Power driving circuit of a thin film transistor liquid crystal display

Abstract: The circuit includes a first analog switching circuit (AS1) for turning ON or OFF a first power signal and a second analog switching circuit (AS2) for turning ON or OFF a second power signal applied from an inverse signal corresponding to each level of an inverse signal and a non-inverse signal; a first Darlington circuit (3) for generating low level of waveform Von by turning OFF said first analog switching circuit; a second Darlington circuit (5) for generating high level of waveform Voff by turning ON said second analog switching circuit; a first switching circuit (n41, n42) for outputting high level of waveform Von by the first power signal turned ON when the inverse signal is at a high level, and for outputting a low level of waveform Von from the first Darlington circuit when said the inverse signal is at low level; a second switching circuit (n61,n62) for outputting a low level of waveform Voff by the second power signal turned ON when the inverse signal is at low level, and for outputting a high level of waveform Voff from the second Darlington circuit when said inverse signal is at a high level; and a third switching circuit (n71,n72) for outputting ground voltage when-said inverse signal is at a low level, and for outputting a given power.

Description: The present invention relates to a power driving circuit of a thin film transistor liquid crystal display (TFT-LCD). In particular, it relates to a driving circuit which more specifically reduces consumption of power by replacing an operational amplifier, for generating the output voltage, with a Darlington circuit. Up to now two methods of providing a thin film transistor liquid crystal display have generally been used; one is a common electrode constant driving method, another is a common electrode reverse driving method. The common electrode reverse driving method can reduce the extension of grey voltage to half of that of the common electrode constant driving method, thereby enabling the use of an integrated driver circuit having a small size and low price, obtained from a complementary metal oxide semiconductor making process. The common electrode reverse driving method has been proposed in JAPAN DISPLAY'92, pp. 475-478, "An 8.4-in TFT-LCD system for a note size computer using 3-bit digital data drivers" and in NIKKEI MICRODEVICES, Aug. 1993, pp. 64-66, TOSHIBA and HITACHI SEISAKUSHO et al, "5V driving method for low consumption power of TFT color liquid crystal". In such a method, the electric potential of the grey voltage applied to the liquid crystal, and that of the common electrode voltage, vibrate at a predetermined amplitude as cited in the above-mentioned papers. That method has an advantage that it can reduce consumption of power in driving the circuit, by driving the liquid crystal with a low voltage, whereas it has a disadvantage in that the construction of the driving circuit is difficult, because of the complicated driving method. For driving a thin film transistor liquid crystal display using the common electrode reverse driving method, power driving signals having the waveform illustrated in Figures 1A-1B is required. Von shown in Figure 1A, is the input waveform to a gate driver, which causes the thin film transistor to be turned ON periodically. Voff, shown in Figure 1C is the input waveform to a gate driver, which causes all transistors of the thin film transistor to be turned ON, and Vcom, shown in Figure 1B, is the input waveform to a common electrode of a liquid crystal capacitor. (Von, Voff and Vcom are indicated in Figure 2.). Conventionally, to make such a waveform, there has been used a typical type of power driving circuit including two analog switches 1 and 2, three operational amplifiers OP1 to OP3 each operated as a voltage follower, and three push-pull amplifiers P1 to P3 as shown in Figure 2. The conventional power driving circuit is described in more detail below with reference to Figure 2. A RVS signal (inversed signal) is a timing signal for phasing Von, Voff, Vcom, which are input to a thin film transistor liquid crystal display, whereas RVSB signal is an antiphase signal to the RVS signal. RVS and RVSB signals are output from a timing controller. A first analog switching circuit 1 is composed of an analog switch AS1, to which a pair of variable resistances VR11 and VR12 and a pair of resistances R11 and R12 are connected. A second analog switching circuit 2 is composed of an analog switch AS2, to which four variable resistances VR21 to VR24 and four resistances R21 and R24 are connected. The analog switching circuits 1 and 2 are turned on when the RVS signal which controls the switch is high, and are turned off when the RVS signal is low. The operational amplifiers OP1 to OP3, operated as a voltage follower, apply the voltage level which is input to a non-inverting terminal of the operational amplifier to a base terminal of the push-pull amplifiers P1 to P3, regardless of the load condition of the push-pull amplifiers P1 to P3. As a means of illustration only, EP-A-0 599 622 discloses a further example of a prior art power