JP3479572B2 - LCD display drive - 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 display driving device in which a liquid crystal display used in a camera or the like is driven by a general-purpose IC.

[0002]

2. Description of the Related Art In order to drive a liquid crystal display,
It is necessary to apply a voltage of about 1 V to each segment. Therefore, when the liquid crystal display is driven by, for example, 1/3 bias, at least 3 is used as a driving power source.
It is necessary to secure a voltage of V or higher. Currently, a dedicated IC for driving a liquid crystal display, which has a built-in booster circuit, is commercially available, and a dedicated IC is often used for driving the liquid crystal display. However, since the dedicated IC for driving the liquid crystal display is originally a dedicated circuit, it cannot be used for other purposes and is expensive. In the case of a camera having a liquid crystal display, a CPU is generally provided to execute various control operations, and a booster circuit is required to drive the CPU. Therefore, instead of using an expensive dedicated IC including a booster circuit only for driving the liquid crystal display,
There is a demand to use a control CPU and a booster circuit for driving a liquid crystal display.

FIG. 4 shows an example of a conventional circuit in which a control CPU and a booster circuit are used to drive a liquid crystal display. In FIG. 4, 1 is a CPU for performing various control operations, 2 is a dry battery that generates a power supply voltage VB, and 3
Boosts the power supply voltage VB to drive the CPU1 drive line (CP
A booster circuit applied to the U drive line VC), 4 is a liquid crystal display, 5 is a capacitor for absorbing noise generated by the oscillating operation of the booster circuit 3, and 6 is a diode for preventing backflow. Now, generally, CPU drive line V
Since C is higher than the drive line (liquid crystal drive line VL) of the liquid crystal display 4, the CPU drive line VC is connected to the resistor R.
The liquid crystal drive line VL (about 3 V) is obtained by dividing the voltage into, for example, 3/4 by 1 to R4.

In the case of a device such as a camera that depends on the dry battery 2 as its power source, the control signal PO is generated when the CPU 1 is inactive.
It is desired to turn off N to stop the booster circuit 3 and prevent the dry battery 2 from being consumed. However, when the operation of the booster circuit 3 is stopped, the CPU drive line VC drops to the power supply voltage VB as the capacitor 5 discharges, the liquid crystal drive line VL also drops, and the liquid crystal display 4 is turned off. Therefore, in order to turn on the liquid crystal display 4 even when the CPU 1 is not operated, the resistors R1 and R2 are connected.
Is provided with switching means SW1 for short-circuiting the voltage dividing point (that is, the liquid crystal drive line VL) with the CPU drive line VC, and by making the switching means SW1, the level of the liquid crystal drive line VL is set to a predetermined operating point level. To maintain.

[0005]

However, in such a conventional method, when the booster circuit 3 is switched from ON to OFF and the switching means SW1 is switched from OFF to ON, the capacitor 5 is discharged. There is a problem in that the display of the liquid crystal display 4 has a shade. This problem will be described in more detail with reference to the time chart of FIG. That is, when the operation of the booster circuit 3 is stopped, the control signal PON is turned off, but at the same time, the liquid crystal drive line VL.
The making of the switching means SW1 in order to secure the level of is as described above. As described above, the booster circuit 3 is provided with the capacitor 5 for removing noise, and the capacitor 3 is in a fully charged state at the moment when the operation of the booster circuit 3 is stopped. Therefore, the liquid crystal drive line VL is accompanied by the make of the switching means SW1.
When the CPU drive line VC and the CPU drive line VC are short-circuited, the liquid crystal drive line VL instantly rises to the full charge level of the capacitor 5 and then decreases as the capacitor 3 is discharged. Therefore, the display density of the liquid crystal display 4 increases only for the time required for discharging the capacitor 5, and uneven density occurs during that time. Therefore, by inserting a resistor 7 between the capacitor 5 and the power supply 2,
Although the discharge time of the capacitor 5 is accelerated as shown by the broken line in Fig. 5, there is a limit to the resistance value that can be selected as the resistor 7 in order to prevent the efficiency of the booster circuit 3 from decreasing, and when the capacitance of the capacitor 5 is large. Causes an uneven density that is noticeable in the visual discrimination. The density curve in FIG. 5 does not increase instantaneously with the increase in the drive voltage VL of the liquid crystal display 4 because of the response delay of the liquid crystal display 4.

