JPH067233B2 - Liquid crystal display drive circuit power supply circuit - Google Patents

Description

The present invention relates to a power supply circuit of a drive circuit for driving a liquid crystal display device by a voltage averaging drive method.

(B) Conventional technology In liquid crystal display devices used for personal computers and word processors, it is possible to switch between negative display in which selected pixels are brighter than non-selected pixels and positive display in which selected pixels are darker than unselected pixels. It may be.

In this case, from the drive circuit of the liquid crystal display device, for example, a plurality of resistors having a predetermined resistance value are connected in series, and a power supply voltage is applied to the series circuit of the resistors from the power supply circuit to obtain a plurality of biases having different voltage values. A voltage is generated, the bias voltages are combined, and a predetermined voltage is applied to the selected pixel and the non-selected pixel in each display. That is, the voltage V _ON to the selected pixel in the negative display, the unselected pixel voltage V _OFF is applied respectively, and the voltage V _OFF to the selected pixel in the positive display, the voltage V _ON to the unselected pixels, respectively Is applied.

The effective values of the voltage V _ON and the voltage V _OFF are set by the following equations, where the maximum voltage applied to the pixel is V _O bias ratio is a and the number of scanning lines is n.

When the ratio of voltage V _ON to voltage V _OFF is maximum Becomes

(C) Problems to be Solved by the Invention However, in general, the transmittance of a pixel of a liquid crystal display device depends on the voltage applied to the pixel. When the applied voltage to the non-selected pixels in the display is equal, the transmittance ratios in the respective display states are equal, and the contrast in the negative display is lowered. Further, in order to improve the deterioration of the contrast, it is sufficient to adjust each applied voltage in each display to an optimum value, but the work of adjusting the power supply voltage of the power supply circuit becomes complicated.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a power supply circuit for a liquid crystal display device drive circuit capable of generating a bias voltage capable of improving the contrast in the case of negative display. Is.

(D) Means for Solving the Problems In the configuration of the present invention, a switching circuit for converting a power supply voltage into two voltages having different values in a positive display and a negative display of a liquid crystal display device, and the switching circuit A voltage divider circuit that is connected in series to the circuit and divides the two voltages applied from the switching circuit into a plurality of bias voltages of different values,
A power supply circuit for a liquid crystal display device drive circuit, which outputs a bias voltage for making a contrast value of a selected pixel and a non-selected pixel larger in a negative display than in a positive display to a voltage dividing circuit.

(E) Action The switching circuit converts the power supply voltage into two voltages having different values in the positive display and the negative display. This 2
The two voltages are applied to the voltage dividing circuit corresponding to the respective displays, and the voltage dividing circuit acts so as to output the optimum bias voltage for each of the positive display and the negative display.

(F) Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings, but the present invention is not limited to the following embodiments.

In FIG. 1, reference numeral 1 is a switching circuit, and a voltage dividing circuit 2 for outputting a plurality of different bias voltages is connected in series via a variable resistor 3 for voltage adjustment. The switching circuit 1 is composed of a switch means 1a and a resistor 1b which are operated by a signal for switching between positive display and negative display, and applies two voltages having different values to the voltage dividing circuit 2. The switch unit 1a connects the terminal A and the terminal C in the negative display, supplies the first voltage V1 to the voltage dividing circuit 2 through the resistor 1b, and in the positive display, the terminal B and the terminal C are connected. It becomes conductive and supplies the second voltage V2 to the voltage dividing circuit 2. As shown in FIG. 2, the switch means 1a includes two field effect transistors (hereinafter referred to as FETs) which are switching elements.
4) and 5 and a control circuit 6 that controls switching of the FETs 4 and 5. The FETs 4 and 5 and the control circuit 6 are connected via diodes 7 and 8 and capacitors 9 and 10 connected in parallel to the diodes 7 and 8, respectively. The control circuit 6 is composed of two transistors 11, 12 and three resistors 13, 14, 15.

The voltage dividing circuit 2 includes five resistors 16, 17, 18, 19, 2
0 and five OP amplifiers 21, 22, 23, 24 and 25. Each of the OP amplifiers 21 to 25 forms a voltage follower. A plurality (five in this embodiment) of bias voltages V _{B2 to} V _{B6 having} different values are output from the output terminals of the respective OP amplifiers 21 to 25.
Is output to a liquid crystal display device drive circuit (not shown). The drive circuit may be a drive circuit according to a voltage averaging drive method known in the art, and includes a bias voltage synthesis circuit for obtaining a drive voltage, a pixel selection circuit, and the like.

Next, the operation of this embodiment will be described with reference to FIGS.

First, the conversion of the power supply voltage in the case of negative display will be described.

In the series circuit of the switching circuit 1 and the voltage dividing circuit 2, a high-voltage side power source voltage V for producing the bias voltages V _{B1 to} V _B6
DD and the power supply voltage V _EE on the low voltage side are applied. Also,
The voltage V _CC is applied to the resistor 13 connected to the collector of the transistor 11 of the control circuit 6, and the voltage -V ₁ is applied to the resistor 14 connected to the connection point between the collector of the transistor 11 and the base of the transistor 12, and the transistor 12 is connected. A voltage V _EC set to a voltage higher than the voltage -V ₁ is applied to the emitter of the resistor 15 connected to the collector of the transistor 12.
The voltage V _CC set to a voltage higher than the voltage V _EE is applied to each of them. The voltage −V ₁ may be at the ground level.

