# Patent

## Title
Output circuit, liquid crystal driving circuit, and liquid crystal driving method

## Abstract

An output circuit for driving a signal line in, for example, a liquid crystal display panel has an impedance conversion element that generates an output signal from an input signal and a feedback signal. During output periods, a first switch conducts the output signal to the output terminal of the output circuit and a second switch conducts the output signal from the output terminal back to the impedance element as the feedback signal. During non-output periods, the first and second switches are switched off and a third switch conducts the output signal back to the impedance element as the feedback signal from a point between the impedance conversion element and the first switch. This dual feedback scheme enables the signal line to be precharged during non-output periods while avoiding loss of driving speed and accuracy during output periods.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an output circuit employing feedback control, a liquid crystal driving circuit that uses the output circuit to drive a liquid crystal panel, and a liquid crystal driving method that uses the output method of the output circuit to drive a liquid crystal panel.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Application Publication No. 11-30975, the driving speed of a liquid crystal display panel having source lines driven by operational amplifiers can be increased by precharging the source lines. The source lines are precharged by disconnecting them from their drivers (the operational amplifiers) and either interconnecting the source signal lines, or connecting them to a fixed potential such as the common-voltage potential of the liquid crystal display panel.
FIG. 8 illustrates the former precharging scheme in a conventional liquid crystal display including a liquid crystal panel 1, a gate driving circuit 2, a source driving circuit 3, a group of m source lines S1, S2, . . . , Sm, and a group of n gate lines G1, G2, . . . , Gm, where m and n are positive integers, m being equal to or greater than two. The liquid crystal panel 1 includes cell transistors TRij and capacitors CXij (1≦i≦m, 1≦j≦n). The gate driving circuit 2 includes gate drivers GDj (1≦j≦n).
Referring to FIG. 9, the source driving circuit 3 comprises m source drivers SD1, SD2, . . . , SDm, connected through respective analog switches A1, A2, . . . , Am to respective output terminals OUT1, OUT2, . . . , OUTm, a group of m−1 analog switches D1, D2, . . . , Dm−1 by which mutually adjacent source lines are switchably interconnected, and an inverter I. A single output circuit comprises a source driver SDi, the corresponding analog switches Ai, Di, and output terminal OUTi (where i is an arbitrary integer from 1 to m). The source driver SDi is an operational amplifier receiving a source driving signal SSi as its non-inverting input, generating a corresponding output signal for driving source line Si, and feeding the output signal back as its inverting input. Feedback ensures that the output signal has the same potential as the source driving signal SSi. Various other impedance conversion means controlled by feedback can also be used as the source driver SDi.
Analog switches A1 to Am and D1 to Dm−1 are controlled by a switch control signal PC input to inverter I and a complementary switch control signal PCB output from inverter I. When switch control signal PC is ‘0’ and PCB is ‘1’, analog switches A1 to Am all turn on and analog switches D1 to Dm−1 all turn off, so that output terminals OUT1 to OUTm (and source lines S1 to Sm) are connected to the output terminals of respective source drivers SD1 to SDm and the output signals from the source drivers SD1 to SDm are output on source lines S1 to Sm. When switch control signal PC goes to ‘1’ and switch control signal PCB goes to ‘0’, analog switches A1 to Am all turn off and analog switches D1 to Dm−1 all turn on, disconnecting output terminals OUT1 to OUTm (and source lines S1 to Sm) from the source drivers SD1 to SDm and interconnecting all of the output terminals and source lines; the output terminals and source lines are thereby precharged. When switch control signal PC returns to ‘0’ and switch control signal PCB returns to ‘1’, analog switches A1 to Am all turn on and analog switches D1 to Dm−1 all turn off, disconnecting output terminals OUT1 to OUTm (and source lines S1 to Sm) from each other and connecting them to the source drivers SD1 to SDm.
Although the purpose of this precharging scheme is faster driving, to enable the source drivers to receive feedback during the precharging period, the feedback signals must be taken from points between the source drivers and the analog switches A1 to Am. Consequently, during driving periods, the source drivers must drive the on-resistance of these analog switches as well as the capacitance of the capacitors in the liquid crystal panel. Because of the voltage drop due to the on-resistance of the analog switches, the potentials of the output terminals of the source driving circuit 3 differ from the potentials of the signals output by the source drivers. Although the potential difference diminishes and eventually disappears as the capacitors approach and eventually reach the intended charge level, the potential difference slows the approach, thereby limiting the speed with which the liquid crystal panel can be driven. A further problem is that variations in wiring resistance due to variations in the on-resistance of the analog switches and the wiring length of the output paths create unwanted variations in driving potential among the output terminals (and source lines), impairing the accuracy with which the liquid crystal panel 1 is driven, leading to lowered image quality. As the number of pixels increases and the driving frequency increases, driving the liquid crystal panel accurately at the necessary speed becomes a significant challenge.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an output circuit in which an impedance conversion element, switchably connectable to an output terminal, can rapidly generate an output signal at the correct potential level at the output terminal.
A further object is to provide a circuit and method for rapidly and accurately driving a liquid crystal display panel.
The impedance conversion element in the invented output circuit generates an output signal from an input signal and a feedback signal. An output path conducts the output signal from the impedance conversion element to the output terminal of the output circuit. The output path includes a first switch that conducts the output signal during output periods and blocks the output signal during non-output periods. A second switch conducts the output signal from a first point on the output path to the impedance conversion element as the feedback signal during the output periods. A third switch conducts the output signal from a second point on the output path to the impedance conversion element as the feedback signal during the non-output periods. The first point is disposed at the output terminal, or between the first switch and the output terminal; the second point is disposed between the impedance conversion element and the first switch.
The second switch provides feedback of the potential at the output terminal to the impedance conversion element. By comparing the feedback signal with the input signal, the impedance conversion element can quickly and accurately adjust its output so that the desired potential is obtained at the output terminal of the output circuit.
Output circuits of the invented type can be used to drive a liquid crystal display panel accurately at high speed. The output terminals and their connected signal lines can be precharged during the non-output periods.



