JP2001282189A - Liquid crystal display - Google Patents

Abstract

(57) [Summary] [Problem] Even when the voltage range of the input voltage is wide,
Provided is a liquid crystal display device capable of stably operating a power supply circuit. A liquid crystal display device comprising a liquid crystal display element and a power supply circuit, wherein the power supply circuit (25) is a transistor (1) connected between an input terminal supplied with an input voltage and an output terminal. ) And a reactor (3), an oscillation circuit (10), and ON / O of the transistor.
Control means (9) for controlling the FF and varying the ON period of the transistor within one cycle of the oscillation frequency of the oscillation circuit; and a resistance element (1) for determining the oscillation frequency.
2, 20) and the capacitor element (13, 23) are selectively selected according to the voltage value of the input voltage, and the oscillation frequency of the oscillation circuit is varied stepwise according to the voltage value of the input voltage. Oscillation frequency varying means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a liquid crystal display device, a liquid crystal display for mounting a DC / DC converter PWM (P ulse W idth M odulation ) method of converting a DC input voltage into an arbitrary DC voltage It relates to technology that is effective when applied to equipment.

[0002]

BACKGROUND ART STN (S uper T wisted N ematic ) method or a liquid crystal display module of the TFT (T hin F ilm T ransister ), is widely used as a display device such as a notebook personal computer. These LCD modules are
A liquid crystal display panel around which a semiconductor chip (a semiconductor integrated circuit device forming a drain driver or a gate driver) is arranged, a backlight unit for irradiating the liquid crystal display panel, and a power supply for supplying a drive voltage to the semiconductor chip, for example. Circuit (DC / DC converter). Such a technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 11-3
No. 57035.

[0003]

Generally, a DC / DC converter generates an arbitrary DC voltage from an arbitrary DC input voltage. For example, a DC / DC converter having a circuit configuration of a step-down type, a step-up type, an inverting type, or the like. It has been known. On the other hand, IEEE
(I nstitute of E lectrical and E lectoronics E nginee
rs) In the DC / DC converter mounted on the liquid crystal display device having the 1394 interface, an IEEE standard document P1394 Standard for a high Performance Serial Bus
(Draft 8.0 V3, October 16, 1995 Page 84), the input voltage is specified to be 8V to 40V, so that DC / D
A wide range of input DC voltage (Vin) input to the C converter is required.

However, in the conventional DC / DC converter, when the input DC voltage (Vin) becomes 5 V or less,
Since the power efficiency of the entire DC / DC converter is reduced and a constant current always flows through each element of the DC / DC converter, the current load on the circuit element increases, and a discrete element having a large current capacity is required at the design stage. And the cost of parts increases. Thereby, the conventional DC /
A liquid crystal display device equipped with a DC converter has a problem that not only power consumption increases but also costs increase. Conversely, the input DC voltage (Vin) is 25V
In this case, the margin of the phase margin indicating the stability of the feedback loop centered on the control IC decreases, and the control system becomes extremely unstable. The instability of this control system is
In some cases, from negative feedback control during normal operation,
Positive feedback control was performed, and the DC / DC converter sometimes caused abnormal oscillation.

As a result, in the conventional liquid crystal display device equipped with a DC / DC converter, not only an abnormal oscillation sound of the DC / DC converter causes an abnormal sound but also an abnormal sound.
Furthermore, the current load of the diode mounted on the DC / DC converter increases, and as a result, the diode generates heat. The present invention has been made in order to solve the problems of the conventional technology, and an object of the present invention is to provide a liquid crystal display device that stably operates a power supply circuit even when a voltage range of an input voltage is wide. It is an object of the present invention to provide a technology capable of operating. The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0006]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows. That is, the present invention provides a liquid crystal display element,
A liquid crystal display device comprising a power supply circuit, wherein the power supply circuit is a series circuit of a transistor and a reactor connected between an input terminal to which an input voltage is supplied and an output terminal, an oscillation circuit, and the transistor Control means for controlling the ON / OFF of the oscillating circuit, and varying the ON period of the transistor within one cycle of the oscillating frequency of the oscillating circuit; Oscillation frequency varying means for dynamically varying the oscillation frequency. In a preferred embodiment of the present invention, the oscillation circuit, as an oscillation frequency determining element,
A resistor element and a capacitor element, wherein the oscillation frequency varying means varies at least one of a resistance value of the oscillation frequency determination resistor element and a capacitance value of the capacitor element according to a voltage value of the input voltage. The oscillation frequency of the oscillation circuit is varied stepwise.

