JP4720099B2 - Constant current drive device, backlight light source device, and color liquid crystal display device - Google Patents

Description

  The present invention relates to a constant current drive device in which a plurality of elements connected in series, for example, a light emitting diode (LED), is driven with constant current by a pulse width modulation constant current drive circuit, and is driven by this constant current drive device. The present invention relates to a backlight light source device and a color liquid crystal display device.

  In recent years, there has been a trend toward thinner displays as represented by LCD TVs and plasma displays (PDPs), and many of the mobile displays are liquid crystal systems, and faithful color reproducibility is desired. ing. In addition, CCFL (Cold Cathode Fluorescent Lamp) type, which uses fluorescent tubes, is the mainstream backlight for liquid crystal panels, but environmentally mercury-free has been required, and light-emitting diodes etc. are promising as light sources to replace CCFLs. Has been.

  Generally, in a display using light emitting diodes as display pixels, in order to drive the light emitting diodes in a matrix, an XY addressing drive circuit is required for each pixel. Select (addressing) the light-emitting diodes in the, and adjust the brightness by modulating the lighting time (pulse width modulation (PWM) drive) to obtain a display screen with a predetermined gradation Yes. This complicates the driving circuit and increases the cost (for example, see Patent Document 1).

  In addition, light emitting diodes have variations in luminance among individual elements. However, in order to correct variations in individual elements, each element must be driven by an independent drive circuit. The driving mode is very similar to the shape corresponding to a display using the above-described light emitting diode as a display pixel.

  That is, there is a drawback that the complexity of the drive circuit due to addressing is exhibited.

  On the other hand, when a light emitting diode is used as a light source, red (R), green (G), and blue (B) have different luminous efficiencies, and therefore, the currents flowing through the individual colors must also tend to be independent. Furthermore, since the semiconductors used for the respective colors are different, there are differences in efficiency deviation ranges due to manufacturing variations of elements used for the respective colors, and these must be overcome.

JP 2001-272938

  By the way, in order to individually adjust the variation of each element of the light emitting diode, a matrix type drive is required. In addition, in LED driving devices for lighting applications where the power of each light-emitting diode is large, LSIs for driving large power have not been created yet, and this is disadvantageous in terms of cost. Although considered to be used, it is difficult to adjust the current variation of each light emitting diode in the series connection type.

  Therefore, in view of the conventional situation as described above, the object of the present invention is to individually adjust the variation of each element with a simple circuit when driving a plurality of elements connected in series, for example, a light emitting diode, at a low current. An object of the present invention is to provide a constant current driving device, a backlight light source device driven by the constant current driving device, and a color liquid crystal display device.

  Other objects of the present invention and specific advantages obtained by the present invention will become more apparent from the description of embodiments described below.

The present invention is a constant current driving apparatus for driving a plurality of light emitting diodes connected in series by a pulse width modulation constant current driving circuit, wherein a load resistance is connected in parallel to each of the plurality of light emitting diodes connected in series. A plurality of transistors constituting a switch to be connected, a diode connected between the base and emitter of each transistor, a capacitor connected to the base of each transistor, and switching to the base of each transistor via the capacitor A control circuit for supplying a control signal, wherein the control circuit controls switching of the plurality of transistors by supplying an asynchronous pulse width modulation signal as the switching control signal, and the plurality of light emitting diodes connected in series the driving current flowing through the shunted to the load resistor, the pulse width modulation By the duty signal, characterized in that the amount of current shunted to the load resistor so as to variably adjusted.

The present invention is a backlight light source device that illuminates a display panel from the back side, and a plurality of light emitting diodes connected in series and a pulse width modulation constant current drive that drives the plurality of light emitting diodes connected in series at a constant current A circuit, a plurality of transistors constituting a switch for connecting a load resistor in parallel to each of the plurality of light emitting diodes connected in series, a diode connected between a base and an emitter of each transistor, and each of the transistors A capacitor connected to a base; and a control circuit for supplying a switching control signal to the base of each of the transistors via the capacitor. The control circuit supplies an asynchronous pulse width modulation signal as the switching control signal. the plurality of transistors and the switching control, is the series by The driving current flowing through the plurality of light emitting diodes is shunted to the load resistor, the duty of the pulse width modulated signal, characterized in that the amount of current shunted to the load resistor so as to variably adjusted.

