my device can work from external power source as well as from two AA batteries, so voltage in this case is 3V, but external supply is 5v.
I need to protect my circuit and power supplies in case of connecting all power sources together at a time.
I have restricted space in my device, so the circuit cannot be relatively large.
Please suggest what to do in this case.
I would be grateful for any help.
Switching and protecting batteries from external power supplies
batteriesdiodespower supplyswitch-mode-power-supplyswitching
Related Solutions
Yes, that part should work fine. You can follow the example circuit in the datasheet here too (use the SEPIC version if you need to vary below and above the input voltage) If the voltage needs to be adjustable you can use a fixed resistor on the bottom half of the FB pin divider, and a fixed reistor in series with a potentiometer for the top half (calculate values to give an adjustment from 9V-15V for a full sweep of the pot. Something like 5K for bottom, a fixed 30k in series with a 25k pot for the top will get close assuming a 1.25V Vref)
Be sure to read the datasheet well for the details on recommended coil value, divider resistance, compensation, layout, etc.
The 3.3V output will have to be regulated separately if you want the first supply to be adjustable (otherwise you could use a transformer with two secondary windings and feedback from one of the outputs)
A simple fixed buck regulator can do this (Microchip make a few nice cheap ones, just make sure the input voltage and current ratings are suitable)
Please check this circuit (I have not simulated this and used some random mosfets).
CASE I: VBUS-ON EXT5V-OFF: OUTPUT voltage = VBUS - Vf (Forward diode drop of D1, select D1 with low Vf). R1 will pull Gate node of Q2 and Q3 to OUTPUT voltage and turns them OFF , no power will flow to EXT5V.
OUTPUT ~ VBUS
CASE II: VBUS-OFF EXT5V-ON: OUTPUT voltage = EXT5V - Q2 and Q3 drop due to Rdson. Voltage at R2 turns off Q4 and turns on Q1. Q1 pulls Q2 and Q3 Gate nodes to GND potential and turns them ON. D1 blocks power flow to VBUS.
OUTPUT ~ EXT5V
CASE III: VBUS,EXT5V-ON OR Plugging EXT5V after VBUS: In this scenario circuit will prioritise EXT5V over VBUS. Potential at R2 will shutdown VBUS path and Turns on Q2-Q3 path.
OUTPUT ~ EXT5V
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Best Answer
This circuit should do what you want.
The two diodes steer either the output of the boost regulator or the external supply to the load. The typical V\$_{F}\$ of the diodes is 100 mV at a 100 mA load, which should not have much of an effect on your circuitry.
The two diodes are available in surface mount.