JP3077488B2 - LCD driver output circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output circuit used for driving a liquid crystal by applying an AC voltage to a liquid crystal panel.

[0002]

2. Description of the Related Art When driving a liquid crystal panel, a voltage is applied to electrodes provided on both front and rear surfaces of the liquid crystal panel. When a voltage is applied, the liquid crystal is twisted or rotated, and the brightness of the panel display can be switched. However, if the twist or rotation of the liquid crystal is always in only one direction, the liquid crystal itself loses a restoring force against the twist or rotation and causes deterioration of the liquid crystal. Therefore, normally, an AC voltage is applied to the liquid crystal panel so as not to twist or rotate.

Hereinafter, an output circuit used as a conventional AC voltage applying means will be described. FIG. 8 shows a configuration of an output circuit of a conventional liquid crystal driving device. In the figure, 1
01 is a liquid crystal capacitance corresponding to a TFT liquid crystal panel, and 102 is a counter electrode. The potential of the counter electrode 102 is (VDD−V−D), where VDD is the power supply potential and VSS is the ground potential.
SS) / 2. Reference numeral 103 denotes an operational amplifier. The operational amplifier 103 includes a constant current circuit 104, an N-channel MOS transistor 105,
It comprises a constant current circuit 106, P-channel MOS transistors 107 and 108, and N-channel MOS transistors 109 and 110 forming a current mirror. The constant current circuit 104 is composed of a transistor having a relatively large driving ability, and a constant current of several mA is supplied to the point C by applying a constant voltage to the gate electrode of the transistor at all times. . A capacitor 111 is connected between the output terminal C and the inverting input terminal A of the operational amplifier 103 and functions as a negative feedback circuit.
The switch 112 is connected in parallel with the
3 is short-circuited between the output terminal C and the inverting input terminal A. Also,
Reference numerals 113 to 118 denote a plurality of capacitors each having one end connected to the inverting input terminal A of the operational amplifier 103.

The operation of the conventional output circuit configured as described above, particularly the operation of controlling the AC voltage applied to the liquid crystal panel, will be described.

In order to control the AC voltage applied to the liquid crystal panel 101, an N-channel MOS transistor 1
05 is controlled. Capacity 113-118
When the potential of the point A, which is the inverting input terminal, is reduced by reducing the charge amount of the
The function of 110 lowers the potential at point B. That is, the gate potential of the N-channel MOS transistor 105 decreases. When the gate potential of the N-channel MOS transistor 105 becomes sufficiently low, the N-channel MOS transistor 105 becomes non-conductive.
04, the power supply potential VDD is applied to the liquid crystal panel,
The potential can be controlled to be higher than that of the counter electrode 102.

When the potential of the liquid crystal panel 101 is lowered or reversed, the charge amount of the capacitors 113 to 118 is increased to increase the potential at the point A and further increase the potential at the point B. When the potential at the point B rises and the gate potential of the N-channel MOS transistor 105 gradually rises, the N-channel MOS transistor 105 starts conducting, so that the potential at the point C gradually approaches the ground potential VSS by discharging,
The potential applied to the liquid crystal panel gradually decreases. On the other hand, since the potential of the counter electrode is fixed at (VDD-VSS) / 2, when the potential of the liquid crystal panel decreases toward VSS, there is no potential difference between the liquid crystal panel 101 and the counter electrode 102 on the way. When the discharge is further continued, the potential becomes lower than the potential of the counter electrode, and a potential difference is generated in the opposite direction. That is, the potential of the liquid crystal panel with respect to the counter electrode is inverted. As described above, the direction of the voltage applied to the liquid crystal panel is reversed by charging and discharging, thereby realizing an alternating current.

[0007]

However, in such a configuration, when the direction of the voltage applied to the liquid crystal panel is switched, that is, when the N-channel MOS transistor 1 is switched.
The N-channel MOS transistor 105 is turned on when the gate potential of the transistor 05 is raised or when the voltage of the liquid crystal panel is in a stable state. Both of the transistors 105 are simultaneously turned on, and a through current is generated. Further, in the conventional configuration, since a transistor having a large driving capability is used as the constant current circuit 104 in order to speed up the voltage rise of the liquid crystal panel, there is a problem that the through current is increased and the power consumption is increased. Was.

