• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 550 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

BQ24450

BQ24450: BQ24450

geoffrey cohen
Prodigy 50 points
Part Number: BQ24450
Other Parts Discussed in Thread: UC3906, UC2906

I would like help in understanding how the formulas for calculating Rb and Rd on page 15 of the Bq24450 datasheet work.  

The calculation for Rb needs knowledge of the value of Rd and the calculation for Rd needs knowledge of the value of Rb. 

The datasheet for the original version of this controller (the UC3906) uses different equations for these calculations ie :

Rb = Rsum-Ra  where Rsum=(Vf - 2.3V) / Id - where Id is the divider current

Rd = 2.3v * Rsum/(Voc - Vf) where Voc is the overcharge voltage and Vf the float voltage

Using these formulas gives results which differ from the values given for the Bq24450 in the datasheet.

Am I missing something that would explain how to solve the circular formulas in the Bq24450 datasheet ?

  • 0
    TI__Intellectual 1640 points
    Hello,
    You can find the value of Rc first,
    from the formula of Vfloat, you can find the value of Ra+Rb, from the formula of Vboost, you can find the value of Rd,
    From Vth you can get Rb, and Ra=2Rc- Rb, you can get Ra.

    You can also look up the EVM user guide, and on the schematic there value of R12= value of Rb, and Value of R7 = value of Rd.
  • 0 in reply to Upal Patel
    Prodigy 50 points

    Thanks for your response.

    For others who struggle with these maths I enclose the workings below:

    Vboost = Vref * (Ra+Rb+Rc//Rd) ÷ Rc//Rd

    Where // denotes resistors in parallel and the R equivalent is ( R1 * R2) ÷ (R1 + R2)

    As Rc = 46400 and Vboost is 7.35 for a 6V battery

    7.35 = 2.3 * (92800 +46400//Rd) ÷ (46400//Rd)

    7.35 = 2.3 * (92800 + ( (46400 * Rd) ÷ (46400 + Rd))) ÷ (( (46400 * Rd) ÷ (46400 + Rd)))

    If Rd = Z

    7.35 = 2.3 * (92800 + ( (46400 * Z) ÷ (46400 + Z))) ÷ (( (46400 * Z) ÷ (46400 + Z)))

    And using the website wolframalpha to solve for Z

    Z = 474311

    So Rd = 474.3K

    However these results differ from those given by the calculation in the UC2906 datasheet - when Rd = 100.7K