The present invention is a color liquid crystal display device comprising a transmissive color liquid crystal display panel provided with a color filter and a backlight light source device for illuminating the color liquid crystal display panel from the back side. A plurality of light emitting diodes connected in series, a pulse width modulation constant current driving circuit for driving the plurality of light emitting diodes connected in series at a constant current, and a load resistor in parallel with each of the plurality of light emitting diodes connected in series A plurality of transistors constituting a switch for connecting, a diode connected between the base and emitter of each transistor, a capacitor connected to the base of each transistor, and a capacitor connected to the base of each transistor via the capacitor and a control circuit for supplying a switching control signal, the control circuit, the switch The plurality of transistors and the switching control by supplying an asynchronous pulse width modulation signal as a ring control signal, the drive current flowing through the plurality of series-connected light emitting diodes is shunted to the load resistor, the pulse width modulated signal The amount of current shunted to the load resistor is variably adjusted according to the duty .

  In the present invention, when a plurality of elements connected in series are driven with constant current by a pulse width modulation constant current driving circuit, a transistor constituting a switch for connecting a load resistance in parallel to each of the plurality of elements connected in series is provided. By supplying a switching control signal from a control circuit to the base of the transistor via a capacitor and controlling the switching of the transistor, the drive current flowing through the plurality of elements connected in series is shunted to the load resistor. The drive current can be individually adjusted according to the individual luminance characteristics of a plurality of elements connected in series, for example, light emitting diodes.

  Further, a pulse width modulation signal is supplied as the switching control signal from the control circuit, and the amount of current shunted to the load resistor can be variably adjusted according to the duty of the pulse width modulation signal, so that the current flows to a plurality of elements. The drive current can be adjusted individually and with high accuracy.

  Further, by making the duty of the pulse width modulation signal variable corresponding to the weighting of n bits, the amount of current shunted to the load resistance can be adjusted with a resolution of (n + 1) bits.

  Furthermore, unnecessary radiation can be reduced by supplying an asynchronous pulse width modulation signal as the switching control signal from the control circuit.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Needless to say, the present invention is not limited to the following examples, and can be arbitrarily changed without departing from the gist of the present invention.

  The present invention is applied to a backlight type color liquid crystal display device 100 having a configuration as shown in FIG.

  The color liquid crystal display device 100 includes a transmissive color liquid crystal display panel 10 and a backlight light source device 20 provided on the back side of the color liquid crystal display panel 10.

  In the transmissive color liquid crystal display panel 10, two transparent substrates (TFT substrate 11 and counter electrode substrate 12) made of glass or the like are arranged opposite to each other, and, for example, twisted nematic (TN) liquid crystal is enclosed in the gap. The liquid crystal layer 13 is provided. On the TFT substrate 11, signal lines 14 and scanning lines 15 arranged in a matrix and thin film transistors 16 and pixel electrodes 17 as switching elements arranged at the intersections thereof are formed. The thin film transistor 16 is sequentially selected by the scanning line 15 and writes the video signal supplied from the signal line 14 to the corresponding pixel electrode 17. On the other hand, a counter electrode 18 and a color filter 19 are formed on the inner surface of the counter electrode substrate 12.

  In this color liquid crystal display device 100, the transmissive color liquid crystal display panel 10 having such a configuration is sandwiched between two polarizing plates 31 and 32, and white light is irradiated from the back side by the backlight light source device 20. A desired full-color video display can be obtained by driving in an active matrix system.

  The backlight light source device 20 includes a light source 21 and a wavelength selection filter 22, and illuminates the color liquid crystal display panel 10 from the back side through the wavelength selection filter 22 with light emitted from the light source 21.

  The color liquid crystal display device 100 is driven by, for example, a drive circuit 200 having an electrical block configuration shown in FIG.

