Incorrect current measurements with PPK2 when using an nPM2100

[David]

Hello,

I am experiencing issues with the current measurements taken by the PPK2 when using the nPM2100-EK to power a device. The current drawn from the 2.7V power source (PPK2) is lower when using the nPM2100 (configured to output 3.0V) compared to a direct connection without the nPM. This is clearly incorrect. How should the PPK2 be configured to reliably measure our system, including the nPM2100?

The system is a Bluetooth-enabled measurement device that uses an nRF52840 chip. When connected to a Bluetooth master, the system draws an average current of around 530µA at 2.7V, which is quite high for a CRxxxx coin cell primary battery. Therefore, I would like to use the nPM2100 to boost the voltage to a constant value of 3.0V. However, when using the nPM2100-EK for this purpose, the PPK2's current measurement shows large spikes and an average value of only 320µA. As the nPM2100 does not generate energy, this measurement must be incorrect. 

I am using Power Profiler Version 4.2.2 and even a second PPK2 produces similarly incorrect results. I have read about the phantom spikes occurring at the highest sampling rate. I therefore selected a sampling rate of 10kHz, but the values are still incorrect.

Has anyone else experienced this issue? What is wrong with the PPK2 measurements?

[Vidar Berg]

Hello,

Do you have an an ammeter with sufficient resolution to confirm the average current draw? It seems strange that you are consistently measuring lower current when using a boost regulator with the PPK2 voltage source. The spikes you're seeing may be artifacts caused by the PPK2 switching measurement ranges during the charge/discharge spikes. However, the average current reading should still be accurate (within the PPK2's spec.). Are you planning to use a cr2032? Anyway, at least this size seem to be pretty resilient against normal current spikes from RADIO and CPU and allow you to utilize much of the capacity.

Best regards,

Vidar

[David]

Hello Vidar,

Yes, I have an ampere meter with a resolution of 0.1 µA. I tested a very simple case:

I used a PPK2 to power the nPM2100-EK with 2.7V. At its 3.0V output, I only connected a 7.5kΩ resistor that draws 400µA. In an ideal world with no losses, the 2.7V input would draw 444µA.

My ampere meter shows about 535µA. That is OK.

Unfortunately, the PPK2 only measures an average current of around 15µA including spikes of up to 300µA (sampling rate: 10kHz). My second PPK2 gives similarly useless results.

Can you reproduce this behavior? Is there a setting on the PPK2 that would improve its measurements?

Best regards
David

[Vidar Berg]

Hi David,

I tested with a 10 kohm resistor connected between VOUT and GND and measured 327 ua @ 3.3 V:

Is there any chance you might have made the wrong connections?

Best regards,

Vidar

[David]

Hello Vidar,

OK, with a resistor connected directly to the output of the PPK2, there is no issue. My problem is that the PPK2's measurements are incorrect when connecting a resistor to the output of the nPM2100-EK.The PPK2 source meter outputs 2.7V into nPM2100-EK which feeds a resistor through its 3.0V output. 

Let me explain it with a simple schematic

In this configuration the PPK2's measurements are not correct. Can you reproduce the error? Is there an option to improve the measurement of PPK2?

Best regards

David

[runsiv]

Hi David, I will take over the case as Vidar is going on vacation. I have ordered new nPM2100 to test your setup on my side to replicate the issue you are seeing. I suspect you have already read https://docs.nordicsemi.com/bundle/ug_npm2100_ek/page/UG/nPM2100_EK/measurements/measure_ppk2.html 

I will update as soon at I have my hands on the kit and can test it myself. Could you share your setup from the nRF Connect Powerup app so that I can replicate it? 
Regards

Runar

[David]

Hello Runar,

Sorry for the late response. I was on holiday for the last two weeks.

Yes, I have read the notes on taking measurements using the PPK. I used the PPK in source meter mode to supply power to the nPM2100-EK. 

However, I did not use the nRF Connect Powerup application to configure the evaluation kit. Instead, I only used the jumpers available on the board, configured as follows:

P1     all open    BATTERY INPUT
P2     2.7V         VBAT
P3     all open    RESERVOIR CAP
P5     all open    LDO OUT
P6     all open    
P7     1.5V         VEXT LEVEL
P8     3.0V         VSET
P9     7.5kΩ res.VOUT
P10   OFF         BOOT MON
P11   all open    TWI
P12   all open    GPIO
P13   all open    CONFIG
P14   all open    LOAD SEL
SW5  VBAT       VBAT SEL

My EK board version is PCA10170 / 0.8.0 / 2024.50

I hope this information will help you reproduce my issue.

Best regards

David

[Asbjørn]

Hello David,

I've tried replicating your setup here now and I measure larger inrush currents to the capacitors on the nPM2100 DK on P3 if I have one or several of those connected. You write that your P3 is all open, have you removed the jumpers from P3 all together? Do you see a difference if you actually switch back and connect one of the reservoir caps?

I do also see some refresh spikes when I'm disconnecting the reservoir caps, but average current is as expected.

Have you tried switching to using the amp meter function on the output from the nPM2100 to see if the reading you get in that mode make more sense for your setup?

Best regards

Asbjørn

[David]

Hello Asbjørn,

Thank you for your response. You are right; I removed all the jumpers from P3 on the nPM2100-EK. Therefore, there are only two capacitors remain on the Vint line (C6 and C7; C8 is not soldered to the board). When I reconnect some more capacitance using jumpers on P3, the input current spikes increase to up to 50mA, but the average current measured by the PPK2 at 2.7V is still below the output current of the nPM2100 at 3.0V, so the result is still incorrect. However, a digital ammeter connected between the PPK2 and the nPM2100 measures an average current of around 530µA, which seems OK. 

When I use the PPK2 in ammeter mode at the output of the nPM2100-EK, the current measurements are still incorrect. It measures around 30µA flowing through my 7.5kΩ resistor. 

As I have two PPK2s, I tried something else that makes me doubt the accuracy of the PPK2. I configured the first PPK2 in source mode to output 3V and connected the second one in series with a 7.5kΩ resistor in ammeter mode. Both PPK2s measure the same current, but the first one shows around 390µA and the second one only 80µA. When I switch their roles, they measure 350µA and 30µA respectively.

Can you explain that ? Are both of my PPK2s broken ?

Best regards

David