The technical field of this invention is a charge pump used to generate voltages above the power supply voltage.
A truly rail to rail input operational amplifier with a PMOS or PMP input stage requires a bootstrap or charge pump voltage above the supply voltage. Any noise and ripples of the charge pump voltage especially at high frequencies leak to the operational amplifier output due to a mismatch of input devices, such as parasitic capacitance etc.
It is an object of the present invention to provide a charge pump voltage source for use with rail to rail operational amplifier tail current sources that has a low ripple.
The present invention is a charge pump generating a bootstrap voltage, in particular the bootstrap voltage for the tail current of an input stage of an operational amplifier. The charge pump comprises a storage capacitor storing a charge pump voltage and a flying capacitor configured to be charged during a first phase of operation and discharged during a second phase of operation so as to charge the storage capacitor. A current source is coupled to the flying capacitor and a switching means is provided for switching current from the current source through the flying capacitor in a first direction during the first phase and in a second direction opposite during the second phase. Switching current from a current source to charge the flying capacitor in the first phase of operation and to discharge the flying capacitor in the second phase of operation determines the current flowing to and from the flying capacitor. The present invention provides a charge pump voltage that is smoother with a more symmetric and more triangular waveform than the sawtooth output voltage produced by the conventional voltage doubler. This produces less high-frequency content and consequently a reduced high frequency noise in the operational amplifier in which the charge pump is used.
Furthermore, the output voltage level can be controlled by configuring the current source, which can be a variable current source, to provide the right level of current for charging the flying capacitor to the required voltage. Therefore the charge pump output voltage can be tailored to any voltage up to twice the input voltage. If an output voltage twice the supply voltage provided by a conventional voltage-doubling charge pump is too high for a particular application, the voltage can be set to the required level by control of the current source. Providing a current source for charging the flying capacitor also results in the charge pump without large amplitude switching pulses. This generates less noise in the supply bus.
The charge pump according to the present invention preferably includes a control loop with an error amplifier. The error amplifier compares the output voltage with a reference voltage. The error amplifier generates a control signal coupled to control the variable current source based on the difference between the output voltage of the charge pump and the reference voltage. This controls the amount of charging current of the flying capacitor and the output voltage level. The desired output voltage of the charge pump can be set by selection of the reference current source providing the appropriate reference voltage and the capacitance of the flying capacitor. The output voltage can be set in a feedback operation. For example, the charge pump output voltage can be compared with the reference voltage. If the output voltage deviates from the reference voltage, the current supplied by the current source is adjusted. The value of the current becomes equal to two times the load current of the charge pump and the output voltage becomes equal to the voltage level defined by the reference voltage. The reference voltage is set to be equal to the desired output voltage.
If the time constant of the control loop is substantially greater than a period of the switching sequence of the switching means, the current drawn from the current source is substantially constant. Charging and discharging the flying capacitor using a constant current means that the current drawn by the charge pump is constant. This reduces voltage ripples in the power supply.
The charge pump preferably comprises a controller controlling the switching means. The switching means preferably comprises a first switching path for switching current through the flying capacitor in a first direction and a second switching path for switching current through the flying capacitor in a second direction. The second switching path can be controlled by a single control port in the controller. This reduces the complexity of the charge pump circuit. The controller provides a feedback mechanism to the switching arrangement, so that when the storage capacitor has been charged to the required charge pump voltage by the flying capacitor, the current source can be immediately switched to start charging the flying capacitor again.
These and other aspects of this invention are illustrated in the drawings, in which:
Flying capacitor C1 is connected to two switching paths implemented by MOS transistors. The first switching path is operable to connect flying capacitor C1 between positive supply voltage VDD and negative supply voltage VSS, which can be ground, via an NMOS transistor MN0 and a PMOS transistor MP2. Transistors MN0 and MN2 act as switches. The gate terminal of transistor MN0 is connected to port S2 of controller CTRL and the gate terminal of transistor MP2 is connected to port S2a of controller CTRL so that control ports S2 and S2a open and close the switches in the first switching path by applying appropriate gate voltages to transistors MN0 and MP2, respectively. It is also possible to use a single control port at controller CTRL to open and close the switching transistors MN0 and MP2.
The second switching path is operable to connect flying capacitor C1 between positive supply voltage VDD and charge pump voltage rail VCP via two PMOS switching transistors MP0 and MP5. The gate terminals of both transistors MP0 and MP5 are connected to control port S1 of controller CTRL so that control port S1 opens and closes the switches in the second switching path by applying an appropriate gate voltage to both transistors MP0 and MP5.
A current source implemented by a PMOS transistor MP1 is connected between positive supply voltage rail VDD and flying capacitor C1 in both switching paths so that when the first switching path is open the current source MP1 is connected to a first terminal of flying capacitor C1 and when the second switching path is open the current source is connected to a second terminal of flying capacitor C1. The gate terminal of current source transistor MP1 represents voltage controlled current source VCCS shown in
Storage capacitor C2 stores the voltage that is to be applied to a load. Storage capacitor C2 is connected between charge pump voltage rail VCP and positive supply voltage rail VDD.
In the first phase of operation, control ports S2 and S2a of controller CTRL close transistors MN0 and MP2 and control port S1 opens transistors MP0 and MP5. Current from current source transistor MP1 flows through flying capacitor C1 from positive supply voltage rail VDD via current source MP1 and closed transistor MP2 to negative supply voltage rail (ground) via closed transistor MN0. This current flow charges flying capacitor C1. In the second phase of operation, control port S2 and S2a open transistors MN0 and MP2 and control port S1 closes transistors MP0 and MP5. This means that the negative terminal of flying capacitor C1 is now connected to positive supply voltage rail VDD via current source MP1 and closed transistor MP0 and the positive terminal of flying capacitor C1 is connected to charge pump voltage rail VCP via closed transistor MP5. Current from current source transistor MP1 then flows through flying capacitor C1 in the opposite direction to the direction of current flow through flying capacitor C1 during the first phase of operation. This discharges flying capacitor C1. As capacitor C1 discharges, it charges storage capacitor C2 to the required operational amplifier input voltage. When controller CTRL detects that charge pump voltage rail VCP is at the required voltage, controller CTRL shuts off transistors MP0 and MP5 using control port S1 and opens transistors MN0 and MP2 using control ports S2 and S2a, respectively. The first phase of operation of the charge pump then begins again so that flying capacitor C1 performs a charge and discharge cycle and the charge pump operates continuously.
Although the present invention has been described with reference to a particular embodiment, it is not limited to this embodiment and no doubt further alternatives will occur to the skilled person that lie within the scope of the claimed invention.
Number | Date | Country | Kind |
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10 2007 020 999.3 | May 2007 | DE | national |
This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 6/016,676 filed Dec. 26, 2007 and under 35 U.S.C. 119(a) to German Patent Application No. 10 2007 020 999.3 filed May 4, 2007.