BQ2057: datasheet correction on PTC thermistor

Jonathan Navor

Genius 9475 points

Part Number: BQ2057

Hi Team,

Good day! Could you please check the datasheet on page 13 regarding the the temperature sensing calculations.

As per customer below:

I am using a vishay 1k Ohm PTC thermistor with an RTH of 1084 k and an RTC of 900 ohm.

The equation in the datasheet gives a negative resistance for RT2.

As the datasheet gives a negative resistance for RT2 I'm assuming either I'm doing the calculation incorrectly or theres a problem with the equation.

Thats the one. Page 13. By my calculation

Thermistor: PTC Vishay TFPTL10L1001FL2B Thermistor, L10 1kΩ
RTH: 45degC = 1.084 kohm
RTC: 0degC = 900 ohm
From the equation on page 13:
RT1 = 8.837 kohm
RT2 = -1.180 kohm

Thank you in advance.

Best regards,

Jonathan

over 3 years ago

0 Nick Brylski over 3 years ago

TI__Mastermind 19195 points

Hello Jonathan,

I will review the values and get back to you by end of week. It is unlikely that there is any error with the equation, this part has been in production for over 20 years.

Thanks,

Nick

0 Jonathan Navor over 3 years ago in reply to Nick Brylski

TI__Genius 9475 points

Hi Nick,

Apologies for the late response. We just got the customer's answer.

The temps that the customer was aiming for where -20 degC to +45 degC although they would be happy with 0 degC to +45 degC.

The datasheet is: 0900766b8167e7f6.pdf (rs-online.com)

Best regards,

Jonathan

0 Nick Brylski over 3 years ago in reply to Jonathan Navor

TI__Mastermind 19195 points

Hi Jonathan,

If RT1 or RT2 value is coming out negative then they are going to need to choose different Rth / Rtc (a different temp window).

I will take a look and see what ranges are possible with that PTC, get back to you next week.

Thanks,

Nick

+1 Nick Brylski over 3 years ago in reply to Nick Brylski

TI__Mastermind 19195 points

Hi Jonathan,

This part does support PTC's but to ensure the RT2 equation does not go negative:

2*RTH - 7*RTC > 0 or

RTH > 7/2*RTC

The PTC hot temp resistance must be at least 3.5x the cold temp resistance. From the datasheet you shared, the PTC does not have enough resistance variance to work.

Customer will be better off using an NTC like semitec 103AT-2, this is good for a -20 to 45C charging range.

Best regards,

Nick

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BQ2057: datasheet correction on PTC thermistor