MCP73871 voltage drops on OUT with no external power supply connected

Question

Problem

I am using the MCP73871 to manage battery charging while supplying a load to a 3.3 V regulator. the downstream load is pulling on average 150 mA off of the LDO. Everything works fine when 5 V is supplied via USB to the input of the battery charger, however when removing the external supply, the voltage drops to ~2.6 V from 3.3 V.

Schematic

Measurements

So far I have probed the output voltage from the LDO as well as the battery voltage and the USB voltage for both scenarios. Both supply voltages stay stable at BATTERY_VOLTAGE (3.3 V -> 4.2 V) or 5 V. The LDO output remains stable with 5V supply and exhibits voltage drops on the battery supply.

I have also substituted the battery for an external DC power supply and tested with voltages (3.3 V -> 4.4 V), and problems persisted. This is also how I observed ~150 mA current draw.

This leads me to believe there is a problem with the MCP or the battery connection rather than the LDO or something further downstream

Any help is appreciated!

Answer 1

The input voltage of the LDO regulator, U9, should be higher than the nominal output voltage by at least 0.3 VDC.:

$$ \mathrm{V_{IN-U9}\geq 3.3 + 0.3=3.6 \ VDC} $$

^{Check the table on p.6 in the datasheet.}

Because the input and output currents are nearly equal for an LDO, there's going to be a drop of 1.5 VDC across R51 for a downstream load current of 150 mA. Taking this drop into account, we can conclude that the \$V_{CHARGE}\$ voltage should be at least 5.1 VDC to get 3.3 VDC output from the LDO, U9.

That's why you get stable 3.3 VDC output when the main input is the USB port. So it's quite normal to get lower output voltage when the main input is about 4 VDC. Short R51 or at least reduce it to a more reasonable value.