BRIEF DESCRIPTION OF THE DRAWINGS
In the attached drawings:
FIG. 1 is a circuit diagram of a liquid crystal display according to a first embodiment of the invention;
FIG. 2 is a more detailed circuit diagram of the source driving circuit in FIG. 1;
FIG. 3 is a timing waveform diagram illustrating the operation of the source driving circuit;
FIG. 4 is a waveform diagram comparing simulated output waveforms in the first embodiment and prior art;
FIG. 5 is a circuit diagram of the source driving circuit in a second embodiment;
FIG. 6 is a circuit diagram of the source driving circuit in a third embodiment;
FIG. 7 is a timing waveform diagram illustrating the operation of the source driving circuit in the third embodiment;
FIG. 8 is a circuit diagram of a liquid crystal display according to the prior art; and
FIG. 9 is a more detailed circuit diagram of the source driving circuit in FIG. 8.



DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters.
Referring to FIG. 1, a liquid crystal display according to a first embodiment of the invention comprises a liquid crystal panel 1, a gate driving circuit 2, a source driving circuit 10, a group of source lines, and a group of gate lines. FIG. 2 shows the circuit configuration of the source driving circuit 10 in more detail.
The group of source lines comprises m source lines S1, S2, . . . , Sm (where m is an arbitrary integer equal to or greater than two); the group of gate lines comprises n gate lines G1, G2, . . . , Gn (where n is an arbitrary integer equal to or greater than two). The source lines and gate lines form a set of matrix lines for driving an m×n matrix of liquid crystal cell switching transistors.
The liquid crystal panel 1 comprises the m×n switching transistors TR12, TR22, . . . , Tmn and m×n liquid crystal cell capacitors CX11, CX21, . . . , CXm1, CX12, CX22, . . . , CXmn. Switching transistor TRij and liquid crystal cell capacitor CXij form a liquid crystal cell (i is an integer from 1 to m; j is an integer from 1 to n). The liquid crystal panel 1 has a matrix of m×n liquid crystal cells.
The source and drain of switching transistor TRij are connected between source line Si and the cell electrode of liquid crystal cell capacitor CXij; the gate of TRij is connected to gate line Gj. The common electrode of liquid crystal cell capacitor CXij is connected to a common power source Vcom.
The gate driving circuit 2 has n gate drivers GD1, GD2, . . . , GDn. The gate driving circuit 2 uses gate driver GDj to drive gate line Gj.
As shown in FIGS. 1 and 2, the source driving circuit 10 in the first embodiment comprises the m source drivers SD1, SD2, . . . , SDm, an A-group of analog switches (A1 etc.) that control output paths, a B-group of analog switches (B1 etc.) that control first feedback paths, a C-group of analog switches (C1 etc.) that control second feedback paths, a D-group of analog switches (D1 etc.) that control precharging, a group of m output terminals OUT1, OUT2, . . . , OUTm, and an inverter I. Each analog switch comprises a p-channel metal-oxide-semiconductor (PMOS) transistor and an n-channel metal-oxide-semiconductor (NMOS) transistor connected in parallel, as can be seen in FIG. 2.
The i-th source driver SDi is an operational amplifier with a non-inverting input terminal to which a source driving signal SS1 is input, an output terminal from which a signal is output to drive the i-th source line Si to the potential of the input source driving signal SS1, and an inverting input terminal to which the output signal is fed back. The source driver SDi operates as a voltage-follower buffer amplifier with high-impedance input and low-impedance output.
The invention is not limited to the use of operational amplifiers. Various types of impedance conversion means including a buffer or amplifier can be used as the source driver SDi.
The A-group of analog switches comprises m analog switches (MOS switches) A1, A2, . . . , Am. Analog switch Ai is connected between the output terminal of the i-th source driver SDi and the i-th output terminal OUTi of the source driving circuit 10, thus between the output terminal of source driver SDi and source line Si. The gate electrode of the PMOS transistor in analog switch Ai receives a switch control signal PC (the input signal to the inverter I); the gate electrode of the NMOS transistor in analog switch Ai receives a complementary switch control signal PCB (the output signal from the inverter I). Analog switch Ai turns off if switch control signal PC is at the logical ‘1’ level (PC=1, PCB=0), thereby disconnecting the output terminal of source driver SDi from output terminal OUTi (source line Si); analog switch Ai turns on if PC is at the logical ‘0’ level (PC=0, PCB=1), thereby connecting the output terminal of source driver SDi to output terminal OUTi (source line Si). This embodiment assumes that the logical ‘0’ level is low and the logical ‘1’ level is high.
The B-group of analog switches comprises m analog switches (MOS switches) B1, B2, . . . , Bm. Analog switch Bi is connected between the i-th output terminal OUTi (source line Si) of the source driving circuit 10 and the inverting input terminal of source driver SDi. The gate electrode of the PMOS transistor in analog switch Bi receives switch control signal PC; the gate electrode of the NMOS transistor in analog switch Bi receives switch control signal PCB. Analog switch Bi turns off when PC=1 (PCB=0), thereby disconnecting the inverting input terminal of source driver SDi from output terminal OUTi (and source line Si); analog switch Bi turns on when PC=0 (PCB=1), thereby connecting the inverting input terminal of source driver SDi to output terminal OUTi (and source line Si).
The C-group of analog switches comprises m analog switches (MOS switches) C1, C2, . . . , Cm. Analog switch Ci is connected between the output and inverting input terminals of source driver SDi. The gate electrode of the PMOS transistor in analog switch Ci receives switch control signal PCB; the gate electrode of the NMOS transistor in analog switch Ci receives switch control signal PC. Analog switch Ci turns on if switch control signal PC=1 (PCB=0), thereby connecting the output terminal of source driver SDi to the inverting input terminal of source driver SDi; analog switch Ci turns off if switch control signal PC=0 (PCB=1), thereby disconnecting the output terminal of source driver SDi from the inverting input terminal of source driver SDi.
