of symbols

1 3.3V DC power supply 11, 12, 13 Switching regulator controller 21, 22 Liquid crystal illumination source LED requiring a constant current of 20 mA
71, 73 Control circuit 10 in liquid crystal panel that requires a constant voltage of 15V 3-terminal regulator 61, 62, 63, 81, 83, 172, 173 Resistor 91, 93 Transistor 51, 52 Constant voltage diode 41, 42, 43 Rectification Diode 92 Logic inverting elements 31, 32, 33 Inductors 121, 122, 123 MOS transistors 141, 142, 143 Error detection comparators 101, 102, 103 Capacitors

Claims (4)

DC power supply,
A constant voltage power supply unit driven by the DC power supply;
A constant current power supply unit driven by the DC power supply;
A step-up control unit shared by the constant voltage means of the constant voltage power supply unit and the constant current power supply unit;
In a power supply circuit having
The boosting control unit performs a control operation according to a load state of the constant current power supply unit. The power supply circuit according to claim 1,
The power supply circuit according to claim 1, wherein the boost control unit performs a control operation according to a load state of the constant voltage power supply unit when the operation of the constant current power supply unit is stopped. The power supply circuit according to claim 2,
The load of the constant voltage power source is a driver for driving a liquid crystal of the liquid crystal display means, the load of the constant current power source unit is a light emitting diode that illuminates the liquid crystal display unit, and the constant voltage unit is connected to the constant current power source unit A power supply circuit for a liquid crystal display device characterized by being a series regulator. Liquid crystal display means;
Illumination means for illuminating the screen of the liquid crystal display;
In a liquid crystal display device having
A liquid crystal display device that supplies power by the power supply circuit according to claim 1.

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