Patent Grant
7990373
The present invention relates to power supply circuits used in liquid crystal display (LCD) devices; and particularly to a power supply circuit having small electrical energy consumption.
LCD devices are commonly used as displays for compact electronic apparatuses. This is because they not only provide good quality images with little power consumption, but also they are very thin. A typical LCD device includes a power supply circuit, which supplies operating voltages for various kinds of working units in the LCD device.
Referring to
The dividing circuit 130 has a first resistor 131, a second resistor 132, a shunt capacitor 134 and a dividing node 135. The first and the second resistors 131, 132 are connected in series to ground, defining a series branch. The diving node 135 is disposed between the first and the second resistors 131, 132. The shunt capacitor 134 is connected between the diving node 135 and ground, which can prevent the low drop-out linear regulator 110 from increasing a voltage amplification of the output voltage Vout, and inhibit the voltage ripple of the output voltage Vout.
The low drop-out linear regulator 110 includes a voltage input terminal 112, a voltage output terminal 113, and a voltage adjust terminal 114. The input voltage Vin is transmitted to the voltage input terminal 112 after being filtered by the first and the second filter capacitors 121, 122. The voltage output terminal 113 is connected to one end of the series branch of the dividing circuit 130, and the output voltage Vout is supplied to the rear DC/DC converter after being filtered by the third and the fourth filter capacitors 123, 124. The voltage adjust terminal 114 is connected to the dividing node 135, and defines a feedback loop with the dividing circuit 130. The feedback loop provides a reference voltage Vref to the low drop-out linear regulator 110 and adjust the output voltage Vout thereof. The reference voltage Vref is 1.25V voltage difference between the output terminal 113 and the voltage adjust terminal 114 of the low drop-out linear regulator 110, which is defined by the internal circuits of the low drop-out linear regulator 110.
In operation, the input voltage Vin is provided to the low drop-out linear regulator 110 through the voltage input terminal 112, and is modulation transferred to an idea output voltage Vout transmitting out through the output terminal 113. The output voltage Vout is adjusted through the feedback loop of the voltage adjust terminal 114 and the dividing circuit 130, which substantially equals to Vout=Vref(1+R1/R2), wherein R1 is the resistance value of the first resistor 131, and R2 is the resistance value of the second resistor 132. Thus, the adjustment of the output voltage Vout can be realized through the adjusting of the resistance values of the first and the second resistor 131, 132.
However, when the liquid crystal display (LCD) device operates in a stand-by mode, the DC/DC converter 100 keeps supplying output voltage Vout to the rear DC/DC converter of the power supply circuit of the LCD device. Thus, a large quantity of electric energy loss is produced, which makes the power supply circuit have a overlarge power dissipation.
What is needed, therefore, is a power supply circuit that can overcome the above-described deficiencies.
An exemplary power supply circuit for a liquid crystal display device includes a switch control circuit for receiving a control signal from an external control circuit, the control signal controlling the turning on or turning off of the switch control circuit; a first DC/DC converter for adjusting the direct current voltage from an external circuit, outputting an output voltage. The switch control circuit controls switches the power supply of the output voltage to a liquid crystal display panel of the liquid crystal display device.
Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
The emphasis in the drawings is placed upon clearly illustrating the principles of various embodiments of the present invention. Like reference numerals designate corresponding parts throughout various drawings.
Reference will now be made to the drawings to describe preferred embodiments of the present invention in detail.
Referring to
The power supply circuit 26 has a first DC/DC converter 27, a switch control circuit 28 and a second DC/DC converter 29. The first DC/DC converter 27 adjust an input voltage Vin from an external circuit, an provides a working voltage Vdd to the micro control unit 25, and outputs an adjusted output voltage Vout to the second DC/DC converter 29 through the switch control circuit 28. The second DC/DC converter 29 transfers the output voltage to gate working voltages VGH, VGL to the gate driving circuit 22, main working voltage of the time schedule controller 24, and working voltage of the video processing circuits 23. The switch control circuit 28 receives the control signal from the micro control unit 25, the control signal controlling turn-on state or turn-off state of the switch control circuit 28.
Referring to
The dividing circuit 276 has a first resistor 2761, a second resistor 2762, a shunt capacitor 2763 and a dividing node 2764. The first and the second resistors 2761, 2762 are connected in series to ground, defining a series branch. The dividing node 2764 is disposed between the first and the second resistors 2761, 2762. The shunt capacitor 2763 is connected between the diving node 2764 and ground, which can prevent the low drop-out linear regulator 2761 from increasing a voltage amplification of the output voltage Vout, and inhibit the voltage ripple of the output voltage Vout.
