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BQ76907: Calculate current range for BQ76907

Tom Tsao
Expert 1420 points
Part Number: BQ76907

Tool/software:

Hi TI support team,

I would like to seek your advice on how to calculate the maximum and minimum current range using a 300 μΩ Rsense.

 

Current = [(24bit ADC – Current offset) x current gain]/8192

               = [(24bit ADC – 0) x 241]/8192

               = 493568 ?? (Table 5-12 max current should be 32.767A)

 

How can I calculate the current range using the formula?

  • 0
    TI__Genius 14845 points

    Hello Tom,

    Can you read the 0x3A Current() command to see what it’s stating? Also is the value 493568 rounded to 16 bits?

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Expert 1420 points

    I would like to seek your advice on how to calculate the maximum and minimum current ranges using a 300 μΩ Rsense with the formula.

    How to setting Quantized CC1 Gain and Quantized Current Gain with 300 μΩ?

    Thanks!

  • 0 in reply to Tom Tsao
    TI__Genius 14845 points

    Hello Tom, 

    I will double-check with a teammate regarding more specifics of how to calculate the current ranges with that formula specifically. 

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Expert 1420 points

    Hello Alexis,

    Is there any update for it?

    Thanks!

  • 0 in reply to Tom Tsao
    TI__Genius 14845 points

    Hi Tom,

    Apologies, yesterday was a holiday. Thank you for your patience so far.

    To begin with:
    1. To find the current range in general, you would take the max SRP/SRN ranges (-0.2V and 0.2V) and divide by the sense resistor selected (300µΩ) and you would get ±~666.67A. You do not need that formula to find the current range.

    2. The current given in the formula you shared uses a 24-bit ADC count reported by the 0x36 Raw Current() command. This means the largest number it could possibly output given by the 24-bit ADC is 2(23) – 1 = 8388607. Working backwards, you seem to have inputted a random number of ~16777216 to get the value of 493568 through the formula. That number is out of range of the 24-bit ADC count.

    3. Table 5-12’s values in the TRM, are just pre-calculated values done for the user to use already for different LSBs/Gains.

    To find the Gain:
    To use the maximum range of the SRN/SRP pins, the LSBnew value is given by the max ranges the pin sees and the 16-bit signed register à 200mV/32767 = ~6uV. The LSBraw is given as 29nV in the TRM. The gain can then be found by setting LSBraw * Gain/8192 = LSBnew. Because the raw LSB and new LSB have different signed bits, we need to ‘map’ them. We can do that by dividing 6uV/29nV, giving us a value of 207. So 207 * Gain/8192 = 1, giving us a Gain of ~39.5. You could than look at the table’s Quantized Current Gain column for 39 and the Rsense of around 250 to get an idea of the min LSB allowed in mA.

    Best Regards,
    Alexis