Level shifter and panel display using the same

This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0029943 filed on Apr. 29, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.

(a) Field of the Invention

The present invention relates to a level shifter and a display using the same. More specifically, the present invention relates to a level shifter of which power efficiency is increased and a flat panel display using the same.

(b) Description of the Related Art

FIG. 1 shows a circuit diagram for representing a conventional level shifter. The conventional level shifter includes positive-channel metal oxide semiconductor (PMOS) transistors M 1 A, M 2 A, and negative-channel metal oxide semiconductor (NMOS) transistors M 3 A, M 4 A. The transistors M 1 A, M 2 A are cross-connected. An input voltage Vin is applied to a gate of the transistor M 3 A. An inverted voltage Vinb of the input voltage Vin is applied to a gate of the transistor M 4 A.

In the conventional level shifter, a voltage level of output signal Vout is varied according to the variation in values of the input voltages Vin, Vinb applied to the gates of the transistors M 3 A, M 4 A, and therefore it has been a problem that the conventional transistor is sensitive to skew of the input voltages Vin, Vinb.

FIG. 2 shows a circuit diagram for representing another conventional level shifter. The level shifter shown in FIG. 2 is different from the level shifter shown in FIG. 1 in that a transistor M 1 B of the level shifter shown in FIG. 2 is diode-connected and a gate of the transistor M 1 B is coupled to a gate of a transistor M 2 B of the level shifter shown in FIG. 2 .

In the level shifter shown in FIG. 2 , it has been a problem that a path of a static current flow is generated from a power source LVDD to a power source VSS while an output voltage is output according to an input voltage. Accordingly, power consumption of the level shifter is problematically increased.

In accordance with the present invention, a level shifter is provided for preventing an output signal from being affected by skew generated between input signals and having less power consumption.

Also in accordance with the present invention, a flat-panel display using a level shifter which is not affected by skew generated between input signals, and using a level shifter which has low power consumption, is provided.

In one aspect of the present invention, a level shifter receives an input signal alternately having a first-level voltage and a second-level voltage, and generates a third-level voltage and a fourth-level voltage according to the first-level voltage and the second-level voltage. The level shifter includes a first transistor coupled between a first power source and an output terminal; a second transistor coupled between the output terminal and a second power source, in which an inverted signal of the input signal is applied to a gate; a capacitor coupled between the first transistor and the gate of the second transistor; and a switch for applying a voltage corresponding to the first-level voltage to the gate of the first transistor by responding to the first-level voltage and for blocking the input signal to the gate of the first transistor by responding to the second-level voltage.

In another aspect of the present invention, a level shifter receives an input signal alternately having a first-level voltage and a second-level voltage, and generates a third-level voltage and, a fourth-level voltage according to the first-level voltage and the second-level voltage. The level shifter includes a first transistor coupled between a first power source and an output terminal, in which an inverted signal of the input signal is applied to a gate; a second transistor coupled between the output terminal and a second power source; a capacitor coupled between the first transistor and the gate of the second transistor; and a switch for blocking the input signal to the gate of the second transistor by responding to the first-level voltage, and for applying a voltage corresponding to the second-level voltage to the gate of the second transistor by responding to the second-level voltage.

In a further aspect of the present invention, a level shifter receives an input signal alternately having a first-level voltage and a second-level voltage, and generates a third-level voltage and a fourth-level voltage according to the first-level voltage and the second-level voltage. The level shifter includes a first transistor coupled between a first power source and an output terminal; a second transistor coupled between the output terminal and a second power source, in which the input signal is applied to a gate; a capacitor coupled between the first transistor and the gate of the second transistor; and a switch coupled between a third power source and the gate of the first transistor, for applying a voltage corresponding to the voltage of the third power source to the gate of the first transistor when the input signal is the first-level voltage, and for blocking the third power source to the gate of the first transistor when the input signal is the second-level voltage.

FIG. 1 shows a circuit diagram for representing a conventional level shifter.

FIG. 2 shows a circuit diagram for representing another conventional level shifter.

FIG. 3 shows a circuit diagram for representing a level shifter according to a first exemplary embodiment of the present invention.

