JPH1010491A - Power source circuit for driving lcd - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LCD power source circuit which promotes no OFF- segment-floating even when displays are switched and especially flashed and needs no adjustment even when it is used for the display device of products to be mass-produced. SOLUTION: A power source means 9 which generates the voltage by a specified voltage lower than the reference positive voltage (+V) of the LCD controller/driver 1 is provided to supply its output to the VLC3 terminal of the LCD controller/driver 1. The VLC1 terminal is installed with an emitter- follower by use of an NPN transistor 16 and the VLC2 terminal is installed with an emitter-follower by use of a PNP transistor 15 to supply power sources to the respective terminals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LCD drive power supply circuit, and more particularly to an LCD drive power supply circuit for outputting a plurality of bias voltages for driving an LCD.

[0002]

2. Description of the Related Art In general, a liquid crystal display device (hereinafter, referred to as an LCD) is advantageous in that it can be driven with low power and low power consumption, is relatively inexpensive, and is extremely thin. Has greatly expanded the market. Its applications cover a wide variety of applications, such as calculators, watches, and cameras. Among these various applications, the most popular one is the so-called TN (Twisted) using the electro-optic effect in a twisted nematic structure.
Nematic) type LCD.

[0003] The basic structure of a TN-type LCD has a structure in which a twisted liquid crystal cell is arranged on a set of polarizers. By applying or removing a voltage to or from this liquid crystal cell, liquid crystal molecules are formed. Rearrangement occurs, which results in the presence or absence of optical rotation with respect to light, forming a light state or a dark state. The degree of rearrangement of the liquid crystal molecules at this time depends on the strength of the electric field applied to the liquid crystal cell, that is, the voltage value.

[0004] The driving method of such a TN type LCD is determined by a bias method, a duty method, a frame frequency and the like. The duty method includes static, 1/2 duty, 1/3 duty, 1/4
Duty and so on. Assuming that an LCD control / driver having 40 SEG terminals is used, the maximum number of driving pixels in each duty system is 40 pixels static, 80 pixels 1/2 duty, and 1 pixel.
120 pixels at / 3 duty, 16 at 1/4 duty
It becomes 0 pixels.

[0005] As a bias method, there are static, 1/2 bias, 1/3 bias and the like. A difference in driving by the bias method is a voltage value applied to a non-selected pixel, that is, a pixel in an unlit state. In the case of static, no voltage is applied to non-selected pixels, and in the case of 1/2 bias, no voltage is applied. In the case of selection, that is, 1/2 of the voltage applied at the time of lighting is applied.は is always applied at the time of lighting.

As described above, the rearrangement of the liquid crystal molecules depends on the voltage value applied to the liquid crystal cell. Therefore, even when the liquid crystal cell is turned off, the fact that a certain amount of voltage is applied means that the pixel is turned off depending on the viewing angle direction. Means that the density is low but it looks lit. That is, the bias method is changed from static to 1/2 bias and from 1/2 bias to 1 bias.
By setting the bias to / 3, so-called OFF-seg floating is likely to occur, and the excellent viewing angle range of the LCD is narrowed.

The following relationship exists between the duty system and the bias system.・ When static is used as the duty method The bias method is static. • When 1/2 duty is used as the duty method Either 1/2 bias or 1/3 bias can be selected as the bias method. • When 1/3 duty is used as the duty method Either 1/2 bias or 1/3 bias can be selected as the bias method. • When 1/4 duty is used as the duty method The bias method is 1/3 bias.

That is, if a large number of pixels are driven with a small number of SEG terminals, the excellent viewing angle range is necessarily narrowed.
However, in recent years, the amount of information required for a display device has become extremely large, and in many cases, drive conditions are determined with priority given to the number of drive pixels. Particularly, in the case of a camera or the like, information required for a display device is The shutter speed, the aperture value, the exposure mode, the feeding mode, the film counter, the battery warning, etc. are various, and the driving is generally performed under the driving conditions of 1/4 duty and 1/3 bias.

Therefore, it is necessary to minimize further deterioration of the excellent viewing angle range. FIG. 3 is a general example of a common voltage waveform and a segment voltage waveform when a TN type LCD is driven using an LCD controller / driver under the driving conditions of 1/4 duty and 1/3 bias. In the output waveform of SEG0, (A) indicates CO
This is a waveform when a pixel consisting of M0 and COM2 is turned on, and FIG. 7B is a waveform when all pixels are turned off.

[0010] As can be seen from FIG. 3, COM 0 to COM 0 regardless of whether the lights are partially turned on or completely turned off.
3 is oscillating at VLCD, VDD-
The light is turned off in a state where the voltage of 1/3 VLCD consisting of VLC1 and VLC2-VLC3 is applied to the unselected pixels. When the outputs of COM0 to COM3 oscillate between VLC1 and VLC2, the light is turned off in the state where the voltage of 1/3 VLCD composed of VLC1 and VLC2 is applied.

