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BQ25570: What storage element is recommended for 3.3V Vout

Alexander Mazany
Prodigy 10 points
Part Number: BQ25570

Hello, I am designing a circuit that will use 3.3V, so I am designing for Vout to be 3.3V. However, it seems that the standard 3.7V li-ion battery may be slightly too low to be used to it's full capacity. If my understanding is correct, the buck regulator won't operate when the Vbat voltage falls into the 3.3-3.6V range, which will greatly limit the amount of energy I can get from the battery. I was wondering if I should simply set the VBAT_OK and VBAT_OK_HYST to be much higher (maybe 3.6V instead of the typical 3V), or if I should use some other storage element. Would it be possible to use NI-Cd batteries in series to reach ~4.8V? Or would that cause problems with charge balancing?

  • 0
    TI__Guru**** 166996 points

    Hi Alexander,

    You could use 3 series NiMH or 2 series 2.7V super caps.  The IC does not provide charge balancing so that might be a concern for long life.  There are 4.35-V LiIon that reach ~90% capacity at 3.8V.

    The buck converter should only need about 200mV headroom, which is VBAT=3.3V+0.2V=3.5V, so you could still use a 3.7-V LiIon battery, that charges up to 4.2V.  Then you could set VBAT_OV = 4.2V, VBAT_OK=3.55V (50mV additional headroom) and VBAT_HYST=3.8V or higher.

    Regards,

    Jeff