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Following from this question, which remains open... if I've got a microcontroller switching the high-side power to a sub-system, using a circuit like this:

enter image description here

... should I add bulk and/or decoupling capacitance on the SENSOR_5V net? I do have bulk and decoupling capacitance on the +5V net. In the broader context of this circuit, SENSOR_5V basically just goes to a connector pin, which supplies power to a separate board through a short cable.

So, now I'm wondering (in light of the referenced prior question) whether it would be a help or a hindrance to add say a 10uF ceramic and a 100nF ceramic to the SENSOR_5V net? Or maybe even a 100uF tantalum perhaps? What do you think?

vicatcu
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  • How much current does the sensor draw? How much of that load is variable? – The Photon Jul 17 '18 at 03:04
  • I'd put a 5V6 zener on the output to ground to catch fast back emfs from the cable inductance. – Andy aka Jul 17 '18 at 08:25
  • @ThePhoton I think it has a peak draw around 300mA at startup, then 40mA average, with bursts of 150mA activity periodically. – vicatcu Jul 17 '18 at 17:22
  • With those numbers I'd think 10's or 100's of uF is not a bad idea. If it were 1-2 mA I'd say 100 uF is probably overkill. – The Photon Jul 17 '18 at 17:26
  • @Andyaka I think I know what you mean, but consider posting an answer with a schematic representation for the benefit of the community? Also consider we're are talking about a 4 inch cable. – vicatcu Jul 17 '18 at 17:37
  • Adding capacitance to SENSOR_5V will cause a surge current when U7P turns on, so I would only capacitance if needed. U7P could be turned on more slowly by adding RC to the gate to reduce the surge current. Even better (but more expensive) use an integrated power switch IC. – EE_socal Jul 17 '18 at 18:19
  • @EE_socal re: integrated power switch IC, can you recommend one, for example? – vicatcu Jul 18 '18 at 02:18

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In case the cable inductance is creating back emfs that lift the 5 volt power supply (via the body diode of the output MOSFET), I'd want to put a 5.6 volt zener diode on the output like this: -

enter image description here

Adding the capacitors is fine but try this first because it could deliver a reason rather than just a blanket solution.

Andy aka
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  • The Zener diode achieves nothing since the capacitance on the +5V net would absorb any current, and the body diode limit the excursion to 5.7V anyway. – Jack Creasey Sep 09 '18 at 17:18