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I am hobbyist who’s been working for two years on a project. I’m currently having an issue with my power circuit.

It consists of three “families” of parts; we have:

  • TP4056 - LiPo charger IC
  • SDB628 - Boost circuit 2 V - 28 V input, 5 V constant output.
  • MIC39301S - Takes 5 V and drops it to 3.3 V constant.

My MCU needs both 3.3 V and 5 V.

Now here comes the issues:

When the device is only on battery power, the SDB628 gives around 3.62 V when I measured. Battery not fully charged. I should see 5 V, not 3.62 V.

When it’s only on USB power (battery disconnected) I get roughly 4.8 V on the other side of my diode (all well and good).

When both the battery and USB is connected, I see almost 8 V which is so strange! Logically speaking, 3.62 + 4.8 is around 8 V.

I have absolutely no idea why. The 3.3 V net works fine as it’s driven by the MIC39301S. My issue is with the 5 V net. The idea is obviously to always have 5 V no matter the battery voltage (3-4 V) or USB cable plugged in or not.

I appreciate any help so that I can learn!

Schematic:

Schematic

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    \$\begingroup\$ Why does C8 connect to the FB pin rather than ground? What type of capacitor are C7 and C8? \$\endgroup\$ Commented Feb 1, 2022 at 23:33
  • \$\begingroup\$ @user2965339 Can you ask a specific question? \$\endgroup\$
    – Voltage Spike
    Commented Feb 1, 2022 at 23:44
  • \$\begingroup\$ @Kevin White thank you for pointing this out. I have now cut the trace for C8 and tied it to ground as it should be. And all my issues are now gone. I get a constant 5.10 V in all scenarios. I truly appreciate your help, you just saved me allot of time. \$\endgroup\$
    – Dogbone06
    Commented Feb 2, 2022 at 9:54

1 Answer 1

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The solution to this problem is like Kevin White pointed out. C8 should be tied to ground instead of to the voltage divider. This small mistake solved the entire issue alltogether. Hopefully this can help someone in the future, the simplest mistakes can sometimes cause huge problems. I wanna take this opportunity to thank everyone who took their time to reply.

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