In a preferred embodiment of the present invention, the oscillation variable means sets the oscillation frequency of the oscillation circuit to a first frequency when the input voltage is lower than a first voltage value. The input voltage is higher than the first voltage;
When the voltage value is lower than the second voltage value, the oscillation frequency of the oscillation circuit is set to a second frequency higher than the first frequency, and the input voltage is higher than the second voltage value. In the case of a voltage value, the oscillation frequency of the oscillation circuit is set to a third frequency higher than the second frequency.

In a more preferred embodiment of the present invention, the oscillating circuit includes a first resistive element as an oscillating frequency determining element and a first resistive element in parallel with the first resistive element via a first switching element. A second resistance element connected to
A first capacitor element and a second capacitor connected in parallel to the first capacitor element via a second switching element.
Wherein the oscillation frequency variable means turns off the first switching element and turns on the second switching element when the input voltage is a voltage value lower than the first voltage value. Setting the oscillation frequency of the oscillation circuit to the first frequency, and when the input voltage is higher than the first voltage and lower than the second voltage value, the first and second frequencies are set. Switching element of OF
As F, the oscillation frequency of the oscillation circuit is set to the second frequency, and when the input voltage is a voltage value higher than the second voltage value, the first switching element is turned off.
N, wherein the second switching element is turned off, and the oscillation frequency of the oscillation circuit is set to the third frequency.

Conventional power supply circuit (DC / DC of PWM system)
Converter), the ON duty ratio of the transistor connected in series to the reactor is determined only by the voltage level of the input voltage Vin. Therefore, when the input voltage Vin exceeds a certain range, the above-described problem occurs. I was However, according to the above means, the ON duty ratio of the transistor connected in series to the reactor is
Since the voltage is made larger or smaller depending on the input voltage Vin, the above-described problem can be solved, and the power supply circuit can operate stably even when the voltage range of the input voltage Vin is wide. it can.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. In all the drawings for describing the embodiments, components having the same functions are denoted by the same reference numerals, and repeated description thereof will be omitted. <Basic Configuration of Liquid Crystal Display Module of Embodiment of the Present Invention> FIG. 1 is a block diagram showing a basic configuration of a liquid crystal display module of a TFT type according to an embodiment of the present invention. As shown in the figure, the liquid crystal display module of the present embodiment
It comprises a liquid crystal display panel 100, a display control device 110, a power supply circuit 120, a drain driver unit 130, and a gate driver unit 140. Liquid crystal display panel 10
0 is a TF on which a pixel electrode, a thin film transistor and the like are formed
A T substrate and a filter substrate on which a counter electrode, a color filter and the like are formed are overlapped with a predetermined gap therebetween, and both substrates are bonded together by a frame-shaped sealing material provided near a peripheral portion between the two substrates. At the same time, the liquid crystal is sealed and sealed inside the sealing material between the two substrates from the liquid crystal sealing opening provided in a part of the sealing material, and further, a polarizing plate is attached to the outside of both the substrates. Further, the drain driver section 130 and the gate driver section 140 are respectively composed of a plurality of drain drivers (not shown) and a plurality of gate drivers (not shown).

<Outline of Operation of Liquid Crystal Display Module of Present Embodiment> The display control device 110 is composed of one semiconductor integrated circuit (LSI), and receives a clock signal, a display timing signal, and a horizontal signal transmitted from a computer main body. The drain driver and the gate driver of the drain driver unit 130 and the gate driver unit 140 are controlled based on the display control signals of the synchronization signal and the vertical synchronization signal and the display data (RGB). Drive. The gate driver sequentially applies High to each gate signal line (G) of the liquid crystal display panel 100 every horizontal scanning time based on the frame start instruction signal (FLM) and the shift clock (CL3) sent from the display control device 110.
Supply level select scan voltage. Thereby, the plurality of thin film transistors (TFTs) connected to each gate signal line (G) of the liquid crystal display panel 100 conduct for one horizontal scanning time. The drain driver is connected to the display control device 110.
The display data transmitted from the display control device 110 is sequentially latched based on a start pulse (display data capture start signal) transmitted from the display control device 110 and a display data latch clock (CL2). Further, the drain driver supplies a gray scale voltage corresponding to the latched display data to each drain signal line (D) based on the output timing control clock (CL1) sent from the display control device 110. By the above operation, an image is displayed on the liquid crystal display panel 100.