FIG. 4 is a circuit diagram showing a conventional LCD driving power supply circuit. The driving voltage levels VLCD, VLCD shown in FIG.
LC1, VLC2 and VLC3 are supplied. The positive power supply (+ V) used for the LCD control / driver 1 is divided by resistors 2 to 5, and smoothing capacitors 6 to 8 are connected between the voltage generated by the voltage division and the positive power supply + V, respectively. 1 is input to each terminal. Here, the resistor 5 is connected to the LCD control /
Positive power supply used for driver 1 and VLC optimal for turning on LCD
D is optimally adjusted when D is different. If the positive power supply and the optimal VLCD are equal, the resistance 5
May be 0Ω.

[0012]

However, in such a conventional LCD drive power supply circuit, a current necessary for driving the LCD flows through the LCD control / driver 1, so that when the display is switched, it is particularly necessary to switch the display. When the blinking display is performed, the driving current of the LCD changes to the required LCD driving current at each of the voltage levels of VDD, VLC1, VLC2, and VLC3.
5, a change occurs in the bias current flowing through VLC1 and VLC2.
, And VLC3.
This phenomenon occurs when a large LCD is driven or when multiple LCs are
This is even more pronounced when D is driven by a single LCD controller / driver.

When a driving current flows from the VLC1 terminal of the LCD control / driver, the voltage of VLC1 drops, and the currents discharged from the VLC2 and VLC3 terminals raise the voltages of VLC2 and VLC3. As a result, VDD
The voltage consisting of VLC1 and VLC1 is greater than 1 / 3VLCD, and the voltage consisting of VLC2 and VLC3 is 1 / 3VLCD.
It will be a smaller value. As described above, in the driving method using the 1/3 bias, VD is applied to the pixel in the light-off state.
Since the 1/3 VLCD composed of D, VLC1, VLC2, and VLC3 is always applied, increasing this voltage promotes OFF-seg floating, further narrows the excellent viewing angle range, and improves the quality as a display device. There was a problem of lowering.

On the other hand, smoothing capacitors 6 to 8
May be further increased, or the resistance values of the resistors 2 to 5 may be further reduced, and the bias current may be increased so that the influence of the LCD drive current can be ignored. However, the former significantly delays the turning on of the LCD when the power is turned on and the turning off of the LCD when the power is turned off, and increases the current consumption when driving the latter, which is not a sufficient measure.

In the conventional circuit, the LCD controller /
Since the variation of the positive power supply used for the driver is directly reflected as the variation of the VLCD, the power supply that outputs the positive power supply to obtain the optimal VLCD for all the products manufactured is required for products such as cameras There has been a problem in that the accuracy of the apparatus has to be improved or adjustment has to be performed by using the resistor 5, resulting in high cost. Therefore, the present invention solves such a problem at once, and does not promote OFF-seg floating even when performing display switching, especially blinking display, and even when used as a display device of a product assuming mass production. It is an object of the present invention to provide an LCD power supply circuit that does not require adjustment or the like.

[0016]

In order to achieve the above object, an LCD drive power supply circuit according to the present invention has a ground potential as an input and a predetermined positive power supply as a reference voltage.
A power supply unit that outputs a voltage lower than the reference voltage by a predetermined voltage as a minimum bias voltage, and a plurality of bias voltages obtained by dividing a voltage between a positive power supply and a minimum bias voltage are provided. An emitter follower circuit that receives a bias voltage as input and outputs the input with low impedance. Therefore, a voltage lower than the reference voltage by a predetermined voltage is output as the minimum bias voltage by the power supply unit that receives the ground potential as input and uses a predetermined positive power supply as a reference voltage, and outputs a voltage between the positive power supply and the minimum bias voltage. Each bias voltage is output with low impedance by an emitter follower circuit provided for each of a plurality of bias voltages obtained by dividing the bias voltage. Further, there is provided a reference voltage correcting means having a temperature sensing element and outputting a reference voltage obtained by correcting the positive power supply based on the ambient temperature. Therefore, the lowest bias voltage is output based on the reference voltage temperature-compensated by the reference voltage correction means.

[0017]

Next, the present invention will be described with reference to the drawings. FIG. 1 shows an LC according to an embodiment of the present invention.
FIG. 4 is a circuit diagram of a D drive power supply circuit, in which the same reference numerals are given to the same or equivalent parts as described above (see FIG. 3). In FIG. 1, a power supply means 9 is a power supply means typified by a three-terminal regulator for generating a voltage that is negative with respect to a reference voltage, based on a positive power supply (+ V) of the LCD controller / driver 1, Its output is connected to the VLC3 terminal of the LCD controller / driver 1.