The D-group of analog switches comprises m−1 analog switches (MOS switches) D1, D2, . . . , Dm−1 (there is no Dm) The i-th analog switch Di is connected between the i-th output terminal OUTi and the (i+1)-th output terminal OUTi+1 of the source driving circuit 10, thus between source line Si and source line Si+1. The gate electrode of the PMOS transistor in analog switch Di receives switch control signal PCB; the gate electrode of the NMOS transistor in analog switch Di receives switch control signal PC. Analog switch Di turns on if switch control signal PC=1 (PCB=0), thereby establishing a short circuit between source line Si and source line Si+1 through output terminals OUTi and OUTi+1 of the source driving circuit; analog switch Di turns off if switch control signal PC=0 (PCB=1), thereby breaking the short circuit that has been established between source lines Si and Si+1 (and between the corresponding output terminals of the source driving circuit). In the first embodiment, a source line (and the corresponding output terminal of the source driving circuit) is precharged from other source lines (and other output terminals of the source driving circuit).
In the source driving circuit 10 of the first embodiment, source driver SDi, analog switches Ai, Bi, Ci, and Di, and output terminal OUTi form an output circuit.
The operation of the source driving circuit 10 in the first embodiment will be described below with reference to FIG. 3, which shows waveforms of an output signal OUT of the source driving circuit 10 (the signal output from output terminal OUTi to source line Si), the switch control signal PC, and the complementary switch control signal PCB. Td indicates the dot driving cycle time of the liquid crystal display, including both the driving (output) period and the precharging (non-output period); Tp indicates the precharging (non-output) period.
During a precharging period, switch control signal PC is ‘1’ and complementary switch control signal PCB is ‘0’, so the A- and B-group analog switches are all in the off state, while the C- and D-group analog switches are all in the on state.
Since analog switches Ai and Bi are in the off state and analog switches Di−1 and Di are in the on state, output terminal OUTi (and source line Si) is disconnected from the output and inverting input terminals of source driver SDi and is connected via analog switches Di−1 and Di to the adjacent output terminals OUTi−1 (source line Si−1) and OUTi+1 (source line Si+1). All of the output terminals OUTi and source lines Si (1≦i≦m) are mutually interconnected in this way, so all of the output terminals OUTi and source lines Si are precharged to substantially the average output potential in the preceding driving period.
Since analog switch Bi is off and analog switch Ci is on, the output potential of source driver SDi is fed back to the inverting input terminal of source driver SDi via analog switch Ci. Since the input impedance of the inverting input of source driver SDi is extremely high, the potential fed back to the inverting input terminal of source driver SDi becomes equal to the output potential of source driver SDi regardless of the on-resistance in analog switch Ci. Since source driver SDi operates so as to make the potential of its inverting input (the output potential of source driver SDi) equal to the potential of its non-inverting input (source driving signal SSi), the output potential of source driver SDi equals the potential of source driving signal SSi.
At the transition from the precharging period to the driving period, switch control signal PC goes to the ‘0’ logic level and switch control signal PCB goes to the ‘1’ logic level, switching all the C- and D-group analog switches off and all the A- and B-group analog switches on. Analog switches Di−1 and Di+1 accordingly turn off and analog switch Ai turns on, disconnecting output terminal OUTi (source line Si) from the adjacent output terminals OUTi−1 (source line Si−1) and OUTi+1 (source line Si+1) and connecting it to the output terminal of source driver SDi via analog switch Ai.
Analog switch Ci turns off and analog switch Bi turns on, switching from the second feedback path to the first feedback path, thereby feeding back the potential of output terminal OUTi (source line Si) after the voltage drop caused by the on-resistance of analog switch Ai to the inverting input terminal of source driver SDi via analog switch Bi. Since the input impedance at the inverting input terminal of source driver SDi is extremely high, the potential at the inverting input terminal of source driver SDi rapidly becomes equal to the potential of output terminal OUTi (source line Si). Since source driver SDi operates so as to make the potential of its inverting input (the potential of output terminal OUTi or source line Si) equal to the potential of its non-inverting input (source driving signal SSi), the potential of output terminal OUTi (source line Si) rapidly becomes equal to the potential of source driving signal SSi.
At the precharging-to-driving transition, accordingly, the source driving circuit 10 in the first embodiment switches the feedback potential of the i-th output circuit from the potential at a point preceding analog switch Ai to the potential at a point following analog switch Ai, thereby compensating for the voltage drop due to the on-resistance of analog switch Ai so that the potential of output terminal OUTi (source line Si) quickly becomes equal to the potential of source driving signal SSi (the input potential to source driver SDi). This feedback arrangement also compensates for variations in voltage drop due to variations in on-resistance, resulting in both faster and more accurate driving of the source lines.
