JPS61240291A - Power source circuit for driving liquid crystal - 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 Technical Field The present invention relates to a power supply circuit for driving a liquid crystal that can adopt a time-divisional driving method with a relatively high duty.

<Prior art> Conventionally, there are static drive and dynamic drive as liquid crystal drive methods, but the former requires a complicated liquid crystal electrode pattern and increases the number of terminals of the drive LSI, which increases the overall price. Currently, dynamic drives are the mainstream, except for those that are used only in watches with a relatively small number of digits.

In particular, a dynamic drive method developed to reduce and simplify the number of drive signals is used in liquid crystal displays, etc., and in the case of liquid crystal displays, time-division selection is performed using threshold voltage characteristics.

Since this threshold voltage depends on temperature, frequency, etc., it is difficult to obtain a high degree of time division (duty ratio).

Such a dynamic drive method utilizes this threshold characteristic in two ways, ie, a voltage averaging method and a two-frequency method.

The former voltage averaging method is a method that directly utilizes the threshold voltage characteristics, and the threshold voltage (v
The driving force type is such that a driving pulse of 100% or more is applied, and only a pulse of less than 100% vth is applied to the half-selective electrode.

The latter two-frequency method utilizes the fact that the threshold voltage characteristic depends on frequency, and uses the frequency at which the threshold voltage suddenly increases, that is, a kind of cut-off frequency (fc).
This method uses a larger frequency for part of the drive, and performs time division selection by applying a frequency lower than fc to the selection electrode and a frequency higher than fc to the half selection electrode.

The present invention is particularly suitable for adopting the former driving method.

In the voltage averaging method, since the threshold voltage varies considerably depending on various factors, various measures have been taken to obtain stable characteristics within the range of environmental conditions required for electronic desktop calculators and the like.

In general, a driving pulse with a pulse height of ■ is applied to both electrodes so that a pulse of 2V is applied to the selective electrode and a pulse of ■ is applied to the half-selective electrode, and 2V>Vth. A method of setting ■ to meet the condition of >V is used. However, this method cannot keep up with fluctuations in the threshold voltage due to changes in environmental conditions.

As a method to solve this interlayer, we considered the so-called bias applied to the conventional half-select, and the selection electrode 172,
A 1/2 bias method and a 1/3 bias method are often used. On the other hand, the degree of time division (D
The selection of uty) is limited to a range that satisfies stability due to the relationship with the specified threshold voltage.

Therefore, the main method is 1/2 duty-1/2 bias.
s, 1/3 duty (/3 bias, 1/4 duty-
1/3 bias etc. are commonly used.

As can be seen from the above, considering the number of liquid crystal driving terminals, it is desirable to make the duty as high as possible.

However, the accuracy of the power supply voltage is strictly required due to various driving voltage margins on the liquid crystal side.

FIG. 1 is a diagram showing a comparison of characteristics of power supply voltage margin (margin) with respect to ambient temperature at each duty.
a) = (b) - (e) are 1/3, 1/4, 1/ respectively
The case of 7 duty is shown.

In the figure, the AII region is the so-called malfunction region where the contrast of the lit segment is insufficient, the Bix region is the so-called malfunction region where the non-lighted segment is lit, and the shaded region is the drive margin region of the power supply voltage suitable for driving the liquid crystal. be.

What can be seen from the figure is that the power supply voltage margin region becomes narrower as the duty becomes higher, and also has temperature characteristics and has a negative slope with respect to temperature.Also, if the drive voltage is a constant voltage, 1 /3duty/4duty allows design within the margin area, but for example, at 1/7 duty, the power supply voltage (drive voltage) protrudes outside the shaded area, which causes so-called margin cracking.In other words, dut
As y increases, the margin area becomes narrower due to the response characteristics of the liquid crystal, and if the power supply voltage is fixed, the power supply voltage (drive voltage) will exceed at a certain temperature, and conversely, if the voltage is fixed and temperature compensation is not performed, the liquid crystal The usable temperature range of the display device tends to be narrow.Here, the temperature range Ta=O~40
℃.

As described above, when performing high-duty driving, it is difficult to use a constant voltage power supply because it causes Marnon cracking, especially at low or high temperatures.

In order to eliminate such drawbacks, it is necessary to use a circuit in which the drive voltage is temperature-compensated, and the applicant previously filed the Utility Model Application No. 53
-Proposed the power supply circuit of No. 119576. However, although this temperature compensation circuit can be implemented using a temperature sensor such as a thermistor, the margin range is narrow, so the temperature compensation circuit requires an accuracy of ±5% of the voltage or less, and the liquid crystal drive method has low accuracy. This is aimed at increasing power consumption, and a circuit that minimizes power consumption in the temperature correction circuit is required.

