Electronic – MCP73871 <=> MCU interface

battery-charginginterfacelow-power

I am designing a small device powered by a lithium cell and I'd like to use the Microchip's MCP73871 battery charger to enable charging it from an USB port. The charger IC has three status indicator outputs: /PG, STAT1 and STAT2. In the datasheet, all are claimed to be open collector outputs capable of driving a LED directly (see typical application schematic). I'd like to use these outputs to indicate the status of the battery on the device's LCD display via the controlling PIC microcontroller.

Provided the outputs are just plain open collectors, there is no problem to use a pull-up resistor (say 50 kOhm) tied to the MCU's VDD and all should work. A problem might come, if there is a CMOS-style diode protection network on the outputs (especially the upper diode tied to VIn) – the pull-ups would power the charger IC from battery via this diode when no USB power cord is present. I know the current would be negligible if the pull-up resistor were sufficiently small. But a plain BJT in Common-Base configuration could effectively separate the "power domains".

I have used such circuit before in a similar situation (FT232R that would short the communication bus to ground when not connected to the host) and it worked fine. However, if this is not needed there, I'd prefer not to make mess and "keep it simple and stupid". I've gone through some ANs, but I have found no actual schematic of MCP73871 connected to a MCU (there are always just indicator LEDs).

So, my question is: May I rely on the assumption that these outputs have no protection circuits or anything else that would sink current when no voltage is present on the wall outlet side?

Best Answer

the pull-ups would power the charger IC from battery via this diode when no USB power cord is present.

The charger IC is already "powered" by either the input voltage or the battery voltage, internally, as the data sheet explains:

The MCP73871 device automatically obtains power for the system load from a single-cell Li-Ion battery or an input power source

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So, connecting pull-up resistors to either of these voltage inputs or the output is absolutely fine.

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