If we need to make a board that use two voltage levels (3.3V and 5.0V) and the board can be powered both from the USB or from an external 5V power bus, then good practice is setting a diode on the USB power to prevent a reverse current to host. With this diode on the board, the "5.0V" power bus will have only 4.5-4.7V because of the diode voltage drop. If we need a true 5.0V, we should use a DC-DC step-up converter.
- Is a current reverse protecion diode so nessesary?
- Is it a lesser evil to remove a current reverse protecion diode if a true 5.0V is needed?
- If good practice requires a current reverse protecion diode, a 5.0-to-3.3 LDO and a DC-DC step-up converter, what is the best cascading of this devices:
- 5.0USB->diode->4.7V->LDO->3.3 and 5.0USB->diode->4.7V->DCDC->5.0PCB
- 5.0USB->diode->4.7V->LDO->3.3 and 3.3->DCDC->5.0PCB
- 5.0USB->diode->4.7V->DCDC->5.0PCB and 5.0PCB->LDO->3.3
- If we need a very clean 5.0V power bus, do we need to step-up the voltage to 5.5V and then use a pecision LDO to cut 5.5V to 5.0V?