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BQ24610: Charging Current Issue

Mikayla Diesch
Prodigy 80 points
Part Number: BQ24610

I have been attempting to validate the BQ24610 charging profile. I was attempting to get a charge current of 2.5A, according to the data sheet that required a ISET1 Voltage of approximately 0.5V.This is based on the equation: 

I_CHARGE = VISET1/(20*RSNS) 

I attempted to achieve this with a voltage divider consisting 100K Resistor (R1) tied to VREF and an 18K resistor (R2) tied to GND. I am using an RSNS resistor of 10mOhm. I measured the voltage of ISET1 to be 0.48V but were only getting a charge current of 1.5A.

To test our design and check for potential pin assignment issues I added to 100K in parallel to R1. This put the voltage of ISET1 to 1.14V and the charging current closer to what we wanted, at ~2.4A. According to the data sheet, though, I should be seeing a charging current closer to 5A. 


I am unsure what the issue as, it appears that the voltage of ISET1 is controlling the charging current but it's not following the equation outlined in the data sheet.

  • 0
    TI__Mastermind 25800 points
    Hey Mikayla,

    Two questions:
    1) What was your battery voltage during your testing and what is your battery regulation voltage programmed to?
    2) What is the VACSET programmed to, what is the RAC value, and what is your input voltage?


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 80 points
    We're running a 3s2P configuration and we've tested this on a stack voltage ranging from 8.1V to 12.6V and we're seeing consistent current draw. We're supplying a 14V power supply.

    VACSET is programmed to 0.5V or equivalent 2.5A RAC is 10mOhm as well
  • 0 in reply to Mikayla Diesch
    TI__Mastermind 25800 points
    Hey Mikayla,

    It does seem odd to me that you see the wrong current as our charger does follow that equation for setting ICHG.

    I wanted to see if during your test the battery voltage was close to the regulation voltage. Whenever the charger is in Constant Voltage (CV) mode, the converter regulates the voltage to the maximum regulation voltage programmed by the feedback resistor divider network. In this mode, the current into the battery will decrease exponentially as the cell voltage gets closer to the regulation voltage.

    Can you verify on a board where you saw the wrong current that the voltage as measured on the SRN pin of the charger is between 9V and 11.5V? If the charger is deep into CV mode, you will measure what I assume is your max set voltage of 12.6V.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 80 points

    Joel,

    Thank you for your follow up. 

    We've been seeing the system in CC and switch to CV once it gets close to the regulation voltage - which you're correct is set to 12.6V.  I went and re-measured everything on the system and got the following results:

    • Stack Battery Voltage : 10.6V
    • Vin : 14V
    • Iin : 1.49A
    • Vsrn : 10.99V
    • Vsrp - Vsrn : 17.9mV
    • Viset1 : 0.502V
    • Viset2 : 0.299V
    • Vacset : 0.5019V

    Best,

    Mikayla

  • 0 in reply to Mikayla Diesch
    TI__Mastermind 25800 points
    Hey Mikayla,

    So at this point, let's double confirm the voltage across the AC resistors as well, right at the charger's pins (Vacp-Vacn). Same for Vsrp-Vsrn, make sure this is sensed at the pins of the charger and not across your actual sense resistor.

    We are double checking this to confirm that the measured voltage from the charger's perspective is the same as that which is across either of the sense resistor.

    One last thing I want to try is increasing the input voltage from 14V to something like 17V and see if that changes your results.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 80 points
    Hey Joel,

    Repeating the test looking right at the pins rather than the SNS resistor, I'm seeing the following:

    Vsrn - 11.0704V
    (Vsrp - Vsrn) : 26mV - which appears to be a symptom of our current problem.

    We are also seeing she VFB and SRP sink close to 5mA from the VBATT line, which according to the data sheet should be at max 15uA. We confirmed our components are within spec and are not sure why we're seeing such a high sink on those (relatively) high impedance lines.

    Best,
    Mikayla
  • 0 in reply to Mikayla Diesch
    TI__Mastermind 25800 points
    Hey Mikayla,

    I am starting to suspect the IC may be damaged. Have you seen this behavior on multiple boards/ICs, or only on a single IC?

    If on only 1 IC, can you try swapping with another IC and see if the issue remains?


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 80 points
    Joel,

    It was across multiple ICs, we were thinking it may be the VBATT trace into the VFB and SRN pins as it is relatively narrow and long in the layout. We measured the trace on our LRC and got ~300nH Ls, 80mF Cs, and 180mOhm Rs
  • 0 in reply to Mikayla Diesch
    Prodigy 80 points

    Joel,

    Wanted to follow up with the testing we've done today

    I cut the trace on 3 different boards and placed  a large airwire from +VBATT to the resistor divider network that goes into VFB and the cap into SRN. That airwire made the LCR readings more favorable, but we're now getting new odd behavior. Two of the boards had previously been powered on and had seen that large current draw into VFB and SRN. When I switched to this airwire the voltage on VFB went from ~1.9V to 0V, and the system is no longer charging at all. 

    For the board that had never been powered on I placed in the batteries and measured the current from the +VBATT line to the VFB pin and measured ~0.5mA. I physcially removed the resistors on the divider network from one of the boards reading VFB = 0 and confirmed they were in fact the high impedance values we had intended (500kOhm and 100kOhm). I am a little stumped as to what may be going on here as this board shouldn't have received any damage beyond the cut trace. 

    Please let me know if there additional tests you think maybe helpful in tracking this down.

    Best,

    Mikayla

  • 0 in reply to Mikayla Diesch
    TI__Mastermind 25800 points
    Hey Mikayla,

    I am also confused. I would recheck your layout to confirm incorrect traces or paths to the charger. You may also want to check the impedance from VFB to GND when those same resistors are removed.

    As far as the airwire, I would confirm on your layout what those cut traces did to the connection to the charger? Also, make sure your measurement of VFB is at the pin of the charger and not being cut out somehow when you cut the traces.

    Also, I noticed from your previous test that you measured 26mB across SRN-SRP. I thought this was your intended value of 2.5A?


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 80 points

    Joel,

    Thank you for the response

    We have been looking through the layout and schematic for any potential issues and have found no issues beyond the long trace into VFB/SRN. I have confirmed the only nets the cut trace impacted are the VFB and SRN pins, to which the airwire connects. It looks like the R(VFB - GND) was on the order of 300KOhm, though this is on a board that has VFB = 0V. 

    I was measuring the 26mV from the SRP of the sense resistor to the actual pin SRN, but if I measure across the sense resistor I'm getting ~15mV. I believe this is why we're seeing the charging current that we are, though I am unsure why there is such a drastic difference in the voltage even though the system is on the same net. I am also stumped by the fact the pins are sinking so much current when the data sheet only specifies 15uA. 

    Best,

    Mikayla

  • 0 in reply to Mikayla Diesch
    TI__Mastermind 25800 points
    Mikayla,

    Do you have a schematic you can share?

    It is odd that much current is being pulled from the SRN node. It looks like there is some other leakage path there that is not being accounted for.


    Regards,
    Joel H