nPM2100 EK single cell AAA - IBALIM

[Jiri0]

Hello,

We use the nPM2100 EK for getting familiar with nPM2100 for its potential use in our project where we aim to use single AAA battery for powering it.

Our usage profile is simulated in the measurement attached below, where:

• top chart represents output voltage ~3.3V and input voltage (AAA battery)
• middle chart represents current drawn from the battery
• bottom chart represents an approximated AAA battery internal resistance (rough estimate - it shouldn't be compared to standard DS values)

Long story short, the battery is periodically loaded with ~50mA @ VOUT (~3.3V) - 1minute ON, 1 minute OFF. During the ON state, in the middle, the load is increased (for 5s) to ~100mA @ VOUT (~3.3V) which is the maximum we expect to draw from the battery.

We test when the output voltage drops below the desired 3.3V level which happens at the end of the measurement. At this state, the output voltage drops and the battery current increases.

Question:

Could you advise why the measured battery current saturates ~600mA when we have the IBALIM set to 800mA?

IMHO:

• In case the bottleneck was the battery, its voltage would drop further, right? It doesn't do that.
• The inductor selected inside the EK seems to have good saturation current rating.

We also used the default config of 600mA, the measured values were lower ~540mA.

Our measurement is performed with INA228 with 0.1Ohm shunt resistor, ~1 sample/second. It's quite under sampled. On the other hand, since IBATLIM is "Battery (valley) current limit setting", I believe the 800mA we set is actually the threshold when the ON state is re-activated again (the inductor gets charged again). So the average value would be higher than this level BUT it's lower...

Thank you!

[Maria Gilje]

Hello,

I have forwarded your findings internally and I'll forward the feedback I get there to you once I get it. Thank you for your patience. 

Best regards,

Maria

[Maria Gilje]

Hi again, 

In case the bottleneck was the battery, its voltage would drop further, right? It doesn't do that.

It could be the case that the voltage would drop further, but we should verify. We'll be able to test this on our end after the weekend, but you may be able to test this yourself like this: 

Use a voltage supply that can deliver well above 800mA instead of the battery for testing. Then, when you hit the current saturation you will know for certain that it's the nPM2100 limiting the current.

If the saturation current is still far from what's selected in IBATLIM, we can investigate further internally. 

Also worth noting, but not necessarily a limiting factor for the current, is that at the end of your sequence, you reach 600/3.3 ~180 mA on VOUT, which exceeds the value for I_{VOUT_MAX} (150mA) documented in the boost electrical specification. Please keep this and other maximum operational boundaries in mind going forward. 

Best regards,

Maria

[Jiri0]

Hello, thanks for the replies.

The suggested test with stabilized laboratory power supply is very good hint. We will try that as soon as we have a chance.

Concerning the load @ VOUT (~3.3V). It can be:

• open,
• 68Ω, or
• 33Ω.

It's purely resistive. As a result the I_VOUT can be:

• 0mA,
• 48.5 mA (~50mA), or
• 100 mA respectively.

With VOUT decreasing the I_VOUT decreases too.

The efficiency curve is also very interesting, for I_VOUT:

• 50mA: starts ~90%, ends ~88%
• 100mA: starts ~84%, ends ~58%

See the attachment below (it's not from the same measurement as the plot above, it's the same battery type but different item).

There's is a positive feedback:

• as the input voltage drops (due to high current and high battery ESR) => more current needed => voltage drops => more current needed...
• as the input voltage drops (due to high current and high battery ESR) => the nPM2100 efficiency drops => more current needed => voltage drops => efficiency drops...

It would be great if you could reproduce it at your side and/or perform the simulation e.g. in PSpice/LTSpice, etc.

Thanks & BR,

Jiri

[Maria Gilje]

Hi Jiri, 

Jiri0 said:

The suggested test with stabilized laboratory power supply is very good hint. We will try that as soon as we have a chance.

One of our PMIC devs performed this test with the results below. I'm sharing them so you can compare your findings with ours.

The IBAT is limited as expected when supplied from a power analyser. On the highest IBATLIM setting (800mA), the VOUT starts collapsing seemingly before that limit is reached, but the PMIC remains operational until that limit with lower VOUT. Exceeding any set limit eventually leads to brown-out. 

Conclusion: the limit appears to be accurate. 

Comment: The observed VOUT collapse may be due to pushing IBAT high and exceeding the boost output rating of 150mA. 

Regarding your positive feedback observations, I have shared this with the team to take a look at. 

Best regards,

Maria

[Jiri0]

Hi Maria,

thanks for the answer.

Maria Gilje said:

One of our PMIC devs performed this test with the results below. I'm sharing them so you can compare your findings with ours.

Could you please share more details (any kind of measurement protocol incl. nPM2100 configuration VIN/VOUT/IOUT values with VIN ESR, etc.)? We can connect directly if better.

As I wrote, the load at output is purely resistive and it is 33Ω minimum. But I can rather double check this. I'm not sure what the "boost output rating of 150mA" means exactly. In our case, we use 100mA as maximum output current.

Best regards,

Jiri

[Maria Gilje]

Hi Jiri, 

Currently we are facing a short wait until another team member returns to the office. I will give a follow up here when they return next week.

Jiri0 said:

I'm not sure what the "boost output rating of 150mA" means exactly.

Poor wording on my end. I was only referring to the maximum value for the boost output.

Best regards,

Maria

[Jiri0]

Hi Maria,

Thank you. Let me know as soon as you have new input.

At our end, we repeated the tests using a laboratory power supply instead of a battery, to rule out battery ESR/current-limiting as a factor (the PSU's current limit was set well above 1.5A throughout, so it was never the constraint). Two main results:

1. Maximum IBAT reached: ~720 mA (BOOST @ 3.3V into 20Ω, LDO @ 3.0V into 60Ω). This occurred as VBAT was lowered to ~1.0–1.1V, at which point VOUT had already dropped significantly below its 3.3V target — to 2.5V and below. So the ~720mA figure is not a stable, in-regulation operating point; it's the current drawn right as the boost is losing regulation.
2. We could not reach IBATLIM=800mA with this loading. Instead, we found a genuine steady state at VBAT=1.44V, IBAT=670mA, where VOUT settled at 3.07V (already ~7% below the 3.3V target rather than fully regulating). As we lowered VBAT further from there, VOUT continued dropping — down to roughly 2.2–2.3V — while IBAT only rose to the ~720mA peak noted above; it never approached 800mA.

One difference worth flagging versus our earlier battery-based tests: those used BOOST @ 3.3V into 33Ω only (no LDO load), whereas this round used 20Ω on BOOST plus 60Ω on the LDO. So the two data sets aren't directly comparable on load — but in both cases we've been unable to get anywhere near 800mA while VOUT stays in regulation.

Our question: under what conditions is it actually possible to reach IBAT close to the 800mA IBATLIM ceiling?

So far it seams that in our original question, the limitation wasn't realized by the IBATLIM protection but rather by nPM efficiency.

Best regards,
Jiri