  The driving circuit 200 includes a power supply unit 110 that supplies driving power to the color liquid crystal display panel 10 and the backlight light source device 20, an X driver circuit 120 and a Y driver circuit 130 that drive the color liquid crystal display panel 10, and external video signals. An RGB process processing unit 150 supplied via an input terminal 140, a video memory 160 and a control unit 170 connected to the RGB process processing unit 150, a backlight drive control unit 180 for controlling the drive of the backlight light source device 20, and the like. Prepare.

  In this drive circuit 200, the video signal input through the input terminal 140 is subjected to signal processing such as chroma processing by the RGB process processing unit 150, and further, RGB signals suitable for driving the color liquid crystal display panel 10 from the composite signal. It is converted into a separate signal, supplied to the control unit 170, and also supplied to the X driver 120 via the image memory 160. In addition, the control unit 170 controls the X driver 120 and the Y driver circuit 130 at a predetermined timing according to the RGB separate signal, and performs color processing using the RGB separate signal supplied to the X driver 120 via the image memory 160. By driving the liquid crystal display panel 10, an image corresponding to the RGB separate signal is displayed.

  Here, the color filter 19 is divided into a plurality of segments corresponding to the pixel electrodes 17. For example, as shown in FIG. 3A, three segments of three primary colors, a red filter CFR, a green filter CFG, and a blue filter CFB, and as shown in FIG. 3B, the three primary colors (RGB) are cyan ( Four segments of red filter CFR, cyan filter CFC, green filter CFG, and blue filter CFB with C) added, or cyan (C) and yellow as three primary colors (RGB) as shown in FIG. It is divided into five segments: a red filter CFR to which (Y) is added, a cyan filter CFC, a blue filter CFG, a yellow filter CFY, and a blue filter CFB.

  Here, the backlight light source device 20 employs an area light type light source 21 that irradiates the transmissive color liquid crystal display panel 10 with a plurality of light emitting diodes (LEDs) disposed on the back surface. ing.

  The arrangement of the light emitting diodes in the light source 21 of the backlight light source device 20 will be described.

  FIG. 4 shows an example of the arrangement of the light emitting diodes, using two red light emitting diodes 1, two green light emitting diodes 2, and blue light emitting diodes 3 for each of the unit cells 4-1 and 4-2. The light emitting diodes are arranged in a line.

  In this arrangement example, there are six. However, in order to make the mixed color white light with a good balance according to the rating of the light emitting diode to be used, light emission efficiency, etc., it is necessary to adjust the light output balance. There can be other variations.

  In the arrangement example shown in FIG. 4, the unit cell 4-1 and the unit cell 4-2 are exactly the same, and are connected by a double-ended arrow portion at the center. Further, FIG. 5 shows a shape in which the unit cell 4-1 and the unit cell 4-2 are connected by a diode mark of an electric circuit diagram symbol. In this example, each light-emitting diode, that is, red light-emitting diode 1, green light-emitting diode 2, and blue light-emitting diode 3 are connected in series with the polarity in the direction in which current flows from left to right.

  Here, two red light emitting diodes 1, green light emitting diodes 2, and blue light emitting diodes 3 are used, and unit cells 4 in which a total of six light emitting diodes are arranged in a row are patterned by the number of light emitting diodes of each color. When expressed, it becomes (2G 2R 2B) as shown in FIG. That is, (2G 2R 2B) indicates that the basic unit is a total of six patterns of two each of green, red, and blue. As shown in FIG. 7, when three unit cells 4 of the basic unit are connected in succession, the symbol is 3 * (2G 2R 2B), and the pattern is expressed by the number of light emitting diodes (6G 6R 6B). Indicated.

  Next, an actual arrangement example of light emitting diodes in the light source 21 of the backlight light source device 20 will be described based on the notation of FIG.

  As shown in FIG. 8, the light source 21 includes three times the basic unit (2G 2R 2B) of the above-described light emitting diode as one middle unit (6G 6R 6B), 4 rows vertically, 5 columns horizontally, total 360 light-emitting diodes are arranged.

