MAX17335 Thermistor 2 and FETTheta

We are much mesmerized in understanding the utilization of Thermistor 2 on the MAX17335 (and its charge capable brethren such as the MAX17332 and MAX17330)!!!!

FetTheta: FetTheta is used to calculate actual Junction temp with only observing DieTemp and Thermistor 2 during charging. FET Junction temperature is calculated with the following equation:

FETTemp = TH2_Temp+ (TH2_Temp - DieTemp) x FetTheta.
The FetTheta configuration range is 0 to 3.875 with 0.125 steps. FetTheta is a scalar and does not have units. If PFail pin is used, or Thermistor 2 is not used, DieTemp is used as FET temperature.

Most intriguing is the FETTheta parameter and its use in the FETTemp equation above.  Particularly, we're distraught about the DataSheet's FetTheta is a scalar and does not have units. statement!

 

1) In all my career Theta (Θ) has always had units of °C/W or K/W!  What does Maxim means by "FetTheta is a scalar and does not have units"?  What is the user supposed to enter for FETTheta in the nChgCfg1 (1CBh) register?  Got no idea on what to put in there!

 

This is an example of the CSD83325LTI FET used in our design:

 

2) We can't even fathom how the FETTemp equation relates to physics!  Can you please enlighten us on how such an equation was conceived?

THCfg: THCfg sets the Thermistor behavior.

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3) Most intriguing is why a mode such as 0b11 wasn't defined to use Thermistor 1 for the battery and Thermistor 2 [alone] for the FET!  Placing Thermistor 2 [alone] next to the FET is a very easy layout task.  Placing the whole MAX17335 next to the FET necessarily isn't so!

Thanks so much,

Georg A. Mussenden