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BQ24075T as per schematic is R8 needed?

ajayt
Genius 9555 points
Other Parts Discussed in Thread: BQ24075T

 

Hi, working out the resistor values for the NTC circuit

 

 I thought I'd do the sums to calculate the resistor divider values.

 

My input parameters are as follows:

Vin = 4.77V

Rhot = 4.926k;   45degC threshold from NTC datasheet

Rcold = 28.416k;  0degC threshold from NTC datasheet

Vcold = 0.25 * Vin = 0.25 * 4.77 = 1.1925V

Vhot = 0.125 * Vin = 0.125 * 4.77 = 0.59625V

 

Using the magic formula from section 10.2.1.2.4 I get:

R7 = 11.036k   (11k will do)

R6 = 23.846k   (24k will do)

 

However, since R6 is less than 100k, the datasheet in this same section says:

"Since the calculated values for R6 is less than 100kΩ, a 100kΩ resistor for R8 must be used"

 

This refers to figure 18 on P20 which also states that the function of this resistor is to 'protect the IC from 28V inputs'.

 

Question:

Since our input is fixed at 4.77V, well below the maximum rating of 7V, do we really need to fit R8 ?

 

 

 

  • 0
    TI__Genius 11480 points
    Ajay,


    The bq24075T will still function without R8.

    If this is a one-off for school, DIY project, etc it will probably be fine as long as you don't mind potentially replacing an IC once in a blue moon.


    If it is a commercial product I would recommend placing the extra resistor, even for redundancy (people tend to do crazy things). Single resistors (especially for this case) can be cheap and small enough to be almost negligible on BOM and layout that it would most likely be worth it.