Charge Pump
A charge pump transfers energy through capacitors and switches. Depending on the circuit it can invert, multiply or reduce a voltage, and some devices regulate their output. Avoiding an inductor does not eliminate switching ripple, losses or layout requirements.
A flying capacitor charges in one switching state and connects differently in another, transferring charge to an output reservoir. Switch resistance, capacitor characteristics, frequency and load affect the delivered voltage and current. Regulated devices can change switching behavior or conversion mode; charge pumps are not all fixed-ratio unregulated inverters.
In plain terms
A bucket brigade: switches move the connections of a charged capacitor, then repeat the cycle to replenish an output reservoir. The bucket does not move physically, and each transfer has losses.
Why designers use it
- Generate a negative bias rail from a positive input.
- Provide a compact supply for a small circuit block without an energy-transfer inductor.
- Generate an appropriate programming or bias voltage when the selected converter's limits fit.
Best for
- Bias supplies
- Compact voltage conversion
- Appropriate memory-programming supplies
Key specifications
- Conversion mode: Device- and topology-specific (Check inversion, multiplication, reduction and regulation behavior.)
- Current and ripple: Use the complete operating conditions (Do not infer a family-wide current ceiling or noise level.)
- External capacitors: Follow the selected device's guidance (Capacitance, ESR, voltage rating and placement matter.)
When not to use it
- When load current, ripple, efficiency or voltage range exceeds the chosen design's capability.
- When a specific conversion mode cannot produce the required output across the input range.
Common mistakes
- Assuming all charge pumps share a 100 mA limit or all lack output regulation.
- Ignoring the recommended flying/output capacitors, their voltage ratings and actual polarity in the circuit.
- Treating inductor-free as noise-free, or copying a PCB layout without considering switching-current paths.
Where you will find it
- The cited Analog Devices article shows a charge-pump supply followed by a linear regulator, and a separate internally regulated MAX619 example. Their stated operating conditions and component choices belong to those circuits.
- The same article discusses charge pumps for flash-memory programming voltages and small bias supplies. That does not mean every flash device or interface chip uses the same voltage or architecture.
A short history
Switched-capacitor converters use a flying capacitor and a reservoir capacitor rather than an inductor to transfer and store energy. The cited Analog Devices article describes basic inverting/doubling arrangements and regulated designs that change their switching behavior. These are different implementations, not one universal circuit or current limit.
Good to know
- Flying describes the capacitor's changing electrical connections, not physical motion.
- Some charge pumps change conversion modes to suit the input voltage. An inductor-free converter can still include feedback regulation.