JP3043178B2 - LCD power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal power supply circuit suitable for a simple matrix type liquid crystal.

[0002]

2. Description of the Related Art Conventionally, in a liquid crystal display device using a so-called simple matrix type liquid crystal cell, a driving circuit for time-division driving is connected to the liquid crystal cell, and a bias voltage by a voltage averaging method is applied to the driving circuit. In supplying such a bias voltage, Japanese Patent Application Laid-Open No. 60-212
As described in Japanese Unexamined Patent Publication No. 735 and the like, a liquid crystal power supply circuit is configured using a D / A converter and the like. In this case, as an external power supply, a +5 volt power supply of a logic circuit system,
D / A supplied with -12 volt power supply for driving liquid crystal
A predetermined liquid crystal voltage V0 is obtained by forming a voltage stabilizing circuit by a converter or the like.

[0003]

However, when the power supply fluctuates in such a liquid crystal voltage circuit, the liquid crystal voltage V0
Although the voltage is stabilized, the voltage actually applied to the liquid crystal cell may be affected by power supply fluctuation. This is because the power supply of the logic circuit system exclusively supplies power to the logic circuits other than the display.
This is due to the fact that the power is obtained by a DC converter or the like, so that the power is hardly fluctuated. In particular, in the case of a portable device, the power supply of the logic circuit system is often obtained from a portable power source, so the fluctuation is remarkable. The power supply of the logic circuit system is used for the liquid crystal driving voltage, which causes a contrast fluctuation due to the voltage fluctuation, particularly a significant decrease in the contrast due to the voltage drop of the logic circuit system power supply, and lowers the display quality.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above points, and is directed to a liquid crystal power supply circuit for obtaining a liquid crystal voltage from a logic circuit power supply and another voltage power supply. A reference voltage of a liquid crystal voltage is supplied by a reference voltage forming circuit having a differential input between a reference voltage from a connected constant voltage element and a divided voltage depending on a logic circuit system voltage.
0 is obtained.

[0005]

As a result, the liquid crystal voltage also fluctuates according to the fluctuation of the power supply voltage of the logic circuit system, and the voltage applied to the liquid crystal cell becomes substantially constant because it is not affected by other fluctuations.

[0006]

FIG. 1 is a circuit diagram of a liquid crystal power supply circuit according to an embodiment of the present invention. FIG. 1 shows a liquid crystal power supply circuit for obtaining a liquid crystal voltage V0 from a logic circuit power supply B and another voltage power supply VL. The logic circuit power supply B receives a voltage of +5 volts from a battery, an inverter, etc. Voltage system power supply VL is, for example,-
It is obtained at 12 volts from a logic circuit power supply or the like by a DCDC converter or the like, and all are supplied from equipment incorporating a display device. These logic circuit power supplies B
A D / A converter 1 (for example, AD7524 manufactured by Analog Devices, Inc.) is used as a constant voltage circuit on the basis of the voltage source VL and another voltage system power supply VL. To obtain the liquid crystal voltage V0. And the reference voltage forming circuit 2
Obtains the reference voltage vr by giving the reference voltage va of the constant voltage element 21 and the divided voltage vb depending on the logic circuit system voltage B obtained by the resistance dividing circuit 22 to the operational amplifier 23 as differential inputs. .

In such a circuit, generally, when a reference voltage is formed, only the reference voltage va is used by a series circuit of a resistor and a Zener diode or the like. In such a conventional method, when the power supply B of the logic circuit system is changed from +5.0 volts to +4.5 volts, the liquid crystal voltage V0 is set to 0.5.
The voltage (B-V0) applied to the liquid crystal is 0.41 to 0.11 volt, while the voltage (B-V0) applied to the liquid crystal is 0.41 to 0.11 volt.
0.61 volt fluctuation and 10-3 contrast
It fluctuates by 0%. However, when the reference voltage vr is formed from the reference voltage va and the divided voltage vb by the constant voltage element as described above, the liquid crystal voltage changes within 0.30 volts with respect to the same change in the logic circuit power supply, and Can be suppressed to about 0.2 volts or less.

Incidentally, such a reference voltage vr of the liquid crystal voltage is used.
If the compensation is made only for the voltage drop of the portable power supply, the response speed of the operational amplifier 23 is made smaller than the response speed of the D / A converter 1 or the operational amplifier 3 so that the fluctuation of the reference voltage vr becomes slow. If it is also slowed down, voltage fluctuations due to load fluctuations and noise are instantaneously absorbed as a stabilized power supply circuit, and a gradual voltage drop due to power consumption is compensated by changing the reference voltage vr. It is more preferable because it can be performed. Such adjustment of the response speed can be performed by, for example, connecting a capacitive load in parallel to an external component of the operational amplifier or adjusting the clock frequency of the D / A converter.

[0009]

As described above, in the present invention, the reference voltage is obtained based on the reference voltage by the constant voltage element and the divided voltage depending on the logic circuit system voltage to stabilize the liquid crystal voltage. Although the influence of the fluctuation in the power supply voltage is absorbed, the voltage applied to the liquid crystal cell becomes substantially constant because the liquid crystal voltage also fluctuates according to the change in accordance with the fluctuation in the power supply voltage of the logic circuit system. Therefore, a decrease in contrast due to a decrease in the voltage of the logic circuit power supply is reduced, and high display quality can be maintained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a liquid crystal power supply circuit according to an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 A / D converter 2 Reference voltage forming circuit 21 Constant voltage element 22 Resistance dividing circuit 23 Operational amplifier 3 Operational amplifier

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Satoru Hiraga 3-201 Minamiyoshikata, Tottori City, Tottori Prefecture Tottori Sanyo Electric Co., Ltd. Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/133 G09G 3/18 G09G 3/36

Claims (1)

(57) [Claims] 1. A liquid crystal power supply circuit for obtaining a liquid crystal voltage from a logic circuit power supply and another voltage power supply.
A reference voltage of a liquid crystal voltage is obtained by a reference voltage forming circuit for differentially inputting a reference voltage by a constant voltage element connected to and a divided voltage depending on a logic circuit system voltage, and the reference voltage of the liquid crystal voltage is supplied to a stabilizing circuit. A liquid crystal power supply circuit characterized by being supplied.