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BQ24725A - ACDRV does not drive ACFET

Nathaniel Gaskin
Prodigy 140 points
Other Parts Discussed in Thread: BQ24725A, CSD17308Q3

Hi Community,

I've been using a bq24725A part for a 4 cell battery charger circuit, and I recently included the suggested reverse polarity circuit (datasheet figure 18) in the design. I now encounter an interesting problem.

When my battery depletes to a particular voltage, and I plug in the external supply/adaptor, ACDRV will drop after a few seconds. This prevents me from charging the battery. If the battery is high enough, 15/16V, this behavior does not occur. But when the battery is very low, or exhausted completely, ACDRV will not continue to drive the ACFET. In an older design this existed worked, without the reverse protections (Figure 18 Q6 and D2). But I can't tell why they would matter?

Anyone else have issues, or suggestions how to handle this particular case?

Thanks.

  • 0
    TI__Mastermind 20570 points
    Can you show your schematic? Also please attach the scope capture on the issue where battery voltage, ACDRV, ACOK?
  • 0 in reply to Jing Zou
    Prodigy 140 points
    Hi Jing,

    Can I send you directly the schematic or scope capture? I have both.

    Nathaniel
  • 0 in reply to Nathaniel Gaskin
    Prodigy 140 points

    Below is a screen capture that shows:

    Yellow - DC IN, which is 18V

    Teal - BAT+, which is ~14.3V.

    Purple- VCC at the BQ

    Blue - ACDRV, which does to 24V for 2.5s before dropping.

    ACOK is high when the DC IN is high.

    The schematic follows Figure 18 in the datasheet.

  • 0 in reply to Nathaniel Gaskin
    Prodigy 140 points

    I've added a graph with ACOK. ACOK remains high. I also have part of the schematic.

    Below is a screen capture that shows:

    Yellow - DC IN, which is 18V, trigger set to DC_IN going high

    Teal - BAT+, which is ~14.3V.

    Purple- ACOK

    Blue - ACDRV, which does to 24V for 2.5s before dropping.

    Shown below is a schematic of the BQ part of the schematic.

  • 0 in reply to Nathaniel Gaskin
    Genius 3295 points
    I'm chiming in, someone who is working with Nathaniel on this problem. After some additional testing, the question is boiling down to wanting to understand what additional factors could cause the chip to stop driving ACDRV. We believe that we're meeting the conditions described in the datasheet for ACDRV to be active, but sometimes the chip stops driving it. Thank you!
  • 0 in reply to Scott Brenneman
    TI__Mastermind 20570 points
    Can you please share the register settings?

    Have you tried on our EVM with your test set up?

    IF you adjust VIN voltage, does it affect the behavior?
  • 0 in reply to Jing Zou
    Prodigy 140 points

    Control Register Settings have been set to 0xF912. I've attempted charging with 19 and 18V. Our battery wants a 17.4 V charging voltage. I have not worked with the EVM recently.

  • 0 in reply to Nathaniel Gaskin
    TI__Mastermind 20570 points

    Can you please try your test set up on the EVM? I have tested on the EVM board, but cannot duplicate the issue.

  • 0 in reply to Jing Zou
    Prodigy 140 points
    What you see on the EVM board is what I would expect. In fact, I have other boards that do function per the datasheet specification: ACDRV is held high. However, most of the boards have issues where the ACDRV and CMSRC fall after some period of time, 1-3 seconds. I have inspected the boards for assembly defects, and have not seen or probed anything that would indicate an assembly problem.

    Are there undocumented error conditions I should be checking for? Per the datasheet, I satisfy the ACDRV condition, and IFAULTconditions do not seem to occur.
  • 0 in reply to Nathaniel Gaskin
    TI__Mastermind 20570 points

    ACDRV has a current limit. If external Cgs is too large, the ACDRV-CMSRC voltage may still go low after the 20ms turn on time window is expired.

    In your scope capture, the 1~3 secs may be the deglitch time for next ACDRV turn-on.

    Can you please change the register setting below to see if this changes the behavior? If so, I suggest to reduce the Cgs capacitance.

  • 0 in reply to Jing Zou
    Prodigy 140 points

    Thanks for the pointer to this condition. However, I've looked at several boards with the problem, and setting ACOK does not seem to effectively modify the behavior.

    There are times when the ACDRV does indeed stay high, but it is not repeatable. In both cases when ACDRV remains high and when it falls within 1-3 seconds, all conditions are met for ACDRV to be high.

    Also, when ACDRV is high, the observed IOUT (20x charge current) is close to 0mV. So even with ACDRV being high, the charging does not start. 

    Again, I have a few other boards where ACDRV behaves per datasheet and charge commences.

