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BQ25601: Issue when BATFET disabled

Mark Guastaferro
Genius 11645 points
Part Number: BQ25601

While running evaluation of the bq25601 EVM an unexpected SYS output is observed. In this test, there is no source connected to VBUS, but a charged battery is connected to VBAT.  

With a 10 ohm resistor load connected SYS, the proper battery voltage is observed at the SYS pin. If the BATFET is disabled, SYS voltage drops to 0V as expected.

However, with an active load consisting of a switching boost converter connected to SYS and set to output 5V, the response is different. When the charger BATFET is disabled, the SYS voltage drops to about 1V, and output of the boost is still 5V. If a 10 ohm resistor is placed between SYS and ground, the SYS voltage drops to 0V.  But if the resistor is removed,  SYS voltage returns to 1V. If the 10 ohm resistor is placed on the output of the boost,  the SYS voltage remains at 1V (with BATFET disabled). 

If the boost converter is disabled and then the BATFET is disabled, the SYS voltage is 1.3V. Boost converter is designed to isolate its input from the output when disabled.

Any ideas why disabling the BATFET does not completely turn off the load? 

  • 0
    TI__Guru 57611 points

    Hi,

    1. Is valid VBUS present or not during the test?

    2. Not sure how an active load consisting of a switching boost converter and 10 ohm resistor are connected to the charger. Please explain further.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    TI__Genius 11645 points

    Hi Ning,

    Sorry if my first explanation was not clear. 

    1. There is nothing connected to VBUS during these tests.  Only a battery is connected to the BAT pin of the bq25601.
    2. There were four different experiments tried to check the voltage on the SYS pin when the BATFET is disabled (set to ship mode).
      1. Place only a 10 ohm resistor to ground on SYS.  The voltage at SYS behaves as expected. Battery voltage present when BATFET is on, SYS voltage drops to 0V when BATFET disabled. 
      2. Remove resistor load and connect boost converting module (not a TI device). Boost converter is enabled through 1M pull-up to SYS voltage and set to output 5V. When BATFET is disabled, SYS voltage drops to about 1V and boost output is still 5V. no load on boost converter.
        1. Place 10 ohm resistor load on output of boost converter. When BATFET disabled, SYS voltage is still 1V.
      3. Move 10 ohm resistor from boost output to boost input (SYS pin). When BATFET disabled, SYS voltage drops to 0V, as desired.
      4. Remove resistor load. Disable boost module grounding its Enable pin. This places a 1M ohm resistor from SYS to ground. When BATFET is disabled, SYS voltage is now 1.3V. If boost is enabled, SYS voltage returns to 1V. It doesn't matter whether boost is disabled before or after BATFET is disabled, SYS voltage response is the same: = 1V when boost enabled and 1.3V when boost disabled with BATFET disabled.

  • 0 in reply to Mark Guastaferro
    TI__Guru 57611 points

    Mark,

    When BATFET is turned off, there's still battery leakage flowing to SYS.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    TI__Genius 11645 points

    Hi Ning,

    Thanks for the note. I'm sure this is what's happening. and with high resistance load,  the voltage is building up higher than expected. 

    This spec describes the leakage current from the battery's perspective (which is really what's important in ship mode). I assume the BQ25601 consumes some of the leakage current and not all of it ends up at the SYS pin. Our tests only show a few uA out the SYS pin. 

  • 0 in reply to Mark Guastaferro
    TI__Guru 57611 points

    Mark,

    It is good that the issue is resolved. The thread is considered closed.

    Thanks,

    Ning.