I am relatively new to circuit design and would appreciate your feedback on this :)
I am designing a power supply circuit that uses the TI TPS61030 voltage regulator. It features a 4A internal switch connected to ENBL. The regulator is ON if ENBL is high (+BATT) and OFF if ENBL is low (GND).
The power circuit also features a Li-Po charger (MCP73871), which uses USB power to charge the battery. It also has allows simultaneous battery charging and system power output, which is a feature I want to bypass.
Essentially, I want the system to power down as soon as the battery starts charging – when the USB is plugged in. The flow chart below summarizes the desired behaviour.
I chose a PNP transistor with an R1 of 4K7 and and R2 of 47K. I then worked out the logic, shown in the table & equations below. When VBUS is GND, the circuit between the base and emitter acts as a voltage divider. Otherwise, I used Kirchoff's law.
So here are my questions:
- Will this work as illustrated in the flow chart?
- When the usb is not plugged in, will VBUS be at 0V? and thus a voltage divider circuit exists between the base and emitter?
P.S. J4 is to bypass the transistor in case I wanted to charge and power the system simultaneously. VE < VB > VC (USB ON) -> Cut-off VE > VB < VC (USB OFF) -> Saturation.
Thank you in advance!