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BQ25886: BQ25886: Heat generating when Vsys_min < Vbat

user5974868
Prodigy 95 points
Part Number: BQ25886
Other Parts Discussed in Thread: BQ25883

Hi,

I want to run my load as long as possible though the power-path (Vsys and Isys), but I have few question related to what happens during Vbat < Vsys_min.

1. As I understand the regulation(maintain systems voltage at ~6.2V) will only activate when Vbat is lower than Vsys_min, is it correct?

2. The max load connected to the power path is 27W, which will generate high currents. I want to run my load as long as possible and want the battery going down to Vbat=5.4V before shutting down. 
This will generate high currents at Ibat and Isys and I would like to receive some recommendations that i need to consider (design related). Any tips? 
Or maybe its not an good idea..? 

3. What is the an estimated efficiency related to nr2?  

thanks in advance

 

  • 0
    TI__Mastermind 40375 points

    1. That is correct. This minimum system voltage is for when VBAT is lower than VSYS_MIN.

    2. The max discharge current (RMS) the battery FET can handle is 5 A. Make sure that as the battery is depleted the discharge current is within the Absolute Maximum Rating. Following the layout guidelines in the datasheet, make sure the SYS capacitors are as close to SYS and GND as possible. You will need to make sure you turn off the load when the battery voltage reaches the level you wish to terminate supplement mode at.

    3. What efficiency are you looking for?

    Thanks,

    Mike Emanuel

    Please click "This resolved my issue" button if this post answers your question.

  • 0
    TI__Guru**** 166626 points

    Hi,

    Regarding 1, yes.

    Regarding 2, Your concern is valid.  The 27W output power is a bit high for this device.  With VBUS=5V and max input current=3.3A, the charger's max average input power is 16.5W.  Output power is less than that due to losses.  If the 27W only occurs during infrequent transients that can use the battery to supplement the current, no issue.  If not, then I recommend changing to the BQ25713A I2C controlled charger with external FETS.  Regardless, when V(BAT) < VSYSMIN is charging, the charger is least efficient due to the drop across the internal battery FET.  Losses across the BATFET = (VSYSMIN-VBAT)*ICHRG = (6.4V-5.6V)*ICHRG where the 886's VBAT_LOWV falling = 5.6V is the transition threshold from fast charge to precharge.  The charger has thermal regulation and will protect itself by lower its output power (charge current reducing first then system load current).  If you change to BQ25883 I2C controlled version, you have more control of VSYSMIN and could improve efficiency by lower VSYSMIN.

    Regarding 3, unfortunately, I don't have any efficiency data down that low.  But, converter efficiency improves as the difference between input and output reduces.  So, I would expect efficiency to be in low 90% range down to VBAT=6.4V.

    Regards,

    Jeff