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BQ25892: Voltage drop/brownout issue on sys output

Lauren Mayfield
Expert 8015 points
Part Number: BQ25892
Other Parts Discussed in Thread: TPS61022

Hi team,

I have a couple questions regarding this chip. It is driving a TPS61022. I have a block diagram and schematic to share as well, let me know if you need those and I will share internally. 

1. If this chip is turned on and not programmed by the host MCU, what's the default supply current on the Sys output? Assuming you have a 4.2V input coming in from the fuel gauge with the battery.

2. What is a recommended ILIM resistor value?

3. What could be causing the drop? The output is going to both a 3.3V LDO and the 4.2V output TPS61022, perhaps it is not able to supply enough current? The dropout occurs as soon as the TPS61022 is enabled.

  • 0
    TI__Guru**** 171616 points

    Hi Lauren,

    If battery is attached at BAT and the BATFET has not been turned off via I2C, then the output power on SYS will be VBUS*ILIM*efficiency plus power supplemented by the battery.  

    ILIM typical is used to set an input current limit clamp to prevent the charger from crashing its input source at system power up, assuming you have PSEL set to provide the highest current limit.  After system power and the USB controller can enumerate the determine the input source's true output current capability, the host can write the I2C registers to disable ILIM pin clamp and set input current limit register to the highest value.

    If battery is attached, I would expect the battery to be able to supplement any surge current required by the TPS61022.  Do you know the TPS61022 max input surge current at start up?  The charger protects against SYS short circuits only if ISYS > 9A, by turning off the BATFET.  If that happens the charger will report that event if the I2C register.  Has that happened?  The battery pack protector also has a protector FET that turns off at a certain current. 

    Regards,

    Jeff

  • 0
    TI__Genius 10385 points

    Lauren,

    1. If this chip is turned on and not programmed by the host MCU, what's the default supply current on the Sys output? Assuming you have a 4.2V input coming in from the fuel gauge with the battery.

    In default mode (no host), with a 4.2V battery connected, SYS should output be 4.2V


    2. What is a recommended ILIM resistor value?

    This would depend on your application. What is your adapter current limit?

    3. What could be causing the drop? The output is going to both a 3.3V LDO and the 4.2V output TPS61022, perhaps it is not able to supply enough current? The dropout occurs as soon as the TPS61022 is enabled.

    As you said it is observed when the TPS device is enabled, so there might be some kind of inrush current. You might be tripping an overcurrent protection out of the fuel gauge. Potentially you could solve this with more input capacitance on the input of the TPS device. I do notice something odd about your block diagram. The TPS device is a boost converter that you have set to output 4.2V, but your TPS input voltage range from 3.5V to 4.2V. Wouldn't this be an issue?

    Best Regards,

    Ricardo