This feedback arrangement can also compensate for the voltage drop due to the resistance of the signal line from the output terminal of source driver SDi to the point at which analog switches Ai and Bi are interconnected, which accounts for most of the wiring resistance on the signal path from the output terminal of source driver SDi to output terminal OUTi. This means that, if there are variations in wiring resistance (or wiring length) on the output paths, they can be compensated for completely, or almost completely, by interconnecting the analog switches Ai and Bi at output terminal OUTi or at a point located as near as possible to output terminal OUTi.
During the driving period, switch control signal PC is ‘0’ and switch control signal PCB is ‘1’, so the A- and B-group analog switches are all in the on state and the C- and D-group analog switches are all in the off state.
Analog switches Di−1 and Di+1 are in the off state, and analog switch Ai is in the on state, disconnecting output terminal OUTi (and source line Si) from the adjacent output terminals OUTi−1 and OUTi+1 (and source lines Si−1 and Si+1) and connecting it to the output terminal of source driver SDi via analog switch Ai.
Analog switch Ci is in the off state and analog switch Bi is in the on state, feeding the potential of output terminal OUTi (source line Si) back via analog switch Bi on the first feedback path to the inverting input terminal of source driver SDi, thereby keeping the potential of output terminal OUTi (source line Si) equal to the potential of the non-inverting input (source driving signal SSi) of source driver SDi.
At the transition from the driving period to the next precharging period, switch control signal PC goes to ‘1’ and switch control signal PCB goes to ‘0’, switching all the A- and B-group analog switches off and all the C- and D-group analog switches on.
Analog switches Ai and Bi turn off and analog switches Di−1 and Di+1 turn on, disconnecting output terminal OUTi (and source line Si) from the output and inverting input terminals of source driver SDi, and connecting output terminal OUTi to adjacent output terminals OUTi−1 and OUTi+1 (and source lines Si−1 and Si+1) via analog switches Di−1 and Di+1, thereby precharging source line Si.
Analog switch Bi turns off and analog switch Ci turns on, changing the feedback path from the first feedback path to the second feedback path, thereby feeding the output potential of source driver SDi back to the inverting input terminal of source driver SDi−1 via analog switch Ci.
FIG. 4 shows simulated waveforms of the switch control signal PC, an output signal OUTA of the source driving circuit 10 in the first embodiment, and an output signal OUTB of the conventional source driving circuit 3. Td indicates the dot driving cycle time of the liquid crystal display; Tp indicates the precharging period. In the simulation shown in FIG. 4, dots are driven alternately positive and negative with respect to the common voltage Vcom, and for simplicity, all dots are driven to the same potential, so precharging does not alter the potential.
As is evident from FIG. 4, the simulated output waveform OUTA in the first embodiment rises nearly ten percent (10%) faster than the simulated output waveform OUTB in the prior art. This improvement in rise time is particularly noticeable at intermediate driving potentials (potentials near the common voltage Vcom).
As described above, the first embodiment provides a first feedback path from a point following the A-group analog switch to the source driver during the driving period and a second feedback path from a point preceding the A-group analog switch to the source driver during the precharging period, and switches the feedback path at transitions from the driving period to the precharging period and vice versa, thereby compensating for the voltage drop due to the on-resistance of the analog switch, and further compensating for variations in on-resistance and wiring resistance of the output path. The first embodiment thereby achieves fast and highly accurate liquid crystal driving. By precharging the source lines from adjacent source lines, the first embodiment also conserves power and eliminates the need for a special precharging power source.
Second Embodiment
Referring to FIG. 5, the source driving circuit 20 in the second embodiment comprises m source drivers SD1, SD2, . . . , SDm, an A-group of analog switches that control output paths, a B-group of analog switches that control first feedback paths, a C-group of analog switches that control second feedback paths, an E-group of analog switches that control precharging, an a-group of protective resistors, a b-group of feedback resistors, a group of m output terminals OUT1, OUT2, . . . , OUTm, and an inverter I, where m is an even number.
The source driving circuit 20 accordingly adds protective resistors and feedback resistors to the source driving circuit 10 in the first embodiment, and alters the group of analog switches that control precharging. The source driving circuit 20 in the second embodiment also arranges the feedback paths during the driving period so that they branch from points following the protective resistors.
The E-group of analog switches comprises m/2 analog switches (MOS switches) E1, E3, . . . , Em−3, Em−1. The i-th analog switch Ei (i being an odd number) interconnects source lines Si and Si+1 through output terminals OUTi and OUTi+1 of the source driving circuit, also being located between analog switches Ai and Ai+1; no analog switch is provided to interconnect source lines Si+1 and Si+2 (analog switches Ai+1 and Ai+2). The number of analog switches in the E-group is therefore half the number of source lines, each analog switch in this group interconnecting two adjacent source lines.
The gate electrode of the PMOS transistor in analog switch Ei receives switch control signal PCB (the output signal from inverter I); the gate electrode of an NMOS transistor in analog switch Ei receives switch control signal PC (the input signal to inverter I). Analog switch Ei turns on if switch control signal PC=1 (PCB=0), thereby establishing a short circuit between source Si and Si+1 through output terminals OUTi and OUTi+1 of the source driving circuit; analog switch Ei turns off if switch control signal PC=0 (PCB=1), thereby breaking the short circuit that has been established between source lines Si and Si+1 (and between the corresponding output terminals of the source driving circuit).