An object of the output circuit of the present invention is to minimize the generation of through current and reduce power consumption.

[0009]

The output circuit of the liquid crystal driving device according to the present invention controls the precharge control means for charging the liquid crystal panel independently of the discharge control means and drives the precharge control means when the applied voltage rises. The output potential to the liquid crystal panel is controlled by discharge control means for discharging the liquid crystal panel.

[0010]

According to the present invention, since the precharge control means is provided separately from the discharge means as the discharge means, a large through current is not generated, and a desired potential can be obtained at a desired timing by the structure of the precharge control means. The potential of the liquid crystal panel can be raised.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration of an output circuit of a liquid crystal display device according to an embodiment of the present invention. In FIG. 1, 1 is T
A liquid crystal capacitor 2 corresponding to an FT (thin film transistor) liquid crystal panel is a counter electrode, and the potential of the counter electrode 2 is (VDD, where VDD is the power supply potential and VSS is the ground potential.
−VSS) / 2. Reference numeral 3 denotes an operational amplifier. The operational amplifier 3 includes a constant current circuit 4, an N-channel MOS transistor 5, a constant current circuit 6, P-channel MOS transistors 7, 8, and a current mirror N. It comprises channel MOS transistors 9 and 10. The constant current circuit 4 is composed of a transistor having a relatively small driving ability, and the gate electrode potential of this transistor is fixed, and is always several μA.
Is supplied to the point C. Also N channel MO
The S-type transistor 5 is a transistor having a relatively large driving capability and can control a current of several mA. 11 is connected between the output terminal C and the inverting input terminal A of the operational amplifier 3;
This is a capacitor functioning as a negative feedback circuit, and a switch 12 is connected in parallel with the capacitor 11 to short-circuit the output terminal C and the inverting input terminal A of the operational amplifier 3. Also, 13-
Reference numeral 18 denotes a plurality of capacitors each having one end connected to the inverting input terminal A of the operational amplifier 3. Reference numeral 19 denotes a precharge control unit which applies a voltage to the liquid crystal panel 1 by external control.

The operation of the output circuit of the present invention configured as described above will be described. The operation of the output circuit includes a charging operation (precharge) and a discharging operation (discharge) for the liquid crystal panel. First, in the charging operation, a voltage is applied to the liquid crystal panel 1 by the precharge control means 19, and the liquid crystal panel 1 is raised to a desired potential (for example, VDD). At this time, the potential of the liquid crystal panel 1 (VD
D) is higher than the potential of the counter electrode 2 ((VDD−VSS) / 2). When displaying on the liquid crystal panel 1,
The brightness of the display is switched depending on whether a difference is made between the potential of the counter electrode 2 and the potential of the liquid crystal panel 1. That is, the potential precharged to the liquid crystal panel 1 is maintained as it is,
The display becomes bright (or dark) if the potential difference between the counter electrode 2 and the counter electrode 2 is substantially eliminated by discharging the precharged potential of the liquid crystal panel 1 by a fixed amount. (Or light).

In order to realize the AC conversion, the liquid crystal panel 1
Is further discharged. The potential of the liquid crystal panel 1 is further reduced by discharging (for example, VS
S), and a potential difference (a potential difference in the opposite direction) is again generated between the liquid crystal panel 1 and the counter electrode 2 to make the display light (or dark) again. As described above, the display is switched based on whether or not a potential difference is applied to the liquid crystal panel 1 with reference to the potential of the counter electrode 2, and the direction of the applied potential difference is reversed to realize AC.

The control of the discharging operation is performed by controlling the N-channel MOS transistor 5.

