Weird power multiplexer behaviour

I have a strange bug/feature with some FPF1320 Power Multiplexers. It’s a very simple setup, with 5V coming in to VINB, and 2xAA batteries (2.4V) hooked up to VINA.

Most of the ones I’ve got work as expected, using the 5V when it’s connected, and then switching across relatively seamlessly to the AA batteries when the 5V is removed. However a couple of them won’t switch across unless I disconnect the batteries (so nothing connected). Then, when the batteries are reconnected, they’ll work.

Any ideas what’s going on? I’m going to pull one apart soon to check my soldering.

Hi Martin,

It’s worth noting VBUS is the USB 5 V supply and is connected to VINB by default through surface-mount jumper.

With that in mind, VINB will remain the choice if USB is connected.


This might not be your scenario, though I am putting it out there just in case.

Hi Graham

Thanks for the info, but in this case, some of these multiplexers are refusing to switch across to VINA, even if both VINB and USB (which I’m not using) are disconnected. To get VINA working I need to disconnect and reconnect VINA.

Hi Martin,

Ok, worth throwing that out there anyhow :slight_smile:

Some other questions that come to mind:

  • What’s connected to SEL; is the 100K internal resistor still on the board and/or is there a voltage divider setup?
  • What load is connected to VOUT?
  • Are there any modifications to the board or external circuit, other than shown below?


Board has not been modified, so the 100k resistor should still be there. I’ll check when I get back.
Load is just a small motor through a PWM controller. IIRC, around 0.1 A.
VinA is 2.4V from a couple of Eneloops.
VinB is 5V from an external source. USB is not used.

I’ve just taken the gadget apart and taken a look at the board. It looks fine. All resistors and caps seem in place as per the product pic.
There’s a bit of rough soldering. Maybe I’ll reseat the solder joints and see if that makes a difference - particular on the lower leg of that diode.

More interesting behaviour. When I plug in the battery into VinA alone (so nothing else connected, then I see 2.6V at the diode, and 2.3V at R3 (as expected with diode voltage drop). More importantly I see nothing on the VinB side.

However, if I plug in battery (VinA) and external 5V (VinB), then disconnect the external, then I see 2.6 V on the VinA side, but I also see around 0.9V on the VinB side, presumably leaking over from VinA somewhere, which is presumably enough for Sel to select VinB, which has nowhere near enough voltage to do anything useful.

Any ideas?

I’ve had a look through the ones I’ve installed in things. So far I’ve got 3 that display this behaviour, and will probably need to be replaced. Any one have any idea what could cause this behaviour. Looks like something is toggled on or starts leaking from VinA when VinB is active, which then keeps Sel high and VinA deactivated.

Hi Martin,

We’ve definitely covered all of the moving parts; thanks for helping out with testing along the way!

It would be best to reply to your order confirmation email and link to this forum topic. We’ll be able to get a replacement underway for you.

Just an update on this. I ordered 10 of these recently, and I’ve now built a tester. All 10 passed, so hopefully, whatever issue was occurring has now been resolved.

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