JPH0553535A - Source driver circuit - Google Patents

Abstract

PURPOSE:To extend an effective video period by reducing the power consumption of the source driver circuit. CONSTITUTION:When a video signal is high, a p channel field effect type transistor(FET) 6 is turned off and an n channel FET 5 is selected to discharge electric charges from a power source VDD to the source line 11 of a liquid crystal panel. When the video signal is low, the n channel FET 6 is turned off and the p channel FET 6 is selected to discharge electric charges from the source line 11 of the liquid crystal panel. The electric charges are charged and discharged only when the video signal varies, and a stand-by current is extremely small, so the current consumption is reduced. The video signal VSH nearly matches an output V0, so the effective output video period is extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a source driver circuit for an active matrix type liquid crystal panel.

[0002]

2. Description of the Related Art As shown in FIG. 5, a source driver circuit for an active matrix type liquid crystal panel comprises a shift register, a level shifter, a multiplexer and a sample and hold circuit, and a video signal sent in series is sampled and held. Samples are sampled at, rearranged in parallel, and then output to the liquid crystal panel. FIG. 3 shows an output stage circuit of a conventional driver circuit connected to one source line, that is, an output circuit used in the output stage of a sample-and-hold circuit. Further, the waveform of each part of the circuit of FIG. 3 is shown in the time chart of FIG.

In FIG. 3, 1 is an operational amplifier, 6 is a p-channel field effect transistor, 9 is a bias resistor,
Reference numeral 11 is a source line of the liquid crystal panel, 12 is a capacitance of the source line, and 13 is a switch. Further, V _SH is a sampled and held video signal, C is a control signal of the switch 13, and V _SH is
_DD is the power supply voltage and V ₀ is the output of the driver circuit. In the output stage of the actual driver circuit, a large number of the circuits shown in FIG. 3 are used in parallel.

The operation of the conventional circuit will be described with reference to the time charts of FIGS. 1 for the video signal V _SH in FIG.
The 1H line inversion drive is employed in which the locality is inverted with respect to the center value of the amplitude for each scanning line (1H). This 1H
It is known that the line inversion drive has an effect of suppressing flicker. In FIG. 3, the video signal V _SH is input to the positive input terminal (+) of the operational amplifier 1. The output of the operational amplifier 1 is connected to the gate of the p-channel field effect transistor 6. The drain of the p-channel field effect transistor 6 is grounded and the source is connected to the negative input terminal of the operational amplifier 1 to form a closed loop. Source is output V ₀
Also, a current flows from the power source V _DD through the resistor 9 to the source or the source line of the liquid crystal panel. Basically, the operational amplifier circuit 1, the p-channel field effect transistor 6 and the resistor 9 constitute a buffer circuit. When the video signal V _SH increases, the output of the operational amplifier 1 also increases, the drain current of the p-channel field effect transistor 6 decreases, and the current flowing through the resistor 9 charges the source line capacitance 12 to increase the source line potential. To do. On the contrary, when the video signal V _SH becomes low, the output of the operational amplifier 1 becomes low, the drain current of the p-channel field effect transistor 6 increases, and the electric charge is discharged from the source line capacitance 12, so the source line potential is low. Become. In FIG. 4, V _SH is a video signal, C is a charging pulse, and V ₀ is an output potential.

The size of the source line capacitance 12 is several tens to several hundreds pF depending on the panel size, and in order to repeatedly charge and discharge this capacitance, it is necessary to increase the current flowing through the resistor 9. In addition, it is necessary to increase the current capacity of the p-channel field effect transistor 6 as well. In this case, even if the video signal V _SH does not change, the power supply V
_{Since a} standby current flows from _DD through the resistor 9 and the p-channel field effect transistor 6, power consumption increases. Therefore, in order to reduce the standby current, as shown in FIG. 4, after the charge pulse switch 13 is charged to V _DD when the video signal V _SH changes, the p-channel field effect transistor 6 is used to change the source line capacitance 12 from the source line capacitance 12. The charge is discharged and V ₀ is lowered to V _SH . For this behavior,
The standby current flowing through the resistor 9 can be reduced to about 2-10 microamps.

[0006]

When the circuit of FIG. 3 is operated with the waveform shown in FIG. 4, the amount of charge Q _S that moves in the output circuit during charging and discharging is the value of the source line capacitance 12 C _S.
Then, it is calculated as follows.

Q _S = C _S · (V _DD −V _SH ) ... (1) The electric charge expressed by the equation (1) is charged to the source line capacitance through the switch 13 at the time of charging, and the p-channel field effect transistor 6 at the time of discharging. Be discharged through.

As described above, even in this circuit in which the standby current is reduced, the power supply voltage is charged and discharged every time the video signal changes, so that the power consumption is still large and it is difficult to reduce the power consumption. It was

Further, since the output V ₀ is the power supply voltage while the discharge pulse C in FIG. 4 is on, the video signal V _SH
However, there is a drawback that the period during which is output to the liquid crystal panel is shortened.

It is an object of the present invention to provide a liquid crystal panel source driver circuit with lower power consumption.