The potential state at point A is determined based on the charged amount of capacitors 13-18. When the charge amount of the capacitors 13 to 18 is sufficiently small and the potential at the point A is low, the P channel M
The OS type transistor 7 becomes conductive, and the constant current circuit 6
, A current from the power supply flows into the P-channel MOS transistor 7 side. At this time, almost no current flows into the P-channel MOS transistor 8, so that the potential at the point B (that is, the gate potential of the N-channel MOS transistor) is low, and the N-channel MOS transistor 5 is turned off. . When the N-channel MOS transistor 5 becomes non-conductive, the point C becomes high potential due to the current flowing from the constant current circuit 4. Therefore, in this case, the liquid crystal panel 1 and the counter electrode 2 continue to have a potential difference without being discharged.

If the capacity of the capacitors 13 to 18 is changed and increased, the potential at the point A increases, and the conduction of the P-channel MOS transistor 7 is limited. P channel MOS
When the conduction of the type transistor 7 is limited, the amount of current flowing into the P-channel MOS type transistor 8 increases,
The potential at point B gradually increases. The potential at point B (ie, N
As the gate potential of the channel MOS transistor increases, the resistance value of the N-channel MOS transistor 5 decreases, and the charge charged in the liquid crystal panel 1 flows into the ground via the N-channel MOS transistor 5 to discharge. Done. Therefore, the potential of the point C, that is, the potential of the liquid crystal panel gradually decreases. Note that, even if the resistance value of the N-channel MOS transistor 5 decreases, the driving capability of the constant current circuit 4 is small and is about several μA.
The through current flowing into the ground via the N-channel MOS transistor 5 does not increase.

As described above, the conduction state of the N-channel MOS type transistor 5 can be controlled in accordance with the charge amount of the capacitors 13 to 18, and the potential of the liquid crystal panel 1 can be lowered by increasing the conduction state. By reducing the conduction state, the potential of the liquid crystal panel 1 can be increased.

As described above, in this embodiment, the precharge control means for driving a large current is controlled and driven independently of the discharge control means (N-channel MOS transistor 5). Even if a discharging operation is performed to implement the conversion, a large through current does not occur, and power consumption can be reduced.

A more specific embodiment will be described with reference to the drawings. FIG. 2 shows the output circuit of FIG. 1 more specifically. In FIG. 2, the same components as those in FIG. FIG. 3 is different from FIG. 1 in that the configuration of the precharge circuit is shown more specifically. That is, the precharge circuit 19 includes a constant current circuit 20 connected to a power supply and having a large driving ability, and a switch S3 controlled from the outside.

FIG. 3 is a timing chart showing the operation of the output circuit of FIG. 2, S1 to S3 indicate opening and closing operations of switches (electronic switches) S1 to S3 in FIG. 2, and V0 indicates an output waveform of a voltage applied to the liquid crystal panel 1. Also, periods T0 to T2, T2 to T4, T
4 to T6 and T6 to T8 respectively indicate data of one line of the liquid crystal panel, and timing T4 indicates a timing of switching from one frame to the next frame. Actually, about 400 to 100 per frame
Although there are zero lines, two lines are shown in each frame for the sake of simplicity.

First, the timing T0 to T1 is a precharge period, and the switches S1 and S3 are closed.
S2 is an open state (in the timing chart of FIG. 2, the closed state of the switch is represented by a high level, and the open state is represented by a low level). S1 is closed S2
Is opened, the inverting input terminal A and the output terminal C are short-circuited, so that the operational amplifier 3 forms a voltage follower circuit,
The data before the capacity 11 is erased. On the other hand, capacity 13-1
8 is charged. When S3 is closed, the constant current circuit 20
, A large current flows into the liquid crystal panel 1, and the potential V0 of the liquid crystal panel 1 rises instantaneously.

Next, timings T1 and T2 are output periods to the liquid crystal panel, and switches S1 and S3 are opened.
S2 is in a closed state. In this period, the capacity is 13-18
The potential state of the liquid crystal panel 1 (that is, the amount of discharge) is determined by the charge amount of the liquid crystal panel 1.
Shows an example in which V.sub.0 does not discharge and V0 maintains a high potential state.

At timing T2, data display of the next line is started. First, a precharge is performed again in a period of T2 to T3, and after the precharge, a discharging operation is performed in an output period of T3 to T4 to make the potential of the liquid crystal panel substantially equal to that of the counter electrode, thereby reducing the brightness of the liquid crystal panel display. Switch.

