1. Field of the Invention
The present invention relates to a regulated charge pump, and more particularly, to a regulated charge pump having a level shift circuit composed of a plurality of MOS transistors.
2. Description of the Prior Art
Since a voltage source fabricated in an integrated circuit needs a bulk of capacity, an integrated circuit can be provided with a charge pump, which is capable of transforming a (or a plurality of) first voltage into a second voltage of a predetermined level to provide voltages of a plurality of levels to reduce the bulk and cost.
Please refer to FIG. 1, which is a circuit diagram of a charge pump 10 according to the prior art. The charge pump 10 comprises a plurality of serially connected charging cells 12, 14, 16 and 18 and an isolation diode 20, each of the charging cells comprises a diode and a capacitor. The charge pump 10 further comprises an input end 40 connected to a first voltage V1. Capacitors 30 and 34 are connected to a first clock CLK1. A second clock CLK2, complementary to the first clock CLK1, is connected to capacitors 32 and 36. The first and second clocks CLK1, CLK2 both have a high level higher than the first voltage V1.
Through the control function of the first and second clocks CLK1 and CLK2 by alternatively turning on and off the diodes 22, 24, 26 and 28 of the charging cells 12, 14, 16 and 18 respectively, a voltage at the output end 42 of the charge pump 10 in pumped higher and higher. Please refer to an equation 1 of a relation between the first and second voltage V1 and V2: V2=V1+(V1−Vt)*N (Eq.1), where Vt is a cutoff voltage of the diodes of the charging cells, and N is the number of charging cells in the charging pump 10.
It can be seen from Eq.1 that the charge pump 10 can adjust N and generate the second voltage V2 of a predetermined level according to the first voltage V1. Therefore, an integrated circuit comprising the charge pump 10 can use the first and second voltages V1 and V2 of different levels simultaneously.
The charge pump 10 is required to not only pull the first voltage V1 up to the second voltage V2 of a predetermined level, the charge pump 10 is also required to keep the second voltage V2 at the predetermined level thereafter. Please refer to FIG. 2, which is a circuit diagram of a regulated charge pump 50 according to the prior art. The regulated charge pump 50 can not only pull the first voltage V1 up to the second voltage V2 of a predetermined level, the regulated charge pump 50 can also keep the second voltage V2 at the predetermined level. The regulated charge pump 50 comprises a charge pump 52, an oscillator 54 connected to the charge pump 52, a differential amplifier 56 connected to the oscillator 54, and a level shift circuit 58 connected between the charge pump 52 and the differential amplifier 56. The level shift circuit 58 comprises a first input end 66 connected to a first reference voltage Vref1 a second input end 68 connected to an output end 74 of the charge pump 52, an output end 64 connected to a negative input end 70 of the differential amplifier 56, a first resistor 60 connected between the first input end 66 and the output end 64, and a second resistor 62 connected between the output end 64 and the second input end 68. The differential amplifier 56 further comprises an output end 76 and a positive input end 72 connected to a second reference voltage Vref2.
The differential amplifier 56 outputs a compare signal at the output end 76 when a voltage at the negative input end 70 is lower than that at the positive input end 72. After receiving the compare signal, the oscillator 54 outputs an oscillation signal at an output end 78 of the oscillator 54. The charge pump 52 continues charging a voltage at the output end 74 only when the charge pump 52 receives the oscillation signal.
In the beginning of operation, a voltage at the output end 74 of the charge pump 52 keeps rising, and a voltage at the output end 64 of the level shift circuit 58 keeps rising accordingly. Since a voltage at the negative input end 70 (connected to the output end 64 of the level shift circuit 58) of the differential amplifier 56 is still lower than the second reference voltage Vref2, the differential amplifier 56 continues to output the compare signal at the output end 76, the oscillator 54 outputs the oscillation signal at the output end 78 accordingly, and the charge pump 52 continues to pump a voltage at the output end 74. As the voltage at the output end 74 of the charge pump 52 is pumped up to a voltage enabling a voltage at the output end 64 of the level shift circuit 58 to be higher than the second reference voltage Vref2, a voltage at the negative input end 70 of the differential amplifier 56 exceeds the second reference voltage Vref2, which is applied to the positive input end 72 of the differential amplifier 56. The differential amplifier 56 at this moment stops outputting the compare signal, the oscillator 54 stops outputting the oscillation signal accordingly, and the charge pump 52 stops to pump the voltage at the output end 74 and keeps the voltage at the predetermined level.
The level shift circuit 58 of the regulated charge pump 50 is composed of a plurality of resistors 60 and 62. Too large a chip area is required to accommodate these resistors increasing the bulk of an integrated circuit comprising the prior art regulated charge pump 50.