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Original question:

BQ25887: BQ25887 Valid USB power detection

BQ25887: How do I get the rated 2.2A charge current?

Randy Richardson
Prodigy 40 points
Part Number: BQ25887
Other Parts Discussed in Thread: , EV2400
I am designing a system that I plan to use the BQ25887 to charge a 2S battery pack (2Sx4P 4.2V batteries).  I bought the BQ25887EVM to test the charger design before commiting my system circuit board to fabrication.  When I connected the system straight out of the box, I got 0.5A current from my 5V bench supply.  I found the I had to move the JP4 jumper to connect PSEL to GND.  This increased the bench supply current to 1A.  I am expecting 3 ~ 3.3A.  I get corresponding readings on the 0.01 ohm series current sense resistor on to the battery pack.  I found the the potentiometer (R5) was not set to zero.  Setting this to zero got me to 2.5A input (1.55A charge to battery).  I shorted past the 383 ohm resisitor to GND the ILIM input.  No improvement. 
I have a 10k NTC thermistor from TS on the evaluation board to GND.  I did not change the TS setup on the board (TS 5.23K to RGEN and 30.1K to GND).
Do I need to change register values?  Is there something I need to set to get to the 2.2A battery charge current?  I don't have the EV2400 to interface to a PC, but I can hook up an I2C system if that is needed.  I would need some documentation on how to address the registers on the chip to read and write them.
  • 0
    TI__Genius 10385 points

    Randy,

    I few things come to mind. Reading the registers would help provide some insight as to what is going on.

    • The device could be in VINDPM. This protection reduces the input current to regulate the input voltage to a minimum value (by default 4.3V)
      • This could be diagnosed two ways:
        • Reading REG0B[5]
        • Measuring TP7 on the EVM to see if the chip is regulating to 4.3V
      • This could be alleviated a few ways:
        • Using 4-wire sensing to ensure the chip does not fall into VINDPM
        • Using shorter cables to prevent conduction loss on VBUS
        • Setting VBUS to a lower threshold 
    • The battery is in CV mode. In CV mode the battery reaches VCELLREG and current begins to exponentially decay until terminations
      • This could be diagnosed by checking the CHRG_STAT bits or measuring TP15

    I think it is worth looking into find a way to read your registers. The support I can offer on getting that configured is limited, but I can tell you how I would go about it. There is a package for Linux SoCs called i2c-tools. That package has several i2c debug tools.

    You could hookup the EVM to an SoC dev board, configure i2c, use i2cdetect to confirm the presence of the BQ25887, and use i2cdump to show the contents of the registers.

    Alternatively you could purchase an EV2400 and I could offer more support on that

    Some resources:

    https://manpages.debian.org/unstable/i2c-tools/i2cdetect.8.en.html

    https://manpages.debian.org/unstable/i2c-tools/i2cdump.8.en.html

  • 0 in reply to Ricardo
    Prodigy 40 points

    TP7 measures 4.92V.  This seems to indicate it is not in VINDPM.

    TP15 voltage increases as the battery voltage increases during charging.  TP15 is slightly greater than the battery voltage by the voltage across the current sense resistor.  This only make sense. 

    If CV is constant voltage, it is not in that mode either.  The current is changing and the voltge is increasing with charge time.  When it gets close to +8.4 volts it reduces charge to about 0.9 mA.  this seems to be good safe operation.  When I try to charge an only slightly discharged battery pack, it charges slowly to top it off and cuts out as expected.  Overall, it works great.

    I am purchasing a EC2400.  You can guide me through that in a few days.  

    I have run a full charge/discharge cycle.  I am pleased with the performance in all areas except this one we are discussing.  I am getting good enough results to find the part acceptable.  Now just to tweek it a bit to get maximum performance.

  • 0 in reply to Randy Richardson
    TI__Genius 10385 points

    Randy,

    I will keep the thread open in the meantime.

    Thanks,

    Ricardo