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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?

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  • \$\begingroup\$ i was about to post a generic answer, but that would most likely not help. Please provide more details, schematics, etc... \$\endgroup\$
    – bobflux
    Commented Aug 16, 2017 at 20:06
  • \$\begingroup\$ You are better off to have UVP or DCOK signal for power up. \$\endgroup\$ Commented Aug 16, 2017 at 20:07
  • \$\begingroup\$ Can you post more details about the module? Link to datasheet? Can you enable/disable it? Would be good to leave it connected and enable/disable on demand if possible. Can you hack it? Sounds like it has huge amount of input bulk capacitance that maybe can be reduced. Of course adding more bulk capacitance on your system will reduce the voltage drop when the other module is added. Can you add a FET circuit? A MOSFET that is always on might seem useless but it serves to limit inrush because the MOSFET will go into constant current mode. Need details to provide a good answer. \$\endgroup\$ Commented Aug 16, 2017 at 20:28
  • \$\begingroup\$ @VincePatron I have a (recently added) FET circuit for switching the high side power in place, and that's actually what revealed this problem to me. When I try to cycle power to the module, my MCU takes a brownout reset on enabling power to the module. I'm going to try lowering the BODLEVEL on my MCU tomorrow. \$\endgroup\$
    – vicatcu
    Commented Aug 18, 2017 at 0:43

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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!

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  • \$\begingroup\$ Agreed. I would definitely try adding capacitance to the power rails, although if that doesn't work the soft start is a good idea as well \$\endgroup\$
    – Redja
    Commented Aug 16, 2017 at 20:25
  • \$\begingroup\$ All good suggestions... tomorrow I'm going to try reducing the BODLEVEL setting on my MCU... failing that I may try the large capacitor maybe inline with the cable. \$\endgroup\$
    – vicatcu
    Commented Aug 17, 2017 at 23:48

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