Brownout Mitigation

Question

I've got a system problem where the power up of one module causes the system at large to brown out, ostensibly because of inrush current to that module. Things kind of recover into a stable equilibrium after an indeterminate number of brownouts, but I'd like to find a solution that results in brownout-free startup.

The module which causes me the problem accepts input voltage 5 to 9VDC, and according to its datasheet draws 300mA peak during startup, though it doesn't indicate if that is dependent on input voltage (though I can't see how it wouldn't depend). Presumably, this module has some bulk capacitance and SMPS circuits on it generate its own operating voltage domains. I'm currently supplying it with power on the low side of that range (about 5VDC). My power source is a 2A 5V wall wart, so I think I have plenty of head room, but still I see evidence of brownout.

Ideally, I would just use a physically independent power source for that module, but that's not a reasonable solution for my design space. So, instead, I was thinking maybe I should integrate a boost converter to step my 5V up to 9V for consumption by the badly-behaving module, in hopes of reducing the inrush current demand on the 5V subsystem, and thereby avoiding brownout. Is that sound thinking about how I might address the problem? What other ways can I mitigate brownout due to a module powering up?

Answer 1

First draft should always be to add local capacitance at the power pins. Simply take a 100uF capacitor and place it near the power pins of the offending module.

Also, be sure to add 0.1uF decoupling near any other potentially sensitive circuitry power leads.

When all else fails, perhaps try a hot-plug circuit strategy. This usually involves a P-channel MOSFET with resistors and caps arranged around it so that it turns on slowly, limiting inrush. You can google and find a half-dozen examples.

Here is one: https://www.maximintegrated.com/en/app-notes/index.mvp/id/2158

If the problem is bulk-capacitance, going to a higher voltage will cause higher currents to be drawn on the 9V rail, causing even higher currents on the 5V rail. If you are feeding a switching converter, then bumping up to a 9V rail might solve your problem as the switching converter will fold back a bit.

Overall, I would try it in the order presented:

1. more local capacitance
2. soft start circuit
3. boost regulator

Good luck!