The primary input is connected to the ground potential GND, so that VLCD is kept constant even when the positive power supply fluctuates. The VLCD output from the power supply means 9 is connected to the resistors 10 and 11
And a PNP is connected to the connection point of these resistors 10 and 11.
The base of the transistor 15 is connected, and P
The collector of the NP transistor 15 is connected to the output terminal of the power supply means 9, and the emitter is connected to the VLC2 terminal of the LCD control / driver 1. At this time, VLC2
The voltage dividing ratio of the resistors 10 and 11 is determined in consideration of the voltage between the base and the emitter of the PNP transistor so that the voltage between -VLC3 becomes 1 / 3VLCD.

Further, the voltage between VDD and VLC2 is changed by a resistor 1
The base of the NPN transistor 16 is connected to a point where the resistors 12 and 13 are connected.
The collector of the NPN transistor 16 is connected to the positive power supply, and the emitter is the V
It is connected to the LC1 terminal and to the output of the power supply means via the resistor 14. Note that the voltage division ratio of the resistors 12 and 13 is the same as that of the resistors 10 and 11 when the voltage between VDD and VLC1 is 1 unit.
The voltage is determined in consideration of the base-emitter voltage of the NPN transistor 16 so as to be / 3 VLCD. LCD
Smoothing capacitors 6, 7, and 8 are connected between the VLC1, VLC2, and VLC3 terminals of the controller / driver and the positive power supply.

Next, the operation of the first embodiment of the present invention will be described with reference to FIG. When the LCD is turned on and off by the drive method of 1/4 duty and 1/3 bias, LC is turned on at the timing when all lights are turned on and all lights are turned off.
The VLC1 terminal of the D controller / driver 1 draws a large current, and the VLC2 terminal discharges a current. On the other hand, in the circuit of the present invention, NP1 is connected to the VLC1 terminal.
Since the emitter follower is provided by the N transistor 16 and the emitter follower is provided at the VLC2 terminal by the PNP transistor 15, it is possible to cope with the fluctuation of the LCD drive current with a very small bias current.
The voltages of VLC1 and VLC2 do not change.

In addition, at the timing when all lights are turned off to all lights, the VLC1 terminal discharges current and the VLC2 terminal sinks current. However, as the display state,
Since the direction in which the pixels that are turned off disappears, the resistance 14
Alternatively, even if the suction ability or the discharge ability is at the level of the resistors 12 and 13, the floating of the OFF segment is not promoted. Since VDD and VLC3 are outputs of the power supply or the power supply means, the output impedance is sufficiently low.

In the above description, the driving method using 1/4 duty and 1/3 bias has been described.
It is apparent that the same effect can be obtained by the circuit of the present invention even when the 1/2 duty system and the 1/3 duty system are employed. In addition, for the bias system, VLC
It is clear that the same effect can be obtained by the circuit of the present invention even in a system in which D is divided.

Next, a second embodiment of the present invention will be described with reference to FIG. The difference between FIG. 2 and FIG. 1 is that the temperature sensitive element 17 is connected between the power
The point is that temperature compensation of D is performed. That is, the power supply means 9 includes a bias current flowing through the parallel circuit of the temperature-sensitive element 17 and the resistance 18 typified by thermistors and posistors having different resistance values depending on the environmental temperature and the resistances 19 and 11, and the self-consumption of the power supply means 9. The operation is performed based on the voltage dropped from the positive power supply of the LCD controller / driver 1 by the voltage difference ΔV determined by the current.

The output of the power supply means 9 is connected to the VLC3 terminal of the LCD controller / driver 1;
The secondary input is connected to GND. As a result, even if the positive power supply fluctuates, VLCD is kept constant, and VLCD changes so as to keep the display density of LCD constant depending on the environmental temperature. The VLCD formed by the power supply means 9 is divided by a temperature sensing element 17, resistors 18, 19 and a resistor 11, and a PNP transistor 1 is connected to a connection point between the resistors 19 and 11.
5 bases are connected.

The collector of the PNP transistor 15 is connected to the output terminal of the power supply means 9, and the emitter is connected to the VLC2 terminal of the LCD control / driver 1. At this time, the temperature sensing element 17 and the voltage dividing ratio of the resistors 18 and 19 and the resistor 11 are determined in consideration of the base-emitter voltage of the PNP transistor so that the voltage between VLC2 and VLC3 becomes 1/3 VLCD. . Also, due to the temperature characteristic of the base-emitter voltage of the PNP transistor, even if VLCD changes due to environmental temperature, VLC2-VLC3
The inter-voltage is always set to 1/3 VLCD.

