Constant-Current Capacitor Charge Time

Formula

Description

When a capacitor is charged with a constant current source (rather than through a resistor), the voltage rises linearly with time. This is the fundamental relationship Q = C×V differentiated with respect to time: I = C × dV/dt, rearranged to find the time to charge from one voltage to another. Constant-current charging is used in current-source LED drivers, ramp generators for ADCs, timing circuits, and battery charging (CC phase of CC/CV charging). The linear voltage ramp makes this technique ideal for precision timing applications.

Variables

• t — Charge time (s)
• C — Capacitance (F)
• ΔV — Voltage change across the capacitor (V)
• I — Constant charging current (A)

Practical Notes

This assumes a perfectly constant current source. In practice, current sources have finite output impedance and compliance voltage limits. The charge time is independent of the starting voltage — charging from 0 V to 1 V takes the same time as charging from 4 V to 5 V with the same current and capacitance.