  Since it is not easy to perform individual addressing on all the 360 light emitting diodes, the backlight light source device 20 has a driving configuration as shown in FIG.

  That is, the RGB pairs g1 to gn corresponding to each of the n columns are configured such that each of the RGB light emitting diodes is independently connected in series to each column, and a constant current is passed by the DC-DC converter 7. ing.

  With reference to FIG. 10, a specific configuration example for flowing a constant current through the LED series connection substrates m1 and m2 will be described.

  That is, the LED array 40 in which a plurality of light emitting diodes are connected in series has one end connected to the DC-DC converter 7 via the detection resistor (Rc) 5 and the other end grounded via the FET 6.

  The DC-DC converter 7 detects a voltage drop due to the detection resistor 5 with respect to the setting of the output voltage Vcc, and configures a feedback loop so that a predetermined constant current ILED flows through the LED strings connected in series. Yes. In this example, the voltage drop due to the detection resistor 5 is sampled and held by the capacitor 9 via the sample and hold switch 8 and fed back to the DC-DC converter 7.

  In addition, a current PWM (Pulse Width Modulation) signal applied to the gate of the FET 6 from the driver IC 181 provided in the backlight drive control unit 180 is turned on and off for a predetermined period by a main PWM (Pulse Width Modulation) signal. The light emission amount of the light emitting diode is increased or decreased.

  That is, in the backlight light source device 20, the FET 6 is switched by a main PWM signal supplied from the driver IC 181 provided in the backlight drive control unit 180, and a plurality of light emitting diodes 41A to 41E are connected in series. The light emitting diodes 41 </ b> A to 41 </ b> E are driven by pulse width modulation constant current by turning on and off the drive current supplied to the LED array 40 by the DC-DC converter 7.

  In this example, in order to control the constant current with the peak value, the current detection feedback loop is provided with a sample hold, but this is one example, and other methods may be used.

  In this example, a sample pulse generated by the sample hold switch drive circuit 183 based on the main PWM signal is supplied to the sample hold switch 8.

  Note that the group of LED rows 40 shown in FIG. 10 corresponds to one row of RGB pairs g1 to gn corresponding to each of the n rows shown in FIG. Therefore, in this example, a similar circuit is required for gn columns × 3 times (for RGB).

  Next, a configuration for individually adjusting the variation of each element of the light emitting diode will be specifically described with reference to FIG.

  Since the number of light emitting diodes 41 in the group of LED rows 40 shown in FIG. 10 varies from the viewpoint of light quantity balance, various cases are conceivable. In particular, in recent years, since the input power of each element has been increased in order to reduce the total number, it is necessary to overcome variations in luminance characteristics of each element by adjustment.

  As shown in FIG. 11, the five light emitting diodes 41A to 41E connected in series are connected to transistors 82A to 82E constituting switches for connecting load resistors 81A to 81E individually in parallel. Clamping diodes 83A to 83E are connected between the bases and emitters of 82A to 82E, and coupling capacitors 84A to 84E are connected to the bases of the transistors 82A to 82E.

  The five light-emitting diodes 41A to 41E connected in series have individual voltage drops from Vfa to Vfe from the top to the bottom, and vary depending on the production lot. The five light emitting diodes 41A to 41E connected in series are PWM driven by the FET 6.

  In the drive circuit having such a configuration, the bases of the transistors 82A to 82E are connected to the sub_PWM via the coupling capacitors 84A to 84E as switching control signals from the drive control circuit 182 provided in the backlight drive control unit 180. Signals a to e are supplied. The sub_PWM signals a to e input to the coupling capacitors 84A to 84E can be handled as AC signals because the emitter potentials of the transistors 82A to 82E are clamped by the diodes 83A to 83E. Therefore, the transistors 82A to 82E can be turned on and off without considering the potential even in series connection.

  When the transistors 82A to 82E are turned on, the anodes and cathodes of the light emitting diodes 41A to 41E are bypassed by the resistors 81A to 81E, and a part of the drive current is shunted to the resistors 81A to 81E in addition to the light emitting diodes 41A to 41E. Therefore, the light output of the light emitting diodes 41A to 41E having the light emitting function is lowered.

