What is a mother-daughter power switch configuration, and why would you stage the switch turn-on?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A mother-daughter switch configuration uses two power switch cells of different strength in series stages — a weaker 'daughter' switch that turns on first, followed by a stronger 'mother' switch — to bring a domain's supply up gradually instead of all at once.
Mentor Explanation
The mother-daughter switch has two control inputs (NSLEEPIN1, NSLEEPIN2) and two outputs (NSLEEPOUT1, NSLEEPOUT2). The daughter switch, which is weaker and has a larger on-resistance, turns on first via NSLEEPIN1; only afterward does the mother switch, which is stronger with lower on-resistance, turn on via NSLEEPIN2. Staging the turn-on this way controls in-rush current in two phases rather than letting the full-strength switch dump a large surge of charging current into the domain's decoupling capacitance all at once. This connects directly to the broader rush-current problem: turning many parallel power-gating cells on simultaneously can momentarily sag the supply voltage for other already-on domains sharing that rail.
Example
Instead of snapping a domain's full-strength header switch on instantly and drawing a huge inrush current, the daughter switch pre-charges the domain's rail partway, then the mother switch finishes the job with much less current spike.
Why It Matters
This tests whether candidates connect a specific library mechanism (mother-daughter cells) to the general physical problem it solves (rush current / IR drop during wake-up).
Common Beginner Mistake
Treating mother-daughter switches as just 'two switches for redundancy' rather than understanding the staged, weak-then-strong turn-on sequence that controls in-rush current.
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