About nPM1300, how to calculated NTC temperature by measured voltage

[Poki Huang]

Hi 

Could you provide nPM1300 NTC pin inside structure?(pull up resistor value, pull up voltage, others..)

And I want to know how to calculated NTC temperature by measured voltage

For example, if we use 10kohm at 25 degree NTC(B-constant is 3380), and we measured nPM1300 NTC pin (D3 pin) voltage is 0.688V

What is the 0.688V mapping value of ADCNTCRESULTMSB & ADCGP0RESULTLSBS[3:2]?

Thank you

Poki

[Szabolcs]

Sorry for misunderstanding your question earlier.

Inside the IC, there is a pull-up resistor that pulls the NTC pin to 1.5V internal reference. This is the max range of the ADC. The pullup resistor's value is normally configured to match the NTC resistors 25 degree resistance.

So the ADC measurement value is given by: 1024 * V_NTC / 1.5V.

I your example case, 0.688V results in 1024*0.688/1.5 = 470 over 10 bits, 8 bits of MSBs (0111 0101) in ADCNTCRESULTMSB  and 2 bits (10) of LSBs in ADCGP0RESULTLSBS[3:2].

(At 25 degrees, the pull-up and the NTC form a 1:1 voltage divider, so the measured voltage is 0.75V = 512 reg. value).

[Poki Huang]

Hi Szabolcs:

So the pull-up resistor is a fixed value resistor or a variable value resistor?

For our example, the NTC at 25 degrees is 10Kohm, so the pull-up resistor is a fixed 10Kohm resistor for any temperature?

Thank you

Poki

[Szabolcs]

Hi Poki,

yes, there are several pull-up resistors that are selectable by ADCNTCRSEL, but all of them are fixed value resistors, independently of temperature. (as much as fixed-value resistors usually are)
So, if you have a 10K NTC, and select the 10K resistor, your voltage on the pin will be 1.5V*R_NTC/(10K+R_NTC)