The a-group of protective resistors comprises m protective resistors a1, a2, . . . , am. The i-th protective resistor ai is connected between analog switch Ai and output terminal OUTi (source line Si) of the source driving circuit 20, and provides protection for analog switch Ai, analog switch Ei or Ei−1, and source driver SDi.
The b-group of feedback resistors comprises m feedback resistors b1, b2, . . . , bm. The i-th feedback resistor bi is connected between analog switch Bi and output terminal OUTi (source line Si) of the source driving circuit 20, and provides protection for analog switch Bi and source driver SDi.
In the source driving circuit 20 of the second embodiment, source driver SDi, analog switches Ai, Bi, Ci, and Ei, protective resistor ai, feedback resistor bi, and output terminal OUTi form an output circuit.
The operation of the source driving circuit 20 in the second embodiment will be described below with reference to FIG. 3, which shows waveforms of an output signal OUT of the source driving circuit 20 (the signal output from output terminal OUTi to source line Si), the switch control signal PC and the complementary switch control signal PCB. Td indicates the dot driving cycle time of the liquid crystal display; Tp indicates the precharging period.
During a precharging period, switch control signal PC is ‘1’ and switch control signal PCB is ‘0’, so the A- and B-group analog switches are all in the off state, while the C- and E-group analog switches are all in the on state.
Since analog switches Ai and Bi are in the off state and analog switch Ei (or Ei−1) is in the on state, output terminal OUTi (source line Si) is disconnected from the output and inverting input terminals of source driver SDi and is connected via analog switch Ei (or Ei−1) to the adjacent output terminal OUTi+1 (source line Si+1) or OUTi−1 (source line Si−1), thereby being precharged.
Since analog switch Bi is off and analog switch Ci is on, the output potential of source driver SDi is fed back to the inverting input terminal of source driver SDi via analog switch Ci. Since the input impedance of the inverting input of source driver SDi is extremely high, the potential at the inverting input terminal of source driver SDi becomes equal to the output potential of source driver SDi regardless of the on-resistance in analog switch Ci. Since source driver SDi operates so as to make the potential of its inverting input (the output potential of source driver SDi) equal to the potential of its non-inverting input (source driving signal SSi), the output potential of source driver SDi equals the potential of source driving signal SSi.
At the transition from the precharging period to the driving period, switch control signal PC goes to the ‘0’ logic level and switch control signal PCB goes to the ‘1’ logic level, switching all the C- and E-group analog switches off and all the A- and B-group analog switches on.
Analog switch Ei (or Ei−1) accordingly turns off and analog switch Ai turns on, disconnecting output terminal OUTi (source line Si) from the adjacent output terminal OUTi+1 (source line Si+1) or OUTi−1 (source line Si−1) and connecting it to the output terminal of source driver SDi via analog switch Ai and protective resistor ai.
Analog switch Ci turns off and analog switch Bi turns on, switching from the second feedback path to the first feedback path, thereby feeding back the potential of output terminal OUTi (source line Si) after the voltage drop caused by the on-resistance of analog switch Ai and the resistance of the protective resistor ai to the inverting input terminal of source driver SDi via analog switch Bi. Since the input impedance at the inverting input terminal of source driver SDi is extremely high, the potential at the inverting input terminal of source driver SDi rapidly becomes equal to the potential of output terminal OUTi (source line Si) despite the presence of feedback resistor bi. Since source driver SDi operates so as to make the potential of its inverting input (the potential of output terminal OUTi or source line Si) equal to the potential of its non-inverting input (source driving signal SSi), the potential of output terminal OUTi (source line Si) rapidly becomes equal to the potential of source driving signal SSi.
At the precharging-to-driving transition, accordingly, the source driving circuit 20 in the second embodiment switches the feedback potential of the i-th output circuit from the potential at a point preceding analog switch Ai to the potential at a point following protective resistor ai, thereby compensating for the voltage drop due to the on-resistance of analog switch Ai and protective resistor ai, so that the potential of output terminal OUTi (source line Si) quickly becomes equal to the potential of source driving signal SSi (the output potential of source driver SDi). This feedback arrangement also compensates for variations in voltage drop due to variations in the resistance of the protective resistors and the on-resistance of the analog switches, resulting in both faster and more accurate driving of the source lines.
This feedback arrangement can also compensate for the voltage drop due to the resistance of the signal line from the output terminal of source driver SDi to the point at which analog switches Ai and Bi are interconnected, which accounts for most of the wiring resistance on the signal path from the output terminal of source driver SDi to output terminal OUTi. This means that, if there are variations in wiring resistance (or wiring length) on the output paths, they can be compensated for completely, or almost completely, by interconnecting the analog switches Ai and Bi at output terminal OUTi or at a point located as near as possible to output terminal OUTi.
During the driving period, switch control signal PC is ‘0’ and switch control signal PCB is ‘1’, so the A- and B-group analog switches are all in the on state and the C- and E-group analog switches are all in the off state.
