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SND9039 for 2S battery cells system

Antony Lin
Genius 13480 points

Hi,

Currently we need to supply a 2S battery system using SND9039.  There'll be a 5V buck to generate 5V as VSYS input to SND9039, but we still need to sense 2S battery voltage through VBATSENSE pin by internal GPADC.  So, the questions are

  1. What is the suggested resisitive divider ratio and corresponding resistor values on VBATSENSE pin?
  2. Is there any concern that VBATSENSE and BUCK/LDO inputs are from "2S battery with resistive divider" and "fixed 5V from a buck" individually?

Thanks!

Antony

  • 0
    TI__Mastermind 25665 points

    Hello Antony,

    To answer the first question, it would be helpful to know the following:

    1. What battery voltage do you want be considered fully charged? For a 2S system, I am thinking 8.7V. If so, a ration of 1:1.24 would match well to the 4.8V max of VBATTsense. Using larger resisters would be better to reduce the amount of current flowing through it, so 1Mohm and 1.24Mohm.

    2. What battery voltage do you want cause Vbatt Power Up, Low Batt Detect, or Power Down event? From a PMIC standpoint, these events have programmable voltage thresholds but if resistor divider doesn't allow the VBATTsense pin to reach a value in the selectable range this could cause a problem.

    I do not see a problem with VBATTsense being supplied separately from BUCK/LDO input but it must be understood that the PMIC cold boot and cold off sequences are triggered by the voltage on VBATTsense.

  • 0 in reply to Michael Gambrill
    TI__Genius 13480 points
    Hi Michael,

    Thanks. 8.7V is correct for 2S battery.

    I remember we also have internal resistive devider for ADC on VBATSENSE (160kohm vs 160kohm ??). When you mentioned 1Mohmm and 1.24Mohm, have you considered the internal resistive divider?

    Thanks!

    Antony
  • 0 in reply to Antony Lin
    TI__Mastermind 25665 points

    I apologize, Antony, I failed to consider the internal resistive divider. I will do that, but first can you answer question number 2 from above? This will play bigger role in determining resistor sizes.

    Regards,

    Mike

  • 0 in reply to Michael Gambrill
    TI__Genius 13480 points

    Hi Mike,

    Regarding question number 2, customer will use default register setting for their application.  Please provide your proposal for VBATSENSE external resistor accordingly.

    Thanks a lot!

    Antony

  • 0 in reply to Antony Lin
    TI__Mastermind 25665 points
    Hello Antony,
    Taking into account the internal resistors for the ADC, I recommend using 200Kohm for the top resistor and 1Mohm for the bottom of the resistive divider from your 8.7V 2S battery. There is an internal resistance of 320Kohm to ground, so this will give you the 4.8V at the VBattsense pin to match with a full charge.

    If you use the recommended divider and default register settings the threshold for LowBATT Detection will be ~5.48V, VBAT PowerUp about 6.03V, and VBAT UVLO ~4.93V.