generating circuit comprising an operational amplifier and a push-pull circuit. In such an operation, power is expressed as follows: V GG (+25V) > V CC (+8V) > V DD (+5V) > GND(OV) > V EE (-8V), where the numbers in the parentheses are typical potentials. The following describes the steps of generating the waveform Von. When the RVS signal is high, the RVSB signal is low. At this time, the analog switch AS1 outputs the voltage set up by the variable resistance VR12, which is input to the base terminal of the push-pull amplifier P1 through the operational amplifier OP1. The input voltage falls as much as the voltage V BE , which amounts to the voltage level V ghl . Differently from the above, when the RVS signal is low, the RVSB signal is high. At this time, the analog switch AS1 outputs the voltage set up by the variable resistance VR11, which is input to the base terminal of the push-pull amplifier P1, through the operational amplifier OP1, as in the above-mentioned case. Then, the push-pull amplifier P1 outputs the voltage V gh2 which is lowered as much as V BE . Waveform Vcom is obtained by the same method. In this case, the level V c1 is adjusted by the variable resistance VR22, while the level V c2 is adjusted by VR21. In waveform Voff, the level V gLI is adjusted by the variable resistance VR24N, while the level V gL2 is adjusted by the variable resistance VR23. However, there are two disadvantages when constructing a power driving circuit as above: First, power consumption is considerably large. This is why, as cited in the above papers of TOSHIBA and HITACHI SEISAKUSHO, power consumption to the circuit has increased in generating power driving waveform caused by large power consumption of the operating amplifier. Second, the power voltage level cannot be output, because the voltage is lowered by the off-set voltage of the operational amplifier and the applied voltage to the base-emitter of the push-pull amplifier. In other words, although it is desirable that the level V gL2 be the voltage V EE , the circuit of the conventional art outputs the attenuated voltage as much as the off-set voltage of the operational amplifier and the applied voltage to base-emitter of the push-pull amplifier. Whereas the ideal waveform Vcom is a swing between the ground potential GND and the voltage V DD , this waveform requires and thus leads to increased power consumption. In view of the above it is an object of the present invention to provide a circuit for driving a thin film transistor liquid crystal display (TFT-LCD) with minimal power capable of reducing power consumption and having an off-set voltage. According to the present invention there is provided a power driving circuit for a thin film transistor liquid crystal display, arranged to provide waveforms Von and Voff for input to a gate driver to cause thin film transistors to be turned on or off respectively, and waveform Vcom for input to a common electrode of a liquid crystal capacitor, comprising: analogue switching circuits including a first analogue switching circuit (AS1) for production of first ON and OFF control signals for turning ON or OFF a first power signal applied from a first potential (V GG ) and a second analogue switching circuit (AS2) for production of second OFF and ON control signals for turning ON or OFF a second power signal applied from a second potential (V EE ) of opposite polarity to that of the first power signal, each said ON control signal corresponding to each high level of a timing signal (RVS) for phasing Von, Voff and Vcom; a first switching circuit (4), arranged to receive said first control signals, for outputting high level of waveform Von (Vgh1) of the first power signal responsive to an ON control signal at the high level of timing signal (RVS), and a low level of waveform Von (Vgh2) at the low level of timing signal (RVS); a second switching circuit (6), arranged to receive said second control signals, for outputting a low level of waveform Voff (VgL2) of the second power signal responsive to an OFF control signal at the low level of timing signal (RVS), and a high level of waveform Voff (VgL1) at the high level of said timing signal (RVS); a third switching circuit (7), arranged to receive said second control signals, for outputting ground voltage at the low level of said timing signal (RVS), and a given power voltage level (VDD) at the high level of said timing signal (RVS); said power driving circuit further comprising : a first Darlington circuit (3) for generating said low level of waveform Von (Vgh2) from its emitter electrode responsive to an OFF control signal output by said first analogue switching circuit; and a second Darlington circuit (5) for generating said high level of waveform Voff(VgL1) from its emitter electrode responsive to an ON control signal output by said second analogue switching circuit; wherein said first and second Darlington circuits each include respective means for adjusting the voltage level of each said first and second power signals by voltage dropping to generate respective low level of Von and high level of Voff. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a conventional waveform diagram of a power driving signal for driving a thin film transistor liquid crystal display; Figure 2 is a detailed circuit diagram of a power driving circuit for driving a thin film transistor liquid crystal display in accordance with the prior art; and Figure 3 is a detailed circuit diagram of a power driving circuit for driving a thin film transistor liquid crystal display in accordance with a preferred embodiment of the present invention. A preferred embodiment of the present invention is described with reference to Figure 3 of the accompanying drawings. As shown in Figure 3, a circuit for forming waveforms such as Von, Voff, Vcom comprises analog switching circuits including a first analog switching circuit AS1 and a second analog switching circuit AS2 which are analog multiplexers; Darlington circuits 3 and 5; and first, second and third switching circuits of which each input terminal is connected to each output terminal of the analog switches AS1 and AS2 and the Darlington circuit 3 and 5. The first, second and third switching circuits each include a respective pair of N-MOS transistors. Further, it is possible that the first, second and third switching circuits each include a respective pair of P-MOS transistors. In the conventional circuit of Figure 2, an analog switch is used for outputting a voltage level which determines the level of output voltage, while in the circuit of the present invention the analog switch is used for outputting the electric potential which makes the N-MOS transistors turn ON or OFF. Each pair of N-MOS transistors n41 and n42, n61 and n62, n71 and n72 which are turned ON or OFF by the output from the analog switches AS1 and AS2 can be replaced with P-MOS transistors. The Darlington circuits 3 and 5 including Darlington transistors D3 and D4 and adjustment resistances VR3 and VR5, respectively, are characterised by the way they output the levels V gh2 and V gL1 through N-MOS transistors n42 and n62. That is, the first and second Darlington circuits each include a variable resistor for adjusting the voltage of said first or second power signal by voltage dropping, and a Darlington transistor for dropping voltage as much as its base-emitter voltage from the adjusted voltage, and for outputting the dropped voltage to the corresponding switching circuit. Next, there is a described a method for forming waveform Von. The analog switch AS 1 is turned ON when the RVS signal is high, while it is turned OFF when the RVS signal is low. Accordingly, provided that the RVS signal in a high state is output, the power signal VGG is applied to a gate of the N-MOS transistor n41, whereby Von becomes V GG -Vth. Simultaneously, ground level is applied to a gate of the N-MOS transistor n42, whereby the N-MOS transistor n42 is turned OFF. Provided that the RVSB signal in a high state is output, V GG is applied to a gate of the N-MOS transistor n42, which is turned ON subsequently, and level V gh2 , determined by adjustment resistance VR3 is output. At this time, ground level is applied to the gate of the N-MOS transistor n41, and the N-MOS transistor n41 is turned off. As a result, in case that the RVS signal is high or low, V gh1 or V gh2 is output, respectively, through the output terminal of waveform Von. The method for forming the waveform Voff is described below. When the RVS signal is high, the analog switch AS2 applies the power signal Vcc to the gate of the N-MOS transistor n62 to be turned ON, and the potential V gL1 , decreased as much as 2V BE at V B adjusted by variable resistance VR5, is output to the source end of the N-MOS transistor n62. In that event, the N-MOS transistor n61 is turned OFF by applying V EE to the gate of the N-MOS transistor n61. Also, the power voltage level is applied to the gate of the N-MOS transistor n62, whereby the N-MOS transistor n62 is turned OFF. As a result, in case that RVS signal is high or low, V gh1 or V gh2 is output respectively through the output terminal of the waveform Voff. The method of forming the waveform Vcom is described next. The gate terminals of the N-MOS transistors n61 and n62 are connected to those of the N-MOS transistors n71 and n72, respectively. From this, the N-MOS transistor n72 is turned ON when the N-MOS transistor n61 is turned ON, and zero potential level (GND) is output through the output terminal of waveform Vcom. Simultaneously, the N-MOS transistor n71 is turned ON when the N-MOS transistor n62 is turned ON, so that power voltage level V DD is output through the output terminal of waveform Vcom. That is, when the RVS signal is high or low, V CI (V DD ) or V C2 (GND) is output, respectively. As described above, this embodiment of the present invention requires power consumption power of about 0.5W less than the prior art. Further, according to the present invention, the voltage level V gL2 can be replaced with the power voltage level V EE , so that the thin film transistor receives the voltage of the waveform Voff sufficiently. From this, it is possible to obtain a circuit for driving a thin film transistor liquid crystal display capable of improving the quality of picture in a liquid crystal display.

