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BQ25601: VSYS current limit when there is no Battery

Balazs Demeny
Prodigy 50 points
Part Number: BQ25601
Other Parts Discussed in Thread: BQ24272

Hello,

We have encountered some issues with the BQ25601 VSYS output while there is no Battery and charging is enabled.

Our Load is a GSM modem, which draws burst current up to 1.5-2A.

While the charging is not enabled (CE PIN is Pulled Up) everything works fine, few mV drop on the VSYS output.

After we enable the charging (CE PIN grounded) we see the charging sequence, which tries to get the battery voltage to VREG (4.1V used), but while if a burst is encountered on the VSYS output the voltage drops below 3V which makes the GSM modem to stop working. It seems while VREG is applied for some reason there is current limit but we are not sure what happens. We have tested if we put a totally depleted battery it works fine, even if that battery probably is not capable of supplying 1.5-2A burst. We tried all kind of setting(REGITER setup), but nothing seemed to solve this issue, which is quite critical, because the battery is mounted by the distributor/client. Also the software is running on the GSM modem, so it's not like an external controller will disable the GSM modem till it checks if the battery is mounted or not.

Attaching a screenshot to be as clear as possible with my description.

Best regards,

Balazs

  • 0
    TI__Mastermind 46551 points

    Balazs,

    1.During termination mode of the charger (charge disabled), the system voltage is being regulated as the active loop which is the fast response loop;

    2.During the constant voltage (CV) mode (charge enabled), the battery voltage is being regulated, which does not require fast response. Therefore, the input current is not pulled fast enough and the system voltage drops.

     

    The most effective walk around solution is to disable charging when the battery is not inserted.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Prodigy 50 points

    Hello Ning,

    Thank you for the answer, which is not that great to hear.

    We thought that this way we can manage the battery detection, by measuring the VSYS transient voltage change, because as I mentioned it's not always plugged in and we need to detect that.

    Also there are cases when the battery is depleted and the battery built in safety circuit shuts down the output, so you need to apply voltage in order to have some voltage at VBAT. In any case the charging must be started.

    In the past we used the BQ24272, which did detect the battery and didn't had this unexpected regulation issue.

    Can you suggest some other component similar to BQ24272, because as I see BQ25601 will not do the job for us and from the datasheet this was not clear at all.

    Thank you.

    Balazs

  • 0 in reply to Balazs Demeny
    TI__Mastermind 46551 points

    Balazs,

    I know little about BQ24272. Please submit a new e2e thread with BQ24272 on the subject line and the BQ24272 support will be able to help with your questions.

    Thank you very much,

    Ning

  • 0 in reply to Ning T
    Prodigy 50 points

    Hello Ning,

    I want to add a short description on how I managed to make a "work around" regarding this and how I've made the battery detection to work, for other interested users as well.

    1) Measure the VBAT voltage, to detect if it has a battery plugged in or not with Charging disabled.

    2) Then for a small amount of time ~500uS (in my case 558us) charging is enabled, this makes the VBAT rise to ~3.7V. This is good in order to wake up the LiPo battery if it has a short circuit protection or shut down circuit in the battery itself.

    3) Measure the VBAT voltage again, if VBAT is present (not 0V) then it means it's plugged in and you have successfully detected the battery. You can continue to charge it, or not, depends on what you want to do.

    4) In case VBAT is not present (0V) you can repeat the whole detection cycle over again with ~500uS ON Charging time every few seconds/minutes depending on how fast you want to detect a depleted, short protected battery, because it won't effect the the fast response loop. I've tested with an Electronic Load simulating 1.6A GSM burst (580uS ON, 4mS OFF) and it worked fine.

    Also attaching a simulation screenshot with continuous GSM burst and battery detection (Charge ON time). I've run the test for several hours and it worked well without the current loop to crash.

    Balazs