JPS6173196A - Liquid crystal driving circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a liquid crystal drive circuit that can be used in a liquid crystal display device.

Conventional Structure and its Problems In recent years, liquid crystal display devices have shown a tendency to become larger. The number of time divisions (duty ratio) of dot matrix type liquid crystal display devices is also increasing, and the number of time divisions (duty ratio) has increased from 1/16 to 1/16.
32, 1/64 and even 1A28, higher duty displays are being put into practical use.

A conventional liquid crystal drive circuit will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of a voltage generating section of a conventional liquid crystal drive circuit.

In Figure 1, 1.2, 3, 4.5 are resistances, 6 is PN
It is a P transistor. VDD is the positive power supply, vK (
is the negative side power supply, vc is the control voltage, vo, vl
, v2. v3. v4. v5 indicates the driving voltage of the liquid crystal. Resistance 1. Resistance 2. Resistance 3. Assuming that the resistance values of the resistors 4° and 5 are R1, R2, R5, R4°R5, and the number of time divisions of the liquid crystal display is N, the following relationship holds true when the voltage averaging method is used as is well known.

R,: R2: R5: R4: R5=1=1:(J
T-3):1:1 vo=vflD is usually sV, V is V, K Yo! ] In case of high duty display, for example vs”-10
It takes a value of about V.

Using the liquid crystal display device configured as described above, V in FIG. When using a voltage with an e value of -V as a driving voltage, there are the following problems. That is, the liquid crystal is a capacitive load, and the resistance of the transparent electrode is also large. A large charging/discharging current flows at the moment when the six-value voltage applied to the capacitive load liquid crystal is switched, and as a result, the drive voltage waveform becomes distorted. When the drive voltage waveform is distorted, the image quality of the character 9 graphics displayed on the liquid crystal is deteriorated. As one method for eliminating this drawback, the waveform distortion of the drive voltage can be eliminated by reducing the resistance values of the resistors 1 to 5 and increasing the bias current flowing through the resistors 1 to 5.

However, by reducing the resistance values of resistors 1 to 6,
Since the power consumption increases, the advantage of the liquid crystal as a low power consumption device is abandoned.

FIG. 2 shows a circuit diagram of another conventional example that eliminates the above-mentioned drawbacks.

In Figure 2, 1 to 6 are the same as in Figure 1, and 7.8
, 9.10 is an operational amplifier, 11 is an operational amplifier 7.8,9
．． This is an IC with 4 circuits of 10 built-in. The positive power terminal of IC11 is vo, and the negative power terminal is v51.
Connected to either vCl vo.

Since the operational amplifiers 7.8 and 9.10 each have a voltage follower configuration, input and output are equally transmitted. Therefore, the connection point of resistors 1 and 2, the connection point of resistors 2 and 3,
The voltage at the connection point of resistors 3 and 4 and the connection point of resistor 4.6 is v+ + v through operational amplifiers 7゜8.9 and 10, respectively.
7 r v5 + v4.

Even if the resistance values R1 to R5 of the resistors 1 to 6 are increased, the output impedance of the operational amplifiers 7 to 1o is extremely small, so that the waveform distortion of the signal voltage can be reduced. Furthermore, since the power consumption of the operational amplifier is relatively low, the display image quality can be improved without increasing the power consumption of the entire liquid crystal display device.

However, the configuration shown in FIG. 2 has the following problems.

In FIG. 2, vo-7s=7p. Using the voltage averaging method, vo-v, =V, -V2=v, -V4=44-V5=
14.V.

However, a=f "-1-1 (N: number of time divisions)".

For example, v. =sv, v5=-1ov, N:12
8, a = 12.3'/a Vp = 1.22 V.

On the other hand, FIG. 3 shows an example of the input/output characteristics of an operational amplifier connected as a voltage follower. In the example in Figure 3, the positive side power supply of the operational amplifier is VDD, and the negative side power supply is V□
It is said that From this example, when the input is V5 or higher, the output is constant (V3). In other words, unless it is saturated, the input and output are not transmitted as equal voltages between s and VDD.

