JPH03241315A - Lighting method for liquid crystal display backlight - Google Patents

Abstract

PURPOSE:To surely execute the design to a frequency in which an interference fringe is invisible and to eliminate the need of adjustment, etc. by generating an oscillation trigger signal of an external inverter circuit for lighting a light source by frequency-dividing a horizontal synchronizing signal or a clock signal being a driving signal of a liquid crystal display. CONSTITUTION:An oscillation trigger signal of an external inverter circuit for energizing a backlight is generated by frequency-dividing a horizontal synchronizing signal or a clock signal being a driving signal of a liquid crystal display (LCD). In such a way, since a lighting frequency is determined by a trigger signal for utilizing a driving signal of the LCD, the lighting frequency for the backlight can be set easily to a frequency in which an interference fringe generated by the driving signal of the LCD is invisible, and also, since the oscillation trigger signal is generated by frequency-dividing the driving signal of the LCD, a difference between the LCD frame frequency and the energizing frequency of the backlight always becomes constant, and it does not occur that the interference fringe becomes visible due to a temperature variation and a change with the lapse of time, or a flicker is generated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for lighting a backlight of a liquid crystal display (hereinafter referred to as LCD), and particularly to an EL display.

The present invention relates to a lighting method when using a light source that emits light in a blinking manner, such as a discharge lamp.

A conventional backlight lighting method will be described using an EL as an example.

As shown in FIG. 5, the conventional EL lighting circuit 4 is a DC-AC
It has an inverter 1 and an oscillation circuit 2, causes the oscillation circuit 2 to oscillate at an arbitrary frequency, uses its output signal Trg as a voltage switching signal for the DC-AC inverter 1, creates an alternating current voltage (2), and lights up the EL3. The voltage (2) applied to EL at this time and the light emission waveform of EL3 are shown in FIG.

On the other hand, in a line-sequentially driven LCD, the bright transmittance at a certain scanning line section is maximum during scanning, and then
It tends to gradually decrease until the next scan is performed (see Figure 7). Therefore, the lighting frequency of EL3 is set to fELl.
If the frame frequency of the LCD is fLCD, then fEL=-XfLCD (Hz) where n is an integer. When the relationship holds, stationary bright and dark interference fringes are visible on the screen. Furthermore, at a frequency slightly deviated from the above frequency, the interference fringes appear to scroll up or down on the screen.

Furthermore, as fEL deviates from the above relationship, the interference fringes begin to look like flicker to the human eye, and furthermore, flicker is no longer perceived by the human eye. The relationship between both frequencies at this time is as follows. If we examine this state in detail, we will see the result shown in Figure 8. Therefore, the EL lighting frequency fEL needs to be set to a frequency where interference fringes generated between the LCD frame frequency f and the LCD are not visible.

However, in the conventional EL lighting method, the EL lighting oscillator is independent of the LCD drive signal, so the frame frequency varies depending on the LCD.
Designing and adjusting an oscillation circuit that does not cause interference fringes is a hassle.

■ The oscillation frequency depends on changes in element characteristics due to temperature changes,
Due to the influence of changes over time, it may drift and become difficult to adjust, causing interference fringes or flickering.

There was a problem.

- Therefore, the present invention provides an LCD backlight, characterized in that an oscillation trigger signal of a separately excited inverter circuit for lighting the backlight is generated by frequency-dividing a horizontal synchronization signal or a clock signal, which is a drive signal of the LCD. This is a method of lighting the light.

Since the lighting frequency is determined by a trigger signal using the LCD drive signal, the lighting frequency of the backlight is L.
It is easy to set the frequency to a level where interference fringes caused by the CD drive signal are not visible, and since the LCD drive signal is created by dividing the frequency, the difference between the LCD frame frequency and the backlight lighting frequency is It is always constant, and no interference fringes become visible or flicker occurs due to temperature changes or changes over time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Below, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows an EL lighting circuit 4 when the original signal to be frequency-divided in the present invention is a horizontal synchronization signal of an LCD drive circuit 10.
An example of a is shown below.

Components that are the same as those in the conventional example are given the same reference numerals and explanations will be omitted.

First, for the LCD, the frame frequency is f LC
D (Hz), and the frequency of the horizontal synchronizing signal HD is fHD (
Hz), and the number of horizontal synchronization signals required for one frame is L(
).

Next, regarding the EL, the lighting frequency of the EL is f EL
(Hz).

Since EL lights up twice in one cycle, the relationship between frequencies where interference fringes are not visible is n+0.5 f EL= X f LCD (Hz) ”=
It is expressed as (1). At this time, n is an appropriate integer.

