I am a hobbyist and new to this, but I am not sure if my circuit it right. The goal is:
- To power the rest of the circuit from a DC source, a USB, or a battery, in that order.
- When there is no DC or USB, then the battery should supply the circuit.
- When there is DC or USB, also charge the battery (MAX1551/1555).
- Avoid a large voltage drop from the battery of USB when those are powering the circuit.
So, my main question is if I am using MOSFETs correctly here: My intention with Q1 is to only allow the battery to supply current when the DC or USB are off. Since they will either be 0V or ~+5V, then when either are on, the V should be greater than Vgs and the switch will be open. When those sources are off, Vgs is greater and the switch will be closed allowing the battery to power the circuit. Is that correct?
Q2 is to allow the USB to power the circuit only if the DC power is off. I was planning to set the output of the DC-DC voltage converter to be such that after the schottky diode it would be a little higher than the max USB level.
Is this diagram correct?