A load switch is an integrated P-channel or N-channel MOSFET with a built-in gate driver, designed to gate a power rail on and off under logic control. The naive approach — driving a MOSFET gate hard from a GPIO — turns the switch on in microseconds, which sounds great until you look at the downstream bulk capacitance. A 100 µF cap charged to 3.3 V in 1 µs demands I = C·dV/dt = 100 µF × 3.3 V / 1 µs = 330 A. Your battery sags, your upstream rail collapses, and your brownout detector trips.

The fix is a controlled slew rate. Load switch ICs (TI's TPS22918, ON Semi's NCP45560, DiodesInc AP22804) include a small capacitor from gate to source, or an internal current-limited gate driver, that forces the gate voltage to ramp linearly. Because V_GS ramps slowly through the Miller plateau, V_DS also ramps down slowly, and the output rail rises in a controlled fashion — typically 0.1 to 10 ms.

The design equation: for a desired output rise time t_r charging a load capacitance C_L from a supply V_IN, the peak inrush current is simply:

• I_inrush = C_L × V_IN / t_r

Example: powering a Raspberry Pi module drawing 1 A steady-state, with 220 µF of decoupling on the 5 V rail. Pick t_r = 5 ms. Then I_inrush = 220 µF × 5 V / 5 ms = 220 mA. Add that to any resistive pre-load, verify it stays below the switch's current limit, and you're safe.

On a load switch IC with an external slew-rate capacitor (the CT pin), the relationship is roughly t_r ≈ C_T × (V_IN / I_gate), where I_gate is the internal charging current (often 1–10 µA). A 10 nF cap with a 2 µA driver at V_IN = 5 V gives t_r ≈ 25 ms.

Real-world use: multi-rail SoCs (Xilinx Zynq, NVIDIA Jetson) require rails to power up in a specific sequence with defined ramp times to avoid latchup in the internal ESD structures. A stack of load switches with staggered enables and controlled slew rates handles this without a dedicated PMIC.

Gotcha: during the ramp, the MOSFET dissipates real power (V_DS × I_load). Long ramps into heavy loads can cook the switch — always check the SOA (Safe Operating Area) curve, not just steady-state R_DS(on).