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BQ24765: Low VIN operation, 5V VIN and not 7V VIN

Mark Pearson
Genius 12705 points
Part Number: BQ24765
Other Parts Discussed in Thread: BQ24171, BQ24172, BQ24780S

Battery charger team - any issues with using the BQ24765 with a 5V input source and not the 'minimum 7V' source in the datasheet.

I see that the 5V source passes all of the UVLO requirements for operation.

My only concern is the 6V internal VDDP LDO that feeds the BOOT pin (FET Gate bias)...

The charging current will be a maximum of 500mA fro a USB source. The load is a 2-cell NiMH AA stack (each cell V,max is 1.6V).

I accept that the BQ24765 will run less efficient (higher RDS(on)) with a lower BOOT voltage... any concerns about this?

To mitigate this a little bit - can we tie the USB 5V to the Anode of D1, figure 17 in the datasheet (disconnect the VDDP pin)? At least this way there's no LDO drop-out voltage degradation in doing this.

Thanks!

  • 0
    TI__Guru*** 146695 points
    Hi Mark,

    Unfortunately, this device was only tested at 7V minimum and so we cannot warranty operation below 7V. Why was this device selected? It is overpowered for 500mA charge current. I assume it is because the charger has charge voltage regulation setting down to 3.2V? Is a host controlled device required? If not, then bq24171 with external resistors to set charge voltage regulation might be an option.
  • 0 in reply to Jeff F
    TI__Genius 12705 points
    Part of the problem is that this is for NiMH batteries and not Li+ or Li-Polymer.
    We would like a stand-alone solution like the BQ200x devices, but with power-path control, like the BQ24765.
    -mark-
  • 0 in reply to Mark Pearson
    TI__Guru*** 146695 points
    Mark,

    Can you clarify the power path control requirement? The bq24765 lets the host control whether the system is powered by the input voltage or battery. The bq24172 ( I mistyped bq24171 above) does the same thing automatically or could be done via host control as well. Its minimum is 4.5V.
  • 0 in reply to Jeff F
    Prodigy 10 points

    Does the BQ24172 & BQ24171 work with NiMH?

  • 0 in reply to Rami Azer
    TI__Genius 12705 points
    Rami,
    The bq24171/2 cannot be used for NiMH for 2 reasons: 1) they have a specific charge and termination scheme for Li+/polymer, and 2) they have fixed terminal voltages, namely 4.1V or 4.2V or multiples of these.
    Another alternative to the BQ24765 would be the BQ24780S. It has external FETs, whereas the BQ24765 has internal FETs. The bq24780S also has a minimum input operating voltage of 4.5V. This is odd to me, because both ICs ('765 & '780S) have 6V internal LDOs for the gate bias. So - if you go with the '780S, choose external FETs with good switching characteristics at a 4.5V VGS level.
  • 0 in reply to Mark Pearson
    TI__Guru*** 146695 points
    Mark,

    The BQ24172 uses external resistors to set the CV voltage to a voltage higher than 2.1V. Termination (and the safety timer) can be disabled by tying the TTC pin low.