  Here, if the values of the resistors 81A to 81E are appropriately selected, it is possible to arbitrarily set the amount of current to be shunted when bypassed. However, the bypass time can be adjusted by operating the on-off period ratio. Then, the average amount of current to be shunted can be adjusted, and as a result, the light amounts of the light emitting diodes 41A to 41E can be individually controlled.

  At this time, the sub_PWM signals a to e used for driving the transistor have a configuration that can be selected independently of the main_PWM signal, and thus have a high degree of freedom. Further, by increasing the frequency of the sub_PWM signals a to e, the resolution can be increased and fine adjustment is possible.

  That is, in the backlight light source device 20, the FET 6 is switched by a main PWM signal supplied from the driver IC 181 provided in the backlight drive control unit 180, and a plurality of light emitting diodes 41A to 41E are connected in series. By turning on and off the drive current supplied to the LED array 40 by the DC-DC converter 7, the plurality of light emitting diodes 41 </ b> A to 41 </ b> A connected in series when the light emitting diodes 41 </ b> A to 41 </ b> E are driven with pulse width modulation constant current. Transistors 82A to 82E constituting switches for connecting load resistors 81A to 81E are provided in parallel with each of 41E, and the bases of the transistors 82A to 82E are provided in the backlight drive control unit 180 via capacitors 84A to 84E. Drive system A switching control signal is supplied from the circuit 182 to control the switching of the transistors 82A to 82E, so that the drive current flowing through the plurality of light emitting diodes 41A to 41E connected in series is shunted to the load resistors 81A to 81E. It has become.

  In this way, by dividing the drive current flowing through the plurality of light emitting diodes 41A to 41E connected in series to the load resistors 81A to 81E, it corresponds to the individual luminance characteristics of the plurality of light emitting diodes 41A to 41E connected in series. The drive current can be adjusted individually.

  Here, the drive control circuit 182 supplies sub_PWM signals a to e as the switching control signals, and variably adjusts the amount of current divided to the load resistors 81A to 81E according to the duty of the sub_PWM signals a to e. The drive currents flowing through the plurality of light emitting diodes 41A to 41E can be adjusted individually and with high accuracy.

  Further, the drive control circuit 182 changes the duty of the sub_PWM signals a to e in accordance with the n-bit weighting, thereby reducing the amount of current shunted to the load resistors 81A to 81E by (n + 1) bits. The resolution can be adjusted.

  Further, the drive control circuit 182 can reduce unnecessary radiation by supplying asynchronous sub_PWM signals a to e as the switching control signals.

It is a typical perspective view which shows the structure of the color liquid crystal display device of the backlight system to which this invention is applied. It is a block diagram which shows the structure of the drive circuit of the said color liquid crystal display device. It is a typical top view which shows the structure of the color filter provided in the color liquid crystal panel in the said color liquid crystal display device. It is a figure which shows typically the example of arrangement | positioning of the light emitting diode in the backlight light source device which comprises the said color liquid crystal display device. It is the figure which showed typically the form where each light emitting diode in the example of arrangement | positioning of the said light emitting diode was connected by the diode mark of the electric circuit diagram symbol. A unit cell in which two red light emitting diodes, two green light emitting diodes and two blue light emitting diodes are used and a total of six light emitting diodes are arranged in a row is schematically shown by pattern notation with the number of light emitting diodes of each color. It is a figure. It is the figure which showed typically the case where the unit cell 4 of a basic unit was connected continuously by pattern description with the number of light emitting diodes. It is the figure which showed typically the example of arrangement | positioning of the light emitting diode in the light source 21 of the said backlight light source device by pattern description with the number of light emitting diodes. It is a figure which shows typically the drive structure of the light emitting diode in the said backlight light source device. It is a figure which shows typically the example of a concrete structure for sending a constant current to the some light emitting diode connected in series in the said backlight light source device. It is a figure which shows typically the example of a concrete structure for adjusting individually the dispersion | variation in each element of the some light emitting diode connected in series in the said backlight light source device.