Further, the voltage between VDD and VLC2 is changed by a resistor 1
2 and 13 and the connection point between the resistors 12 and 13 is N
The base of the PN transistor 16 is connected, and NP
The collector of the N-transistor 16 is connected to the positive power supply, the emitter is connected to the VLC1 terminal of the LCD controller / driver 1, and is connected via the resistor 14 to the output of the power supply means. Note that the voltage division ratio of the resistors 12 and 13 is also equal to VDD−V
The voltage between LC1 and １ ／ VLCD is determined in consideration of the voltage between the base and the emitter of NPN transistor 16.

The voltage between VDD and VLC1 is always set to 1/3 VLCD irrespective of the temperature change of VLCD due to the temperature characteristic of the base-emitter voltage of the NPN transistor 16. Smoothing capacitors 6, 7, and 8 are connected between the VLC1, VLC2, and VLC3 terminals of the LCD controller / driver and the positive power supply.

Next, an operation according to the second embodiment of the present invention will be described with reference to FIG. When the LCD is turned on and off by the drive method of 1/4 duty and 1/3 bias, LC is turned on at the timing when all lights are turned on and all lights are turned off.
The D controller / driver's VLC1 terminal draws a large current, and the VLC2 terminal discharges a current. On the other hand, in the circuit shown in FIG. 2, an emitter follower using an NPN transistor 16 is provided at the VLC1 terminal,
Since the emitter follower of the PNP transistor 15 is provided at the LC2 terminal, the fluctuation of the LCD driving current can be coped with with a very small bias current, and the voltages of VLC1 and VLC2 do not fluctuate.

In addition, at the timing when all lights are turned off to all lights, the VLC1 terminal discharges current and the VLC2 terminal sinks current. However, as the display state,
Since the direction in which the pixels that are turned off disappears, the resistance 14
Even if the suction ability or the discharge ability is at the level of the resistors 12 and 13, the floating of the OFF segment is not promoted. Note that VDD and VLC3 have sufficiently low output impedance because they are outputs of the power supply or the power supply means.

In the above description, the driving method using 1/4 duty and 1/3 bias has been described.
It is apparent that the same effect can be obtained by the circuit of the present invention even when the 1/2 duty system and the 1/3 duty system are employed. It is also apparent that the same effect can be obtained by the circuit of the present invention even if the VLCD is divided into three or more bias systems.

[0031]

As described above, according to the present invention, the ground potential is input and the predetermined positive power supply is used as the reference voltage.
Power supply means for outputting a voltage lower than the reference voltage by a predetermined voltage as the minimum bias voltage is provided, and a plurality of bias voltages obtained by dividing the voltage between the positive power supply and the minimum bias voltage are individually converted into individual bias voltages. The output is low impedance from the emitter follower circuit, so the display does not promote OFF-seg floating even when switching display, especially blinking display, and display of products for mass production etc. Even when used as an apparatus, a remarkable effect that no adjustment or the like is required can be obtained. Further, a reference voltage correction means having a temperature sensing element and outputting a reference voltage obtained by correcting the positive power supply based on the ambient temperature is provided, and a minimum bias is provided by the power supply means based on the reference voltage temperature compensated by the reference voltage correction means. Since the voltage is output, it is possible to perform temperature compensation collectively for each bias voltage.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an LCD drive power supply circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an LCD drive power supply circuit according to another embodiment of the present invention.

FIG. 3 is a signal waveform diagram showing a general LCD drive voltage waveform.

FIG. 4 is a circuit diagram showing a conventional LCD drive circuit.

[Explanation of symbols]

1: LCD controller / driver, 6 to 8: capacitor, 9: power supply means, 10 to 14, 18, 19: resistor, 1
5 ... PNP transistor, 16 ... NPN transistor,
17: temperature sensing element, + V: positive power supply.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenji Isono 3-2-2, Marunouchi, Chiyoda-ku, Tokyo Nikon Corporation (72) Inventor Takeharu Kato 3-2-2, Marunouchi, Chiyoda-ku, Tokyo Stock Company Nikon company

Claims (2)

[Claims] 1. An LCD driving power supply circuit for outputting a plurality of bias voltages for driving an LCD, wherein a ground potential is input, a predetermined positive power supply is used as a reference voltage, and a voltage lower than the reference voltage by a predetermined voltage. And a plurality of bias voltages obtained by dividing a voltage between the positive power supply and the lowest bias voltage, and output with a low impedance with each bias voltage as an input. An LCD drive power supply circuit comprising: an emitter follower circuit. 2. The LCD drive power supply circuit according to claim 1, further comprising: a reference voltage correction means having a temperature sensing element and outputting a reference voltage obtained by correcting a positive power supply based on an ambient temperature. Power circuit.