Here, the frame frequency of the LCD is fLCDc, and
Substituting equation 0 into equation (1) yields L. In other words, the horizontal synchronization signal HD is (2n+1)/
A frequency dividing circuit 12 that divides the frequency by 4L may be provided and provided as the trigger signal HD' of the DC-AC inverter 1. In reality, it is rare for (2n+1)/4L to be exactly 1/(integer), and it is difficult to accurately create equation (3) by frequency division. However, as shown in Figure 6, it has been experimentally confirmed that the human eye does not perceive interference fringes within ±to (Hz) of the calculated value, so fEL is within the ideal frequency ±10
(Hz).

In other words, it is easy to create it by dividing the frequency, as it only needs to be 4L.

A second embodiment of the EL lighting circuit 4b according to the present invention in which the original signal to be frequency-divided is used as the clock signal of the LCD drive circuit 10.
As shown in the figure.

First, the frame frequency for LCD is f LCD(H
z), and the clock frequency is f CLK (H2),
Assume that the number of clock signals required for one frame is K (numbers).

Next, let the lighting frequency of EL be f EL (Hz).

Frame frequency f of LCD When LCD is expressed by clock signal frequency f CLK, the following is obtained. Therefore, using equation (1) of Example 1 and equation (4) above, the EL lighting frequency at which no interference fringes occur is expressed as follows.

Therefore, it is sufficient to provide a circuit 22 that divides the frequency of CLK in FIG. In this example, as in Example 1, fEL is the ideal frequency ±10 (Hz).
Designed to fit.

The above description has been made using EL as an example, but the same applies to fluorescent lamps and other discharge lamps.

However, if the lighting frequency is increased, the interference becomes higher-order and the interference fringes become less noticeable, but when using EL, the higher the lighting frequency is, the shorter the life is, so the lighting frequency cannot be increased. Therefore, the present invention is particularly useful when EL is used as a backlight.

Next, a description will be given of another embodiment that is further improved from the one in which the LCD displays six levels.

When viewing a color display screen on a black and white LCD, gradation display is required. The LCD gradation display method uses an LCD whose transmittance changes depending on the applied voltage, and either creates gradation using the voltage, or thins out the signal of the gradation display pixel to reduce the number of times it passes through a certain period of time. This is done by lowering the transmittance. However, since the circuit is easy to construct, the latter method, which reduces the apparent transmittance by thinning out the signals, is often used.

When gradation display is added to the LCD display, the apparent frame frequency changes, so the ratio with the EL lighting frequency changes, causing interference fringes to become noticeable.

Therefore, in this embodiment, a function is added to the frequency dividing circuit that determines the lighting frequency of the backlight to change the frequency division ratio depending on the display state of the LCD. This method of lighting an LCD backlight is characterized by lighting the backlight at a frequency at which interference fringes are not visible even when the frequency is changed.

This will be explained below with reference to the drawings.

FIG. 3 shows a trigger signal output circuit IO which generates an oscillation trigger signal OUT by frequency-dividing the clock signal CLK of the LCD drive circuit at a frequency division ratio determined by the LCD display state signal Dp, which is a characteristic part of this embodiment. An example of this is shown below. FIG. 4 shows an embodiment of the entire EL lighting circuit of the present invention.

Components that are the same as those in the conventional example are given the same reference numerals and explanations will be omitted.

The EL lighting frequency fEL1 at which interference fringes do not occur when there is no gradation display is (5
) is expressed as 4. Here n is an arbitrary integer. However,
In a portion where the LCD includes a gradation display and the apparent frame frequency of the LCD is reduced to 1/2 of the normal frequency, the frequency at which no interference fringes are visible is as follows. Here, m is an arbitrary integer. In other words, in the case of normal display, the frequency is divided by a division ratio that satisfies the formula 0, and when gradation display is included, the frequency is divided by a division ratio that satisfies the formula (5) and the formula 0. As described in the first embodiment, the frequency at which interference fringes are not visible has a range, and is equal to (2n+1)/4 in equation (5) and (2n+1)/4 in equation 0.
There is no need to divide the frequency exactly by 2n+1)/(4KX2) as long as the interference fringes are not visible. Therefore, (2n
+1)/4 and 1/ close to (2n+1)/(4KX2)
(an integer), and it is easy to configure a frequency dividing circuit.

The frequency that normally lights up the EL is 600 to 800 (Hz)
It has been confirmed that Equations (5) and (0) also give a margin of about ±to (Hz). Similarly, when there are many gradations, according to changes in frame frequency, , it is sufficient to set a frequency division ratio that satisfies all of , at each stage.