In this state, when a positive pulse signal indicating negative display is input to the base of the transistor 11, the transistor 11 and the transistor 12 are turned on, and the connection point P between the collector of the transistor 12 and the respective diodes 7 and 8 Becomes almost 0, a voltage is applied between the terminal A and the connection point P, and the terminals A and C, that is, the FET 4 are switched to be in a conductive state. As a result, the first voltage V1 stepped down by the resistor 1b and the variable resistor 3 is applied to the voltage dividing circuit 2.

Next, when a negative pulse signal indicating positive display is input to the base of the transistor 11, the transistor 11 is turned off.
FF, a voltage lower than the voltage V _{EC obtained} by dividing the voltage V _CC and the voltage −V ₁ by the resistors 13 and 14 is applied to the base of the transistor 12, and the transistor 12 is turned off. As a result, the voltage V _CC is applied to the connection point P, the FET 5 is turned on, and the terminals B and C are electrically connected. Therefore, the second voltage V _{2 which} is not stepped down by the resistor 1b is applied to the voltage dividing circuit 2.

The resistance values of the resistors 16, 17, 18, 19, 20 of the voltage dividing circuit 2 are R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , the resistance value of the variable resistor 3 is R ₆ , and the switching circuit is 1 resistor 1b
If the resistance value of R is R ₇ , the first voltage V1 and the second voltage V2 are obtained by the following equations.

As is clear from the equations (2-1) and (2-2), a lower voltage is applied to the voltage dividing circuit 2 in the negative display than in the positive display.

The voltage dividing circuit 2 divides the voltage V1 and the voltage V2 by the resistors 16 to 20 corresponding to each display, and outputs the bias voltage V2.
_{B2 to} V _B6 are output. The bias voltages V _{B1 to} V _B6 are bias ratios that maximize the ratio between the voltage applied to the selected pixel and the voltage applied to the non-selected pixel at a duty of 1/200 in order to improve the contrast 1. It is set to obtain the applied voltage by the / 9 bias.

FIG. 3 is a graph showing the waveform of the voltage applied to the pixel of the liquid crystal display device with a 1/9 bias, where T is the scanning time,
t is the time per scanning line (T / n), V ₀ is the voltage (maximum voltage) applied to the selected pixel, 7 / 9V ₀ is the voltage applied to the half-selected pixel, and 1 / 9V ₀ is unselected It is the voltage applied to the pixel. In this embodiment, V ₀ is the bias voltage V _B1 .

Here, the applied voltages V _ON and V _OFF to the selected pixel and the applied voltages V _OFF ′ and V _ON ′ to the non-selected pixel and the applied voltages V _ON , V _OFF and V _OFF ′ in the negative display and the positive display, respectively. , V _ON ′, the relationship between the transmittance and the voltages V ₁ and V ₂ will be described.

If the number of pixels of the driven liquid crystal display device is 640 × 400,
The number of scans is 200 in the upper and lower divided scans. If the ratio of applied voltage V _ON / V _OFF in the case of negative display is 1.078, V _ON =
2.20Vrms, is set to V _OFF = 2.04Vrms. When the transmittance for this value is obtained from the graph of FIG. 4, the applied voltage V
_The transmittance T ₁ is 90% for _ON and the transmittance T ₂ is 20% for the applied voltage V _OFF . Therefore, the contrast value T ₁ / T ₂ is 4.5. In this case, the voltage V1 is 22.61V.

On the other hand, in the case of positive display, V _OFF ′ = 2.08 Vrms, V
'With the setting = 2.24Vrms, the applied voltage V _{ON' ON} transmittance T ₁ '= 70%, the applied voltage V _OFF' against the transmittance T ₂ '= 27% relative. Therefore, the contrast value T ₁ ′ / T ₂ ′ becomes approximately 2.60. In this case, the voltage V2 is
It is 23.1V. As is clear from the above, the contrast of the negative display is improved as compared with the case of the positive display.

(G) Effect of the Invention According to the present invention, it is possible to supply a plurality of bias voltages to the liquid crystal display device drive circuit corresponding to positive display and negative display, so as to obtain optimum contrast in each display.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of an embodiment of the present invention, FIG. 2 is an electric circuit diagram of a switching circuit of the same embodiment, and FIG. 3 is applied to each pixel of a liquid crystal display device obtained from the bias voltage of the embodiment. FIG. 4 is a graph showing the relationship between the voltage applied to the pixel (applied voltage) and the transmittance. 1 ... Switching circuit, 2 ... Voltage dividing circuit.

Claims (1)

[Claims] 1. A switching circuit for converting a power supply voltage into two voltages having different values in positive display and negative display of a liquid crystal display device, and a switching circuit connected in series to the switching circuit and applied from the switching circuit. A voltage dividing circuit for dividing the two voltages into a plurality of bias voltages of different values, and in the case of a negative display, the contrast value of the selected pixel and the non-selected pixel is made larger than that in the case of the positive display. A power supply circuit for a liquid crystal display device drive circuit, which outputs a bias voltage to a voltage dividing circuit.