STNS01 not shutting down properly

[miket6000]

Hello, I have a simple circuit with an STNS01 charging a 150mAh single cell battery and am using the LDO to power an STM32F042 and a couple of other small devices. It mostly seems to work OK, however if I pull the SD line high using the micro (or an external short to the 3.1V LDO output), the output of the LDO drops to 1.28V, rather than zero. Some devices on the LDO output can still operate at this voltage so the drain on the battery doesn't drop to the <500nA mentioned in the datasheet, instead I'm seeing a battery draw of ~550uA. I'm not sure why the LDO isn't being fully disabled.

If I unplug the battery and reconnect it the current draw drops down to about 10nA so I'm sure the STNS01 is capable of fully disabling the LDO, but for some reason it isn't.

In a test I set all the signals in the attached schematic (nCHARGE, CHARGE_EN, SHUTDOWN, BATT_SENSE and nSENSE_EN) to High-Z and then used a jumper on J3 to short SHUTDOWN to 3V3 (actually 3V1, just mis-labled). This had the exact same result, of 550uA draw.

Any ideas for debugging or advice would be greatly appreciated.

Cheers,
Mike

[Peter BENSCH]

Welcome @miket6000, to the community!

I am sorry that the pin designation SD is somewhat misleading. However, the data sheet explicitly mentions that:

„… when the input voltage (VIN) is not valid makes the device enter shutdown mode.“

The shutdown mode is therefore only activated if both SD is active and VIN is invalid. The explanation of "invalid" is then unfortunately a little later: Vuvlo < Vin < Vinovp. Vin must therefore be outside this range in order to switch off the device together with activated SD.

Hope that helps?

Regards
/Peter

[miket6000]

Thanks @Peter BENSCH , unfortunately I don't think that's the issue in this case. When the SHUTDOWN line is being asserted, there is no power supply on VIN.

I measured VIN anyway, just in case there was some unexpected leakage, however with a multimeter across C7 (VIN) I was seeing ~1mV. To definitively rule this out, I shorted VIN to GND and then asserted the SHUTDOWN pin, unfortunately this made no difference to the battery current draw, I'm still seeing more than 500uA.

In cases where the system attempts to shutdown while power is supplied to the 5V rail the output stays on fully, which is what I expect given the datasheet and your description of the shutdown pin, not the strange half dead state where there's ~1.3V on the LDO output.

If you have any other ideas for things to check I'd be very grateful.

Regards,
Mike.