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I use mcp73871 for charging LiPo battery.
This is my schematic:
enter image description here
Some settings:
-charge current 100mA via R15
-charge stop current 10mA via R14
-Charge enable high
-Timer enable set pull-up via R13(I'm not sure whether pull-up is OK for this)

When Battery disconnected VBAT output shows 0V (Only USB connected)

When Battery connected(also USB connected), VOUT is 5V, this means source selection succesful. But the circuit which is connected to MCP73871 consume power slowly and battery voltage decreases gradually. No signs of charging.

Indicator outputs aren't routed on PCB except STAT1. Is there any missing or wrong connection on schematic, why the IC doesn't charge the battery?

Berker Işık
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1 Answers1

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Quickly looking into part 3.15 of the datasheet (please, do always link those when you expect some help regarding configuration!); are you sure the THERM pin is being used correctly?

The IC expects a 10k NTC thermistor there. Seeing a 1k resistance there would probably make the IC think the battery is being seriously overheated:

The MCP73871 device continuously monitors battery temperature during a charge cycle by measuring the voltage between the THERM and VSS pins.

An internal 50 μA current source provides the bias for most common 10 kOhm Negative Temperature Coefficient (NTC) thermistors.

Your 1 kOhm there would only produce 50 mV or 0.05V with 50 µA passing through it.

The MCP73871 device compares the voltage at the THERM pin to factory set thresholds of 1.24V and 0.25V, typically. Once a voltage outside the thresholds is detected during a charge cycle, the MCP73871 device immediately suspends the charge cycle. The charge cycle resumes when the voltage at the THERM pin returns to the normal range.

Which is outside of this boundary, making the charger shutdown its charging cycle.

Richard the Spacecat
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  • Recommended value of Rtherm between 2k and 50k in the datasheet. I'll change it with an 10k resistor. – Berker Işık Nov 05 '19 at 12:48
  • I don't think that would really be a recommendation as such. Seeing it's in the Conditions column next to the current source specifications, I'd assume it's what the design of the current source just counts with; meaning the Itherm would be outside of the spec shall the Rtherm to ground be outside of this range. Just use either an actual 10k NTC thermistor or a random 10k resistor indeed. – Richard the Spacecat Nov 05 '19 at 12:54
  • Sorry, I mean Conditions as well. It's not a recommendations. – Berker Işık Nov 05 '19 at 13:24
  • As you said above, reason of the problem was the value of Rtherm. I changed it with a 10k resistor. IC is able to charge now. Thanks. – Berker Işık Nov 05 '19 at 13:27
  • Glad it worked! – Richard the Spacecat Nov 05 '19 at 13:31