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BQ24025, Floating AC input

Trilogis
Prodigy 170 points
Other Parts Discussed in Thread: BQ24025

I'm using the BQ24025 with a Varta EasyPack Li-Polymer cell (3.7V, 1000mAh).

I don't use the AC input (floating) but only the USB input.

When charging the battery with a 100mA current (ISET2 low) the status on STAT1 and STAT2 is 'fast charge in progress'. Measuring TS shows 860mV (battery at 25°C). Slow but OK.

When charging the battery with a 500mA current (ISET2 high), the status on STAT1 and STAT2 is toggling between 'fast charge in progress' and 'charge done' (500ms charging, 1000ms done). The battery voltage remains steady, not charging at all. TS is toggling as well between 350mV and 860mV (thermal alarm is issuing the toggling....).

I fear that this is due to my floating AC input.

Is it mandatory to connect AC to GND ?

May I connect AC and USB together ?

 

 

 

  • 0
    TI__Mastermind 31120 points

    You are correct...I connected the circuit in my lab and got the same result.  There is a leakage current from the USB to the AC pin and this pin will float up if not grounded with a 1k or lower resistance.  The AC voltage floats up above the battery if the battery is low enough (in my case it was 3.3V) and switches to AC mode and then drops out since there is no source there.  A 4.5k resistance (pot reduced from 10k) form the AC to ground  kept the AC float votlage from becoming ACPG and stopped the cycling from AC mode to USB mode.  This does not seem to happen on the ICs without the TS pin.  Grounding the AC in is also acceptable.

  • 0 in reply to Charles Mauney
    Prodigy 170 points

    Thanks for your answer.

    In the documentation for bq24025, on page 12, one can read:

    "As default, the bqTINY-II attempts to charge from the AC input. If AC input is not present, the USB is selected."

    I can't find anything about grounding USB or similar.

    Documentation quality is usually much better for TI products. For example, in the same document, the figure 2 on page 9 in truncated and the taper time is missing on the very right side.

     

    I have a whole production lot that have this problem (AC floating). Do you see any other solution ? Do I have to correct all PCBs and connect the AC pin to GND ?