When the next frame is changed at timing T4, the precharge operation in T4 to T5 is performed in the same manner as in the previous frame, but in the next output period T5 to T6, the potential of the liquid crystal panel is set to the opposite electrode. And the direction of the potential difference is reversed to perform AC conversion. That is, the potential of the liquid crystal panel is made lower than that of the counter electrode by performing more discharges than in the case of the previous frame.

Similarly, in T7 to T8, the potential of the liquid crystal panel is controlled in a range lower than the potential of the counter electrode.

As described above, in the present embodiment, precharging is performed for each line, and the potential of the liquid crystal panel is inverted for each frame to achieve alternating current.

Next, another specific embodiment will be described with reference to the drawings. FIG. 4 shows the configuration of the output circuit in this embodiment. In FIG. 4, the same components as those in FIG. 2 is different from FIG. 2 in that the precharge circuit 1
9. That is, a constant current circuit 21 having a large driving capability connected to the power supply of the potential V1, a switch S4 externally controlled, a constant current circuit 22 having a large driving capability connected to the power supply of the potential V2, and a switch externally controlled. The precharge circuit 19 is formed from S5. Also,
Opening and closing of these switches S4 and S5 are controlled via two AND circuits 23 and 24 and an inverter 25.

FIG. 5 is a timing chart showing the operation of the output circuit of FIG. In the figure, S1, S2, S
4 and S5 show the opening and closing operations of the respective switches in FIG. 4, and V0 shows the output waveform of the voltage applied to the liquid crystal panel 1. Also, periods T0 to T2, T2 to T4, T4 to
T6, T6 to T8 each indicate data for one line of the liquid crystal panel, and timing T4 indicates the timing of switching from one frame to the next frame.

The operation shown in the chart of FIG. 5 is almost the same as the operation shown in FIG. 3, for example, the precharge operation is performed for each line, but using two types of voltage values applied to the liquid crystal panel during the precharge period. They differ in the way they are used.
That is, as the precharge potential, a high potential (VDD)
When two types of potentials, V1 and V2, are used, and the potential of the liquid crystal panel is controlled in a range of potential higher than that of the counter electrode,
That is, when inverting the potential of the liquid crystal panel, V1 is used as the precharge potential (T0 to T1, T2 to T1).
3) When controlling in a potential range lower than the counter electrode, V2 is used as the precharge potential (T4 to T5, T4
6-T7). In the present embodiment, since alternating current is performed in units of frames (that is, the potential of the liquid crystal panel is inverted), V1 and V2 are switched at timing T4 when the frame changes. As in the present embodiment, in the case of the inversion control, a low precharge potential V2 is sufficient, and it is possible to reduce wasteful power consumption at the time of precharge as compared with the case shown in FIG. It has the effect of.

As described above, in the operation shown in FIG. 3, the precharge potential is always the same, but in the present embodiment, the power consumption of the output circuit can be further increased by selectively using the high precharge potential and the low precharge potential. Can be reduced.

Next, still another specific embodiment will be described with reference to the drawings. FIG. 6 shows the configuration of the output circuit in this embodiment. This embodiment is particularly an embodiment in the case where the inversion (alternating) of the applied voltage is performed not in units of frames but in units of lines. 6, the same components as those in FIG. 2 are denoted by the same reference numerals, and the description will be omitted. 2 differs from FIG. 2 in the manner of controlling the precharge circuit 19. That is, the precharge circuit 19 includes a constant current circuit 20 connected to a power supply and having a large driving ability, and a switch S3.
3 is controlled via an AND circuit 26. The precharge signal and the inversion control signal are input to the AND circuit 26. The inversion control signal is a signal whose high level / low level is inverted for each line, and is a signal already used in a general liquid crystal driving device.

FIG. 7 is a timing chart showing the operation of the output circuit of FIG. 6A shows a precharge signal, FIG. 6B shows an inversion control signal, and S3 shows a signal shown in FIG.
Shows the opening and closing operation of the switch S3 in FIG. V0 indicates an output waveform of a voltage applied to the liquid crystal panel 1. T0-T
The meaning of 8 is the same as in the other embodiments.