[0011]

According to the present invention, a video signal is input to a positive input terminal of an operational amplifier in an output stage of a source driver circuit of a thin film transistor driving liquid crystal panel, and the output of the operational amplifier is input to two switches. The switch connected to a terminal, one output terminal of the two switches is grounded via a pull-down resistor, the other output terminal is connected to a power supply via a pull-up resistor, and is grounded via the pull-down resistor. Is connected to the gate of an n-channel field effect transistor, and the output of the switch, which is connected to the power source through the pull-up resistor, is connected to the gate of a p-channel field effect transistor. The drain of the field effect transistor is connected to the power supply, and the p-channel field effect transistor is connected. The drain is grounded, the source of the n-channel field effect transistor is connected to the source of the p-channel field effect transistor, and a resistor is provided between the power source and the sources of the n and p channel field effect transistors. Are connected to each other, the sources of the n- and p-channel field effect transistors are grounded through another resistor, and the sources of the n- and p-channel field effect transistors are connected to the negative input terminal of the operational amplifier. The two switches are connected, and one of the two switches is selected and the switch is closed.

[0012]

1 is a circuit diagram of an output stage of a liquid crystal panel source driver circuit according to the present invention. FIG. 2 is a time chart explaining the operation of FIG. Video signal V _{SH of} FIG.
Is positive, the n-channel field effect transistor is selected, and the charge is supplied from the power supply side to the liquid crystal panel side to charge the source line capacitance. When the video signal V _SH is on the-side, the p-channel field effect transistor is selected, the charge is discharged from the liquid crystal panel side, and the source line potential is lowered.
Unlike the conventional circuit, it is not necessary to charge the power supply voltage once each time the next video signal is sent and to lower the voltage to a predetermined voltage again, so that the current consumption is reduced. Further, in the past, there was a period in which the power supply voltage was output to the output due to the charging pulse, but according to the present invention, since the output substantially matches the input video signal, the video signal applied to the source line of the liquid crystal panel is The effective period increases.

[0013]

1 is a circuit diagram of an embodiment of an output stage of a liquid crystal panel source driver circuit according to the present invention, in which 1 is an operational amplifier, 2 and 3 are selection switches, 4 is an inverting circuit, 5
Is an n-channel field effect transistor, 6 is a p-channel field effect transistor, 7 is a pull-up resistor, 8 is a pull-down resistor, 9 and 10 are standby current resistors, 11 is a source line of a liquid crystal panel, and 12 is a liquid crystal panel. Source line capacitance,
V _SH is the video signal sent from the sample hold circuit, V
_DD is a power source, S is a selection pulse, and V ₀ is an output. Also,
FIG. 2 is a time chart for explaining the operation of FIG. 1. V _SH is a video signal sent from the sample hold circuit, V ₀ is an output, and in the present invention, V _SH and V ₀ have substantially the same waveform. S is a selection pulse.

The operation of the circuit according to the present invention will be described with reference to the circuit of FIG. 1 and the time chart of FIG. The video signal V _SH in FIG. 1 employs 1H line inversion drive in which the polarity is inverted with respect to the center value of the amplitude for each scanning line (1H). In FIG. 1, the video signal V _SH is input to the positive input terminal (+) of the operational amplifier 1. The output of the operational amplifier 1 is connected to the selection switches 2 to 3. Selection switches 2 and 3
Is selected by the selection pulse and the inverting circuit 4, and the switch is turned on. The output of the selection switch 2 is connected to the gate of the n-channel field effect transistor 5. The output of the selection switch 3 is connected to the gate of the p-channel field effect transistor 6. The drain of the n-channel field effect transistor 5 is connected to the power supply V _DD , and p
The drain of the channel field effect transistor 6 is grounded, and the sources of the n and p type field effect transistors 5 and 6 are connected to each other. Source is operational amplifier 1
It is connected to the negative input terminal of and forms a closed loop. The source is also connected to the output V ₀ . A current flows from the power source V _DD through the resistor 9 to the source and the source line 11 to the resistor 10 of the liquid crystal panel, and the current from the source and the source line 11 to the resistor 9 of the liquid crystal panel is discharged to the ground side through the resistor 10. The resistors 9 and 10 may be active resistors using field effect transistors.

