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Original question:

BQ25792: Using the BQ25792 in a portable device with removable batteries

BQ25792: VSYS output current and ILIM

oneil
Prodigy 115 points
Part Number: BQ25792

Hi TI team,

I have two issues related to the BQ25792:

  1. Is there a spec for VSYS output current while on battery? I see IBAT_OCP is 9.3A, so I assume that VSYS can output at least that much current?
  2. I'm a bit confused on the use of VREGN to set IINDPM. The TIDA-0500047 schematic shows a voltage divider with a note saying it sets IINDPM to 500mA.
    1. If I use the equation on page 28 of the BQ25792 datasheet and the resistor values on the reference design, I find that VREGN would have to be about 3.3V to set IINDPM to 500mA. This is confusing because VREGN should be regulated to about 4.8V according to the datasheet.
    2. I am also confused about how VREGN could be used to accurately set IINDPM since it varies so much (between 4.6V and 5.2V depending on the test conditions). Does VREGN have a different value at startup that is more consistent?

Thanks!

  • 0
    TI__Guru**** 165966 points

    Hi Oneil,

    Regarding 1, max converter output current is 5A per

    8.3 Recommended Operating Conditions.  The battery charging and discharge current specs are below:

    The 9.3A is the battery overdischarge protection.

    Regarding 2, ILIM clamp is set by resistor divider from REGN.  You are correct that VREGN varies if input voltage is at 5V since the linear regulator powering REGN is in dropout.  IINDPM is the lower of the ILIM clamp setting or D+D- determined input current limit.  The host can change the IINDPM value or disable ILIM using EN_ILIM bit.

    To set ILIM clamp using the equation on page 28, the ILIM_HIZ pin is adjusted using a voltage divider from REGN to ILIM_HIZ to GND.   

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 115 points

    Hi Jeff,

    Thanks for the prompt response. You've cleared up my first issue, but I am still confused on the second.

    Assuming the lowest VREGN value in the datasheet, 4.6V, the voltage at VILIM_HIZ on the reference design should be 4.6V * (100k/(100k+137k)) = 1.94V

    Using the equation on page 28, a 1.94V VILIM_HIZ gives an ILIM of 1.17A, which is in disagreement with the note on the reference design. Am I misinterpreting how to use these equations or is the note on the reference design incorrect?

    Thanks

  • +1 in reply to oneil
    TI__Guru**** 165966 points

    Hi Oneil

    The datasheet equation below  

    V(ILIM_HIZ) = 1V + 800mΩ × IINDPMclamp = V(REGN)*Rbottom/(Rtop+Rbottom) is correct and works for the BQ25792EVM if you set V(REGN) = 5V:

    I will ask the team who made that reference design to update the equation.

    Regards,

    Jeff