FIG. 4 shows waveforms of a first input voltage Vin and an output voltage Vout of a level shifter according to the first exemplary embodiment of the present invention.

FIG. 5 shows a circuit diagram for representing a level shifter according to a second exemplary embodiment of the present invention.

FIG. 6 shows waveforms of a second input voltage Vinb and an output voltage Vout of a level shifter according to the second exemplary embodiment of the present invention.

FIG. 7 shows a circuit diagram for representing a level shifter according to a third exemplary embodiment of the present invention.

FIG. 8 shows waveforms of a first input voltage Vin and an output voltage Vout of a level shifter according to the third exemplary embodiment of the present invention.

FIG. 9 shows a circuit diagram for representing a level shifter according to a fourth exemplary embodiment of the present invention.

FIG. 10 shows waveforms of a second input voltage Vin and an output voltage Vout of a level shifter according to the fourth exemplary embodiment of the present invention.

FIG. 11 shows a diagram for representing a flat panel display using the level shifter according to the exemplary embodiments of the present invention.

A level shifter according to a first exemplary embodiment of the present invention will be described with reference to FIG. 3 and FIG. 4 .

As shown in FIG. 3 , the level shifter according to the first exemplary embodiment of the present invention includes transistors M 1 C, M 2 C, M 3 C and a capacitor C 1 C. According to the first exemplary embodiment of the present invention, the transistors M 1 C, M 3 C are P-channel transistors, and the transistor M 2 C are an N-channel transistor.

The transistors M 1 C, M 2 C are coupled to each other in series between a power source for supplying a voltage LVDD and a power source for supplying a voltage VSS. The voltage LVDD is applied to a source of the transistor M 1 C and a drain of the transistor M 1 C is coupled to a drain of the transistor M 2 C. The voltage VSS is applied to a source of the transistor M 2 C.

A voltage at a node of the drain of the transistor M 1 C and the drain of the transistor M 2 C is an output voltage Vout at the level shifter according to the first exemplary embodiment of the present invention.

The transistor M 3 C is a diode-connected transistor, and a drain of the transistor M 3 C is coupled to a gate of the transistor M 1 C. According to the first exemplary embodiment of the present invention, the transistor M 3 C is forward or reverse biased by a first input voltage Vin, and therefore transmits the first input voltage Vin to the gate of the transistor M 1 C or blocks the first input voltage Vin.

The capacitor C 1 C is coupled between the gate of the transistor M 1 C and a gate of the transistor M 2 C, and maintains a voltage between the gates of the transistors M 1 C, M 2 C.

According to the first exemplary embodiment of the present invention, the first input voltage Vin is applied to a source of the transistor M 3 C, and a second input voltage Vinb is applied to the gate of the transistor M 2 C. The second input voltage Vinb is an inverted voltage of the first input voltage Vin.

An operation of the level shifter according to the first exemplary embodiment of the present invention will be described with reference to FIG. 4 . The first input voltage Vin alternately has a high-level voltage VDD and a low-level voltage VSS. According to the first exemplary embodiment of the present invention, the high-level voltage VDD is lower than a voltage LVDD. Herein, the voltage VSS may be a ground voltage.

The voltage VDD is assumed such that the transistor M 1 C is turned on by a difference (VDD−Vt 3 ) between a voltage VDD and an absolute value (Vt 3 ) of a threshold voltage of the transistor M 3 C as given in Equation 1.

VDD−Vt 3< LVDD−Vt 1  [EQUATION 1]

where Vt 1 denotes the absolute value of the threshold voltage of the transistor M 1 C.

The voltage VDD is higher than the threshold voltage Vt 3 of the transistor M 3 C. The voltage VDD is assumed such that the transistor M 1 C is turned off by a difference value (2VDD−Vt 3 ) between twice the voltage VDD and the threshold voltage Vt 3 of the transistor M 3 C as given in Equation 2.

2 VDD−Vt 3> LVDD−Vt 1  [EQUATION 2]

When the first input voltage Vin is the high-level voltage VDD in a period of T 1 , the transistor M 3 C is forward-biased, and the first input voltage Vin is transmitted to the gate of the transistor M 1 C. At this time, the transistor M 3 C is diode-connected, and therefore a voltage applied to the gate of the transistor M 1 C is given to be a voltage (VDD−Vt 3 ).

As assumed above, the difference (VDD−Vt 3 ) between the voltage VDD and the absolute value (Vt 3 ) of the threshold voltage of the transistor M 3 C is a value at a level for turning on the transistor M 1 C, and therefore the transistor M 1 C is turned on in the period of T 1 .

The second input voltage Vinb is applied to the gate of the transistor M 2 C, and the transistor M 2 C is turned off because the second input voltage Vinb is the low-level voltage VSS.

Accordingly, the output voltage Vout substantially corresponds to the high-level voltage LVDD in the period of T 1 .

The voltage (VDD−Vt 3 ) is applied to an electrode AC of the capacitor C 1 C, the voltage VSS is applied to another electrode BC of the capacitor C 1 C, and therefore, the capacitor C 1 C is charged with a charge corresponding to a voltage difference between both electrodes.

When the first input voltage Vin becomes the low-level voltage VSS in a period of T 2 , the transistor M 3 C is reverse-biased, and the first input voltage Vin is not transmitted to the gate of the transistor M 1 C. The transistor M 2 C is turned on because the second input voltage Vinb becomes the high-level voltage VDD.

At this time, the voltage applied to the electrode BC of the capacitor C 1 C is switched from the low-level voltage VSS to the high-level voltage VDD, and the charge of the capacitor C 1 C is maintained because the electrode AC is floated (that is, no current flows through the electrode AC). Therefore, a voltage variation value of the electrode BC of the capacitor C 1 C affects status of the voltage of the electrode AC because the voltages of both terminals are maintained.

That is, when the low-level voltage VSS is a ground voltage, the voltage variation value of the electrode BC of the capacitor C 1 C is VDD, and therefore the voltage of the electrode AC of the capacitor C 1 C is increased by the voltage VDD from the voltage (VDD−VT 3 ) in the period of T 1 .

Accordingly, the voltage applied to the electrode AC of the capacitor C 1 C is given by Equation 3.

V CA =( VDD−Vt 3)+ VDD= 2 VDD−Vt 3  [EQUATION 3]

where V CA denotes a voltage of the electrode AC of the capacitor C 1 C, and Vt 3 denotes an absolute value of the threshold voltage of the transistor M 3 C.

As described above, the voltage (2VDD−Vt 3 ) is a voltage for turning off the transistor M 1 C, and therefore the transistor M 1 C is turned off by the voltage (2VDD−Vt 3 ).

Accordingly, the transistor M 2 C is turned on, the transistor M 1 C is turned off, and therefore the output voltage Vout substantially corresponds to the low-level voltage VSS.

The level shifter according to the first exemplary embodiment of the present invention outputs the voltage LVDD when the input voltage Vin is the high-level voltage VDD, and outputs the voltage VSS when the input voltage Vin is the low-level voltage VSS. The voltage LVDD is higher than the voltage VDD, and therefore the level shifter according to the first exemplary embodiment of the present invention is a level-up shifter for increasing the level of the input voltage Vin and outputting the input voltage.

According to the first exemplary embodiment of the present invention, one of the transistors M 1 C, M 2 C is interrupted while the level of the first input voltage Vin is converted and the input voltage is output as the output voltage Vout. Therefore, power consumption caused by static currents is not substantially generated and the power consumption of the level shifter is reduced.

The level shifter according to the first exemplary embodiment of the present invention has been described above. The transistor M 3 C has been described above as a P-channel transistor. However, the invention is not limited to the predetermined channel type of the transistor M 3 C, and the transistor M 3 C may also be an N-channel transistor, which will be described in third and a fourth exemplary embodiments. With such a configuration, a leakage current flowing through the transistor M 3 C is reduced. That is, an increase of the reverse currents according to an increase of the reverse bias voltage is immaterial when an N-channel TFT (thin film transistor) uses an LDD (lightly doped drain) structure. The leakage currents caused by the leakage of charge of the capacitor are reduced when the N-channel TFT is used instead of the P-channel TFT in which the reverse currents are greatly increased according to the increase of the reverse bias voltage.

A level shifter according to a second exemplary embodiment of the present invention will be described with reference to FIG. 5 and FIG. 6 . FIG. 5 shows a circuit diagram representing a level shifter according to a second exemplary embodiment of the present invention, and FIG. 6 shows waveforms of a second input voltage Vinb and an output voltage Vout of a level shifter according to the second exemplary embodiment of the present invention.

The level shifter according to the second exemplary embodiment of the present invention is different from the level shifter according to the first exemplary embodiment of the present invention in that the high-level voltage VDD is applied to the source of the transistor M 3 D.

An operation of the level shifter according to the second exemplary embodiment of the present invention will be described.

The transistor M 2 D is turned off when the second input voltage Vinb is the low-level voltage VSS. However, the transistor M 3 D is forward-biased by the high-level voltage VDD, and a voltage corresponding to a difference between the voltage VDD and the absolute value (Vt 3 ) of the threshold voltage of the transistor M 3 D is applied to the gate of the transistor M 1 D. Accordingly, the transistor M 1 D is turned on.

The output voltage Vout of the level shifter according to the second exemplary embodiment of the present invention is the high-level voltage LVDD in the period of T 1 .

A voltage applied to an electrode AD of the capacitor C 1 D in the period of T 1 is the voltage (VDD−Vt 3 ), and a voltage applied to another electrode BD of the capacitor C 1 D is the voltage VSS, and therefore the capacitor C 1 D is charged with charge corresponding to a voltage difference between both electrodes.

The transistor M 2 D is turned on when the second input voltage Vinb becomes the high-level voltage VDD in a period of T 2 .

The voltage of the electrode BD of the capacitor C 1 D is switched from the low-level voltage VSS to the high-level voltage VDD and the charge of the capacitor C 1 D is maintained because the electrode AD is floated. Accordingly, the voltages of both terminals of the capacitor C 1 D are maintained, and the voltage of the electrode AD of the capacitor C 1 D is varied by as much as the voltage variation value of the electrode BD. That is, the voltage variation value of the electrode BD of the capacitor C 1 D is VDD when the voltage VSS is a ground voltage, and therefore the voltage of the electrode AD of the capacitor C 1 D is increased by the voltage VDD from the voltage (VDD−Vt 3 ).

Accordingly, the voltage applied to the gate of the transistor M 1 D reaches (2VDD−Vt 3 ), and the transistor M 1 D is turned off.

The output voltage Vout of the level shifter in the period of T 2 is the low-level voltage VSS.

As described above, the level shifter is operated by the second input voltage Vinb which is the inverted voltage of the first input voltage Vin, and the problem of skew between the first input voltage Vin and the second input voltage Vinb is solved according to the second exemplary embodiment of the present invention. While the level of the second input voltage Vinb is switched to be output as the output voltage Vout, one of the transistors M 1 D, M 2 D is interrupted and therefore a path for the static current flow is interrupted. Accordingly, the power consumption of the level shifter is reduced because substantially no power consumption caused by the static currents is generated.

A level shifter according to a third exemplary embodiment of the present invention will be described with reference to FIG. 7 and FIG. 8 .

FIG. 7 shows a circuit diagram for representing a level shifter according to a third exemplary embodiment of the present invention, and FIG. 8 shows waveforms of a first input voltage Vin and an output voltage Vout of a level shifter according to the third exemplary embodiment of the present invention.

The level shifter according to the third exemplary embodiment of the present invention is different from the level shifter according to the first exemplary embodiment of the present invention in that the voltage VDD is applied to the source of the transistor M 1 E, the voltage LVSS is applied to the source of the transistor M 2 E, and the transistor M 3 E is coupled to the gate of the transistor M 2 E. The first input voltage Vin is applied to the source of the transistor M 3 E, and the second input voltage Vinb is applied to the gate of the transistor M 1 E.

The voltage LVSS which is a lower voltage than the voltage VSS is assumed to be a voltage such that the transistor M 2 E is turned on by a sum of the voltage VSS and the threshold voltage Vt 3 of the transistor M 3 E in a like manner of Equation 4.

VSS+Vt 3> LVSS+Vt 2  [EQUATION 4]

It is assumed that the transistor M 2 E is turned off by a value obtained by subtracting the voltage VDD from a sum of the voltage VSS and the threshold voltage Vt 3 of the transistor M 3 E in a like manner of Equation 5.

