BQ76952: BQ76952 TS1 query

Star Xu

Part Number: BQ76952

The customer design with BQ76952, TS1 is configured as a 0x07 thermistor. Use the command 0x70 to get the temperature of TS1. When they disconnect TS1, the temperature value read is -60°C after conversion, This is correct. But when they short-circuit TS1 , the temperature read is -31℃.

It is normal if TS1 is not short-circuited. The disconnection value is about -60, but the short-circuit display is -31. The short-circuit should be a large value.

Does it normal？

Thanks

Star

8 months ago

0 Maxwell Verboncoeur 8 months ago

TI__Genius 16050 points

Hi Star,

Just to confirm whether this is an issue with the ADC calculation, can you provide the raw ADC measurements from while the device is exhibiting this behavior?

You can find the registers to get these values from Section 4.6: Subcommands 0x0075–0x0077 DASTATUS5–7(), Additional Measurements of the TRM.

Regards,

Max Verboncoeur

0 Star Xu 8 months ago in reply to Maxwell Verboncoeur

TI__Genius 9645 points

Hi Max

At normal temperature around 26°C, use 0x0076 to read bytes 24 to 27 in the TS1 block. The displayed AD count value is 0x001C5570. After shorting TS1, the displayed AD count is 0x22138.

Thanks

Star

0 Maxwell Verboncoeur 8 months ago in reply to Star Xu

TI__Genius 16050 points

Hi Star,

With some rough calculations, 0x1C5570 equates to ~0.447V across the thermistor. (ADC counts * ADC Gain) / 2^24, (I'm assuming ADC Gain = 4041 as that is a common gain value).

Assuming the internal pullup is an ideal 18k, this would equate to ~6kOhms on the thermistor. ( 0.447V / (1.8V - 0.447V) ) * 18kOhms.

Does that sound about right for ~26C?

Using the same calculations, 0x22138 equals ~0.034V and 346 Ohms. This sounds about right for a short since the calculations expect ideal components.

Could you walk me through step by step what the customer's process is to acquire the temperature measurement?

Also, in parallel to that, could the customer see if trying the example code has any impact on the measurement?

Regards,

Max Verboncoeur

0 Star Xu 8 months ago in reply to Maxwell Verboncoeur

TI__Genius 9645 points

Hi Max

Thanks for your reply.

The customer uses the default semitec 103-AT 10k thermistor, only use TS1, and configure it to 0x07, and then use the command 0x70 to get the TS1 temperature value, and they found that when increased the temperature of the TS1 to 125 ℃, if the temperature continues to rise, its value will become -126 ℃, short the TS1 will become -26 ℃.The result is same when using the sample code.

Waiting for your reply.

Thanks

Star

0 Maxwell Verboncoeur 8 months ago in reply to Star Xu

TI__Genius 16050 points

Hi Star,

It may be that the default polynomial doesn't fit as well at such high temperatures.

In the datasheet the error is only spec'ed up to ~110C:

You may want to try using the thermistor coefficient calculator to recalculate a polynomial that fits the range of temperatures that you want.

This FAQ explains the process: [FAQ] BQ76952: How to configure the temperature model of BQ769x2 using the thermistor coefficient calculator tool? - Power management forum - Power management - TI E2E support forums.

Regards,

Max Verboncoeur