RE: nPM1300 Vbus running on 4V

PuffinEgg — Fri, 02 May 2025 18:08:02 GMT

Thank you for the in-depth explanation. Yes, I think the main reason this is happening is because I have the termination voltage of the battery set to 4.35V. This explains why the charge current doesn't kick in fully when Vbus is < 4.35 + 150mV.

RE: nPM1300 Vbus running on 4V

Sigurd Hellesvik — Fri, 02 May 2025 12:05:25 GMT

Key Requirements for Charging:
VBUS > VBAT + 150mV (headroom requirement)
VBUS > 3.6V (minimum voltage threshold)
Also initial VBUS must exceed 4.0V due to the ~400mV hysteresis in the undervoltage comparator of the VBUS.

So there can be several potential reasons for this behaviour,

If your battery is already at 3.85V or higher, charging won't initiate with 4V VBUS since the 150mV headroom requirement can't be met.

Even with a lower battery voltage (e.g., 3.75V), system current draw can pull VBUS below 4V during operation. If VBUS drops below VBAT+150mV, charging will disable.

If VBUS never rises above 4V during power-up, the undervoltage comparator won't trip, and charging won't start at all, regardless of battery voltage.

That being said, limited charging is still possible when the battery is at a lower voltage (around 3V) and with a 4V VBUS input. The 1V headroom would allow some charging current, though at a reduced CC current rate compared to normal specifications.

As the battery voltage increases closer to 4V, the charging current would progressively decrease and eventually stop due to insufficient headroom. This is characteristic of linear charger designs, which require sufficient voltage differential to operate.

No register settings or tweaks can fully optimize this scenario, since it's a fundamental limitation of the linear charger architecture.

See the typical electrical characteristics for VBUS response against loading (IBUS) for more info.

https://docs.nordicsemi.com/bundle/ps_npm1300/page/vbusin.html#ariaid-title8

RE: nPM1300 Vbus running on 4V

Sigurd Hellesvik — Fri, 02 May 2025 10:10:10 GMT

Hi,

[quote user=""] However, charging only starts when Vbus is above 4.4V, and the full charge current is only reached at 5V (during CC phase).[/quote]

Are these observations that you have made?

Regards,
Sigurd Hellesvik