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bq77PL900 start-up circuit

Dylan Davis
Prodigy 10 points
Other Parts Discussed in Thread: BQ77PL900, BQ24610

I'm using the bq77PL900 with the bq24610 to provide Li-Po battery charging and protection in an autonomous system, and am currently designing a PCB for this purpose. This board needs to automatically activate as soon as a battery pack is connected, however the bq77PL900 requires a >7.5V start-up voltage to activate. How can I provide this voltage automatically?

  • 0
    TI__Guru 54715 points

    The wake up voltage is applied to the device PACK pin which also serves as a power input and a sense input for the charge FET source. Since power can also come from the BAT pin, you may be able to  provide the higher of the system's PACK+ or ~8V to the PACK pin. This would allow the part to control the charge FET normally when voltages are high, when the discharge and charge FET turn off and the system's PACK+ voltage falls, the PACK pin would fall to the ~8V level.  The part would remain awake. Since it would see the PACK pin at the ~8V level, it would drive the CHG output to this level when it wants the CHG FET off, so your charge FET and any transient clamping should be selected to accommodate this situation.

    Your 8V would likely need to be generated from your cells with some low current protected regulator circuit. You would want to enable the regulator after cells are connected since if the bq77PL900 wakes up with cells partically connected it will begin pulling on the cell inputs and could develop a large differential voltage on the inputs which could be damaging. The 8V can then be left on to prevent shutdown in the bq77PL900 standalone mode.

    If only a momentary wake up signal is needed, a resistive switch from the Battery+ to the PACK pin may be sufficient. The part will shutdown if the cells go under voltage in standalone mode.  If the device is operated in host mode, the host can determine whether to shutdown, so a momentary wake up signal may be sufficient. Depending on your system perhaps this could be provided at an assembly test fixture.