I'm trying to design a circuit that could be powered by an USB host OR by a wide voltage range power input. These are my requirements:
- the USB rail (VUSB) should be obviously protected from the auxiliary power input (VIN)
- VIN should have priority over VUSB
- THRU could be used to power a sibling device by connecting THRU(unit1)->VIN(unit2)
- voltage drops should be negligible (in particular when using VUSB)
- VIN should offer polarity inversion protection
- downstream voltage: 5VDC
I'm currently using two ideal diode controllers LM74700 (D1, D2) and a DC/DC stepdown converter LMZM23601V5SILR (U1), the circuit above synthesizes the power input stage. D3 is a clamp contained in a ST USB6B1.
So far, all the requirements are satisfied, with the exception of (5): by applying a reverse voltage to VIN/GND, I end up with a short.
This happens no matter whether I use the LM74700 or a diode, so I suspect I'm missing something really basic here. I added D3 because so far it's the only potential shunt on the upstream side of the circuit but I can't explain why is being engaged.
Does anybody have an explanation and a solution to fulfill all the reqs?
Any help is wildly appreciated!