As shown in FIG. 3A, the precharge signal goes high each time the line is switched, and as shown in FIG. 3B, the inversion control signal repeats high / low for each line. Therefore, the output of the AND circuit 26, that is, the opening and closing of S3, is configured to perform the precharge only when the potential of the liquid crystal panel is controlled in a potential range higher than the counter electrode (T0 to T1, T4 to T5).

In the configuration in which alternating current is performed in units of lines as in the present embodiment, precharging is sufficient for two lines, so that the configuration shown in FIG. 6 can be employed. In this embodiment, the number of precharges can be reduced as compared with the other embodiments, so that the power consumption can be further reduced.

In this embodiment, an AND circuit is used as a logic circuit for controlling the switch S3. However, the present invention is not limited to this, and another logic circuit such as an OR circuit may be used.

[0037]

According to the present invention, a through current during a discharging operation can be suppressed as much as possible, power consumption can be reduced, and an output circuit of a liquid crystal panel excellent in practicality can be provided.

Further, power consumption can be reduced by controlling the precharge voltage at the time of precharge to a necessary minimum.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an output circuit of a liquid crystal driving device according to the present invention.

FIG. 2 is a configuration diagram showing a specific example of an output circuit of the liquid crystal driving device of the present invention.

FIG. 3 is a timing chart illustrating an operation of the output circuit illustrated in FIG. 2;

FIG. 4 is a block diagram showing another specific embodiment of the output circuit of the liquid crystal driving device of the present invention.

FIG. 5 is a timing chart showing the operation of the output circuit shown in FIG.

FIG. 6 is a block diagram showing another specific embodiment of the output circuit of the liquid crystal driving device of the present invention.

FIG. 7 is a timing chart showing the operation of the output circuit shown in FIG.

FIG. 8 is a configuration diagram showing an output circuit of a conventional liquid crystal driving device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1,101 Liquid crystal panel 2,102 Counter electrode 3,103 Differential amplifier 4,104 Constant current circuit 5,105 N-channel MOS transistor 6,106 Constant current circuit 7,107 P-channel MOS transistor 8,108 P-channel MOS Type transistor 9,109 N-channel MOS transistor 10,110 N-channel MOS transistor 11,111 Capacitance 112 Switch 13 ~ 18,113 ~ 118 Capacitance 19 Precharge control means 20 Constant current circuit 21,22 Constant current circuit 23,24 AND circuit 25 Inverter 26 AND circuit S1 to S5 Switch

────────────────────────────────────────────────── ─── Continued on the front page (72) Osamu Sarai 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP 5-303353 (JP, A) JP 5-A 313603 (JP, A) JP-A-5-134627 (JP, A) JP-A-7-56529 (JP, A) JP-A-7-181924 (JP, A) JP-A-6-202583 (JP, A) 63-162328 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/133 520 G09G 3/36

Claims (4)

(57) [Claims] 1. An output circuit for applying an AC voltage to a liquid crystal panel, comprising: precharge control means for charging the liquid crystal panel; and discharge control means for discharging the liquid crystal panel. The discharge control means is controlled independently of the discharge control means and is driven when the voltage applied to the liquid crystal panel rises, and the discharge control means has a relatively small driving capability.
Controls the conduction state of the charge discharging transistor
The output circuit of the liquid crystal driving device which is characterized by controlling the output voltage to the liquid crystal panel by. 2. The output circuit according to claim 1, wherein said precharge control means charges said liquid crystal panel to one of two kinds of potentials V1 and V2. Wherein said precharge control means, the output circuit of the liquid crystal drive apparatus of claim 1, wherein the controller controls the inversion control signal switched for each of the liquid crystal panel display of the line. 4. An output circuit for applying an AC voltage to a liquid crystal panel, comprising: two charge-charging transistors;
It has one transistor for charge discharging, and the two transistors for charge charging are composed of a transistor having a large driving ability and a transistor having a small driving ability, and driving the transistor having the large driving ability at the time of raising the voltage applied to the liquid crystal panel. Characteristic output circuit of liquid crystal driving device .