When the video signal V _SH is on the + side, the selection pulse S
Thereby, the switch 2 is selected and the buffer circuit of the system of the n-channel field effect transistor 5 operates. That is, the n-channel field effect transistor 5 operates, electric charges are supplied from the power source V _DD to the source line 11, and the output V ₀ is fielded back to the negative input terminal (−) of the operational amplifier 1. In this case, the switch 3 is off, and the power supply voltage V _DD is applied to the gate of the p-channel field effect transistor 6 by the pull-up resistor 7, so the p-channel field effect transistor 6 is off. The operation of the buffer circuit is such that when the video signal V _SH becomes high, the operational amplifier 1
Output increases, the gate voltage of the n-channel field effect transistor 5 increases, the drain current also increases, the source line capacitance 12 is charged, and the source line potential increases. On the contrary, when the video signal V _SH is on the _minus side, the switch 3 is selected by the selection pulse S and the inverting circuit 4 and the buffer circuit of the system of the p-channel field effect transistor 6 operates. That is, the p-channel field effect transistor 6 operates, and the electric charge is discharged from the source line capacitance 12 as a drain current. The output V ₀ is field-backed to the negative input terminal (−) of the operational amplifier 1. In this case, the switch 2 is off, and the gate of the n-channel field effect transistor 5 is grounded by the pull-down resistor 8. Therefore, the n-channel field effect transistor 5 is off. As for the operation of the buffer circuit, when the video signal V _SH becomes low, the output of the operational amplifier 1 becomes low, the drain current of the p-channel field effect transistor 6 increases, and the source line capacitance 12 discharges electric charges. The potential of the line 11 becomes low.

The charge transfer amount Q _SS in the liquid crystal panel source driver output stage according to the present invention is expressed by the following formula, where C _S is the capacitance per liquid crystal panel source line and V _SH-P is the amplitude of the video signal V _SH . It is shown by the following formula.

Q _SS = C _S · V _SH-P (2) Since the moving charges are consumed by the resistance component of the transistor or the like, it is preferable that the moving amount of the charges is small. The value is compared with the conventional value using a specific numerical value. For a 10 inch liquid crystal panel, C _S is about 200 pF. As the power supply voltage, V _{DD is set} to 15V. The center of the amplitude of the video signal is V _DD
In the case of the formula (1), V _{SH is set} to 7.5 V as an average value. Thus the conventional charge transfer amount is the sum the time of discharging and _{charging, Q S = 2 · 200 ×} 10- 12 · (15-7.5) = 3 × 10 -9 ( Coulomb) ... (3) is there. The coefficient 2 is for taking the absolute value of the movement amount during charging and discharging. As for the amplitude of the video signal, if the maximum voltage required to apply to the liquid crystal is ± 4V, the average amplitude is 4V with respect to the median value.

[0018] ^{_{Q SS = 200 × 10- 12 ·}} 4 = 8 × 10- 10 ( Coulomb) ... (4) than in the conventional charge transfer amount, the amount of the charge transfer circuit of the present invention (3), (4) Comparing the formulas, it is reduced to about 27%.

The current consumption of the source driver circuit according to the present invention and the conventional source driver circuit is 64 pixels.
When actually measured using a 0 × 400 10-inch liquid crystal panel, the current consumption of about 180 mA in the entire circuit (the entire source driver circuit such as the sample hold circuit) is reduced to 70 mA in the present invention, which is about 40%. did.

Further, conventionally, 5 microseconds were required as the period for applying the charging pulse, but according to the present invention, since it is not necessary to charge, it is effective in most of 40 microseconds of one scanning line period. Since the video signal can be applied to the liquid crystal pulse, there is a margin in the on-current characteristics of the thin film field effect transistor which is a switch element in the liquid crystal panel.

[0021]

As described above, the source driver circuit for a liquid crystal panel of the present invention can reduce the current consumption and can effectively apply the video signal to the liquid crystal panel within the scanning period. It is valid.

[Brief description of drawings]

FIG. 1 is an output circuit diagram according to the present invention.

FIG. 2 is a time chart illustrating a circuit of the present invention.

FIG. 3 is a conventional output circuit diagram.

FIG. 4 is a time chart illustrating a conventional circuit.

FIG. 5 is a block diagram of a source driver circuit.

[Explanation of symbols]

 1 Operational Amplifier Circuit 2, 3 Selection Switch 4 Inversion Circuit 5 n-Channel Thin Film Field Effect Transistor 6 n-Channel Thin Film Field Effect Transistor 7 Pull-up Resistor 8 Pull-down Resistor 9, 10 Resistor 11 Liquid Crystal Panel Source Line 12 Source Line Capacitance 13 Switch

Claims (1)

[Claims] 1. A source driver circuit comprising a shift register, a level shifter, a multiplexer, and a sample-and-hold circuit, wherein an output circuit constituting an output stage of the sample-and-hold circuit outputs a video signal to a positive input terminal of an operational amplifier. The output of the operational amplifier is connected to the input terminals of two switches, one output terminal of the two switches is grounded via a pull-down resistor, and the other output terminal is connected to a power source via a pull-up resistor. The output of the switch, which is connected and grounded through the pull-down resistor, is connected to the gate of an n-channel field effect transistor, and the output of the switch which is connected to the power supply through the pull-up resistor is p The gate of the channel field effect transistor is connected to the n-channel field effect transistor. The drain of the transistor is connected to the power supply, the drain of the p-channel field effect transistor is grounded, the source of the n-channel field effect transistor and the source of the p-channel field effect transistor are connected to each other, and A resistor is connected between the source of the n-channel and p-channel field effect transistor, and the source of the n-channel p-channel field effect transistor is grounded via another resistor. The source driver circuit is characterized in that the sources of both channel field effect transistors are connected to a negative input terminal of the operational amplifier, and one of the two switches is selected to close the switch.