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I have a battery powered circuit using a MCU and several peripherial circuits (eg. an SD Card slot, small audio amplifier).

The peripherials are rarely used, but the MCU is constantly running. So the peripherials are switched on and off by low side FETs to stop them draining the batteries.

However, the inrush current from switching a SD-Card or audio amplifier creates a temporary voltage drop, that causes the MCU to restart by brownout / blackout detection.

I don't like to add capacitance to the main circuit, as it's leakage would drain the batteries while the peripherials are off.

So what else can I do?

Experimentally adding resistors to the peripherial power lines do stop MCU resets, but I don't want to risk flaky behaviour / incompatibilities of the SD-Cards of course, or audio distortions.

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    \$\begingroup\$ What inrush current causes a problem and for how long does this inrush last? \$\endgroup\$
    – Andy aka
    Commented Nov 29, 2016 at 15:11
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    \$\begingroup\$ Is the leakage current of a capacitor really an issue compared to the current drawn by the microcontroller? The MCU will be drawing milliamps when active, but a capacitor will leak micro- or nano-amps. Just seems like a made up problem in this case. \$\endgroup\$
    – Brendan Simpson
    Commented Nov 29, 2016 at 15:50
  • \$\begingroup\$ Use a switch with a controlled slew rate (soft-start). Here's an article that discusses such load switches (although it deals with high-side switches). Here's a related thread on slowing down the switching time of a low-side MOSFET. \$\endgroup\$
    – Nick Alexeev
    Commented Nov 29, 2016 at 16:41

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Small coins cells may have an ESR of 1~3kΩ , but I can only guess what you have.

You can use ceramic caps which although not as good as FILM but low enough leakage often C＞0.047µF:More than 500Ω·F so 1uF = 500MΩ....in good quality ceramic (eg Murata)

use a 1:10 cap divider ratio to reduce load to source regulation error.

e.g. 1 uF source , 0.1uf switched load and add ferrite beads to reduce Voltage dip further.

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