That is, a circuit that has both high voltage accuracy and low power loss has a disadvantage in that it occupies a high cost in terms of cost, and the cost of the entire system increases significantly.

<Object of the invention> The present invention has been made in order to eliminate the above-mentioned drawbacks.
It is an object of the present invention to provide a power supply circuit for driving a liquid crystal, in which a plurality of power supply voltages are set within a power supply voltage margin region, and the plurality of power supply voltages are sequentially switched using a single switching key.

<Example> An embodiment of the present invention will be described below with reference to the drawings.

Fig. 2 is a circuit configuration diagram showing one embodiment of the same, where E is a DC voltage, S, -S are switches, R2-R9 are resistors, RL is a load resistance (including a liquid crystal load), and V LCD is a liquid crystal. This is the driving voltage. It is assumed that one of the switches 81 to S is turned on and all the others are turned off.

Now, if Ri<Ri+1 (i=1 to 4), when switch 5i (i=1 to 5) is ON, vt, an(i)=
ExRL/ (Ri+RL) To't Lua'l'
La, VLCD(i) > VLCD(i+1) (i=1~
The relationship 4) holds true. Therefore, by appropriately determining the value of Ri for a constant load resistance RL, the configuration is such that VLCD(i) can be determined within the power supply voltage margin region as shown in FIG.

That is, in order to selectively supply n voltages Ei (i=1-m) (Ei≦E) to the load resistor RL, n resistors Ri, n switches Si, and the load resistor RL are connected. established,
One of the switches Si is connected to the power supply E, the other to the corresponding Ri, one of each resistor Ri is connected to the resistor RL, and the other is connected to the power supply via a switch that is selectively turned on. Then, the output voltage is obtained from the connection point between the resistor Ri and the load resistor RL, and n types of output voltages Ei (i = 1 to n) are obtained by turning on any one of the switches Si and turning off all others. It is designed so that it can be obtained.

As described above, the circuit of the present invention can be realized with a simple configuration of several switches S and resistors R, and since there is no current loss other than the load current, it is possible to obtain low cost and low power characteristics. I can do it.

Further, since the above-mentioned switch can be constructed from a semiconductor element such as a MOS transistor, it can be built into an LSI and can be used in a calculator or the like. Moreover, it goes without saying that the number of switches and resistors can be selected arbitrarily.

FIG. 4 shows another embodiment in which the switch can be freely selected and changed from outside the calculator or the like using a slide switch or the like.

FIG. 5 shows still another embodiment, in which a ring counter RC
This is achieved using Here, K is a switching key, and AS is an analog switch. Each time the key is pressed, the ON position of the analog switch As moves, allowing the operator to select the LCD drive voltage most suitable for the environment (ambient) temperature at that time. This allows for easy selection.

For example, when b is at LoIll level and others are at High level, if you press key K, C becomes L1 and others become H. If you press the key again, d becomes low and others become H, and if you repeat this, it goes to L level. The position of is 6-e a→l)-+ c→d→e
→B-41) →... becomes a fairy tale. Here, a P-channel type MOSFET is used as the run transistor, and the date is L+,
: 1r ONL, VLCDIm generates a voltage of Ex RL/(Ri+RL) due to the resistance ratio of the resistor R connected to the drain of the MOSFET and the load resistor RL.

<Effects> As described above, according to the liquid crystal drive power supply circuit of the present invention, a plurality of power supply voltages (drive voltages) set within the power supply voltage margin area can be sequentially switched using a single switching key. The author can change the display contrast with a simple operation.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a comparison of the characteristics of the power supply voltage margin with respect to the ambient temperature at each duty, Fig. 2 is a circuit configuration diagram showing an embodiment of the liquid crystal driving power supply circuit according to the present invention, and Fig. 3 is 4 and 5 are circuit configuration diagrams showing other embodiments of the present invention. In the figure, E: DC power supply, Si: switch, Ri: resistor. RL: load resistance, VLCD: liquid crystal drive voltage, SS Nislide switch, NI: switching key, RC: ring counter. AS: Analog switch. Agent Patent attorney Takeshi Sugiyama (and 1 other person) Procedural amendment (method) 1985 ≠ month aθ day

Claims (1)

[Claims] 1. When driving in a time division manner with relatively high duty,
In a liquid crystal drive system in which a power supply voltage margin region is narrow with respect to ambient temperature characteristics, a setting means for setting a plurality of power supply voltages within the power supply voltage margin region and a single switch for switching the power supply voltage are provided. 1. A power supply circuit for driving a liquid crystal, comprising: a switch key, and switching means for sequentially switching the set power supply voltage in response to pressing of the single switch key.