Board Current Consumption Changes when Touched

[GeorgeIoak]

I've got a bit of an odd situation so I thought I'd post and see if anyone has any ideas. I've got some boards we built up which have a PSoC, an accelerometer, battery charger, and a LM3671-ADJ regulator. The regulator is set to produce 3.1V and has with the feedback network set with the proper poles (see schematic).

Most boards come in and out of sleep just fine but some are not able to go below pulling ~5mA. While debugging and probing the circuit I noticed that if I touched one of the resistors or caps in the feedback network the current consumption would drop to ~100uA (which is correct) and stay at this level. If I then activate the vibration motor the circuit will return to pulling 5mA instead of ~100uA. I can touch the filter circuit, the current drops and I can repeat this cycle.

I can monitor the switching node of the LM3671 and see it switch from PWM to PFM mode when the current consumption changes. Everything on these boards works as expected except for the current draw not able to drop under 5mA. To confuse matters this doesn't happen on all cold power starts (plug and unplug the battery).

The board is very small so when I touch I cannot say exactly which part is being touched but if I touch the motor circuit I don't get the current drop so it seems like it's the feedback network on the LM3671 is impacted by the finger touch.

Hopefully someone has some good ideas on how I can figure this issue out!

[poorchava]

My guess would be, that the circuit is on the edge of being unstable and by touching it you introduce additional parasitics which push the circuit into oscillations, which causes additional current draw.

[GeorgeIoak]

I was thinking along those lines that perhaps the setting of the poles is more critical in low current situations. I'm not too sure though how to check and adjust other than trial and error which is difficult to do when this doesn't happen all the time.