Analog switch Ei (or Ei−1) is in the off state, and analog switch Ai is in the on state, disconnecting output terminal OUTi (source line Si) from the adjacent output terminal OUTi+1 (source line Si+1) or OUT1−1 (source line Si−1) and connecting it to the output terminal of source driver SDi via analog switch Ai.
Analog switch Ci is in the off state and analog switch Bi is in the on state, feeding the potential of output terminal OUTi (source line Si) back via feedback resistor bi and analog switch Bi on the first feedback path to the inverting input terminal of source driver SDi, thereby keeping the potential of output terminal OUTi (source line Si) equal to the potential of the non-inverting input (source driving signal SSi) of source driver SDi.
At the transition from the driving period to the next precharging period, switch control signal PC goes to ‘1’ and switch control signal PCB goes to ‘0’, switching all the A- and B-group analog switches off and all the C- and E-group analog switches on.
Analog switches Ai and Bi turn off and analog switch Ei (or Ei−1) turns on, disconnecting output terminal OUTi (source line Si) from the output and inverting input terminals of source driver SDi, and connecting output terminal OUTi to adjacent output terminal OUTi+1 (source line Si+1) or OUTi−1 (source line Si−1) via analog switch Ei (or Ei−1), thereby precharging source line Si to the average potential of source line Si (output terminal OUTi) and the adjacent source line Si+1 or Si−1 (output terminal OUTi+1 or OUTi−1) during the preceding driving period.
Analog switch Bi turns off and analog switch Ci turns on, switching the feedback path from the first feedback path to the second feedback path, thereby feeding the output potential of source driver SDi back to the inverting input terminal of source driver SDi via analog switch Ci.
As described above, the second embodiment provides a first feedback path from a point following the protective resistor to the source driver during the driving period and a second feedback path from a point preceding the A-group analog switch to the source driver during the precharging period, and switches the feedback path at transitions from the driving period to the precharging period and vice versa, thereby compensating for the voltage drop due to the on-resistance of the analog switch and the resistance of the protective resistor, and further compensating for variations in on-resistance and wiring resistance of the output path. The second embodiment thereby achieves fast and highly accurate liquid crystal driving. The second embodiment also conserves power by precharging each source line from an adjacent source line, and reduces the number of analog switches that control precharging by providing only one such switch for each two source lines.
Third Embodiment
Referring to FIG. 6, the source driving circuit 30 in the third embodiment comprises m source drivers SD1, SD2, . . . , SDm, an A-group of analog switches that control output paths, a B-group of analog switches that control first feedback paths, a C-group of analog switches that control second feedback paths, an F-group of analog switches that control precharging, a group of m output terminals OUT1, OUT2, . . . , OUTm, and an inverter I, where m is an arbitrary integer equal to or greater than two.
The source driving circuit 30 in the third embodiment accordingly alters the group of analog switches that control precharging in the source driving circuit 10 (see FIGS. 1 and 2) in the first embodiment.
The F-group of analog switches comprises m analog switches (MOS switches) F1, F2, . . . , Fm. Analog switch Fi is connected between the i-th output terminal OUTi (source line Si) of the source driving circuit 30 and the common voltage Vcom (the potential of the common electrode of the liquid crystal capacitors). The gate electrode of the PMOS transistor in analog switch Fi receives the switch control signal PCB output from the inverter I; the gate electrode of the NMOS transistor in analog switch Fi receives switch control signal PC. Analog switch Fi turns on when PC=1 (PCB=0), thereby connecting output terminal OUTi (source line Si) to the common voltage Vcom; analog switch Fi turns off when PC=0 (PCB=1), thereby disconnecting output terminal OUTi (source line Si) from the common voltage Vcom. The third embodiment uses the common voltage Vcom for precharging the source lines (the output terminals of the source driving circuit). The common voltage Vcom is, for example, half the potential of the power supply voltage supplied to source drivers SD1 to SDm, this being the midpoint potential in the output range of source drivers SD1 to SDm.
In the source driving circuit 30 of the third embodiment, source driver SDi, analog switches Ai, Bi, Ci, and Fi, and output terminal OUTi form an output circuit.
The operation of the source driving circuit 30 in the third embodiment will be described below with reference to FIG. 7, which shows waveforms of an output signal OUT of the source driving circuit 30 (the signal output from output terminal OUTi to source line Si), the switch control signal PC and the complementary switch control signal PCB. Td indicates the dot driving cycle time of the liquid crystal display; Tp indicates the precharging period.
During a precharging period, switch control signal PC (the input signal to inverter I) is ‘1’ and switch control signal PCB (the output signal from inverter I) is ‘0’, so the A- and B-group analog switches are all in the off state, while the C- and F-group analog switches are all in the on state.
Since analog switches Ai and Bi are in the off state and analog switch Fi is in the on state, output terminal OUTi (source line Si) is disconnected from the output and inverting input terminals of source driver SDi and is connected via analog switches Fi to the common voltage Vcom, thereby being precharged to the Vcom potential.
