NPM1304 PCM lockout

[BillB]

Hi,

We're looking to integrate the NPM1304 in a product with a very small battery. As per the NPM datasheet we've selected a battery with a PCM.

We've previously seen the PCM trip in a couple of prototypes; we are not sure why seems likely to be overcurrent.

Using the EK we triggered a trip with a momentary short; we see the battery output at 1.25V after this and we've measured 85uA still going into the NPM EK. This current seems too high for the PCM to reset; if we momentarily disconnect the battery it's back up to 4.1V on reconnection and everything works.

Problem is this voltage seems to be too low for any of the NPM1304 functions like button reset or shipping mode to function, so as far as we can tell there would be no way for the user to get the product out of this state.

Any suggestions? Seems like we need the NPM's UVLO current to drop below the battery PCM's reset threshold but no idea how to go about it.

Thanks,

Bill

[Elfving]

Hi Bill,

Could you get us the schematics of the board? Does it recover from this state by plugging in USB? Could we also get the datasheet of the PCM?

I'll be happy to make the case private if you want me to.

Regards,

Elfving

[BillB]

Hi Elfving,

I'm sorry I'm so delayed. We haven't had luck getting the PCM datasheet yet, but we'll keep pushing because I had the same thought when it happened. I need to look into sharing the schematics, change of management here so still trying to work out processes.

In the meantime, my test so far was just hooking up the battery (with PCM) to J3 on the NPM1304-EK.

I'm trying to remember if I tried USB in that state. I think I better try again and get back to you.
I also need to think about if that's enough of a solution even if it does; it feels a bit weird to ask a user to plug in a wearable that they might expect to be charged already? I.e. in my mind if the battery still has enough juice the product should be able to recover itself.

Thanks,

Bill

[BillB]

Hi Elfving,

I just realised I had a quick way to answer the USB question.

The answer is NO, it doesn't recover.

Steps to reproduce:

• Connect battery with PCM on NPM1304-EK J3
• Measure VSYS, VOUT1, VOUT2 - OK
• Measure Vbatt = 4.1V
• Momentarily short Vbatt
• Measure Vbatt = 1.25V
• Measure VSYS, VOUT1, VOUT2 - discharging
• Plug in USB
• Measure VSYS, VOUT1, VOUT2 - OK
• Measure Vbatt = 3.94V
• Measure Iusb = 0mA
• Unplug USB
• Measure Vbatt = 1.25V
• Measure VSYS, VOUT1, VOUT2 - discharging
• Disconnect battery - its terminals measure 4.1V

I then repeated this test but this time I held the button on the EK to trigger a hard reset while USB was connected. I did see the rails drop momentarily so it appears the NPM is active, but it didn't change the final result - i.e. the PCM was still locked out.

I do note that with USB connected a CPU connected to VOUT is probably running and it may be possible to use it to reset the PCM? But I'm not sure the best way to go about that and as I said in the other post it doesn't feel like a good solution.

Surely there's a way for the NPM1304 to do this on its own? It's practically the use-case it's been designed for. 

Thanks,

Bill

[BillB]

Same result with the battery that's included in the NPM1304-EK connected to J2.

That should be the easiest option for you guys to reproduce.

[Tharaka]

Hi Bill

Thanks for the detailed summary. I could also replicate the scenario with nPM1304-EK battery LP181917 which has a PCM with DW01-P battery protection IC

The behavior you're observing is the correct and expected operation for a battery with a standard Protection Circuit Module (PCM), which typically uses an IC like the DW01. After a severe short circuit, the PCM enters a "latched" safety state. This is a design feature that prevents the battery from reconnecting to a potentially faulty device. 

Since this is not a very common use case, you could try enabling the charger when USB power is connected. This should also provide a trigger signal to the PCM, allowing it to reliably reset from its latched state.

Regards 

Tharaka