I'm trying to do a simple, low power, low cost power "switch" for a custom board (I know, it will be complicated).
The device will be powered by default by a coin cell battery (3V3 and lower) and can also be powered by USB (5V). I will use a boost converter (TPS61291) for the battery, it will allow a constant 3V3, with a bypass for sleep mode controlled by a MCU.
This is a part of the power rails I designed:
simulate this circuit – Schematic created using CircuitLab
The battery is protected by the schottky diode (some nA can flow into it, but it is accepted by the manufacturer)
If the VUSB is not present, the diode D2 will avoid reverse current in USB connector. D4 is protecting the LDO itself from the reverse current by bypassing it.
The boost bypass will be activated by the MCU when it is in active or when USB is plugged.
Do you think it will work correctly?
Best Answer
Electronic industry already has a solid solution for this case. It is solved by a special class of devices called "power distribution switch", within PMIC group of IC. Search Digi-key for this type, "PMIC - Power Distribution Switches, Load Drivers", then select, say, "2:1" in the column "Ratio - Input:Output".
Essentially these are switches based on FETs, as "ideal diodes". Here is an example, FPF1320UCX.
A similar line of "PowerPath, Ideal Diodes & Load Switches" is offered by Analog Devices, see something like LTC4413
I am certain that many other manufacturers offer similar line of products. When comparing cost of solutions, you need to take into account not only the cost of individual components, but also the cost PCB area, and placing/soldering costs.