Charge pumping using adjustable capacitor

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.