BQ76972: Clarification on Using 4th-Order Polynomial Coefficients in BQ76972 for NTC Thermistor Temperature Estimation

shiva kumar

Part Number: BQ76972

Tool/software:

Hi TI Team,

I’m working on configuring the BQ76972 AFE to support NTC thermistor-based temperature estimation using the internal 4th-order polynomial model.

What I’ve Done So Far:

Collected Temp-Voltage Data
I measured thermistor voltage (V_ntc) across a wide temperature range and computed the scaled voltage as per the AFE’s internal 1.8V reference (V_scaled = V_ntc × 1.8 / 2.459).
Fitted a 4th-Order Polynomial
In Excel, I plotted Temperature (°C) vs V_scaled and used a trendline to extract the coefficients for this polynomial:
y = 63.258x4 - 259.79x3 + 385.98x2 - 301.04x + 116.96
Extracted Coefficients:
- A4 = 63.2576665
- A3 = -259.7937656
- A2 = 385.9757121
- A1 = -301.0384332
- A0 = 116.9573288
Formatted These Coefficients in Excel
I set up a dynamic spreadsheet to auto-update polynomial coefficients whenever thermistor characteristics or voltage scaling changes.

My Request / Questions:

Can you please confirm that this is the correct approach for configuring the BQ76972 to interpret thermistor voltage as temperature using the internal polynomial model?
Do you have any official documentation or application note that explains:
- The format (e.g., Q-format) required for loading these coefficients into the AFE’s data memory
- The register locations or DF addresses for these coefficients in the BQ76972
- Any tool or script provided by TI to assist in converting float → hex format for the coefficients?

NTC_103JT_4thOrder_Poly_Model.xlsx

4 months ago

0 shiva kumar 4 months ago

Prodigy 100 points

Hi TI Team,

I’m following up as I’m currently waiting for your reply regarding the earlier communication. Kindly provide an update

0 Luis H. S. 4 months ago in reply to shiva kumar

TI__Mastermind 42060 points

Hello Shiva,

I'd advice to see the following FAQ's on the topic:

shiva kumar said:
The format (e.g., Q-format) required for loading these coefficients into the AFE’s data memory

It is in 2's complement.

shiva kumar said:
The register locations or DF addresses for these coefficients in the BQ76972

See the Technical Reference Manual for details here.

shiva kumar said:
Any tool or script provided by TI to assist in converting float → hex format for the coefficients?

Coefficients are not float, they are integers in 2's complement. There are many resources online that talk about how to convert a negative value to hexadecimal using 2's complement

Best Regards,

Luis Hernandez Salomon

0 shiva kumar 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hello Luis H. S.,

Thank you for your valuable input.

I have a few questions regarding my calculation approach:

In my calculations for the polynomial model, I used V_scaled (V) vs Temperature (°C) because I have an external voltage divider with a pull-up voltage of 2.495V.
Since the AFE ADC reference voltage is 1.8V, I scaled the NTC voltage as follows:

Vscaled = VNTC × (1.8 / 2.495)

Could you please confirm if this approach is correct?

Looking forward to your guidance.

0 Luis H. S. 4 months ago in reply to shiva kumar

TI__Mastermind 42060 points

Hello Shiva,

That sounds fine. Ensure that you are using the input as an ADCIN mode, since you are using an external value.

Best Regards,

Luis Hernandez Salomon

0 shiva kumar 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hello Luis H. S.,

Thank you. I will perform the testing, and if I need any further assistance, I will get back to you.

0 SAI VARUN PENDELA 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hi Luis,

In this case, I have an external voltage divider with a pull-up voltage of 2.495v.
Since the AFE ADC reference voltage is 1.8v, I scaled the NTC voltage as follows:

Vscaled = VNTC × (1.8 / 2.495)

Now,

1. How can the DFETOFF Pin be configured as a Temperature Sense Pin when I do not use any internal pull-up?

2. What is the significance of the two polynomials where [a1, a2, a3, a4, a5] are the coefficients of the first polynomial and [b1, b2, b3, b4] are the coefficients of the second polynomial?

Best Regards,

SAI VARUN

0 SAI VARUN PENDELA 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hello Luis,

When I am using the External Pull-Up and connect the thermistors to the Temperature sense pins then how to configure the following parameters?

What is the significance of the two polynomials, and how to calculate the coefficients for those two polynomials?

Best Regards,

SAI VARUN

0 Luis H. S. 4 months ago in reply to SAI VARUN PENDELA

TI__Mastermind 42060 points

Hello Sai,

You cannot use the pins as temperature sense pins when using an external pull-up. If you want to use the polynomial and use the pins for temperature purposes without the involvement of the MCU, you will need to use the internal pull-up instead.

I would recommend that as well, because it should be more accurate than the ADCIN one.

Best Regards,

Luis Hernandez Salomon

0 SAI VARUN PENDELA 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hi Luis,

I got your point. Can you tell me in general why we need two polynomial equations here? One equation with Co-efficients {a1, a2, a3, a4, a5} and another with Co-efficients {b1, b2, b3, b4} and what is ADC0 and Rc0?

Best Regards

SAI VARUN

0 Luis H. S. 4 months ago in reply to SAI VARUN PENDELA

TI__Mastermind 42060 points

Hello Sai,

These are not two polynomial equations. These are both part of the same polynomial equation. One for the numerator, one for the denominator.

ADC0 & RC0 are related to internal components of the device.

Best Regards,

Luis Hernandez Salomon

0 SAI VARUN PENDELA 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hello Luis,

Can you share details about how exactly the calculation is happening, like the formula being used internally and the formula to calculate ADC0 and Rc0?

Best Regards,

SAI VARUN

0 Luis H. S. 4 months ago in reply to SAI VARUN PENDELA

TI__Mastermind 42060 points

Hello Sai,

No. This is internal information that we cannot share.

Best Regards,

Luis Hernandez Salomon

0 SAI VARUN PENDELA 4 months ago in reply to Luis H. S.

Prodigy 100 points

Hi Luis,

I understood your point. If you don't share the information, how can we use the Custom Temperature model with an internal 18k Pull-Up? Or else tell how to calculate the Co-efficients [a1, a2, a3, a4, a5], [b1, b2, b3, b4], adc0 and Rc0 values because I have used the GPC tool to calculate those values, which are giving negative temperature values. I have also gone through the Technical Reference Manual, but no use.

So, kindly tell us how to calculate the coefficients of both the polynomials and adc0, Rc0.

Best Regards,

SAI VARUN

0 Luis H. S. 4 months ago in reply to SAI VARUN PENDELA

TI__Mastermind 42060 points

Use the 18k model if you are using a single thermistor type.

The FAQ I shared explains all of these.

Best Regards,

Luis Hernandez Salomon

Power management

Power management forum

BQ76972: Clarification on Using 4th-Order Polynomial Coefficients in BQ76972 for NTC Thermistor Temperature Estimation

What I’ve Done So Far:

My Request / Questions: