Top-off charge pump and methods of operating same
A charge pump includes a first power source having a voltage V REG generated from a regulated and circuit-limiter supply, a second power source having a voltage V BRG and a top-off capacitor adapted to be charged to a voltage of the high of V REG or V BRG to a limit of a voltage clamp across the top-off capacitor.
1. A charge pump circuit, comprising:
a first power source having a voltage V REG ;
a second power source having a voltage V BRG ; and
a top-off capacitor adapted to be charged to a voltage of the high of V REG or V BRG to a limit of a voltage clamp across the top-off capacitors;
wherein the second voltage source comprises an always-on current mirror.
2. The charge pump circuit of claim 1 comprising a voltage clamp disposed in parallel with the top-off capacitor.
3. The charge pump circuit of claim 1 wherein the first voltage source comprises a regulated and circuit-limited supply.
4. The charge pump circuit of claim 1 comprising a Zener diode to provide a voltage clamp at an input of the first power supply.
5. The charge pump circuit of claim 1 wherein a top plate of the top-off capacitor is adapted to provide a voltage of V REG and V BRG up to a limit of V BRG and V CHARGE , where V CHARGE is a clamp voltage across the top-off capacitor.
6. The charge pump circuit of claim 1 wherein the top-off capacitor includes a top plate and a bottom plate, the top plate connected to the first power source through a first diode, the top plate also connected to an output of the charge pump through a second diode.
7. The charge pump circuit of claim 6 comprising a first switch having an input and an output, the input connected to the bottom plate of the top-off capacitor and the output connected to a reference potential.
8. The charge pump circuit of claim 7 comprising a second switch having an input and an output, the input connected to the output of the second power source and the input connected to the bottom plate of the top-off capacitor.
9. The charge pump circuit of claim 8 comprising a diode having an anode connected to the output of the second power supply and a cathode connected to the top plate of the top-off capacitor.
10. A charge pump circuit comprising:
a first power source, having an output voltage V REG ;
a second power source having an output voltage V BRG ;
a top-off capacitor having a top plate and a bottom plate, the top plate connected to the first power source through a first diode and adapted to be charged to a voltage the high of V REG or V BRG , the top plate also connected to an output of the charge pump through a second diode;
a first switch having an input and an output, the input connected to the bottom plate of the top-off capacitor and the output connected to a reference potential;
a Zener diode disposed in parallel with the top-off capacitor;
a second switch having an input and an output, the input coupled to the output of the second power source and the input connected to the bottom plate of the top-off capacitor; and
a diode having an anode connected to the output of the second power supply and a cathode connected to the top plate of the top-off capacitor;
wherein the second voltage source comprises an always-on current mirror.
11. The charge pump circuit of claim 10 wherein the first power source is a regulated and circuit-limited supply.
12. The charge pump circuit of claim 10 comprising a Zener diode to provide a voltage clamp at an input of the first power supply.
13. The charge pump circuit of claim 10 comprising a diode connected in parallel with the first switch.
14. The charge pump circuit of claim 10 comprising a diode connected in parallel with the second switch.
15. The charge pump circuit of claim 10 wherein a top plate of the top-off capacitor is adapted to provide a voltage of V REG and V BRG .
16. A method of operating a charge pump comprising:
providing a first power source having a voltage V REG ;
providing a second power source having a voltage V BRG , the second voltage source comprising an always-on current mirror;
charging a top-off capacitor to the voltage level of V REG ;
pumping the top-off capacitor to increase the voltage level at a top plate of the top-off capacitor by the voltage V BRG ; and
providing at an output of the charge pump an output voltage having a voltage V REG and V BRG .
17. The method of claim 16 wherein the output voltage is at least the higher of V REG or V BRG .
18. The method of claim 17 wherein the output voltage is limited by a voltage clamp across the top-off capacitor.
19. The method of claim 16 comprising providing protection circuitry to protect the charge pump when the voltage V BRG goes negative.
20. The method of claim 16 comprising providing a Zener diode to provide a voltage clamp at an input of the first power supply.