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BQ25798: Or BQ25792 low power application with small form factor solar and infrequent 24VDC input

Jacob Towe
Prodigy 10 points
Part Number: BQ25798
Other Parts Discussed in Thread: BQ25792,

As stated in the title we are looking to use a BQ25792 or BQ25798 to power a device that will require 3V at around 50 micro amps most of the time with a high current draw around 300-400mA infrequently (less than 1 time a day for a short period) on battery power.  These ICs were chosen because the application is outdoor and requires the batteries to be monitored for temperature during a charge to stay within recommended limits.

We need to use a very small PV panel only capable of supplying around 1-1.5w of power due to space constraints.  Currently looking at a 9V 100mA to stay above the 3.5 Vmin for a longer period of time during daylight.  Could potentially go a little bit larger on the PV panel but it would still be well under 500mA.

So, the first question is, can a Lion or LiPo be "trickle" charged from this small of a solar cell using either of these ICs?  Or if not able to charge the battery, at least be able to offset power usage of the board during the day?

The board will also randomly be hooked to a 24VDC source that can be as high as 28-30 volts depending on the alternator/batteries connected.

Should a regulator be installed before the chip to trim that voltage down to the Vmax of 24 or can they use this power supply as is below the absolute Vmax of 30?

Links to a similar reference design would be extremely helpful as well.

Thanks,

Jacob

  • 0
    TI__Guru*** 143880 points

    Hi Jacob,

    BQ25798 min input voltage is 3.6V and max operating (charging) voltage of 24V with abs max voltage of (is not damaged up to) 30V.  So, the solar panel needs to provide at least 3.6V and ~100mA for the charger to power up and provide trickle/precharge/fast charge current.  A regulator or clamp is required in front of the regulator to step down 28-30V input. The BQ25798 has MPP feature that periodically resets the charger's VINDPM (input voltage regulation loop) to prevent crashing the solar panel. 

    Also the charger SYS output follows the battery up.  So if it charges up a LiIon battery to 4.2V, a step down regulator or switching converter is required to provide 3.0V to your device.

    Unfortunately, I do not have any reference designs.

    Regards,
    Jeff