A power supply circuit 120 shown in FIG. 1 supplies a positive gray scale reference voltage and a negative gray scale reference voltage to each drain driver, and supplies a gate electrode of a thin film transistor (TFT) to a gate driver. Is supplied with a scanning drive voltage to be applied to the scanning. The power supply circuit 120 shown in FIG.
It is composed of an M type DC / DC converter. Hereinafter, before describing the circuit configuration of the power supply circuit 120 of the present embodiment, the circuit configuration of the power supply circuit 120 of the conventional liquid crystal display module will be described.

<Power circuit 1 of conventional liquid crystal display module>
FIG. 4 is a circuit diagram showing a circuit configuration of a DC / DC converter constituting a power supply circuit 120 of a conventional liquid crystal display module. FIG. 4 shows, as a representative example, a circuit configuration of a step-down D / DC converter based on the PWM method. The DC / DC converter 41 shown in FIG. 4 is a PNP bipolar transistor (hereinafter simply referred to as “PNP bipolar transistor”).
It is called a PNP transistor. ) 27, a reactor 29 connected in series with the PNP transistor 27, and a PN
A diode 28 connected between the collector electrode of the P transistor 27 and the ground potential, a smoothing capacitor 30 connected between the output terminal of the reactor 29 and the ground potential,
A control IC (hereinafter, referred to as IC) 39 for controlling ON / OFF of the PNP transistor 27.

The IC 39 includes an oscillator (OSC) 32 that oscillates with a time constant (τ; τ = C × R) of a resistor 37 (R) and a capacitor 38 (C), and an O (O) of the PNP transistor 27.
An output control circuit 34 for fixing the N / OFF switching frequency, changing the duty ratio and controlling the output voltage is built in. The PNP transistor 27 has a base electrode connected to an output control circuit 34 via a resistor 33,
A DC bias resistor 31 is connected between the emitter electrode and the base electrode. An output voltage divided by the feedback resistor 35 and the resistor 36 is input to the output control circuit 34, and the DC / DC converter 4
1 is determined.

FIG. 5 shows a collector voltage waveform (vertical axis: voltage, horizontal axis: time) of the PNP transistor 27 and current waveforms (A), (B) flowing through the reactor 29 in the DC / DC converter 41 shown in FIG. (Vertical axis: current, horizontal axis: time). Hereinafter, the operation of the DC / DC converter 41 shown in FIG. 4 will be described. DC / DC converter 4
Reference numeral 1 denotes a circuit that generates and outputs an arbitrary voltage value (hereinafter, referred to as Vo) lower than the input voltage Vin supplied to the load 42 from the voltage value of the input DC voltage 40 (hereinafter, referred to as Vin). . The output control circuit 34 in the IC 39 determines the magnitude relationship between the voltage level of the triangular wave output from the oscillator (OSC) 32 and the voltage level of the smoothing capacitor 30 divided by the feedback resistor 35 and the resistor 36. A PWM pulse having a constant period (T) and a different duty ratio (ratio of the high-level period (T1) to the period (T); T1 / T) is output. The PNP transistor 27 to which the PWM pulse is applied to the base electrode
ON / OFF is repeated by M pulses.

During the period when the PNP transistor 27 is turned on (Ton period in FIG. 5), the input DC voltage 41 → PNP
A current flows through a path from the transistor 27 to the reactor 29. At this time, if the flowing current value changes, the reactor 2
9, a back electromotive force (hereinafter referred to as VL) is generated. D
The output voltage Vo of the C / DC converter 41 is (Vin−
VL), and a similar voltage is applied to the smoothing capacitor 30 to accumulate electric charges. The current flowing through the reactor 29 may be one of the current waveforms (A) and (B) shown in FIG. 5 depending on the oscillation frequency of the oscillator 32 and the constant value of the reactor 29. In general, the former current waveform (current waveform (A ))
When the reactor 29 is controlled by the discontinuous current mode,
The case where the reactor 29 is controlled with the latter current waveform (current waveform (B)) is called a continuous current mode.