Explanation of symbols

  1, 2, 3, 41, 41A to 41E Light-emitting diode, 4-1, 4-2 Unit cell, 5 Detector resistor, 6 FET, 7 DC-DC converter, 8 Sample hold switch, 9 Capacitor, 10 Color liquid crystal display panel , 11 TFT substrate, 12 Counter electrode substrate, 13 Liquid crystal layer, 14 Signal line, 15 Scan line, 16 Thin film transistor, 17 Pixel electrode, 18 Counter electrode, 19 Color filter, 20 Back light source device, 21 Light source 21, 22 Wavelength selection Filter, 31, 32 Polarizing plate, 40 LED array, 81A to 81E load resistance, 82A to 82E transistor, 83A to 83E diode, 84A to 84E capacitor, 100 color liquid crystal display device, 110 power supply unit, 120 X driver circuit, 130 Y Driver circuit, 140 input terminals 40,150 RGB process processing unit, 160 video memory, 170 control unit, 180 a backlight driving control unit, 181 a driver IC, 182 drive control circuit, 183 a sample hold switch driving circuit, 200 a driving circuit

Claims (6)

A constant current driving device that drives a plurality of light emitting diodes connected in series by a pulse width modulation constant current driving circuit,
A plurality of transistors constituting a switch for connecting a load resistor in parallel to each of the plurality of light emitting diodes connected in series;
A diode connected between the base and emitter of each transistor;
A capacitor connected to the base of each of the transistors,
A control circuit for supplying a switching control signal to the base of each transistor through the capacitor;
The control circuit performs switching control of the plurality of transistors by supplying an asynchronous pulse width modulation signal as the switching control signal, and diverts a drive current flowing through the plurality of light emitting diodes connected in series to the load resistor. A constant current driving device in which the amount of current shunted to the load resistor is variably adjusted according to the duty of the pulse width modulation signal . Constant current drive device according to claim 1, wherein the variable corresponding duty of the pulse width modulated signal to the weighting of the n bits. A backlight light source device for illuminating the display panel from the back side,
A plurality of light emitting diodes connected in series;
A pulse width modulation constant current drive circuit for constant current driving the plurality of light emitting diodes connected in series;
A plurality of transistors constituting a switch for connecting a load resistor in parallel to each of the plurality of light emitting diodes connected in series;
A diode connected between the base and emitter of each transistor;
A capacitor connected to the base of each of the transistors,
A control circuit for supplying a switching control signal to the base of each transistor through the capacitor;
The control circuit performs switching control of the plurality of transistors by supplying an asynchronous pulse width modulation signal as the switching control signal, and diverts a drive current flowing through the plurality of light emitting diodes connected in series to the load resistor. A backlight light source device in which the amount of current shunted to the load resistance is variably adjusted according to the duty of the pulse width modulation signal . 4. The backlight source device according to claim 3, wherein the duty of the pulse width modulation signal is variable corresponding to n-bit weighting. A color liquid crystal display device comprising a transmissive color liquid crystal display panel provided with a color filter and a backlight light source device for illuminating the color liquid crystal display panel from the back side,
The backlight light source device is
A plurality of light emitting diodes connected in series, a pulse width modulation constant current driving circuit for driving the plurality of light emitting diodes connected in series at a constant current, and a load resistor in parallel with each of the plurality of light emitting diodes connected in series A plurality of transistors constituting a switch to be connected, a diode connected between the base and emitter of each transistor, a capacitor connected to the base of each transistor, and switching to the base of each transistor via the capacitor A control circuit for supplying a control signal ,
The control circuit performs switching control of the plurality of transistors by supplying an asynchronous pulse width modulation signal as the switching control signal, and diverts a drive current flowing through the plurality of light emitting diodes connected in series to the load resistor. A color liquid crystal display device in which the amount of current shunted to the load resistor is variably adjusted according to the duty of the pulse width modulation signal . 6. A color liquid crystal display device according to claim 5, wherein the duty of the pulse width modulation signal is variable corresponding to n-bit weighting.

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