2026-06-12
When a switching power supply turns on, the output capacitor looks like a short circuit. The control loop slams the duty cycle to maximum trying to regulate, and the inductor current spikes — sometimes tripping overcurrent protection, saturating the transformer, or stressing the input bulk caps. Soft-start circuits solve this by gradually ramping the reference voltage (or duty cycle limit) over milliseconds instead of microseconds, letting the output charge in a controlled fashion.
The basic mechanism: most modern controllers (LM5085, TPS54x, UCC28x) expose an SS pin. You hang a capacitor from SS to ground; an internal current source (typically 1–10 µA) charges it linearly. The error amplifier's reference is clamped to whichever is lower — the true bandgap reference or the SS voltage. As C_ss charges, the effective reference ramps from 0 V up to the bandgap, dragging the output up with it.
Sizing the capacitor: the ramp time is
For a controller with V_ref = 0.8 V and I_ss = 5 µA, a 10 nF cap gives t_ss = (10 nF × 0.8 V) / 5 µA = 1.6 ms. Rule of thumb: aim for 1–10 ms on most converters. Too fast and you defeat the purpose; too slow and the supply may not be ready when downstream rails expect it.
Real-world example: a 12 V → 3.3 V buck powering an FPGA with 470 µF of output bulk capacitance. Without soft-start, charging that to 3.3 V instantly requires I = C·dV/dt — at 10 µs ramp, that's 155 A of inductor current. The MOSFET cooks, OCP trips, and the supply hiccups indefinitely. With a 5 ms soft-start, peak charging current drops to 310 mA — well within normal operating range. The FPGA's POR circuit sees a clean monotonic rise and boots reliably.
DIY soft-start without a dedicated pin: for a linear regulator like an LM317, put an RC network on ADJ. A 10 µF cap from ADJ to ground with the normal feedback resistors gives a ramp time of roughly R2 × C_ss. For a brute-force solution on any supply, an N-channel MOSFET inrush limiter with a gate-RC delay works — the gate charges slowly, VGS traverses the linear region, and the MOSFET behaves momentarily as a variable resistor.
Pitfalls: a soft-start that's slower than your undervoltage lockout (UVLO) recovery causes restart loops. And don't forget that soft-start only controls startup — it doesn't help with load transients or recovery from a fault.