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BQ25896 out of regulation

Tom Lowdermilk
Intellectual 300 points
Other Parts Discussed in Thread: BQ25896, TPS61252

I'm bringing up 9 prototype boards which use the BQ25896. Five of the boards are working but the other four 

are all failing in the following way:

When I attach a USB charger to the BQ25896 the SYS output is around 1.7 V. Referring to the attached doc 

I measured SYS, BTST and SW with a scope,... it looks like the BQ25896 is not in regulation. 

Some other info: I don't have a battery attached, and a visual inspection of the failing board didn't find any 

missing/broken components, solder problems. 

BQ25896_DEBUG.docx

  • 0
    TI__Intellectual 1520 points
    Hi Tom,
    could you provide a bit more information about:
    - load that the device is driving
    - input current limit
    - input voltage and current

    Also, what is changing in the test conditions between failing and not failing boards? Are you running the same tests?
    Thanks.

    Regards,
    Sergio
  • 0 in reply to Sergio Iannace
    Intellectual 300 points

    hello Sergio,

    Thanks for the quick reply. 

    The output of the BQ25896 goes to a +5V reg (TI TPS61252) and a +3.3V reg (Richtek RT6154A).

    The load on the +5V is very light (on the order of 20 mA). I disabled the +5V reg but that didn't make 

    any difference. The load on the +3.3V reg is processor daughtercard; the current requirement varies  

    during bootup but during power up it's typically around 200 to 300 mA.

    I've set the input current limit to its max with a 120  Ohm pulldown resistor (I'll play with reducing the 

    current limit after I send this update).

    The input voltage (coming from a Tenergy USB charger,... which supports Qualcomm QuickCharge 2.0) 

    is about 5.2 V.

    My basic test is to simply plug the USB charger into the system.

    thx!

    Tom

  • 0 in reply to Tom Lowdermilk
    Intellectual 300 points
    quick update,... I changed the current limit resistor from 120 to 200 Ohm,... no change in the problem.
  • 0 in reply to Tom Lowdermilk
    Intellectual 300 points
    some more info from experiments,... the design has the option of enabling another Richtek RT6154A ...
    so my final test configuration is as follows:

    1. TI TPS61252 disabled
    2. BQ25896 supplying power to two Richtek RT6154A (each one set to +3.3V)
    3. no load (I removed the processor daughtercard
    4. battery not attached to BQ25896
    5. Ilim resistor = 200 Ohm

    with this configuration if when I attach the USB charger I see the same original problem; the switcher
    in the BQ25896 appears to be out of regulation
  • 0 in reply to Tom Lowdermilk
    Intellectual 300 points
    hi Sergio,

    I think I found a fix,... I have two 10 uF (X5R, 20 VDC) ceramic caps close to the BQ25896 inductor and another 5 10 uF caps around my
    three DC/DC regs,... if I attach a 100 uF onto one of the two caps close to the chip the switcher works.

    Do you know why this is working now ? (!)

    thx,

    Tom
  • 0 in reply to Tom Lowdermilk
    TI__Intellectual 1520 points
    Hi Tom,
    I am happy it is working. Could you send me the schematic?
    Thanks.

    Sergio
  • 0 in reply to Sergio Iannace
    Intellectual 300 points
    hello Sergio,

    Can you ping me at tlowdermilk@usens.com? I'll send the schematics.

    In the end here's what works,... solder a 100 uF on the SYS output.

    thx,

    Tom