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I am having an issue where there happens to be a voltage dip down to 2.2V and due to the POR of the microcontroller which is set at 2.2V it would've dropped lower.

  • ** What Happens :** 3V3 drops down to 2.2V or lower causing instability in system.

  • When it happens : During transition between VBCAP and 3V3 and vice versa

  • What I am seeking: Eliminate the voltage dips that happens in transitions of voltage rails through the Mosfet and Diode switching circuit

  • What I have tried to do to eliminate but failed : Put a voltage divider to a voltage supervisor set at 4.5V which then controls an N-Channel that is tied to a P-Channel mosfet which disables/enables the VIN of the +3V3 LDO

  • Result: 1 voltage dip at transition from VBCAP to +3V3 and another from +3V3 to VBCAP.

  • What I try to avoid : use BJT or Diode at the path of VBCAP to avoid any losses of battery life while keeping cost of components at minimum if possible.

  • Target Information : Curent load at 3V3 varies between 50mA to 150mA

Below is Analog capture session of a 10 second on 10 second off cycle where the transition happens between either VBCAP or +3V3 taking over and providing Source to the target microcontroller.

enter image description here

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  • \$\begingroup\$ How about some bulk capacitance on the 3V3 rail? \$\endgroup\$ Commented Apr 2 at 16:54
  • \$\begingroup\$ tried 100uF at 3V3 but did not resolve , or is it something bigger ? \$\endgroup\$ Commented Apr 2 at 17:12
  • \$\begingroup\$ Hi, I was facing same problem long time ago and there are some reasons for such dips, I will try to call for main ones. To switch from ldo to cap you need to provide sufficient vgs and also charge gate and miller capacitances as fast as possible. So, ideally you need to discharge gate at the moment when you detect 5v disconnect and provide bulk capacitance at 3v3 node for transient. If you are not doing this - system load requests 50-150mA (from question) but current through mosfet at vgs~=vgsth is in range of microamps, and this creates dip. \$\endgroup\$ Commented Apr 3 at 6:31
  • \$\begingroup\$ +3.3V drops too slow and the MOSFET turns on too late. You need to switch the mosfets on sooner or replace it with a diode. \$\endgroup\$ Commented Apr 3 at 6:32
  • \$\begingroup\$ For solution you can try discharge gate if detecting 5v disconnect and provide bulk capacitance. But I’ve suggest you to switch mosfet with ideal diode (2 bjts and p mosfet, you can google it out). But be aware, that at least dual (matched bjts are even better) required for ideal diode to work properly. And you will need to rethink vcap charging circuit, if you relying on charge through body diode of p mosfet now. \$\endgroup\$ Commented Apr 3 at 6:34

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