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Original question:

BQ25700A: Problems with reading IDCHG value using ADCIBAT

BQ25700A: Issues with LDO and ADC

Guru Dutt S
Prodigy 40 points
Part Number: BQ25700A
Dear TI team,
I have some issues with bq25700a
a) In my case I've tested ChargeOption0 at 0x020a and have disabled LDO and found that ADC is able to measure discharge current while it isn't able to do so is LDO is enabled i.e ChargeOption0 at 0x020e. Is there a reason for this?
c) Why doesn't pmic discharge by software disabling Hi_Z mode, why do I need to physically pull up ILim_HiZ pin and drive it >4.0V? By software disabling I mean setting r REG0x35[7] is set to 0
d)I'm also unable to set InputVoltage register to any value. I always get an error when I try to set it to any voltage level. Why could this be happening?
e) There is a noticeable spike in ICHG pin of about 4A during POR as I've indicated in the attached image. This has caused my RPS to go into OCProtection mode. Is there a reason why this could be happening because I noticed this specifically when LDO is on and didn't when LDO is off.
  • +1
    TI__Expert 4780 points

    Hi Guru,

    Please see my comments/questions below regarding your questions.

    1. The LSB for IDCHG is pretty large (256mA), is it possible the Discharge current is low enough to show as '0' in the situation you describe?
      • With LDO Off, you mentioned the ADC does measure Discharge current. What is the value?
      • With LDO On, do you have a way of measuring the current directly to see what it should be?
    2. The software setting in Register 0x31[7] (EN_EXTILIM) determines whether ILIM_HIZ pin is involved in determining the input current limit. However, this does not disable the HIZ functionality of the pin.
      • Enable ILIM_HIZ pin to set input current limit
        • 0b: Input current limit is set by REG0x3F.
        • 1b: Input current limit is set by the lower value of ILIM_HIZ pin and REG0x3F.
      • Regardless of above setting:
        • When the pin voltage is below 0.4 V, the device enters Hi-Z mode with low quiescent current. When the pin voltage is above 0.8 V, the device is out of Hi-Z mode.
      • Therefore, you still need to set ILIM_HIZ pin to >0.8V to begin charging, when EN_EXTILIM is set to '0'
    3. The Input Voltage Limit register is defined as [VBUS - InputVoltage() Register]. The default value (0x00) is VBUS-1.28V. In a previous thread, I saw that you tried to set 8.192V in the InputVoltage register while having a VBUS of 9.62V. This would translate to [9.62V - 8.192V] which is an invalid setting. If you would like to set an input voltage limit that is ~8.192V with VBUS of 9.62V then you need to set the InputVoltage register to something close to 1.4V.
    4. Can you clarify what your settings are when you experience this 4A spike?
      • What is the VBUS voltage?
      • What is the BAT voltage at startup and when you see the spike to 4A?
      • Is ChargeCurrent() register set to 1A? Can you read it back after the spike to see if it has changed to 4A?

    Thanks,

    Khalid

  • 0 in reply to Khalid Bairuti
    Prodigy 40 points

    Hi Khalid,

    1) What I tried differently before with LDO off helped me measure current at about 0.32 A that I verified using a multimeter. But multimeter reading was different and corresponded to value of ICHG that I had set. But after your suggestions I did set Reg31[7] = 0; got it working

    The sequence that actually worked was disabling Ext_Ilim and Manually setting MinSysVoltage. The MinSysVoltage should automatically detect type of cell but in our case it indicates 4s i.e 12.8 V instead of 1s 3.584 V which I manually set. This got the pmic working properly. Any reason why this could be the case?

    2) and 3) Noted

    4) VBUS = 9.6V

        VBAT = 3.6V

        ChargeCurrent = 128mA and read back value was not changed to 4A  (Note: En_EXTLim was high and LDO was disabled)

  • 0 in reply to Guru Dutt S
    TI__Expert 4780 points

    Hi Guru,

    1. Glad to see the ADC issue is resolved. Regarding your follow up question, cell count is determined based on a resistor divider attached to CELL_BATPRESZ pin, biased to VDDA. For 1S selection, the target voltage on CELL_BATPRESZ is 25% of VDDA.
    2. In the original post you mentioned you are seeing the spike when LDO is connected. Also, the chart you sent showed ICHG at around 1A before spiking to 4A. I am not sure what is causing this, but based on the timescale is in seconds you could be hitting a watchdog timeout. This should reset ChargeCurrent to 0A, but based on the state of your setup when it occurs, it could be causing a spike. You can try writing REG0x12[14:13] = 00 to disable watchdog timer in case that is the root cause.

    Thanks,

    Khalid

  • 0 in reply to Khalid Bairuti
    Prodigy 40 points

    1) Noted Khalid.

    2) I've actually set Reg12 to 0x020e completely disabling WDT. The current charge resistor is not reset in it's value. 

    Thanks, 

    Guru