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LM73 Power supply ramp-up issue

Madhuri Akkenepalli
Expert 1215 points
Other Parts Discussed in Thread: LM73

My customer has been experiencing a problem with the TI LM73 temperature sensor, where it reports incorrect values in the field.  So far, they've been unable to reproduce this on their bench.  From the symptoms, it sounds like a Power supply ramp-up problem that is documented on Page 19 of the LM73 datasheet.  However, the description on Page 19 does not say what ramp-up rate will cause the problem to occur.

As I need a way of getting the LM73 into this state in order to test out the workaround, would your team be able to share the proper slew rate needed to make the problem manifest?

 This is affecting several product in the field so please help asap.

  • 0
    TI__Mastermind 37350 points

    Hi Madhuri,

    Thank you for your inquiry. My guessing is that it could be the slow ramping supply voltage after a POR has occurred. Could you please elaborate what is "incorrect values"? Was the temperature fluctuating? Can you capture the power supply applied to the board?

    Aaron

  • 0 in reply to Aaron Heng
    Expert 1215 points

    Thank you Aaron for your help.

    The LM73 would report temperatures like 147 C and -21.5 C even though ambient was 14.5 C.  Wildly off.

    The reported temperature would fluctuate, jumping between many different values.  It's hard to say exactly what the pattern was, as there seemed to be none.

    The application is a high voltage measurement system. Hence we cannot capture a profile of the power rail, as this all occurred inside a 12 kV electric field.

    thanks

    Madhuri

  • 0 in reply to Madhuri Akkenepalli
    TI__Mastermind 37350 points

    Hi Madhuri,

    Based on the temperature you provided, it seems that you had a slow ramp rate; therefore, the registers wasn't properly loaded into the eeprom. A power-on-reset (POR) will occur at a certain POR voltage.  After this occurs, then the trim code will be loaded from the EEPROM to a register.  If the VDD ramp rate is slower, then the EEPROM is read at a lower voltage.  However, if the ramp rate is faster, then the EEPROM is read a higher voltage.  A slow ramping power supply will result in an inaccurate transfer of data from the EEPROM to a register.  One of the bits that transferred is the bit that sets the I2C address that distinguishes between the LM73-0 and LM73-1.

    There is a work-around for this. Please refer to the datasheet in Application Hints "Power Supply Ramp-up Considerations". Please let me know if this work out for you.

    Aaron