A clock signal input from the LCD is CLK, and a signal indicating the display state is Dp. CLK outputs output signals qn-1...q of the n-bit counter 101. On the other hand, Dp is input to the decoding circuit 102,
The state is decoded and the n-bit output signal Dn-1...
・Output DO. qn-1 and Dn-1, qn-2 and D
n-2.

......, When each of the n combinations of qO and DO match, all EOR arrays EORn-1-40RO are turned on, the AND gate is turned on, and the D flip-flop 103 is synchronized with CLK. and outputs a counter reset signal RESET. The RESET signal is fed back to the n-bit binary counter 1 to reset the counter and start a new counter operation. That is, the frequency division ratio can be changed by the outputs Dn-1, . . . , DO of the decoding circuit 2.

The outputs qn-1...qO of the n-bit counter are also input to the pulse width setting circuit 104. In the pulse width setting circuit 4, the frequency-divided signal RESET is connected to a DC-A signal (not shown).
The SR latch 105 is reset so that the signal OUT has the optimum pulse width for oscillation for the C inverter circuit.

In addition, in the above example, the input to the decoding circuit is 1 bit, and only two types of timing settings can be made, but the input Dp
is composed of multiple bits, and each corresponding Dn-1.
...By setting DO, it is possible to support several types of gradation display. Furthermore, the decoding circuit 2
OM1 parallel I10, registers, PLD, etc. can be used.

By adding a function that changes the oscillation frequency depending on the display state of the LCD, even if the LCD frame frequency changes, the backlight can be turned on at a frequency where interference fringes are not visible between it and the LCD frame frequency. Moreover, since interference fringes do not occur even if the LCD and backlight are not driven at high frequency, they can be driven at low frequency, so low power consumption can be achieved.

Furthermore, by using a ROM or a shift register in the decoding circuit for timing control, all the circuits shown in FIG. 1 can be constructed with one gate array, making it possible to miniaturize the circuit. Furthermore, even when the present invention is used as a backlight lighting circuit for an LCD with a different frame frequency, it can be handled by rewriting only the decoding circuit, and there is no need to design a new circuit.

The lighting circuit of the present invention has the following features: (1) By determining the lighting frequency through calculation, it is possible to reliably design the frequency to a frequency where interference fringes are not visible, so that the lighting circuit does not require any adjustment and is easy to design.

(2) By using the LCD drive signal, even if the LCD frame frequency changes due to external factors such as temperature changes, the lighting frequency will follow it, so it will not deviate from the frequency where interference fringes are not visible.

There is an advantage.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an EL lighting circuit according to a first embodiment of the present invention, which employs an EL lighting method using a frequency-divided signal of a horizontal synchronizing signal of an LCD drive signal as a trigger. FIG. 2 is a block diagram of an EL lighting circuit according to a second embodiment of the present invention, which employs an EL lighting method using a frequency-divided signal of a clock signal of an LCD drive signal as a trigger. FIG. 3 shows the characteristic portion of FIG. 4, which is another embodiment of the present invention. FIG. 4 shows another embodiment of the present invention, which adopts a method in which the clock signal of the LCD drive signal is divided by a frequency division ratio determined according to the LCD display status signal and outputted as a trigger signal to a separately excited inverter circuit. FIG. 2 is a block diagram of an EL lighting circuit. FIG. 5 is a block diagram of an EL lighting circuit according to a conventional EL lighting method. FIG. 6 is a diagram showing the relationship between the AC voltage applied to the EL and the EL light emission waveform. FIG. 7 is a diagram showing the relationship between the change in transmittance of the LCD over time as the LCD scans, and the EL light emission waveform. FIG. 8 illustrates the appearance of interference fringes when the EL frequency is changed with respect to the LCD frequency. 1...DC-AC inverter, Figure 1, Figure 2, Figure 3...EL. 4.4a, 4b...EL lighting circuit, 10...
... LCD drive circuit, 12.22 ... Frequency division circuit, 110 ... Trigger signal output circuit. Fig. 4 5 Fig. 4 EL, i, outside chart □ Time chart - [1g interval a, b, c-...Rig I-machi Abaya

Claims (2)

[Claims] (1) When using a light source that emits light in a flashing manner for the backlight of a liquid crystal display, the oscillation trigger signal of a separately excited inverter circuit that lights up the light source is set to a horizontal synchronization signal or a clock signal that is a drive signal for the liquid crystal display. A method for lighting the backlight of a liquid crystal display, which is characterized by creating a backlight by dividing the frequency. (2) The back of the liquid crystal display according to claim 1, wherein the liquid crystal display is capable of displaying multiple gradations, and the oscillation trigger signal is changed to a frequency according to the gradation state of the display. How to turn on the lights.