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BQ25790: BQ25790 UVP

Part Number: BQ25790

Hi,

I used bq25790 for 2S 18650/3500mAh batteries charging, setting PROG resistor 8.2k.

The batteries were completely charged initially (totally up to 8.4V), providing  the same voltage to 25Ohm system load.

After switching the external PS off they slowly dischaged down to VBAT=5.72V and VSYS=5.71V and didn't stop.

  1. Does bq25790 have hardware / built-in under voltage protection (UVP) and what its default level is?
  2. What internal settings/registers might be used to change the UVP level?

Thanks,

Semion

  • Hi Semion,

    What was the external PS voltage?  The 790 has a 7V VBUS OVP setting that must be increased when using a higher voltage PS.

    Regards,
    Jeff

  • Hi Jeff,

    The problem is that without an external PS voltage, neither internal (BATFET) nor external Ship FET switches-off the battery from the load, so the battery continues discharge and finally destroys.

    I.e., exhausted 2S battery (@VBAT=5.72V) connected to the bq25790 continuing discharging by the load.

    Regards,

    Semion

  • HI Semion,

    The internal BATFET has a body diode that always allows current to flow from SYS to BAT.  The external shipFET can only be turned off for ship or shutdown mode.  Battery chargers are not meant to provide primary overdischarge protection.  The battery pack protector circuitry has over voltage and over discharge protection circuits for that. 

    Regards,

    Jeff 

  • Hi Jeff,

    Thanks for your clarification.

    But I see in the DS two parameters (VBAT_OTGZ and VBAT_UVLO) limiting the battery operation at low voltage.

    Are these the voltages at which the charger stops working?

     I use BQ25790EVM (BMS027) Evaluation Module using ShipFET, Q5 at disconnected jumpers JP5 and JP13.

    And without external PS voltage, at battery connection, Q5 should not stay switched-on constantly, since its Vgs is negative relative to the Vs, i.e., Vgs=Vs-0.75V.

    In this case how the battery stays connected to the BAT pins? I.e., through what circuit?

    Regards,

    Semion

  • Hi Smion,

    VBAT_OTGZ is the battery voltage at which reverse/OTG mode turns off.  VBAT_UVLO is the voltage at which the charger's charge pump for the internal BATFET can longer to turn on the BATFET.

    Regards,

    Jeff

  • Hi Jeff,

    It’s not clear how in the case when PS-off ShipFET, Q5 (N-channel) conducts at its direct connection, i.e. at Vds positive and Vgs negative, providing voltage to pin BAT of the charger.

    Please clarify.

    Thanks,

    Semion

  • ...i.e. in this case charge pump doesn't work.

  • Hi Semion,

    I should have been more clear.  There are two charge pumps, one to drive the internal BATFET and one to provide the SDRV pin voltage to drive the shipFET.  The UVLO's for each are in the table below with the shipFET being the top values.  The shipFET charge pump input is BATP pin.

    Please note that the internal BATFET has a body diode that sets SYS = battery voltage less a diode drop.

    Regards,

    Jeff

  • Thank you, Jeff

    As I see the charge pump capability at PS-on and off is different.

    The charge pump w/o PS voltage is significantly weaker than at VBUS voltage presents.

    Best regards,

    Semion