I'm working on a device using a TPS2112 power mux (fed by USB or an external power input) which then supplies, among other things, a TPS63000 switcher. The TPS63000 is used to produce a 5.25v rail which is lightly loaded (a few mA quiescent). Cout on the TPS2112 is 3x10uF, plus a half dozen 0.1u parts.
I've found that when I enable current limiting in the TPS2112 (400R resistor from ILIM to GND), the TPS63000 regulator doesn't start up successfully; the TPS2112 stays in current limiting with output around 2.0V, and the TPS63000's inductor makes a whining noise. Yikes!
When I short the TPS2112 ILIM pin directly to ground, disabling current limiting, the TPS63000 starts up just fine. However, the inrush current peaks at nearly two amps, which will cause me USB spec compliance problems. Also the TPS2112 datasheet says not to do that. it seems like I could fix this with a slower soft-start on the TPS63000, but that doesn't appear to be controllable. Any ideas for workarounds?
