Free Charge Pump Switched-Capacitor Calculator

How Switched-Capacitor Charge Pumps Work

Switched-capacitor charge pumps generate a negative or multiplied rail (such as generating -5V from +5V for analog op-amps) using only switches and capacitors—completely eliminating bulky inductors and magnetic EMI radiation.

1. The Two Asymptotic Impedance Limits

The output impedance ($R_{out}$) of an unregulated charge pump governs how much output voltage sags under load:

• Slow Switching Limit (SSL): At low switching frequencies, the flying capacitor has plenty of time to fully charge and discharge during each clock phase. Impedance is purely capacitive:
  R_SSL = 1 / [f_sw × C_fly]
• Fast Switching Limit (FSL): At high switching frequencies, the capacitor cannot fully charge because internal MOSFET switch resistance ($R_{on}$) and capacitor Equivalent Series Resistance (ESR) bottleneck the current:
  R_FSL ≈ 4 × R_on + 2 × ESR_fly

The total equivalent output resistance is approximately the sum: $R_{out} approx R_{SSL} + R_{FSL}$.

2. Peak-to-Peak Voltage Ripple

During the discharge phase, the output reservoir capacitor ($C_{out}$) sustains the entire load current for half a cycle ($T / 2$):

Using multi-layer ceramic capacitors (MLCC) with sub-10 mΩ ESR virtually eliminates the resistive ripple component.