The low drop-out linear regulator 271 includes a voltage input terminal 2711, a voltage output terminal 2712, and a voltage adjust terminal 2713. The input voltage Vin is transmitted to the voltage input terminal 2711 after being filtered by the first and the second filter capacitors 272, 273. The voltage output terminal 2712 is connected to one end of the series branch of the dividing circuit 276, and the output voltage Vout is supplied to the micro control unit 25 and the second DC/DC converter 29, respectively, after being filtered by the third and the fourth filter capacitors 274, 275. The voltage adjust terminal 2713 is connected to the dividing node 2764, and defines a feedback loop with the dividing circuit 276. The feedback loop provides a reference voltage Vref to the low drop-out linear regulator 271 and adjust the output voltage Vout thereof. The reference voltage Vref is 1.25V voltage difference between the output terminal 2712 and the voltage adjust terminal 2713 of the low drop-out linear regulator 271, which is defined by the internal circuits of the low drop-out linear regulator 271.
The switch control circuit 28 includes a transistor 281, a field effect transistor (FET) 282, three bias resistors 283, 284, 285, and a postponed starting capacitor 286. The transistor 281 is a NPN transistor, which has a base electrode 2811, a collector electrode 2812, and an emitting electrode 2813. The FET 282 is a P-channel metallic oxide semiconductor field effect transistor (MOSFET), which has a gate electrode 2821, a source electrode 2822, and a drain electrode 2823. The base electrode 2811 of the transistor 281 receives the control signal from the micro-control unit 25 through the first bias resistor 283, the emitting electrode 2813 is grounded, and the collector electrode 2812 is connected to the gate electrode 2821 of the FET 282 through the second bias resistor 284. The source electrode 2822 of the EFT is connected to the voltage output terminal 2712, the drain electrode 2823 output voltage to the second DC/DC converter 29. The third bias resistor 285 is connected between the collector electrode 2812 and the voltage terminal 2712, and the postponed starting capacitor 286 is connected between the gate electrode 2821 and the voltage output terminal 2712.
In operation, the input voltage Vin is provided to the low drop-out linear regulator 271 through the voltage input terminal 2711, and is modulation transferred to an idea output voltage Vout transmitting out through the output terminal 2712. The output voltage Vout is adjusted through the feedback loop of the voltage adjust terminal 2713 and the dividing circuit 276, which substantially equals to Vout=Vref(1+R1/R2), wherein R1 is the resistance value of the first resistor 2761, and R2 is the resistance value of the second resistor 2762. Thus, the adjustment of the output voltage Vout can be realized through the adjusting of the resistance values of the first and the second resistor 2761, 2762. After that, one part of the output voltage Vout is provided to the micro control circuit 25. Because the micro control circuit 25 needs a micro load current, generally less than 50 mA, the electrical energy consumption of the output voltage Vout is less. The other part of the output voltage Vout is provided to the second DC/DC converter 29 through the switch control circuit 28. When the LCD device 2 works normally, the micro control unit 25 sends a high-level control signal to the base electrode 2811 of the transistor 281 and turn on the transistor 281. Thus, the potential of the collector electrode 2812 is nearly equal to zero, and the potential of the gate electrode 2821 of the EFT 282 is lowered to a low-level, and the EFT 282 is turned on, and the output voltage Vout is transmitted to the second DC/DC converter 29 through the drain electrode 2823. On the other hand, when a user inputs a stand-by signal to the micro control unit 25 through the human-computer interaction interface, the control unit 25 sends a low-level control signal to the base electrode 2811 of the transistor 281 and turn off the transistor 281. Thus, the potential of the gate electrode 2821 of the EFT 282 substantially equals to the output voltage Vout, and the EFT 282 is turned off, and the output voltage Vout is just provided to the micro control unit 25.
Comparing to the conventional circuit, the power supply circuit 26 utilizes the switch control circuit 28 to control the transmitting path of the output voltage Vout from the first DC/DC converter 27. Thus, when the LCD device 2 works in a stand-by state, the first DC/DC converter 27 stops supplying output voltage Vout to the second DC/DC converter 29, and only provides it to the micro control unit 25. Because the micro control unit 25 needs a micro load current, generally less than 50 mA, the electrical energy consumption of the output voltage Vout is less. Therefore, the LCD device 2 having the power supply circuit 26 has a small electrical energy consumption when it works at electrical-saving mode.
It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 95220885 U | Nov 2006 | TW | national |
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| Number | Date | Country | |
|---|---|---|---|
| 20080122828 A1 | May 2008 | US |