When the average current flowing through the load 42 of the DC / DC converter 41 is as low as about 0.4 [A] or less, the operation is performed in the discontinuous current mode.
Operate in continuous mode. In recent devices, power saving has been achieved, and the average current flowing to the load has also been reduced. Therefore, the devices are often operated in a discontinuous current mode. In the discontinuous current mode, the collector current of the PNP transistor 27 rises from 0 [A], whereas in the continuous current mode, a constant DC current always flows through the reactor 29. Therefore, in the discontinuous current mode, P
Power loss (= (Vc) × (I) during the time when the collector voltage of the NP transistor 27 rises (Tr in FIG. 3)
c)) is characterized in that it is smaller than in the continuous current mode.

During a period in which the PNP transistor 27 is turned OFF (Toff period in FIG. 5), an electromotive force is generated in the reactor 29 in a direction opposite to the ON period, and the reactor 29 → the smoothing capacitor 30 → the diode 28 The energy stored in the reactor 29 is released, and the smoothing capacitor 30 is charged. Therefore, the output voltage Vo of the DC / DC converter 41 increases from (Vin-VL). On the other hand, since the current flows through the load 42 connected to the DC / DC converter 41, the smoothing capacitor 30
Is discharged, and the output voltage Vo decreases.
The voltage drop of the output voltage Vo is detected by the IC 39 by the feedback resistor 35 and the resistor 36, and the smoothing capacitor 3 is turned on by turning on the PNP transistor 27 again.
Store the charge at zero. Thereafter, the above operation is repeated. Therefore, by keeping the control frequency of the IC 39 constant and controlling the duty ratio of the PNP transistor 27 during the ON period, a desired output voltage Vo can be obtained.

Here, the duty ratio (D1) of the ON period (Ton [s]) of the PNP transistor 27 with respect to the input voltage Vin when operated in the discontinuous current mode.
[%]; Hereinafter, referred to as ON duty ratio. ) Is obtained. However, the collector of the PNP transistor 27
The emitter-to-emitter saturation voltage and the forward voltage of the diode 28 are omitted because their voltage values are small. Generally, the relationship among the reactor 29 (L [H]), load current (Io [A]), input voltage (Vin [V]), and output voltage (Vo [V]) is as follows.
It is represented by the following equation (1). Further, the ON duty ratio (D1) and the oscillation frequency of the oscillator 32 (fosc [H
z]) and the ON period (Ton), the following (2)
In a relationship. Therefore, when the following equation (1) is substituted into the following equation (2) and expanded, the following equation (3) is obtained.

[0020]

L = (Vin−Vo) × (Ton) / (2 × (Io)) (1) D1 = (Ton) × (fosc) × 100 (2) D1 = A1 / B1 (3) where A1 = (2 × (Io) × (L) × (fosc) B1 = (Vin−Vo)) × 100 Io = 0.3 [A], L = 10 [μH], fosc = 3
Assuming that 50 [kHz] and Vo = 3.3 [V], the input voltage Vin is varied from 4.5 [V] to 40 [V], and D
Table 1 shows the result of calculating the ON duty ratio (D1) using the equation (3) when the C / DC converter 41 is operated in the discontinuous mode.

[0021]

[Table 1] As can be seen from Table 1, when the input voltage Vin becomes 5 [V] or less, the calculated ON duty ratio (D1) becomes 10
Exceeds 0 [%]. In this case, the current waveform of the reactor 29 shifts from (A) to (B) in FIG.
Converter 41 operates from the discontinuous current mode to the continuous current mode. In the continuous current mode, as described above, since the power loss during the time when the collector voltage of the PNP transistor 27 rises (Tr in FIG. 3) increases, the DC / DC decreases as the input voltage Vin decreases.
The power supply efficiency of the entire converter 41 decreases. At the same time, since a constant current always flows through the PNP transistor 27 and the reactor 29, the current load on the element increases, and a discrete element having a large current capacity is required at the design stage, and the component cost increases. This not only increases the power consumption of the conventional liquid crystal display module, but also
There was a problem that cost increased.