Since analog switch Bi is off and analog switch Ci is on, the output potential of source driver SDi is fed back to the inverting input terminal of source driver SDi via analog switch Ci. Since the input impedance of the inverting input of source driver SDi is extremely high, the potential at the inverting input terminal of source driver SDi becomes equal to the output potential of source driver SDi regardless of the on-resistance in analog switch Ci. Since source driver SDi operates so as to make the potential of its inverting input (the output potential of source driver SDi) equal to the potential of its non-inverting input (source driving signal SSi), the output potential of source driver SDi equals the potential of source driving signal SSi.
At the transition from the precharging period to the driving period, switch control signal PC goes to the ‘0’ logic level and switch control signal PCB goes to the ‘1’ logic level, switching all the C- and F-group analog switches off and all the A- and B-group analog switches on.
Analog switch Fi accordingly turns off and analog switch Ai turns on, disconnecting output terminal OUTi (source line Si) from the common voltage Vcom and connecting it to the output terminal of source driver SDi via analog switch Ai.
Analog switch Ci turns off and analog switch Bi turns on, switching from the second feedback path to the first feedback path, thereby feeding back the potential of output terminal OUTi (source line Si) after the voltage drop caused by the on-resistance of analog switch Ai to the inverting input terminal of source driver SDi via analog switch Bi. Since the input impedance at the inverting input terminal of source driver SDi is extremely high, the potential at the inverting input terminal of source driver SDi rapidly becomes equal to the potential of output terminal OUTi (source line Si) regardless of the on-resistance of analog switch Bi. Since source driver SDi operates so as to make the potential of its inverting input (the potential of output terminal OUTi or source line Si) equal to the potential of its non-inverting input (source driving signal SSi), the potential of output terminal OUTi (source line Si) rapidly becomes equal to the potential of source driving signal SSi.
At the precharging-to-driving transition, accordingly, the source driving circuit 30 in the third embodiment switches the feedback potential of the i-th output circuit from the potential at a point preceding analog switch Ai to the potential at a point following analog switch Ai, thereby compensating for the voltage drop due to the on-resistance of analog switch Ai so that the potential of output terminal OUTi (source line Si) quickly becomes equal to the potential of source driving signal SSi (the input potential to source driver SDi). This feedback arrangement also compensates for variations in voltage drop due to variations in on-resistance, resulting in both faster and more accurate driving of the source lines.
This feedback arrangement can also compensate for the voltage drop due to the resistance of the signal line from the output terminal of source driver SDi to the point at which analog switches Ai and Bi are interconnected, which accounts for most of the wiring resistance on the signal path from the output terminal of source driver SDi to output terminal OUTi. This means that, if there are variations in wiring resistance (or wiring length) on the output paths, they can be compensated for completely, or almost completely, by interconnecting the analog switches Ai and Bi at output terminal OUTi or at a point located as near as possible to output terminal OUTi.
During the driving period, switch control signal PC is ‘0’ and switch control signal PCB is ‘1’, so the A- and B-group analog switches are all in the on state and the C- and F-group analog switches are all in the off state.
Analog switch Fi is in the off state, and analog switch Ai is in the on state, disconnecting output terminal OUTi (source line Si) from the common voltage Vcom and connecting it to the output terminal of source driver SDi via analog switch Ai.
Analog switch Ci is in the off state and analog switch Bi is in the on state, feeding the potential of output terminal OUTi (source line Si), which is the output potential of source driver SDi minus the voltage drop due to the on-resistance of analog switch Ai, back via analog switch Bi to the inverting input terminal of source driver SDi, thereby keeping the potential of output terminal OUTi (source line Si) equal to the potential of the non-inverting input (source driving signal SSi) of source driver SDi.
At the transition from the driving period to the next precharging period, switch control signal PC goes to ‘1’ and switch control signal PCB goes to ‘0’, switching all the A- and B-group analog switches off and all the C- and F-group analog switches on.
Analog switches Ai and Bi turn off and analog switch Fi turns on, disconnecting output terminal OUTi (source line Si) from the output and inverting input terminals of source driver SDi, and connecting output terminal OUTi to the common voltage Vcom, thereby precharging source line Si to the Vcom potential.
Analog switch Bi turns off and analog switch Ci turns on, switching from the first feedback path to the second feedback path, thereby feeding the output potential of source driver SDi back to the inverting input terminal of source driver SDi via analog switch Ci.
As described above, the third embodiment provides a first feedback path from a point following the A-group analog switch to the source driver during the driving period and a second feedback path from a point preceding the A-group analog switch to the source driver during the precharging period, and switches the feedback path at transitions from the driving period to the precharging period and vice versa, thereby compensating for the voltage drop due to the on-resistance of the analog switch in the driving period, and further compensating for variations in on-resistance and wiring resistance of the output path. The third embodiment thereby achieves fast and highly accurate liquid crystal driving.
Those skilled in the art will recognize that many modifications can be made to the above embodiments within the scope of the invention, which is defined in the appended claims.

