PNP Transistor to switch between Voltage Regulation and Battery Charging

Question

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!

Answer 1

No, when the USB cable is disconnected net VBUS is floating. Its voltage has no defined state. You can add a pull-down on VBUS so it is approximately zero.

However an NPN transistor would be a more suitable choice here.

^{simulate this circuit – Schematic created using CircuitLab}

This is essentially the same schematic as yours, however the EN-pin switching is being done on the defined VBUS state versus the disconnected Hi-Z one.

The override switch as configured in your schematic allows you to disable the regulator at any time. If you however want to override the USB disable function you are adding, all you need is a SPST switch.

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Aside, with the PNP option, your also burning 500 uA continuously during the operate from battery mode (if you added a suitable pull-down on VBUS, but that may already be there internal to charge IC). In the NPN case, the override consumes no quiescent current when USB is disconnected.