LIBRARYPower sequencing and soft-start.

Some chips insist their power rails arrive in a particular order, and a board that switches every rail on at once can latch up or surge. Sequencing brings the rails up in the right order; soft-start brings each one up gently. Together they make a multi-rail board turn on cleanly.

Why the order matters

A part with more than one rail, an FPGA or a processor with a separate core and IO supply, often specifies which rail must come up first. Power the IO before the core, or the other way when the datasheet demands it, and internal protection diodes can conduct or the chip can latch into a stuck, high-current state. The datasheet's sequencing requirement is binding.

The latch-up and surge risk

Latch-up is a parasitic path inside a chip that, once triggered by a bad rail order or an overshoot, shorts power to ground until you remove power entirely. Even short of a full latch-up, slamming several rails on together stacks their inrush surges into one big spike. Sequencing and soft-start defuse both.

Soft-start: ramping a rail up

Soft-start ramps a regulator's output up smoothly over a few milliseconds instead of stepping it on instantly. The gentle ramp limits the inrush into the downstream capacitors and gives the rest of the board time to react. Many regulators have a soft-start pin or a fixed internal ramp for exactly this.

Simple ways to sequence

You do not always need a dedicated sequencer chip. A common trick is an enable chain: the first regulator's power-good output enables the second, which enables the third, so they come up in order by construction. An RC delay on an enable pin staggers a rail by a set time. A board with tight requirements uses a purpose-built sequencer, but many get by with an enable chain.