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BQ24296: How to calculate the thermistor resistance at 40degC ?

Ray Chang
Intellectual 1345 points
Part Number: BQ24296

Hi, Team

My customer want to calculate the thermistor(NTC) resistance at 40degC for BQ24296.

Could you please help to provide the equation or calculation tool for this requirement?

  • 0
    TI__Expert 8230 points
    Hello Ray,

    Can you please clarify the customer's request: does the customer need the equation to calculate the resistor network used on the TS pin in conjunction with the thermistor used? Or does he need the actual resistance value for the thermistor used?

    If the former, please refer to section 9.3.3.3.1 of the bq24296 datasheet. This section includes the equations needed to calculate the values of the resistor network needed based on the application's temperature requirements and thermistor used. Our datasheet provides an example using a 103AT NTC thermistor.

    If the latter, this information is dependent on the thermistor used and it's usually available on the datasheet of the thermistor.
  • 0 in reply to Fernando Lopez
    TI__Intellectual 1345 points
    Hi, Lopez

    Customer need to calculate actual resistance value for thermistor.
    Customer want to use equation 1(section 9.3.3.3.1) to calculate RTHcold and RTHhot.
    For example, if we know the voltage of TS pin, how to calculate actual thermistor value by TS pin voltage?

    one more question,
    Is there minimal load requirement on VREGN to keep the voltage stable?
    We found the VREGN voltage lower than 4.7V when we use the 5V input and there is no load on VREGN.
    Do you have any common for this situation?
  • 0 in reply to Ray Chang
    TI__Expert 8230 points
    Hello Ray,

    The set of equations on 9.3.3.3.1 work under the assumption that the user/designer will provide the initial RTHcold and RTHhot values from the datasheet of the thermistor used in the application for a specific temperature range. This is to calculate the complementary RT1 and RT2 resistors used to form the resistor network on the TS pin for the application's temperature range (for example, 0C to 45C).

    This network is a basic resistor divider with REGN as the source voltage. If the assumption that RT1 and RT2 are already chosen is correct, it is possible to solve the set of equations for RTHcold and RTHhot as a function of VREGN, but this could prove taxing for an embedded host.

    If the objective is to determine temperature during operation, this can be achieve more efficiently by measuring the REGN voltage and the TS pin and calculating the ratio of the TS voltage as a percentage of the REGN voltage. This percentage can be compared with the thresholds provided on the EC table of the datasheet to determine if the device is within the normal operating temperature (cold or hot).

    Regarding the REGN voltage question, there is no minimum load requirement on REGN, however, REGN is assumed not to be used only for internal circuit biasing and as a voltage reference for the TS network pin. If the REGN voltage is less than 4.7V, it might be operating in dropout. What is the VBUS voltage during this? Is the device in input current or input voltage limit?