Patent Application
20090243706
1. Field of Invention
The present invention relates to a charge pump. More particularly, the present invention relates to a voltage regulated negative charge pump.
2. Description of Related Art
A traditional charge pump, which makes use of the charge storing capability of capacitors, is provided to generate an output voltage that is the double or triple of a supply voltage. However, for a gate driver in a liquid crystal display (LCD), it is necessary to provide bias voltages, e.g. Vgh and Vgl, which are more precise to boost the gate driver for driving gate lines. Therefore, a charge pump with voltage regulation capabilities and the ability to generate a regulated output voltage is needed.
To this end, a voltage regulated charge pump, which takes advantage of an error amplifier to regulate the output voltage, is provided, so as to obtain more precise bias voltages to boost the gate driver. However, the liquid crystal display has become bigger, so the voltage regulated charge pump has to provide a higher output voltage for the loading of the large-sized liquid crystal display. In addition, the error amplifier of the conventional voltage regulated charge pump must be designed accordingly to have more sinking and sourcing capabilities to fit in with the large-sized liquid crystal display. Therefore, developing a voltage regulated charge pump for large-sized liquid crystal display is difficult, and the charge pump may have stability problems.
In accordance with one embodiment of the present invention, a voltage regulated charge pump is provided. The voltage regulated charge pump includes a charge pump circuit for receiving an input voltage to generate an output voltage, in which the charge pump circuit further includes a capacitor, a first switch and a second switch. The first switch is coupled between the input voltage and a first end of the capacitor. The second switch is coupled to the first end of the capacitor. The first and second switch are non-simultaneously turned on, so that the capacitor is charged to generate the output voltage. The voltage regulated charge pump further includes a third switch and a voltage regulating circuit. The third switch is coupled to the charge pump circuit. The voltage regulating circuit has an input receiving the output voltage and an output coupled to the third switch. The voltage regulating circuit is configured to modulate the third switch for regulating the output voltage generated by the charge pump circuit.
For the foregoing embodiment of the present invention, the voltage regulated charge pump can be provided to fit in with the large-sized liquid crystal display and output the required and more precise output voltage, so as to solve the stability problems caused when the voltage regulated charge pump outputs a large voltage.
It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.
The invention can be more fully understood by reading the following detailed description of the embodiments, with reference made to the accompanying drawings as follows:
The voltage regulated charge pump 100 further includes a third switch 120 and a voltage regulating circuit 130. The third switch 120 is coupled to the second switch 114. The voltage regulating circuit 130 has an input receiving the output voltage Vneg and an output coupled to the third switch 120. The voltage regulating circuit 130 is configured to modulate the third switch 120 for regulating the output voltage Vneg generated by the charge pump circuit 110. Moreover, the voltage regulating circuit 130 also receives a reference voltage Vref, which is usually set at a level to compare fluctuations in the output voltage Vneg.
In the present embodiment, the first switch 112 is a p-type metal-oxide-semiconductor field effect transistor (MOSFET) M1, and the second switch 114 is an n-type MOSFET M2. The transistor M1 has a gate coupled to the output of the first operational amplifier 116, a source coupled to the input voltage VIN, and a drain coupled to the first end of the capacitor C1. The transistor M2 has a gate coupled to the output of the second operational amplifier 118, a drain coupled to the first end of the capacitor C1, and a source coupled to the third switch 120. In addition, the third switch 120 can be an n-type MOSFET M3, in which the transistor M3 has a drain coupled to the source of the transistor M2, a source coupled to the ground GND, and a gate coupled to the voltage regulating circuit 130.
The voltage regulating circuit 130 further includes an error amplifier 132 and a voltage dividing circuit 134, in which the error amplifier 132 is an operational amplifier and configured to modulate the transistor M3. The error amplifier 132 has a first amplifier input coupled to the voltage dividing circuit 134, a second amplifier input coupled to the ground GND, and an amplifier output coupled to the gate of the transistor M3 so as to control the turn-on state of the transistor M3. The voltage dividing circuit 134 is coupled between the output voltage Vneg and the first amplifier input of the error amplifier 132, and outputs a fraction of the output voltage Vneg to the first amplifier input of the error amplifier 132. Furthermore, the voltage dividing circuit 134 can include two resistors R1 and R2, in which the resistor R1 has an end connected to the output voltage Vneg and another end connected to the first amplifier input of the error amplifier 132, and the resistor R2 has an end connected to the first amplifier input of the error amplifier 132 and another end connected to the reference voltage Vref.
One embodiment of the operation of the voltage regulated charge pump 100 is described as follows. The charge pump circuit 110 is operated during different periods to generate the regulated output voltage Vneg according to the clock signal CLK. During the first period, the transistor M1 is turned on and the transistor M2 is turned off, the voltage of the node Qa is thus charged approximately to the input voltage VIN, and the voltage of the node Qb is at a very low level and close to 0. At that moment, the output voltage Vneg is generated by the capacitor C1 and output to the first amplifier input of the error amplifier 132 through the resistor R1. The error amplifier 132 receives the fraction of the output voltage Vneg to output an error signal to turn on the transistor M3. If the output voltage Vneg changes based on the capacitor C1, the voltage of the amplifier output of the error amplifier 132, i.e. the voltage of the node Qc, will vary accordingly and thus changes the turn-on state of the s transistor M3. Since the turn-on state of the transistor M3 is changed, a turn-on resistance Rds(on) that is related to the turn-on state of the transistor M3 changes accordingly. Therefore, the turn-on resistance Rds(on) of the transistor M3 is modulated according to the error amplifier 132.
After that, during the second period, since the transistor M1 is turned off and the transistor M2 is turned on, a different output voltage Vneg is thus generated by the capacitor C1 and output to the first amplifier input of the error amplifier 132 through the resistor R1. At that moment, the transistor M3 remains turned on, so the current flowing through the transistor M3 is continuous. Since the current flowing through the transistor M3 is continuous, there are fewer ripples appeared in the output voltage Vneg. The output voltage Vneg can be regulated only by modulating the turn-on resistance Rds(on) of the transistor M3. In other words, the charge pump can thus provide the regulated output voltage Vneg only by controlling the turn-on state of the transistor M3, without considering the sinking and sourcing capabilities of the error amplifier 132.
Therefore, the output voltage Vneg of the charge pump can be regulated only by controlling the turn-on state of the transistor M3; that is, the charge pump can provide the regulated output voltage Vneg only by modulating the turn-on resistance Rds(on) of the transistor M3, even if the charge pump has to provide a higher output voltage for a large-sized liquid crystal display. The charge pump can be thus operated stably as well.
For the foregoing embodiment of the present invention, the voltage regulated charge pump can be provided to fit in with the large-sized liquid crystal display and output the required and more precise output voltage, so as to solve the stability problems caused when the voltage regulated charge pump outputs the large voltage.
As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.
