BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an LCD according to a first embodiment of the present invention, the LCD including a power supply circuit.
FIG. 2 is a diagram of the power supply circuit of FIG. 1, the power supply circuit including a detecting circuit and a switching circuit.
FIG. 3 is a diagram of the detecting circuit of FIG. 2.
FIG. 4 is a diagram of the switching circuit of FIG. 2.
FIG. 5 is a diagram of a power supply circuit of an LCD according to a second embodiment of the present invention.
FIG. 6 is a diagram of a conventional power supply circuit of an LCD.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference will now be made to the drawings to describe various embodiments of the present invention in detail.
FIG. 1 is a block diagram of an LCD 2 according to a first embodiment of the present invention. The LCD 2 includes a control board 21 and an LCM 22. The LCM 22 includes a plurality of display units (not shown) arranged in a matrix for displaying images. The control board 21 includes a signal processing circuit 211 and a power supply circuit 212. The signal processing circuit 211 is configured to provide a plurality of control signals and a plurality of image signals to the LCM 22. The power supply circuit 212 is configured to provide a plurality of positive voltages and a plurality of negative voltages to the LCM 22.
Referring also to FIG. 2, this is a diagram of the power supply circuit 212. The power supply circuit 212 includes a power source IC 214. The power source IC 214 includes a detecting circuit 2141 and a switching circuit 2142 integrated therein. The power source IC 214 further includes a voltage input 213 configured to receive an external power source Vcc, a positive voltage output 215 configured to provide a positive voltage such as +5.5V, +3.3V or a gate switch on voltage (“VGH”, not shown) to the LCM 22, and a negative voltage output 216 configured to provide a negative voltage such as a gate switch off voltage (“VGL”, not shown) to the LCM 22.
Referring also to FIG. 3, this is a diagram of the detecting circuit 2141. The detecting circuit 2141 includes a comparator 2140. The comparator 2140 includes a first input 2143, a second input 2144, and an output 2145. The first input 2143 of the comparator 2140 is configured to receive the external power source Vcc. The second input 2144 of the comparator 2140 is configured to receive a reference voltage, which is equal to a normal output voltage of the external power source Vcc. The output 2145 of the comparator 2140 is configured to generate control signals according to a result of the comparison of the external power source Vcc and the reference voltage, and provide the control signals to the switching circuit 2142 for switching on or switching off the switching circuit 2142.
Referring also to FIG. 4, this is a diagram of the switching circuit 2142. The switching circuit 2142 includes a negative-positive-negative (NPN) bipolar transistor 2146, a current limiting resistor 2147, and a bias resistor 2148. The NPN bipolar transistor 2146 includes a collector electrode connected to the positive voltage output 215 of the power source IC 214 via the bias resistor 2148, an emitter electrode connected to the negative voltage output 216 of the power source IC 214, and a base electrode connected to the output 2145 of the comparator 2140 via the current limiting resistor 2147 for receiving the control signals.
When the LCD works normally, the comparator 2140 generates a first control signal when the external power source Vcc is equal to the reference voltage. Then the first control signal is provided by the output 2145 of the comparator 2140 to the base electrode of the NPN bipolar transistor 2146 via the current limiting resistor 2147, and the NPN bipolar transistor 2146 is switched off.
When the LCD is turned off, the comparator 2140 generates a second control signal when the external power source Vcc is decreased and is less than the reference voltage. Then the second control signal is provided by the output 2145 of the comparator 2140 to the base electrode of the NPN bipolar transistor 2146 via the current limiting resistor 2147, and the NPN bipolar transistor 2146 is switched on. Thus the positive voltage output 215 of the power source IC 214 is connected to the negative voltage output 216 of the power source IC 214 via the bias resistor 2148 and the activated NPN bipolar transistor 2146 in series. Therefore a voltage of the negative voltage output 216 of the power source IC 214 can be charged to zero voltage by the positive voltage output 215 of the power source IC 214. Thus, electric charge stored in each display unit of the LCM 22 is discharged quickly via the corresponding thin film transistor which is turned on when the voltage of the negative voltage output 216 is approximately equal to zero. Thereby, any residual image produced on a display screen of the LCM 22 may be depressed or even eliminated.
In summary, the LCD 2 includes the detecting circuit 2141 configured to generate a second control signal when the LCD 2 is turned off, and the switch circuit 2142 configured to receive the second control signal and electrically connect the negative voltage output 216 to the positive voltage output 215 in order to quickly increase a potential of the negative voltage output 216. Therefore electric charge stored in each display unit of the LCM 22 is quickly discharged, and any residual image that would otherwise be produced on the LCM 22 can be mitigated or even eliminated.
In an alternative embodiment, the NPN bipolar transistor 2146 may also be replaced by an n-channel metal-oxide-semiconductor field-effect transistor (NMOSFET).
FIG. 5 is a diagram of a power supply circuit 312 of an LCD according to a second embodiment of the present invention. The power supply circuit 312 is similar to the power supply circuit 212 of the LCD 2 of the first embodiment. However, a switching circuit 3142 of the power supply circuit 312 is arranged outside a power source IC 314, at a periphery of the power source IC 314.
It is to be understood, however, that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail, especially in matters of arrangement of parts within the principles of present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
Patent Application
20080042952
