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BQ25895: Battery charge current limited, despite ADC conversion current values not approaching limit

Adam Gulyas
Prodigy 30 points
Part Number: BQ25895

I have a BC25895 that is only charging a 1S Li-Ion 2020 mAh battery at around 200-350mA. The data from the registers show that it knows the input current limit is 3.25 A, the fast charge current limit is 2.048 A, and it is in the fast charge part of the charging cycle. After enabling the conversion ADC, I was able to read that the charge current is only 0.2A. This was confirmed with an external DMM measurement. The IINDPM Status bit is 1, which means it thinks it's drawing too much current from VBUS on the USB input. However, the supply I'm using shows that in total only 350mA is being sourced (150 mA for the rest of the system, 200 mA charging the battery). Charging current increases to 350mA when the rest of the system is turned off. I've seen this total go to 450mA sometimes, but never over 500mA.

The circuit is the same as Figure 47 in the datasheet, except that there is an 8A fuse in series with the battery.

Here's a dump of the register data I'm reading.

REG Hex
00 7F
01 05
02 3D
03 3A
04 20
05 13
06 5E
07 9D
08 03
09 44
0A 93
0B 76
0C 80
0D 17
0E 53
0F 53
10 4F
11 98
12 04
13 46
14 79

Blue values are read after turning on the internal ADC and waiting at least one second.

I've been simulating a USB DCP by connecting a bench supply at 5V and 2.5A to the Vbus pin, and then shorting the D+/D- lines, as per the BC1.2 spec. The detection cycle looks like this:

Top trace is USB Vbus, bottom two traces are D+/D-. I can see the primary and secondary detection happening on D+/D- in the first 0.6V bump, and then it goes to 3.5V upon detecting a DCP as per the BC1.2 spec. However, I'm only guessing that the long period of 0.6V again after that and the return to 3.6V are part of the Adjustable High Voltage Handshake. In any case, the register data shows that it successfully detected a DCP with the proper current limit. The noisy part at the end was when I plugged in the battery.

Why does it think it needs to limit charge current to 200mA when it knows the max input current is 3.25A, max charge current is 2A, and the rest of the system is only drawing 150mA? Is the internal input current sense circuitry broken?

  • 0
    TI__Guru 51941 points

    Hi,

    Referring to page 8 of the d/s, the IINDPM is set to 200mA if VSYS is less than 2.2V.

    If the charger is powered up without the battery, VSYS will start from 0V and may stuck in 200mA IINDPM if SYS tries to pull too much current during start-up.

    Please connect a battery with >2.2V then power up VBUS and check the charger behavior.

    Thanks,

    Ning.

    Please click "This Resolved my issue" button if this post answer your question.

  • 0
    Prodigy 30 points

    I was able to solve this problem.

    This charge chip contains a feature called Input Current Optimization (ICO), described on page 18 of the datasheet. (Also see SLVA812A, a TI app note.) It limits current draw in the charge chip based on voltage droop it sees on the VBUS pin. I found that when the charge current was being limited to a low value, it was often the fault of this feature. By reading REG13[0:5] via I2C from the charge chip, you can see what the limit is set to. For example, in one test the current drawn from the bench supply would be 340mA, the limit would be set to 400mA, and REG13[6] would be 1, indicating that IINDPM was in effect.

    I believe ICO was being triggered by having a current measuring multimeter in series with the bench top power supply I was using to power the system. When I connected directly to the power supply with short wires ICO was not triggered and I was able to charge at up to 2A, the limit imposed by the ILIM pin.