Charge pumping using adjustable capacitor
A voltage supply circuit that generates a voltage on a voltage supply node. The voltage supply circuit includes an adjustable capacitor, an alternating voltage source, a charge source, and an adjusting circuit. When the alternating voltage on the second capacitor terminal transitions low, the voltage on the first capacitor terminal also becomes low, and the charge source provides charge to the first capacitor terminal of the adjustable capacitor. When the alternating voltage on the second capacitor terminal transitions high, the voltage on the first capacitor terminal also becomes high, and charge is thereby pumped from the first capacitor terminal to the voltage supply node. The adjusting circuit periodically samples the voltage on the voltage supply node, and adjusts the capacitance of the adjustable capacitor to increase or decrease that voltage.
1. A voltage supply circuit that is configured to generate a voltage on a voltage supply node, the voltage supply circuit comprising:
a charge source;
an alternating voltage source configured to provide an alternating voltage that alternates between a first voltage and a second voltage; and
an adjustable capacitor having an adjustable capacitance and comprising a first capacitor terminal coupled to the voltage supply node and to the charge source, and having a second capacitor terminal connected to the alternating voltage source to as to receive the alternating voltage from the alternating voltage source; wherein
when the alternating voltage on the second capacitor terminal transitions low, the voltage on the first capacitor terminal also becomes low, and the charge source provides charge to the first capacitor terminal of the adjustable capacitor; and
when the alternating voltage on the second capacitor terminal transitions high, the voltage on the first capacitor terminal also becomes high, and charge is thereby pumped from the first capacitor terminal to the voltage supply node,
the charge source configured to supply charge to the first capacitor terminal when the voltage of the first capacitor terminal drops below a reference voltage, but configured to not supply charge to the first capacitor terminal when the voltage is above the reference voltage,
the charge source comprising a field-effect transistor having a gate-source threshold voltage, a source terminal of the field-effect transistor connected to the first capacitor terminal, a drain terminal connected to a voltage source that provides a voltage greater than the reference voltage, and a gate terminal that is biased at the reference voltage plus the gate-source threshold voltage.
2. The voltage supply circuit according to claim 1 , the voltage supply circuit further comprising an adjusting circuit configured to periodically adjust the adjustable capacitance in response to periodically sampling the voltage on the voltage supply node, wherein
when the adjusting circuit determines that a voltage on the voltage supply node is lower than a first threshold voltage, the adjusting circuit adjusts upwards the adjustable capacitance of the adjustable capacitor, and
when the adjusting circuit determines that a voltage on the voltage supply node is higher than a second threshold voltage, the adjusting circuit adjusts downwards the adjustable capacitance of the adjustable capacitor.
3. The voltage supply circuit according to claim 2 , the first threshold voltage and the second threshold voltage being a same threshold voltage.
4. The voltage supply circuit according to claim 2 , the second threshold voltage being higher than the first threshold voltage.
5. The voltage supply circuit according to claim 4 , wherein
when the adjusting circuit determines that a voltage on the voltage supply node is higher than a first threshold voltage and lower than the second threshold voltage, the adjusting circuit does not change the adjustable capacitance of the adjustable capacitor.
6. The voltage supply circuit according to claim 1 , further comprising:
a set of binary capacitors sized in binary proportions; and
a set of switches, each switch connecting or disconnecting a corresponding binary capacitor of the set of binary capacitors such that a set of connected binary capacitors forms the adjustable capacitor, and such that adjustment of the adjustable capacitance of the adjustable capacitor is performed by selecting binary capacitor to connect, and connected the selected binary capacitors.
7. The voltage supply circuit according to claim 6 , the adjusting circuit configured to adjust the adjustable capacitance by generating a binary code and providing a respective bit of the binary code to each of switches of the set of switches.
8. The voltage supply circuit according to claim 1 , the charge source being the voltage supply node connected to a switching load.
9. The voltage supply circuit according to claim 8 , the charge source configured to provide charge to the first capacitor terminal of the adjustable capacitor through a series connection of a diode and a resistor.
10. The voltage supply circuit according to claim 9 , wherein when the alternating voltage decreases from the second voltage to the first voltage on the second capacitor terminal of the adjustable capacitor, a corresponding voltage decreases on the voltage supply node is induced by forcing charge from the voltage supply node through the diode and resistor and to the first capacitor terminal.
11. The voltage supply circuit according to claim 10 , the diode being a first diode, wherein when the alternating voltage increases from the first voltage to the second voltage on the second capacitor terminal of the adjustable capacitor, a corresponding voltage increase on the voltage supply node is induced by forcing charge from the first capacitor terminal through a second diode to the voltage supply node.
12. The voltage supply circuit according to claim 9 , the diode being a first diode, wherein when the alternating voltage increases from the first voltage to the second voltage on the second capacitor terminal of the adjustable capacitor, a corresponding voltage increase on the voltage supply node is induced by forcing charge from the first capacitor terminal through a second diode to the voltage supply node.
13. The voltage supply circuit according to claim 1 , wherein when the alternating voltage increases from the first voltage to the second voltage on the second capacitor terminal of the adjustable capacitor, a corresponding voltage increase on the voltage supply node is induced, and the voltage supply node is connected to a gate terminal of a power transistor.
14. The voltage supply circuit according to claim 1 , further comprising:
a fixed capacitance capacitor connected between a fixed voltage source and the voltage supply node.