Conversely, when the input voltage Vin becomes 25 V or more, the ON duty ratio (D
1) is 10% or less. D commonly used
In the control IC 39 of the C / DC converter, when the pulse width of the control pulse for turning on the PNP transistor 27 becomes extremely small, the PNP transistor 27 becomes saturated and the ON / OFF switching operation of the PNP transistor 27 cannot be performed. (PNP transistor 27, reactor 29, resistor 35 and resistor 36 for determining output voltage)
, The margin of the phase margin indicating the stability of the loop (e.g., loop) decreases, and the control system becomes extremely unstable. The instability of the control system varies depending on the variation of the elements constituting the circuit. In some cases, the negative feedback control during normal operation is changed to the positive feedback control, and the DC / DC converter 41 causes abnormal oscillation. Thereby, in the conventional liquid crystal display module, the abnormal oscillation of the DC / DC converter 41 causes
Not only an abnormal dial tone is generated but also the ON period of the PNP transistor 27 becomes extremely short,
The current load on the diode 28 increases, and as a result, a problem occurs in that the diode 28 generates heat.

<Circuit Configuration of Power Supply Circuit 120 of Liquid Crystal Display Module of Present Embodiment> FIG. 1 is a circuit diagram showing a circuit configuration of a DC / DC converter circuit forming a power supply circuit 120 of an embodiment of the present invention. is there. PNP transistor 1, reactor 3, diode 2, smoothing capacitor 6, control IC 11, oscillator (OS
C) 10, the output control circuit 9, and the feedback resistors (7, 8) 6 are of the conventional step-down type shown in FIG.
Since the operation and the circuit configuration are the same as those of the C / DC converter 41, the description is omitted. In the present embodiment, the resistors 14 to 20, the resistor 43, the resistor 44, the bipolar NPN transistor (hereinafter, referred to as NPN transistor) 21,
A circuit including the NPN transistor 22, the NPN transistor 45, and the capacitor 23 is newly added. This circuit is oscillation frequency varying means for varying the oscillation frequency of the oscillator 10 when the input voltage Vin reaches a certain level, and its operation will be described below.

The oscillation frequency (fosc) of the oscillator 10 in the control IC 11 is determined by the time constant of the resistor 12 and the capacitor 13 connected to the oscillator 10, and the oscillation frequency (of the oscillator 10) of the general DC / DC converter 11 fo
sc) is obtained by the following equation (4).

Fosc = α / (C × R) (4) where α is an arbitrary positive value, and C is the capacitance of the capacitor. Value, R represents the resistance value of the resistor, and C represents
3. If R is the resistor 12, a certain oscillation frequency (hereinafter referred to as fo) is determined based on the capacitance value of the capacitor 13 and the resistance value of the resistor 12. Therefore, the oscillation frequency (fosc) can be changed to an arbitrary value by increasing or decreasing the resistance value of the resistor 12 or the capacitance value of the capacitor 13.

In this embodiment, when the frequency is increased from a certain oscillation frequency fo, the resistance value of the resistor 12 is decreased, and when the frequency is decreased from a certain oscillation frequency fo,
The capacitance value of the capacitor 13 is increased. Therefore, in the present embodiment, the NPN
One of the transistor 21 and the NPN transistor 22 is turned on, and the other is controlled to be turned off.
The resistor 20 or the capacitor 23 connected to the collector electrode of the N-transistor 22 is connected to a parallel circuit of the resistor 12 and the capacitor 13.

FIG. 3 is a diagram showing the relationship between the oscillation frequency (fosc) of the oscillator 10 of the DC / DC converter 25 of the present embodiment and the input voltage Vin. A general bipolar transistor is turned on when the forward voltage between the base and the emitter reaches about 0.7 [V]. Therefore, in the present embodiment, when the voltage of the input voltage Vin exceeds a certain voltage (Va in FIG. 3), the divided voltage divided by the voltage dividing circuit including the resistors 14 and 15 becomes 0. .7 [V] or more, and the NPN transistor 2
1 is changed from the OFF state to the ON state. As a result, the resistance 20 changes from the open state to the ground state with respect to the ground potential, and the resistance for determining the oscillation frequency is the resistance 12 and the resistance 20 connected in parallel.
The resistance value R in the equation becomes apparently smaller, and the oscillation frequency becomes an oscillation frequency foa higher than a certain oscillation frequency fo.
However, when the NPN transistor 21 is ON, the NPN transistor 22 is in the OFF state.