Here, vDtl-v,=v, and v have a certain value depending on the type of amplifier. For example, if 4 circuit operational amplifiers are built into one IC, 324
324 series operational amplifiers are often used, and the voltage for 324 series operational amplifiers is 1.5V.

Accordingly, in the above example, if 1/&vP = 1.22V, the positive side power supply of the operational amplifier 7 in FIG. 2 is set to V. =
If vDD, the voltage at the connection point between resistor 1 and resistor 2 cannot be obtained as it is at the output terminal of operational amplifier 7, and the voltage vl has a value different from the value determined by the voltage averaging method. As a result, the display quality of the liquid crystal is significantly impaired.

In this way, as the number of time divisions increases, the voltage that can be input to the operational amplifier is limited. One way to avoid this is to
The purpose is to set the positive power supply of the operational amplifier to a voltage higher than vDD+v (this is referred to as V). For example, vH can be created using a DC-DC converter, but the number of components increases, power consumption in the DC-DC converter is added, and power consumption in the operational amplifier increases due to the increase in power supply voltage, so it is difficult to reduce the voltage. This goes against the original purpose of constructing a power-consuming liquid crystal display device.

As described above, in the conventional example shown in FIGS. 1 and 2, in order to reduce the waveform distortion of the signal voltage, the problem is that the power consumption increases and the number of parts increases as a result.

OBJECTS OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks, to provide a high time division liquid crystal drive circuit that consumes low power, eliminates signal voltage waveform distortion, and does not require a power supply voltage higher than VDD. It is about providing.

Structure of the Invention In the liquid crystal drive circuit of the present invention, at least one of the voltages divided by the resistors is connected to an NPN transistor and a PNP l-
The voltage is supplied to the liquid crystal display element via a transistor and the other via an operational amplifier, and the transistor is operated between VDD and ground, and the lowest voltage of the liquid crystal drive voltage is supplied from the operational amplifier. As a result, even if the number of time divisions increases, the liquid crystal drive voltage based on the voltage averaging method can be transmitted accurately and with low impedance, eliminating waveform distortion of the signal voltage and improving image quality. Therefore, it is possible to provide a high time division liquid crystal drive circuit that can reduce power consumption and require a small number of components.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 4 shows an example of a liquid crystal drive circuit of the present invention.

In Figure 4, 1 to 5, 7 to 11 are 12 as in Figure 2.
is an NPN transistor, 13 and 14 are resistors, and 1s is a PN transistor.
It is a P transistor. v0' is a control voltage. IC
The positive and positive power supply terminals of No. 11 are connected to vDD, and the negative power supply terminal is connected to vo.

In the circuit shown in FIG. 4, if the potential at the connection point between resistor 1 and resistor 2 is V, and the V□ of the NPN transistor 12° and the PNP transistor 16 are respectively vBET tV□2, then the emitter potential of the NPNI transistor 12 is Ma1
-v01, so the emitter potential of the PNP transistor 15, ie, V, is v1=v.

'111 + vBx2. NPN) U7 JISTA 1
If a 2°PNP transistor 15 with uniform vBx characteristics is used, vBx, = vBx2, can be obtained.
7,==7. is obtained. Further, since V is the emitter follower output of the PNP transistor 15, the output impedance is extremely small. i.e., NPN) Langimeta12. Resistors 13, 14. PNP transistor 15
This allows you to obtain input/output characteristics equivalent to the voltage follower of an operational amplifier.

Furthermore, as is clear from FIG. 4, the input/output characteristics of the circuit composed of the NPN transistor 12, the resistors 13 and 14, and the PNPI transistor 15 are such that the range of the straight line is the range in which the transistor is in the active region. Wider than that of an op amp's voltage follower. In other words, even if Vo−v becomes very small, it is possible to set v,=ma. Therefore, the number of time divisions of the liquid crystal display increases, and the peak value V of the liquid crystal drive voltage increases. Even if −v5 becomes small, v1=v can be set. Generally, as the number of time divisions for liquid crystal display increases, the peak value V of the liquid crystal drive voltage increases. -V cannot get larger, v2゜V, ,
Each voltage of V4 falls between vIII and vs in FIG.

