SLUAAJ8 January   2023 BQ25180

 

  1.   Abstract
  2.   Trademarks
  3. 1Temperature Sensing
    1. 1.1 Temperature Sensing and JEITA Standard
    2. 1.2 Current Driven TS Function
    3. 1.3 Calculating RS and RP
  4. 2BQ25180 and NTC Monitoring Calculator
  5. 3Example of TS Circuit Design
  6. 4References

1.3 Calculating RS and RP

The series resistor (RS) and parallel resistor (Rp) can be calculated using a few formulas. The resistance of an NTC thermistor is governed by Equation 1:

Equation 1. β = l n R T 1 R C A L 1 T 1 - 1 T C A L

RCAL is the resistance of the thermistor calibrated at TCAL, typically 25°C. β is given in terms of Kelvin so T1 and TCAL are expressed in Kelvin. This equation allows us to calculate the thermistor resistance at different temperatures, notably at the HOT JEITA threshold of 60°C (RH) and at the COLD JEITA threshold 0°C (R­C). The resistor network and bias current of the TS current source produce a voltage calculated by Equation 2:

Equation 2. V T S =   I T S _ B I A S R P | | ( R S + R N T C )

The charger data sheet provides voltage thresholds for the various JEITA temperature thresholds. These voltages, in place of VTS, in Equation 2 allows the use of a system of equations to solve for the two variables needed to create our TS circuit: RP and RS. Because there are only have two variables to compensate the system with, only two JEITA thresholds can be met with total accuracy. Typically, HOT JEITA threshold (V­H) and COLD JEITA threshold (V­­C) are prioritized.

Using known values V­H, V­­C, RH, and RC, as well as Equation 2, we create and provide a system of equations to derive Equation 3 and Equation 4 to calculate RS and RP.

Equation 3. R S = - R H + R C ± R H + R C 2 - 4 R H × R C + V H × V C V H - V C × I T S _ B I A S × R C - R H 2
Equation 4. R P = V H × R H + R S I T S _ B I A S × R H + R S - V H