VSS+Vt 3− TDD<LVSS+Vt 2  [EQUATION 5]

An operation of the level shifter according to the third exemplary embodiment of the present invention will be described with reference to FIG. 8 .

The transistor M 3 E is forward-biased when a first input voltage Vin is a low-level voltage VSS in the period of T 1 . Accordingly, a voltage (VSS+Vt 3 ) corresponding to the sum of the voltage VSS and the threshold voltage Vt 3 of the transistor M 3 E is applied to the gate of the transistor M 2 E, and the transistor M 2 E is turned on as assumed above.

The transistor M 1 E is turned off when a second input voltage Vinb is the high-level voltage VDD.

Accordingly, the output voltage Vout of the level shifter substantially corresponds to the low-level voltage LVSS in the period of T 1 .

At this time, the voltage VDD is applied to an electrode AE of the capacitor C 1 E, the voltage (VSS+Vt 3 ) is applied to another electrode BE, and therefore a charge corresponding to the voltage difference between both electrodes is charged in the capacitor C 1 E.

When the first input voltage Vin is the high-level voltage VDD in the period of T 2 , the transistor M 3 E is reverse-biased, and therefore the first input voltage Vin is not transmitted to the gate of the transistor M 2 E. The transistor M 1 is turned on because the second input voltage Vinb is the low-level voltage VSS.

When the voltage of the electrode AE of the capacitor C 1 E is switched from the high-level voltage VDD to the low-level voltage VSS, the charge of the capacitor C 1 E is maintained because the electrode BE is floated (that is, no current flows through the electrode BE). Accordingly, the voltage of the electrode BE of the capacitor C 1 E is varied corresponding to the voltage variation value of the electrode AE because the voltages of both electrodes are maintained.

In more detail, when the voltage VSS is a ground voltage, the voltage of the electrode BE of the capacitor C 1 E is reduced by the voltage VDD from the voltage (VSS+Vt 3 ) in the period of T 1 because the voltage variation of the electrode AE of the capacitor C 1 E is −VDD. Therefore, the voltage at the electrode BE of the capacitor C 1 E is given in Equation 6.

V CB =( VSS+Vt 3)− VDD=VSS+Vt 3− VDD   [EQUATION 6]

where V CB denotes the voltage at the electrode BE of the capacitor C 1 E.

As assumed above, the transistor M 2 E is turned off by the voltage V CB of the electrode BE of the capacitor C 1 E because the transistor M 2 E is turned off by a level of voltage which is a value obtained by subtracting the voltage VDD from the sum of the voltage VSS and the threshold voltage Vt 3 of the transistor M 3 E.

Accordingly, the level shifter according to the third exemplary embodiment of the present invention outputs the high-level voltage VDD when the first input voltage Vin is a high-level voltage, and outputs the low-level voltage LVSS when the first input voltage Vin is a low-level voltage VSS. The voltage LVSS is lower than the voltage VSS, and therefore the level shifter according to the third exemplary embodiment of the present invention is a level-down shifter.

In the third exemplary embodiment of the present invention, the power consumption of the level shifter is reduced because one of the transistors M 1 E, M 2 E is interrupted while the level of the first input voltage Vin is converted and the input voltage is output as an output voltage Vout.

FIG. 9 shows a circuit diagram for representing a level shifter according to a fourth exemplary embodiment of the present invention, and FIG. 10 shows waveforms of a second input voltage Vin and an output voltage Vout of a level shifter according to a fourth exemplary embodiment of the present invention.

The level shifter according to the fourth exemplary embodiment of the present invention is different from the level shifter according to the third exemplary embodiment of the present invention because the low-level voltage VSS is applied to the source of the transistor M 3 F.

Operation of the level shifter according to the fourth exemplary embodiment of the present invention will be described with reference to FIG. 10 .

The transistor M 1 F is turned off when the second input voltage Vin is the high-level voltage VDD in the period of T 1 .

The transistor M 3 F is forward-biased by the low-level voltage VSS applied to the source of the transistor M 3 F. Therefore, a voltage corresponding to the sum of the voltage VSS and the threshold voltage Vt 3 of the transistor M 3 F is applied to the electrode BF of the capacitor C 1 F, and the transistor M 2 F is turned on as assumed.

