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BQ25504: BQ25504 + TPS78233 design review

Armel Fourreau
Prodigy 20 points
Part Number: BQ25504
Other Parts Discussed in Thread: TPS782

Hello,

We are working on a solar-powered supply for an embedded system running on 3.3V, with max current of about 30mA but <1 mA most o the time. We came up with the design in the picture attached, which is based on the BQ25504 and TPS78233, with a AM-5412CAR solar cell from Panasonic (3.4V open circuit voltage, 2.2V at Pmpp), and a LiPo battery as storage element.

Now we are witnessing an unexplained behaviour: when we connect a fully loaded battery to the system, and set the ENABLE of the TPS78233 to ground, the LDO output is not 0, but 0.7V to 1.4V. When the LDO is enabled, its output is 3.3V as expected.

Would you have any clue about what might be going on?

Thanks

  • 0
    TI__Mastermind 21590 points
    Hi,
    Looking at the BQ25504, the only issue i found is OK_HYST is higher than it should be. This value should be between 50mV and VBAT_OV - VBAT_UV which is about 1V in your case. Regarding the LDO, can you try connecting the ground directly instead of a switch and check if this is still happening.
    Regards,
    Raheem
  • 0 in reply to Raheem
    Prodigy 20 points
    Hi Raheem,

    Thanks for the hints, unfortunately I didn"t have any luck connecting the enable directly to ground, the LDO output is still > 700mV despite being disabled. I just realized there is a single ground (i.e. AVSS not split from the power ground), could there be some sort of ground shifting? Still I am not sure how that could impact the LDO output.
    Any thoughts?
    Thanks,

    Armel
  • 0 in reply to Armel Fourreau
    TI__Mastermind 21590 points
    Hello,
    Any luck on this issue? Can you try removing the 2 output capacitors and checking again to see if the output is zero when you disable the LDO?
    Regards,
    Raheem
  • 0 in reply to Raheem
    TI__Expert 8905 points
    Hi,

    As a follow-up the device may have some low levels of leakage current flowing through the pass device (typically nA), or the output capacitor could still be holding charge.

    If you force the output voltage to 0V (use a short across Vout) does the TPS782 Vout stay at 0V once the short is removed? This would indicate the output cap is still holding charge.

    Regards,
    David