How to set nPM1300 BCHISETDISCHARGEMSB/-LSB

[sillvi]

Hi,

We want to set the PMIC's battery discharge limit via the registers BCHISETDISCHARGEMSB and BCHISETDISCHARGELSB to 1200mA but the nPM1300 PS does not describe how to translate a current to a register setting.

This post that poses the same question:
nPM1300 BCHGISETDISCHARGE - Nordic Q&A - Nordic DevZone - Nordic DevZone

But we don't understand how to do the conversion from that answer. Could you explain how the translation works?

Thank you

/ Silvia

[Audun]

Hi sillvi ,

the nPM1300 datasheet was changed from originally showing a wider range of discharge limits to now only two limits, high (1000 mA) and low (200 mA). The electrical specification states a typical value of 1470 mA for the high limit, but this can be lower due to device variation. The 1000 mA and 200 mA limits are guaranteed however.

Best regards,

Audun

[sillvi]

Hi Audun,

I'm not quite understanding what you mean. Are you saying the high limit is 1000 mA or 1470 mA?

We have tried setting the limit to 1000 mA by following the settings described in the data sheet and have read the registers back to confirm they were set correctly. We have also used only the zephyr device tree property

 

dischg-limit-microamp = <1000000>;

With both of these methods, we have manage to load the system with 1600 mA.

We are using a battery that can handle maximum 1200 mA so this is way too high.

If we can't get exactly 1200 mA, how can we at least get the guaranteed 1000 mA limit?

Best regards,

Silvia

Edit: 

When I first posted we were working with an nPM1300 with hardware version code 'C' and tracking code '2503AG' and on that one we did manage to set a discharge current limit slightly below 1200 mA. Just now we saw that in the new version of our hardware, we have an nPM1300 with hardware version code 'D' and tracking code '2521AA' instead (the rest of the IC marking is the same as before).

Could the hardware version have something to do with why the discharge current limit setting no longer works?

[Michal]

Hello again Silvia,

I have learnt that the 200 mA and 1 A are not hard limits and can also vary quite a bit based on the device. Those are just the lowest values that we guarantee for the limit, but then it may take for example around 290 mA or 1400 mA for the limit to drop the VSYS enough depending on the device.

Also it will cause a reset, so for a proper current limiting you may need some external circuitry (for example in case of a continuous overcurrent).

Best rergards,

Michal

[sillvi]

Hello Michal,

Thank you for the explanation! We have two questions:

1. What is the tolerance of this limit? You say setting 1 A may result in reset at around 1400 mA, but how much higher can we expect the 1 A limit to actually be? We saw no reset even at 1600 mA

2. Is there an errata regarding this?

Best regards,

Silvia

[Michal]

Hello Silvia,

sillvi said:

1. What is the tolerance of this limit? You say setting 1 A may result in reset at around 1400 mA, but how much higher can we expect the 1 A limit to actually be? We saw no reset even at 1600 mA

I don't have any specific data, but we have 290 mA and 1470 mA as typical values for the low/high limits in the electrical specification currently

https://docs.nordicsemi.com/bundle/ps_npm1300/page/chapters/charger.html#ariaid-title12

sillvi said:

2. Is there an errata regarding this?

It is not a silicon issue, but an easy to misunderstand documentation, which we are working on improving, so I am very sorry for that.

Best regards,

Michal

[sillvi]

Hi Michal,

Understood

Just to be sure, is what you're saying that setting the limit to 1A means that it will guaranteed never drop the VSYS when the discharge current is below 1A? The actual limit will be anywhere above 1A and never below?

Best regards,

Silvia

[Michal]

Yes, that is the information I got.

Best regards,

Michal