Accordingly, the output voltage Vout of the level shifter according to the fourth exemplary embodiment of the present invention substantially corresponds to the low-level voltage LVSS in the period of T 1 .

The voltage VDD is applied to the electrode AF of the capacitor C 1 F, the voltage (VSS+Vt 3 ) is applied to the electrode BF, and therefore a charge corresponding to the voltage difference between both electrodes is charged to the capacitor C 1 F.

In the period of T 2 , when the second input voltage Vinb is the low-level voltage VSS, the transistor M 1 F is turned on and the voltage of the electrode AF of the capacitor C 1 F is switched from the high-level voltage VDD to the low-level voltage VSS.

When the voltage VSS is assumed to be a ground voltage, the voltage of the electrode AF of the capacitor C 1 F is reduced by the voltage VDD from the voltage of the electrode AF, and the charge of the capacitor C 1 F is maintained because the electrode BF is floated (that is, no current flows through the electrode BF). Accordingly, the voltage of the electrode BF of the capacitor C 1 F is reduced by the voltage VDD from the voltage in the period of T 1 because the voltages of both electrodes are maintained. Therefore, the transistor M 2 F is turned off because the voltage applied to the gate of the transistor M 2 F is a voltage (VS+Vt 3 −VDD) and the voltage VDD is higher than the voltage Vt 3 .

The level shifter according to the fourth exemplary embodiment of the present invention is affected by the second input voltage Vinb. The level shifter according to the fourth exemplary embodiment of the present invention outputs the low-level voltage LVSS when the second input voltage is the high-level voltage, and outputs the high-level voltage VDD when the second input voltage is the low-level voltage.

The level shifter according to the fourth exemplary embodiment of the present invention eliminates the problem of skew between the input voltages by using an input voltage. The power consumption is reduced because one of the transistors M 1 F and M 2 F is interrupted while the level of the first input voltage Vin is converted and the input voltage is output as an output voltage Vout.

The level shifters according to the exemplary embodiments of the present invention have been described. The level shifter is applied to a flat panel display using an integrated circuit (IC) of another level of voltage to thus convert a voltage level between the IC and the flat panel display. A flat panel display using the level shifter according to the exemplary embodiment of the present invention will now be described with reference to FIG. 11 .

FIG. 11 shows a diagram for representing a flat panel display using the level shifter according to the exemplary embodiment of the present invention.

The flat panel display shown in FIG. 11 includes a timing controller Tcon 100 , a shift register S/R 200 , a data driver 300 , and a display panel 400 . The timing controller 100 generates scan/timing signals and data/timing signals for driving the shift register 200 and the data driver 300 , respectively. The shift register 200 receives the scan/timing signals from the timing controller 100 and sequentially applies scan signals to scanning lines X 1 to Xm formed on the display panel 400 . The data driver 300 applies the data/timing signals to data lines Y 1 to Yn on the display panel 400 according to the timing signals.

When the voltage ranges used in the timing controller 100 and the shift register 200 are assumed to be different from each other, a level shifter L/S 500 in accordance with the exemplary embodiments of the present invention described above is formed between the timing controller 100 and the shift register 200 , and an output voltage range of the timing controller 100 is converted to a voltage range used in the shift register 200 . A buffer (not shown) may be used for the timing signals CLK and /CLK between the level shifter 500 and the shifter register 200 .

When the voltage ranges used in the shift register 200 and the display panel 400 are assumed to be different from each other, a level shifter L/S 600 is included between the shift register 200 and the scanning lines X 1 to Xm of the display panel 400 , and an output voltage range of the shift register 200 is converted to a voltage range used in the display panel 400 . A buffer (not shown) operated according to the voltage range used in the display panel 400 may be included between the level shifter 600 and the display panel 400 .

It has been exemplified that the level shifters are used between the timing controller 100 and the shift register 200 , and between the shift register 200 and the display panel 400 in FIG. 11 . In addition, the present invention is not merely limited to the examples described, but can be applied to other flat display configurations in which a voltage range is converted.

While this invention has been described in connection with what is presently considered to be practical embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.