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BQ25713B: ChargeCurrent and MinimumSystemVollage setting recommendation

David Mckinney
Expert 6830 points
Part Number: BQ25713B
Other Parts Discussed in Thread: BQ25713

The BQ25713 feature that when the ChargeCurrent Register (0x02~0x03) is all zeroes, VSys is regulated at whatever the battery voltage is plus 160mV.  If such register is set to all zeroes at a point that the battery voltage is not at its fully-charged voltage state (e.g. perhaps a battery disconnection is needed because the battery is too hot), this will mean that VSys will be at a lower voltage than that specified at the ChargeVoltage Register (0x04~0x05) which would restrict the amount of available power at VSys.

 

Would the recommended practice then be to increase the MinimumSystemVoltage register (0x0c~0x0D) to a voltage close to (but less to) the ChargeVoltage Register when the ChargeCurrent Register is all 0 and reduce it to the normal lower voltage when not?

  • 0
    TI__Expert 5330 points

    Hi David,

    Generally the MinimumSystemVoltage would be set to the lowest supported discharge voltage of the battery.  I.e. a typical range would be to charge LiIon to 4.2-4.3V per cell and discharge down to 2.8-3.0V per cell.  So if you planned to discharge down to 3.0V and you had a three cell system, you would generally set MinimumSystemVoltage to 9.0V  (whether charging or not). 

    I would not recommend raising MinimumSystemVoltage in the way that you describe when you are not charging.  The reason that VSys is regulated at VBATT + 160 mV is so that this allows the battery to supplement the system voltage if needed without there being a large drop in VSYS.  If your system overloads the adapter, VSYS will begin to fall because adapter cannot provide the full power needed.  In this situation, it will only drop 160 mV before the battery begins discharging to supplement the adapter.  This is one of the primary advantages of the NVDC architecture, that you can supplement the adapter with the battery and not have VSYS drop too far.

    If you instead raise VSYS in the manner you are describing, then when you get a heavy system transient that overloads the adapter, VSYS will drop sharply from wherever you set MinimumSystemVotlage until it hits the battery voltage and the battery can supplement.  Then, when the load releases and the adapter takes back over, VSYS will rise sharply from VBATT back to VSYS min.  This kind of voltage step on VSYS could be disruptive to the downstream system and is generally undesirable.

    Also, I am not sure that having a lower VSYS will reduce your available power dramatically.  It may have a little lower efficiency at the converter stage and a little more resistive loss along the way, but unless you hit a current limit somewhere, you should just have proportionally higher current at the lower voltage and end up with close to the same available power.

    Regards,

    Steve