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TMP112: Predicting the lower operating current

Drew Doran
Prodigy 240 points
Part Number: TMP112
Other Parts Discussed in Thread: TMP102

Hi,

I was wondering if you are able to help answer the following question?: 

"I have been searching for the lowest operating current temperature sensor I can find.  The TMP series of parts from TI have a lot of good candidates.  Most of them have a quiescent current rated with the bus operating and 1 temperature conversion per second.  The TMP 112 is different in that it has about the same current, but at 4 conversions per second.  I am targeting 1 conversion per second or slower.  Is there a way I can predict the lower operating current of the TMP 112 at 1 conversion per second?"

I appreciate your support!

Regards,
Drew

  • 0
    TI__Mastermind 43795 points

    Hi Drew,

    This is a question that has been asked a few times, so please bear with me while I give a thorough explanation that we can re-use.

    There are two modes that our devices can operate in: Shutdown and Continuous Conversion. This is configured by the SD bit in the Configuration register where SD=1 corresponds to Shutdown and SD=0 corresponds to Continuous Conversions.

    In Shutdown, the device does not take any temperature measurements. This means that the on-board ADC is not performing any Conversions. During this time, the current consumed is specified as ISD 0.5uA. If you would like to perform a measurement, you do so by writing OS=1 to the Configuration register. Upon receiving OS=1, the device will perform one temperature measurement (aka ADC Conversion) and then return to Shutdown (if SD=1.) The temperature measurement takes roughly 26ms (specified as Conversion Time.) During this time, the current consumed is about 40uA. This is not specified in Electrical Characteristics table, but it is mentioned in Section 7.4.1 on page 17.

    The Average Current Consumption for TMP102 in Shutdown mode is calculated as follows:

    ActiveCurrent * ConversionTime + ShutdownCurrent * ShutdownTime

    Here it is assumed that a OneShot temperature measurement is triggered once per second, and Average Current is estimated to be 1.5uA:

    40e-6*26e-3+0.5e-6*974e-3

    = 1.527e-6

    In Continuous Conversion, the device makes temperature measurements at regular intervals specified by the Conversion Rate setting. This mode is indicated by SD=0 in the Configuration register, and Conversion Rate is specified using the CR1 and CR0 bits of the Configuration register. During the time that the device is idling between measurements, it consumes about 2.2uA. This is not specified in Electrical Characteristics table, but it is mentioned in Section 7.4.1 on page 17. The current consumed during this time reflects the operation of the oscillator and counter keeping track of time. When the device takes a measurement, it consumes about 40uA for 26ms exactly the same as when a OneShot is triggered.

    The Average Current Consumption for TMP102 in Continuous mode is calculated as follows:

    ActiveCurrent * ConversionTime + DelayCurrent * DelayTime

    Here it is assumed that the Conversion Rate is configured for once per second (CR=01,) and Average Current is estimated to be 3.2uA:

    40e-6*26e-3+2.2e-6*974e-3

    = 3.1828e-6

    By default, TMP102 is configured for Continuous Conversions at a Conversion Rate of 4 times per second. The estimate for this is below, and it comes out slightly below the typical value of 7uA specified in the Electrical Characteristics table:

    40e-6*26e-3*4+2.2e-6*896e-3

    = 6.1312e-6

    Thanks,

    Ren