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BQ25792: BQ25792

Part Number: BQ25792
Other Parts Discussed in Thread: BQ25798

Tool/software:

Using the bq25792 for a single cell. Charge voltage 4.1 VDC  Charge current 250 ma. I am not using the system output but do have the 10 MFDs on the pin.

When my micro sets the register to 1 cell the default VSYSMIN sets to 3500mv.  Charge is inhibited due to VBAT OVP.

If i simply change the VSYSMIN to something higher it starts to charge. 

Can you explain why the default does not work.

  • HI Corbin,

    I assume there is no battery or battery simulator attached at BAT?  If not, with charge enabled (via CE pin or bit) and no fault from TS due to missing thermistor, the BQ25792 sees a BATOVP fault due to overcharging of the default capacitors at BAT pin.  The OVP fault causes the converter to stop switching and SYS falls.  If you disable charge, there should be no OVP.  Increasing SYSMIN does not consistently prevent BATOVP but increasing battery pin capacitor to 200uF does.  The BQ25792 was designed for non-removable battery applications.  The pin-2-pin BQ25798 still shows OVP with no battery and low capacitance but the converter does not stop switching.

    Regards,

    Jeff

  • The screen shots were taken with the battery attached.  also took screen shots of the 16 bit registers.

    This application does have the battery permanently attached to the charger, bit I did not know I couldn't use the same circuit with detachable batteries.

    Again, battery attached, VSYSMIN at default 3500 = no charge/flags set. simply set the VSYSMIN to 5000, write registers, flags clear and charge begins. 

  • Hi Corbin,

    The charger does not support MINSYS > BATREG.  The point of MINSYS is provide the minimum system voltage when the battery is discharged.  Then SYS follows the battery up to BATREG.

    Regards,

    Jeff

  • That is exactly why I am asking the question. As you can see from the captures, I get the fault flags with the default of 3500 mv Charge Voltage at 1 cell,4.1VDC and the battery at 3.7 vdc. Then they clear and everything works when I try a higher MINSYS. This is the opposite of what I expected.

  • That is exactly why I am asking the question. As you can see from the captures, I get the fault flags with the default of 3500 mv. MINSYS, Charge Voltage at 1 cell,4.1VDC and the battery at 3.7 vdc. Then they clear and everything works when I try a higher MINSYS. This is the opposite of what I expected.

  • Hi Corbin,

    I have never this before.  Can you provide an oscilloscope shot of VBUS, VSYS and VBAT, triggered on VBUS coming up, this occurs?

    Regards,

    Jeff