That is, since the voltage divided by the resistors is transmitted as it is by the operational amplifier, there is no practical problem in the circuit example of FIG. 4 of the present invention.

Furthermore, as is clear from the circuit example in FIG.
One end of 3 is connected to ground, and PNP transistor 1
5 is connected to ground, and one end of emitter resistor 14 is connected to vDD. In other words, NPN l−
Since the transistor 12.degree. PNP transistor 15 operates between VDD and ground, the collector loss of the transistor and the power consumption in the resistor are smaller than when the transistor operates between vDD and v0. Further, by increasing the resistance values of the resistors 13 and 14 within a range that does not cause any practical problems, power consumption can be extremely reduced.

Further, as can be seen from the circuit example in FIG. 4, the lowest voltage v5 of the liquid crystal drive voltage is supplied to the operational amplifier 7.

Therefore, when comparing the conventional example shown in FIG. 2 and the circuit example shown in FIG. 4 according to the present invention, the additions in the circuit example shown in FIG. 4 are two resistors and one transistor. These are small in size, and if chip parts are used, an increase in volume will hardly be a problem. Compared to the case of adding a DC-DC converter that generates a high voltage (vH) to the conventional example shown in FIG. 2, the circuit shown in FIG. The advantage of examples is clear.

As described above, according to this embodiment, by using the NPNI transistor 12, the resistors 13 and 14, and the PNP I transistor 15, the number of time divisions can be further increased.
V even when the potential difference between V and V is small.

It has the advantage of being able to accurately transmit data with low impedance, consuming less power, and requiring no DC-DC converter, so it has fewer parts.

In the above embodiment, the PIP transistor is placed after the fPN transistor. :,'Although the structure was constructed by connecting transistors, it goes without saying that the same effect can be obtained even if an NPN transistor is connected after the PNPI-transistor as long as the transistor is in the active region.

Effects of the Invention As is clear from the above explanation, the present invention provides at least one of the voltages divided by the resistors to the liquid crystal display element via the NPN transistor and the PNP l-transistor, and the other via the operational amplifier. Since it is configured to supply
Even if the number of time divisions increases, the LCD drive voltage based on the voltage averaging method can be transmitted accurately and with low impedance, which has the excellent effect of eliminating signal voltage waveform distortion and improving image quality. .

Furthermore, since the present invention is configured to operate the transistor between vDD and ground, an excellent effect of reducing power consumption can be obtained.

Furthermore, since the present invention is configured so that the lowest voltage of the liquid crystal drive voltage is supplied from the operational amplifier, an excellent effect can be obtained in that the number of parts can be reduced.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a part of a conventional liquid crystal drive circuit, Figure 2 is a circuit diagram of a part of another conventional liquid crystal drive circuit, Figure 3 is a diagram showing an example of input/output characteristics of an operational amplifier, FIG. 4 is a circuit diagram of an embodiment of the liquid crystal drive circuit of the present invention. 1.2,3,4,6,13,14...Resistance, 6
゜15...PNP transistor, 12...
...NPN transistor, 7, 8, 9, 10...
...Operational amplifier, 11...IC Name of agent Patent attorney Toshi Nakao and one other person
~c+1
Size>>>>z>
Long-distance Yasu-Kumigo

Claims (3)

[Claims] (1) At least one of the voltages divided by the resistors is NP
A liquid crystal drive circuit characterized in that supply is supplied to a liquid crystal display element through an N transistor and a PNP transistor, and the others through an operational amplifier. (2) The liquid crystal drive circuit according to claim 1, wherein the transistor is operated between a positive voltage V_D_D and ground. (3) The liquid crystal drive circuit according to claim 1, wherein the lowest voltage of the liquid crystal drive voltage is supplied from an operational amplifier.