  • 0 in reply to Nathaniel Gaskin
    TI__Intellectual 1470 points
    Team,Can we please review the above for our mutual customer
  • 0 in reply to Omid (TI)
    TI__Mastermind 20570 points
    We have suggested to use lower Cgs capacitance. Has the customer experiment this?
  • 0 in reply to Jing Zou
    Prodigy 140 points
    The customer has NOT experimented in this, but will soon. Jing, the problem seems to occur after unplugging/plugging in the battery (15V Li-ion) a few times. Does the action of unplugging and then plugging the battery, which DC In is at 19V, stress the bq to break?
  • 0 in reply to Nathaniel Gaskin
    TI__Mastermind 20570 points
    I have tried on the EVM. Plug and unplug the battery does not cause any issue. Can you please get the full register setting from the customer? In the previous post, the customer only provided charge option register setting. What about the other three registers?
  • 0 in reply to Jing Zou
    Prodigy 140 points
    The largest apparent difference between the customer implementation and the the EVM are the SRN/SRP resistors R24 and R4, customer application has none, and a higher Cgs ACDRV fet (Q1 and Q2 in EVM).

    The register settings are the following:
    ChargeOptions: 0xF912
    ChargeVoltage - 0x43f0
    ChargeCurrent: 0x03c0
    InputCurrent - 0x1180
  • 0 in reply to Jing Zou
    Prodigy 140 points
    Are you able to confirm the register settings?

    The ACFET and RBFET were changed to lower Cgs devices (CSD17308Q3) and it did not effect the shut off time. ACDRV still goes low after a period of time.
  • 0 in reply to Nathaniel Gaskin
    TI__Mastermind 20570 points

    Do you mean the customer does not have the sense resistor? The schematic customer has shown before has R68 and R73 as the sense resistor.

    Without the sense resistor, the IC cannot get input current and charge current feedback for correct operation.

  • 0 in reply to Jing Zou
    Prodigy 140 points

    Customer does have sense resistor, which is R68/R73 in customer schematic. Customer does not have optional resistors for better current sensing (Fig 18 R14/15 in data sheet), shown here:

  • 0 in reply to Nathaniel Gaskin
    TI__Mastermind 20570 points
    This should not cause what we are seen here. Regarding on the register setting, does the customer has a gauge? Can you do a read of all register value after the issue occur? Some times the gauge can overwrite the values in the charger register.
  • 0 in reply to Jing Zou
    Genius 3295 points

    To answer your first question, there is no battery gas gauge in the system.  I don't have a register dump to share, but we can get one.

    Regards,

    Scott

  • 0 in reply to Scott Brenneman
    Genius 3295 points
    I have one of the boards and have been experimenting with it. It is 100% consistent in its behavior:
    - No battery installed
    - AC adapter is applied, and ACDRV jumps up to about 2 V.
    - About 160 ms later, ACOK goes high and ACDRV jumps to 6.2 V above CMSRC
    - About 820 ms after that, ACOK is still high, but ACDRV goes low, at which point ACP and ACN fall to near zero.
    - ACOK stays high for as long as there is enough voltage to feed its pull-up resistor.
    - The voltage on ACDET the whole time is about 2.65 V.
    - I see some I2C traffic about 40 ms prior to ACDRV going low. However, there are other I2C devices on the bus, and unfortunately I don't have a logic analyzer handy right now to see exactly what the I2C traffic is saying, and to which device(s) it's directed. I can say that I have another board which consistently keeps ACDRV high without a battery, and it also has I2C traffic. I can't say yet if that traffic is the same between the two boards. Is there an I2C command which can cause ACDRV to go low?

    I know I'm returning to the beginning of the discussion, but I feel like this has to be pretty simple. There are only a few things which can cause the chip to turn off ACDRV. I'm not 100% clear on what those conditions are. Section 8.4.1 and 8.4.3 talk about it, but I'm having trouble being sure that I know all of the conditions are because information is scattered, and conditions which cause ACDRV to turn on may not be fully the inverse of what cause it to turn off once it's already on. Clearly, though, it's not due to ACOK going low! So, here's my (hopefully) simple request: please list for us all possible causes of ACDRV dropping once it's high, with no battery installed, including any undocumented causes, such as a test mode, which we may somehow be tripping over.

    Thanks,
    Scott
  • 0 in reply to Scott Brenneman
    Genius 3295 points
    Sorry, I think I posted the above too hastily. I just did an experiment with holding the MCU in reset so that there is no I2C traffic, and the problem goes away, i.e. ACDRV stays high. So, we need to investigate what that I2C traffic is doing. We may have questions for you about that, if we can't figure it out, but at least we're making progress. Thank you for you hanging in there with us.

    Scott
  • 0 in reply to Scott Brenneman
    Prodigy 140 points
    The BQ24725A stops driving the ACDRV line after it receives an I2C command setting the ChargeVoltage or ChargeCurrent register. Even if we write 0x0000s to that register, the device ceases to drive the the input fet. Can you comment on what the chip may be doing?
  • 0 in reply to Nathaniel Gaskin
    Genius 3295 points

    That's with the battery NOT installed, correct?

    Scott

  • 0 in reply to Scott Brenneman
    Prodigy 140 points

    That is correct. When no battery is preset, if I write setChargeVoltage the BQ immediately shuts off the ACFET. 

  • 0 in reply to Nathaniel Gaskin
    TI__Mastermind 20570 points
    Thought I replied. Since your ACOK stays highly, it seems like when you try to enable charge, ocp protection is tripped and ACDRV latches. Please change reg 0x12h[8:7] to 0,1 and try again?
  • 0 in reply to Jing Zou
    TI__Intellectual 1470 points
    Thanks a bunch Jing!

    We are also working with Jasper to resolve.