Claims: A power driving circuit for a thin film transistor liquid crystal display, arranged to provide waveforms Von and Voff for input to a gate driver to cause thin film transistors to be turned on or off respectively, and waveform Vcom for input to a common electrode of a liquid crystal capacitor, comprising: analogue switching circuits including a first analogue switching circuit (AS1) for production of first ON and OFF control signals for turning ON or OFF a first power signal applied from a first potential (V GG ) and a second analogue switching circuit (AS2) for production of second OFF and ON control signals for turning ON or OFF a second power signal applied from a second potential (V EE ) of opposite polarity to that of the first power signal, each said ON control signal corresponding to each high level of a timing signal (RVS) for phasing Von, Voff and Vcom; a first switching circuit (4), arranged to receive said first control signals, for outputting a high level of waveform Von (Vgh1) of the first power signal responsive to an ON control signal at the high level of timing signal (RVS), and a low level of waveform Von (Vgh2) at the low level of timing signal (RVS); a second switching circuit (6), arranged to receive said second control signals, for outputting a low level of waveform Voff (VgL2) of the second power signal responsive to an OFF control signal at the low level of timing signal (RVS), and a high level of waveform Voff (VgL1) at the high level of said timing signal (RVS); a third switching circuit (7), arranged to receive said second control signals, for outputting ground voltage at the low level of said timing signal (RVS), and a given power voltage level (V DD ) at the high level of said timing signal (RVS); said power driving circuit further comprising : a first Darlington circuit (3) for generating said low level of waveform Von (Vgh2) from its emitter electrode responsive to an OFF control signal output by said first analogue switching circuit; and a second Darlington circuit (5) for generating said high level of waveform Voff (VgL1) from its emitter electrode responsive to an ON control signal output by said second analogue switching circuit; wherein said first and second Darlington circuits each include respective means for adjusting the voltage level of each said first and second power signals by voltage dropping to generate respective low level of Von and high level of Voff. A power driving circuit for a thin film transistor liquid crystal display according to Claim 1, wherein the first and second analog switching circuits are analog multiplex. A power driving circuit for a thin film transistor liquid crystal display according to Claim 1 or Claim 2, wherein the first, second and third switching circuits each include a respective pair of N-MOS transistors. A power driving circuit for a thin film transistor liquid crystal display according to Claim 1 or Claim 2, wherein the first, second and third switching circuits each include a respective pair of P-MOS transistors. A power driving circuit for a thin film transistor liquid crystal display according to any of the preceding claims, wherein the first and second Darlington circuits each include a variable resistor for adjusting the voltage of said first or second power signal by dropping voltage, and a Darlington transistor for dropping voltage as much as its base-emitter voltage from the adjusted voltage, and for outputting the dropped voltage to the corresponding switching circuit

Inventors: MOON SEUNG-HWAN, SHIN KYOUNG-HOON, MOON, SEUNG-HWAN, SHIN, KYOUNG-HOON
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1995
Main classification: G09G3
IPC codes: G02F1, G09G3