Conversely, if the input voltage Vin drops below a certain voltage (Vb in FIG. 3), the resistance 16 and the resistance 1
7. The divided voltage divided by the voltage dividing circuit consisting of.
7 [V] so that the NPN transistor 4
5 is turned off. As a result, the input voltage Vin is applied to the base electrode of the NPN transistor 22, so that the NPN transistor 22 is turned on and the NPN transistor 22 is turned on.
The capacitor 23 connected to the collector of the transistor 22 is grounded to the ground potential. Therefore, since the capacitors that determine the oscillation frequency are the capacitor 23 and the capacitor 13 connected in parallel, the capacitance value C
Becomes apparently larger, and the oscillation frequency becomes an oscillation frequency fob lower than a certain oscillation frequency fo. When the input voltage Vin exceeds a certain voltage (Vb in FIG. 3), the divided voltage divided by the voltage dividing circuit including the resistor 16 and the resistor 17 becomes 0.7 [V] or more. So that
The NPN transistor 45 is changed from the OFF state to the ON state. As a result, the ground potential (0 [V]) is applied to the base electrode of the NPN transistor 22.
The PN transistor 22 is turned off, and the capacitor 23 connected to the collector of the NPN transistor 22 is turned off.
Are open with respect to the ground potential.

In the present embodiment, in the above operation, N
The PN transistor 21 and the NPN transistor 22 have resistors (14 to 14) so that they are turned on at different input voltages Vin.
It is necessary to set the resistance value of 17). For example, when the input voltage Vin is about 25 V or more, the NPN transistor 21 is turned off.
In the case of N, the resistance value of the resistor 14 is set to 34 [KΩ],
The resistance value of the resistor 15 is set to 1.0 [KΩ], and the input voltage Vin is set.
When the NPN transistor 22 is turned on at about 5 [V] or less, the resistance of the resistor 17 is changed to 3.6 [KΩ]
6 The resistance value may be set to 600 [Ω].

In this embodiment, the input voltage Vin =
Fosc = 500 [KHz] at 25 V or more, input voltage V
fosc = 100 [KHz] when in = 5 [V] or less, and fosc = 350 [KH] at other input voltages Vin.
z], the ON duty ratio (D2
Table 2 shows the results of calculating [%]) using the above equation (3). However, other parameters (L, Vo, Io)
Is the same value as in the calculation in Table 1 described above.

[0030]

[Table 2] As can be seen from Table 2, in the present embodiment, the input voltage V
Even when in is 5 V or less, the ON duty ratio becomes 50 [%] or less, so that the DC / DC converter 25 operates stably in the discontinuous current mode and can keep the power supply efficiency constant. Thereby, in the liquid crystal display module of the present embodiment, unlike the conventional liquid crystal display module, the power consumption does not increase and the cost does not increase. Even when the input voltage Vin is 25 V or more, the ON duty ratio is 10% or more, so that the control IC 11 can stably perform the duty control without abnormal oscillation of the control IC 11. As a result, in the liquid crystal display module of the present embodiment, unlike the conventional liquid crystal display module, an abnormal oscillation sound of the DC / DC converter does not generate an abnormal dial tone, and the liquid crystal display module is mounted on the DC / DC converter. The current load on the diode increases, and as a result, the diode does not generate heat.

In the above embodiment, the step-down type DC
Although the embodiment in which the present invention is applied to the / DC converter has been described, the present invention is not limited to this, and it is needless to say that the present invention can be applied to a DC / DC converter such as a boost type or an inverting type. Nor. Further, in the above-described embodiment, an embodiment in which the present invention is applied to a TFT type liquid crystal display module has been described. However, the present invention is not limited to this, and is also applicable to an STN type liquid crystal display module. It goes without saying that it is possible. Further, the DC / DC converter according to the present invention is adapted to the IEEE13
It is needless to say that the present invention can be applied to a small CCD camera device having a 94 standard interface, a video device, and the like. As described above, the invention made by the inventor has been specifically described based on the embodiment.
It is needless to say that the present invention is not limited to the above-described embodiment, but can be variously changed without departing from the gist thereof.

[0032]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows. According to the liquid crystal display device of the present invention, it is possible to operate the power supply circuit stably even when the voltage range of the input DC voltage input from the outside is wide.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a basic configuration of a TFT type liquid crystal display module according to an embodiment of the present invention.

FIG. 2 shows a DC constituting a power supply circuit according to an embodiment of the present invention.
FIG. 3 is a circuit diagram illustrating a circuit configuration of a / DC converter circuit.

