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BQ25731: Adjustable VSYS_MIN and Power Path Control questions

John Pierson
Prodigy 105 points
Part Number: BQ25731
Other Parts Discussed in Thread: BQ25730

Hello, I want to reach out to E2E with some questions regarding the TI BQ25731 Charge Controller IC we are using. First, I will list some requirements we are looking for with the BQ25731 device. 

  • Our battery pack employs NiMH battery cells. The configuration is 12S2P (12 cells in series, 2 of these stacks in parallel)
  • We plan on supporting charging of this battery pack by two different means:
    • An AC powered AC/DC adapter rated 19.5 VDC, 9.23 Amp, 180 W output.
    • Or charger input can be supplied by an Automobile Aux power jack.
  • We want the battery charge current level to be 8 to 10 Amps max.
  • And we want to be able to simultaneously supply additional supplementary power to the load from the charger input while charging the battery.
  • When using an automobile Aux Power jack, we don’t want to discharge the auto battery.
    • To support this, we want to disable battery charging when the charger input voltage is less than 12.6 VDC

 Questions:

 To support disabling the charge controller when input voltage is less than 12.6 VDC. The BQ25731 supports fixed VSY_MIN of 15.4 V with the 5S cell count. So it appears the BQ25731 device cannot support this. Is this correct?

  1. BQ25730 supports programming VSYS_MIN register address 0D/0Ch. Is this the preferred method for supporting a 12.6 V min input voltage?
  2. Regarding the requirement for supplying additional supplemental power to the load while charging. I have observed the BQ25731 charge controller can do this. Is the BQ25730 a better choice for this requirement?
  3. And finally, what is the Narrow VDC feature, and is it better for our requirements?

 

  • 0
    TI__Expert 4780 points

    Hello John,

    The VSYS_Min is adjustable via register write. The values based on cell configuration are just defaults.

    Please note, VSYS refers to the output voltage at the System. VSYS_Min is the minimum voltage that the charger will regulate, if the battery is below that voltage. If you want to limit the input voltage, you can use the VINDPM feature, which is designed to not overload and protect the input source.

    When the input current exceeds the input current setting (IIN_DPM), or the input voltage falls below the input voltage setting (VINDPM), the charger decreases the charge current to provide priority to the system load. As the system current rises, the available charge current drops accordingly towards zero. If the system load keeps increasing after the charge current drops down to zero, the system voltage starts to drop. As the system voltage drops below the battery voltage, the battery will discharge to supplement the heavy system load.

    The difference between BQ25730 and BQ25731 is that the BQ25730 has NVDC architecture while the BQ25731 has direct power path architecture. With the direct power path, the system gets connected directly to the battery, so the system (output) voltage follows the battery voltage. With the NVDC architecture, there is a FET between the battery and the system, so the charger can regulate the system voltage above the battery voltage (start up even with depleted battery). 

    The Narrow VDC buck-boost architecture avoid direct power path from input source to system voltage which implements wide input voltage range and narrow system voltage range at same time. Narrow system voltage range is valuable for faster battery supplement and lower MOSFET rating for next stage converter.

    Based on your requirement for supplying additional supplemental power to the load while charging, the NVDC is the best option for this, so BQ25730 should be a good choice.

    Khalid

  • 0 in reply to Khalid Bairuti
    Prodigy 105 points

    Hi Khalid,

    Apologies for not responding sooner, I have been out of office. Thank you for the information in your response. This resolves all but one of my questions.

    We would like the charge controller to not draw or consume adapter current if the VIN level is less than 13.0 VDC. Regarding VINDPM - From the Register 0B/0Ah description:

    If the input voltage drops more than the InputVoltage register allows, the device enters VINDPM and reduces the charge current.

    If VINDPM is less than 13.0 V, adapter current is still consumed. It looks like we could use register ADCVBUS - 8-bit digital output of input voltage. Could we read this value and disable the charge controller when ADCVBUS is < 13.0 V? 

  • 0
    TI__Prodigy 21 points

    John, have you considered using the independent comparator and the "FORCE_CONV_OFF" register bit? it seems a little simpler than using the ADC, although you have to clear the bit to restart charging.

  • 0 in reply to Eric Southard
    Prodigy 105 points

    Hi Eric,

    I like the independent comparator suggestion. This would be "automatic". Each time an adapter or in this case, an automobile AUX power connection is plugged in, and VBUS_IN is under 13.0 V, the charge converter would be disabled. It is minor for the firmware to clear the FORCE_CONV_OFF bit.

    Thanks for this recommendation and support. This case can be closed.