[0006]

The present invention has been made in view of the above problems, and is premised on a driving device of a liquid crystal display device using a general-purpose IC, the liquid crystal display when the operation of the booster circuit is stopped. It is an object of the present invention to provide a driving device for a liquid crystal display capable of preventing the uneven density of the liquid crystal display. In summary, the liquid crystal display driving device of the present invention includes: a CPU that performs various control operations.
1, a booster circuit 3 for boosting the power supply voltage VB and applying it to a CPU drive line VC which is a drive line for the CPU.
The booster circuit 3 is provided with a capacitive element 5 for absorbing noise generated by the oscillation operation, and the CPU drive line VC is divided and applied to a liquid crystal drive line VL for driving the liquid crystal display 4. Assuming a liquid crystal display drive device: booster circuit operation stopping means PON for stopping the oscillation operation of the booster circuit 3, first switching means SW1 for short-circuiting the CPU drive line VC and the liquid crystal drive line VL, and A second switching means SW2 for electrically connecting the CPU drive line VC to the ground line via the resistance element 8: the step-up circuit operation stopping means in accordance with the switching operation of the CPU1 from the operating state to the non-operating state. The operation of the booster circuit 3 is stopped by the PON to bring the second switching means SW2 into a conductive state, and The first switching means SW1
Is intermittently operated for a predetermined period at a cycle shorter than the response time of the liquid crystal display 4, and then the first switching means SW1 is brought into a short-circuited state so that the shading of the liquid crystal display 4 does not occur.

[0007]

In the present invention, the second switching means SW2 is closed when the operation of the booster circuit 3 is stopped, and the capacitive element 5 is discharged through the resistance element 8. At this time, the resistance element 8 serving as the discharge path does not affect the efficiency of the booster circuit 3, so that there is no restriction on the resistance value and the time required for discharging is shortened. Further, the liquid crystal display 4 has a low responsiveness to the fluctuation of the driving voltage, and a slight time lag occurs after the driving voltage increases until the display density actually increases. The present invention utilizes the response delay of the liquid crystal display 4 so that the density change of the capacitive element 5 during discharge does not appear.
That is, when the first switching means SW1 for short-circuiting the CPU drive line VC and the liquid crystal drive line VL is operated intermittently, the liquid crystal display 4 is accompanied by this intermittent operation.
The charging level of the capacitor 5 and the level obtained by dividing the voltage by 3/4 are alternately applied to the capacitor. By making the period of the intermittent operation shorter than the response time of the liquid crystal display, the density of the liquid crystal during the discharge operation of the capacitive element does not seem to increase or decrease, and the user does not feel uncomfortable.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention, and elements similar to those of the related art described with reference to FIG. 4 are designated by the same reference numerals as those in FIG.
In the drawing, reference numeral 8 is a resistance provided in place of the bypass resistance 7 shown in FIG. 4 for explaining the conventional example, and the resistance 8 is the second switching means SW2 which is broken during the operation of the CPU 1. Is provided between the power supply line VC of the CPU 1 and the ground via the. Resistor 8 is CP
Since the switching means SW2 disconnects the circuit from the circuit during the operation of U1, that is, during the operation of the booster circuit 3, it is possible to select a sufficiently low resistance value without affecting the efficiency of the booster circuit 3 at all. Therefore, the discharge time constant of the capacitor 5 can be made sufficiently fast.

Further, the switching means SW1 for short-circuiting the drive line VC of the CPU 1 and the drive line VL of the liquid crystal display 4 is shown at the same place as in the prior art shown in FIG. 4, but in this embodiment, the switching means SW1 is The liquid crystal display 4 is designed to operate intermittently in a cycle shorter than the response time.

Next, the operation of the above embodiment will be described with reference to the time chart of FIG. First, while the CPU 1 is operating, the control signal PON is at the H level, and the booster circuit 3 boosts the power supply voltage VB to give a drive voltage of about 4V to the CPU drive line VC. Therefore, capacitor 5
Is charged up to the CPU drive line VC. At this time, the switching means SW1 and SW2 both break, and the CPU drive line VC is connected to the resistors R1 to R as the liquid crystal drive line VL for driving the liquid crystal display 4.
A level (about 3 V) divided by 3/4 is applied, and the liquid crystal display 4 is maintained at an appropriate display density.

When the operation of the CPU 1 is stopped from this state, for example, when the photographing operation is completed, the control signal PON is turned off to stop the operation of the booster circuit 3 in order to save power. At the same time, the second switching means SW2 is made to discharge the charges accumulated in the capacitor 3 via the resistor 8 having a sufficiently small time constant. Further, at this time, the first switching means SW1 is made for a minute time T0, and then the switching means SW1 is intermittently operated during the period of T1. The level of the liquid crystal drive line VL is short-circuited with the CPU drive line VC along with the make of the first switching means SW1, and at this point the discharge of the capacitor 5 has hardly progressed, so the level of the liquid crystal drive line VL rises, It acts in the direction of increasing the display density of the liquid crystal display 4.