FIG. 3 is a diagram illustrating a relationship between an oscillation frequency (fosc) of an oscillator of a DC / DC converter according to an embodiment of the present invention and an input voltage Vin.

FIG. 4 is a circuit diagram showing a circuit configuration of a DC / DC converter forming a power supply circuit of a conventional liquid crystal display module.

5 is a diagram showing a collector voltage waveform of a bipolar PNP transistor and a current waveform flowing through a reactor in the DC / DC converter shown in FIG.

[Explanation of symbols]

1,27 ... bipolar PNP transistor, 2,28 ...
Diode, 3,29 ... reactor, 4,5,7,8,
12, 14, 15, 16, 17, 18, 19, 20, 3
1, 33, 35, 36, 37, 43, 44 ... resistance, 6,
30: smoothing capacitor, 9, 34: output control circuit, 1
0, 32: oscillator (OSC), 11, 39: control I
C, 13, 23, 38 ... capacitors, 21, 22, 45
... Bipolar NPN transistor, 24,40 ... Input DC voltage, 25,41 ... Step-down DC / DC converter, 2
6, 42: load, 100: liquid crystal display panel, 110: display control device, 120: power supply circuit, 130: drain driver section, 140: gate driver section 140.

Continuation of the front page (72) Inventor Tomohide Ohira 3300 Hayano Mobara-shi, Chiba F-term (reference) 5C006 AF84 BB16 BF31 BF36 BF37 BF42 BF46 FA47 5C080 AA10 BB05 DD09 DD26 FF03 JJ02 JJ03 5H730A14 AS01 BB13 BB57 DD02 FD01 FD11 FG05 FG07 FG25

Claims (5)

[Claims] 1. A liquid crystal display device comprising a liquid crystal display element and a power supply circuit, wherein the power supply circuit includes a transistor and a reactor connected between an input terminal supplied with an input voltage and an output terminal. A series circuit, an oscillation circuit, and ON / OFF control of the transistor;
Control means for varying the ON period of the transistor within one cycle of the oscillation frequency of the oscillation circuit; and oscillation frequency varying means for varying the oscillation frequency of the oscillation circuit stepwise according to the voltage value of the input voltage. A liquid crystal display device comprising: 2. The oscillating circuit has a resistor element and a capacitor element as oscillating frequency determining elements, and the oscillating frequency varying means controls the oscillating frequency determining element according to a voltage value of the input voltage. By changing at least one of the resistance value of the resistance element and the capacitance value of the capacitor element,
2. The liquid crystal display device according to claim 1, wherein the oscillation frequency of the oscillation circuit is varied stepwise. 3. The oscillating variable means sets an oscillating frequency of the oscillating circuit to a first frequency when the input voltage is lower than a first voltage value, wherein the input voltage is equal to the first voltage. When the input voltage is higher than the first voltage, the oscillation frequency of the oscillation circuit is set to a second frequency higher than the first frequency. 2. The liquid crystal display device according to claim 1, wherein when the voltage value is higher than the voltage value, the oscillation frequency of the oscillation circuit is set to a third frequency higher than the second frequency. 4. The oscillation circuit according to claim 1, further comprising: a first resistance element as an oscillation frequency determination element; and a second resistance element connected in parallel to the first resistance element via a first switching element. A first capacitor element; and a second capacitor element connected in parallel to the first capacitor element via a second switching element. When the voltage value is lower than the first voltage value, the first switching element is turned off and the second switching element is turned on, and the oscillation frequency of the oscillation circuit is set to the first frequency; Is higher than the first voltage and lower than the second voltage value, the first and second switching elements are turned off, and the oscillation frequency of the oscillation circuit is set to the second frequency. When the input voltage is a voltage value higher than the second voltage value, the first switching element is turned on and the second switching element is turned off, and the oscillation frequency of the oscillation circuit is set to the third value. 4. The liquid crystal display device according to claim 3, wherein the frequency is set to: 5. The oscillation frequency varying means has a first resistor voltage divider circuit and a second resistor voltage divider circuit, and when the input voltage is a voltage value lower than the first voltage value,
The second switching element is turned on based on a divided output voltage of the second resistor voltage dividing circuit. When the input voltage is a voltage value higher than the second voltage value, the first resistor is turned on. 5. The liquid crystal display device according to claim 4, wherein the first switching element is turned on based on a divided output voltage of a voltage dividing circuit.