## Claims

1. An output circuit having an impedance conversion element generating an output signal from an input signal and a feedback signal, and an output path that conducts the output signal from the impedance conversion element to an output terminal, the output circuit also comprising:
a first switch disposed on the output path, for conducting the output signal during an output period and blocking the output signal during a non-output period;
a second switch for conducting the output signal from a first point on the output path to the impedance conversion element as the feedback signal during the output period, the first point being disposed at the output terminal or between the first switch and the output terminal; and
a third switch for conducting the output signal from a second point on the output path to the impedance conversion element as the feedback signal during the non-output period, the second point being disposed between the impedance conversion element and the first switch.

2. The output circuit of claim 1, further comprising:
a protective resistor connecting the first point to the first switch; and
a feedback resistor connecting the first point to the second switch.

3. The output circuit of claim 1, further comprising a fourth switch for connecting the output terminal to a fixed power supply during the non-output period.

4. The output circuit of claim 1, wherein the impedance conversion element is an operational amplifier having an inverting input terminal for receiving the feedback signal.

5. The output circuit of claim 1, wherein the first, second, and third switches are analog switches controlled by a switch control signal, the first and second switches being turned on when the switch control signal is at a first logic level and being turned off when the switch control signal is at a second logic level, the third switch being turned off when the switch control signal is at the first logic level and being turned on when the switch control signal is at the second logic level.

6. A liquid crystal driving circuit for driving a liquid crystal panel, the liquid crystal driving circuit comprising a plurality of output circuits as described in claim 1, the liquid crystal panel having a plurality of signal lines connected to the output terminals of the driving circuits, the impedance conversion element of each output circuit thus functioning as a signal line driver.

7. The liquid crystal driving circuit of claim 6, further comprising at least one fourth switch for interconnecting said signal lines during the non-output period.

8. The liquid crystal driving circuit of claim 6, further comprising at least one fourth switch for interconnecting a mutually adjacent pair of said signal lines during the non-output period.

9. The liquid crystal driving circuit of claim 6, further comprising a plurality of fourth switches for connecting said signal lines to a fixed power supply during the non-output period.

10. A method of driving a liquid crystal panel having a plurality of signal lines by using a plurality of drivers generating respective output signals from respective input signals and respective feedback signals, the method comprising:
connecting the drivers to a plurality of output terminals to which said signal lines are connected via respective switches disposed between the drivers and the output terminals, thereby using the output signals of the drivers to drive said signal lines, and returning the output signals from connection points disposed between the switches and the output terminals to the drivers as said feedback signals while the drivers are connected; and
disconnecting the drivers from the output terminals and precharging said signal lines while using the output signals of the drivers as said feedback signals.

11. The method of claim 10, wherein precharging said signal lines comprises interconnecting said signal lines.

12. The method of claim 10, wherein precharging said signal lines comprises interconnecting mutually adjacent pairs of said signal lines.

13. The method of claim 10, wherein precharging said signal lines comprises connecting said signal lines to a fixed power supply.