BQ24070: Excessive Current Draw on the OUT Pins during board power up

Hi Jerry,

Thanks for reaching out. I'll be helping you out, but Garrett is still here.

Jerry Molnar said:

[1] Will this high current draw cause a protection circuit to kick in resulting in only a 2V output from the chip until it is power cycled.

It's possible that you're seeing the output short circuit protection feature of the BQ24070.

Refer to section 8.3.5 of the datasheet:

If VOUT drops 200mV below VBAT, the BATFET will turn off. Then, to attempt to recover from this state, an internal 10mA current source is enabled from BAT to OUT to try and bring VOUT within 200mV of VBAT.

It's possible that your device is getting stuck in short circuit mode due to the voltage drop on VOUT when your capacitive load is switched on. When your load switch is off, the output cap on the charger side will be charged up to VBAT. Then, when your load switch is turned on, the inrush current to your capacitive load and downstream components will likely cause a voltage drop on VOUT, which could trigger the short circuit protection.

You mentioned that you're seeing 2V on VOUT when this issue occurs, which might support the idea that your device is stuck in short circuit mode.

Jerry Molnar said:

[2] Could this inrush current damage the part and result in the failure mode we've seen 3 times now. That is, the device charges the battery, but never recognizes the fully charged state. (as least by the status indicators).

Yes, it's definitely possible that high inrush current is damaging the device. The absolute maximum battery discharge current through the BATFET is 4A (datasheet section 7.1).

While the short circuit protection feature can help keep the steady-state battery discharge current within safe limits for the device, it's not designed to immediately clamp inrush current like some of our newer devices can. If your devices are repeatedly exposed to inrush currents greater than 4A, then your devices could be getting damaged. Damaged devices can behave unpredictably.

Not only that, but connectors and traces on your board could also be getting damaged if they aren't rated for the high inrush currents you're seeing.

I recommend checking out the following Technical Article on reducing inrush current by using TI load switches (there are other great resources that can be found at the end of this article too): 

https://www.ti.com/lit/ta/ssztbx2/ssztbx2.pdf?ts=1742424437574&ref_url=https%253A%252F%252Fwww.google.com%252F#:~:text=You%20can%20reduce%20inrush%20current,rate%20to%20mitigate%20inrush%20current.

I also recommend doing some testing in your application with a fresh unit, a reduced capacitive load, and no boost converter. If the behavior you're currently seeing is indeed the device's short circuit protection kicking in, then reducing the inrush current will likely reduce the output voltage drop, and you may no longer see this behavior. This would support the idea that managing your inrush current could resolve this issue.

Best regards,

Alec

Hi Alec,

Thanks for your detailed response. The info is enlightening and very helpful. We're going to (and able) to put a 1 ohm resistor in series with the switch to limit the inrush. We are lucky that we are able to do that and that the current through the switch during operation is low so that we're not bothered by the IR drop through the resistor.

Thanks and Best Regards. Much appreciated.

Jerry Molnar