However, since the liquid crystal display 4 has a response delay with respect to the fluctuation of the applied voltage, if the minute time T0 for making the first switching means SW1 is sufficiently shorter than the response time of the liquid crystal display 4, the liquid crystal is The switching means SW1 breaks before the display density of the display 4 increases so that it can be visually recognized. T0
When the switching means SW1 breaks and the liquid crystal drive line VL reaches a level obtained by dividing the CPU drive line VC into 3/4 after the period of time elapses, the discharge of the capacitor 5 progresses during this period T0 and the subsequent period T1. Therefore, the liquid crystal drive line VL becomes lower than the standard operating voltage of 3V of the liquid crystal display 4, and acts to decrease the display density of the liquid crystal display 4 below the reference density. However, in the present invention, the switching means SW1 is intermittently driven in the period of T1, and the intermittent period is sufficiently shorter than the response time of the liquid crystal display 4, so that the vibration of the liquid crystal drive line VL accompanying the intermittent switching of the switching means SW1. The increase / decrease in the display density of the liquid crystal display 4 due to is not visible to the naked eye, and does not cause an unpleasant light and shade to the user of the camera.

Then, when the period of T1 has elapsed, the switching means SW1 finally becomes the make state,
The power supply level VB is applied to the liquid crystal drive line VL. At the same time, the switching means SW2 breaks and the capacitor 5
To stop the discharge. Therefore, the time T1 during which the switching means SW1 is intermittently driven may be appropriately selected according to the time required for discharging the capacitor 5.

Next, FIG. 3 is a circuit diagram showing a modified example of the present invention. The same elements as those of the embodiment shown in FIG. In the modification shown in FIG. 3, a resistor 10 for supplying a drive current to a position detecting photo interrupter 9 provided in the camera is used instead of the resistor 8 in FIG. It becomes possible to effectively utilize the element.

[0015]

As described above, according to the present invention, since the capacitor is discharged by using the resistor that does not affect the efficiency of the booster circuit, it is possible to shorten the discharge time and drive the liquid crystal. It becomes possible to promptly bring the line VL to a stable level, and the switching means for short-circuiting the CPU drive line and the liquid crystal drive line during this discharge time is operated intermittently at a cycle shorter than the response time of the liquid crystal. The density of the display density of the liquid crystal display cannot follow the increase and decrease of the liquid crystal drive voltage, the apparent display density becomes stable, and the user does not feel uncomfortable.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a time chart of the embodiment shown in FIG.

FIG. 3 is a circuit diagram showing a modification of the embodiment shown in FIG.

FIG. 4 is a circuit diagram showing an example of a conventional drive circuit.

5 is a time chart of the conventional circuit shown in FIG.

[Explanation of symbols]

1 CPU 2 power supplies 3 Booster circuit 4 LCD display R1 resistance R2 resistance R3 resistance R4 resistance 5 capacitors 8 resistance SW1 First switching means SW2 Second switching means VC CPU drive line VL LCD drive line

Claims (2)

(57) [Claims] 1. A CPU that performs various control operations, a booster circuit that boosts a power supply voltage and applies the booster voltage to a CPU drive line that is a drive line of the CPU, and noise that is generated due to an oscillating operation of the booster circuit. A booster circuit for stopping the oscillation operation of the booster circuit, wherein the booster circuit is provided with a capacitive element for absorbing the voltage and applies the voltage to the liquid crystal drive line for driving the liquid crystal display by dividing the CPU drive line. An operation stop means, a first switching means for short-circuiting the CPU drive line and the liquid crystal drive line, and a second switching means for electrically connecting the CPU drive line to a ground line via a resistance element, With the switching operation of the CPU from the operating state to the non-operating state, the step-up circuit operation stopping means operates the step-up circuit. , The second switching means is turned on, and the first switching means is intermittently operated for a predetermined period at a cycle shorter than the response time of the liquid crystal display, and then the first switching means is short-circuited. A liquid crystal display drive device characterized by being set in a state. 2. The liquid crystal display driving device according to claim 1, wherein the capacitive element is the second element during the predetermined period.
The liquid crystal display drive device is characterized in that the time required for discharging through the switching means to a level at which it can be regarded as substantially the same as the power supply voltage.