The LMT87 device is a precision CMOS analog temperature sensor with a linear output voltage that is inversely proportional to temperature. "A push-pull output gives the LMT87 the ability to sink and source significant current."
Here is the datasheet for this device: https://www.ti.com/lit/ds/symlink/lmt87.pdf
MD5 SHA-1
17f1702e1056186f86021e10f9e8e44c 0d0be8f79b70a96b1b968fea38c75f04f5e643b2 LMT87.pdf
Main TI.com landing for LMT87: https://www.ti.com/product/LMT87
This project was created to help enigneers, technicians, and hobbyist quicky get the LMT87 low voltage temperature sensor working in their own projects.
The characteristic equations below should make it relatively easy and quick for you to design your own circuits around this device.
Characteristic Equation 1, FOR 0 (uA) <= Iout <= 50 (uA), LMT87 is sourcing current:
Vout = -0.007779402248826279 * Temperature_In_Fahrenheit + 0.0009733172601552494 * VDD + 0.000028261925286710542 * Iout_In_uA + 2.8933598263741938
The coefficient of determination (r-squared) for this Characteristic Equation 1 is 0.9882776686812974.
NOTE 1: Iout_In_uA must be entered into the characteristic equations in uA (1 microAmp = 1x10^-06 Amps). See examples below.
NOTE 2: Regarding these characteristic equations and the schematic above, I am defining Iout to be postiive (+) when Iout is flowing out of the LMT87 (sourcing current) , and I am defining Iout to be negative (-) when Iout is flowing into the LMT87 (sinking current).
NOTE 3: From the manufacturer's datasheet, Iout must be: -50uA <= Iout <= 50uA for normal temperature sensing operation.
Example 1a:
If temperature is 77 degree Fahrenheit, VDD = 4.1, Iout = +25uA, then
Vout = -0.007779402248826279 * (77) + 0.0009733172601552494 * (4.1) + 0.000028261925286710542 * (25) + 2.8933598263741938 = 2.29904300211337460309 Volts
Vout ~ 2.299 Volts
Example 1b:
If temperature is 77 degree Fahrenheit, VDD = 4.1, Iout = 0uA, then
Vout = -0.007779402248826279 * (77) + 0.0009733172601552494 * (4.1) + 0.000028261925286710542 * (0) + 2.8933598263741938 = 2.29833645398120683954 Volts
Vout ~ 2.298 Volts
The following form below of Characteristic Equation 1 may be easier for you if you know the DC Thevenin equivalent of the circuit your LMT87 will drive:
Let Iout (in uA) = [(Vout - VL) / RL] * 1,000,000, then an equivalent alternative form of Characteristic Equation 1 is:
Vout = [a * Temperature_In_Fahrenheit + b * VDD - ((c * 1,000,000 * VL)/RL) + e]/[1 - (c * 1,000,000/RL)]
Vout = [-0.007779402248826279 * (Temperature_In_Fahrenheit) + 0.0009733172601552494 * (VDD) - ((0.000028261925286710542 * (1,000,000) * (VL)) / RL) + 2.8933598263741938]/[1 - (0.000028261925286710542 * (1,000,000) / RL)]
Example 1c:
If the ambient temperature is 77 degrees Fahrenheit, VDD = 4.1 Volts, Iout = +25uA, VL = 0, then
RL = ((2.29904300211337460309 - 0) / 25) * 1,000,000 = 91,961.7200845349841236 Ohms.
Vout = (-0.007779402248826279 * (77) + 0.0009733172601552494 * (4.1) - ((0.000028261925286710542 * 1,000,000*(0)) / 91,961.7200845349841236) + 2.8933598263741938) / (1 - (0.000028261925286710542 * 1,000,000 / 91,961.7200845349841236)) = 2.29904300211337460309 Volts
*Compare to result of Example 1a. They are the same.
Characteristic Equation 2, FOR -50 (uA) <= Iout <= 0 (uA), LMT87 is sinking current:
Vout = -0.0075608961627175765 * Temperature_In_Fahrenheit + 0.0009416100793021476 * VDD + -0.000018778567101973768 * Iout_In_uA + 2.87419156362495
The coefficient of determination (r-squared) for Characteristic Equation 2 is 0.990896061949286.
NOTE 1: Iout_In_uA must be entered into the characteristic equations in uA (1 microAmp = 1x10^-06 Amps). See examples below.
NOTE 2: Regarding these characteristic equations and the schematic above, I am defining Iout to be postiive (+) when Iout is flowing out of the LMT87 (sourcing current) , and I am defining Iout to be negative (-) when Iout is flowing into the LMT87 (sinking current).
NOTE 3: From the manufacturer's datasheet, Iout must be: -50uA <= Iout <= 50uA for normal temperature sensing operation.
Example 2a:
If temperature is 77 degree Fahrenheit, VDD = 4.1, Iout = -25uA, then
Vout = -0.0075608961627175765 * (77) + 0.0009416100793021476 * (4.1) + -0.000018778567101973768 * (-25) + 2.87419156362495 = 2.29633262459838475886 Volts
Vout ~ 2.296 Volts
Example 2b:
If temperature is 77 degree Fahrenheit, VDD = 4.1, Iout = 0uA, then
Vout = -0.0075608961627175765 * (77) + 0.0009416100793021476 * (4.1) + -0.000018778567101973768 * (0) + 2.87419156362495 = 2.29586316042083541466 Volts
Vout ~ 2.296 Volts
The following form below of Characteristic Equation 2 may be easier for you if you know the DC Thevenin equivalent of the circuit your LMT87 will drive:
Let Iout (in uA) = [(Vout - VL) / RL] * 1,000,000, then an equivalent alternative form of Characteristic Equation 1 is:
Vout = [a * Temperature_In_Fahrenheit + b * VDD - ((c * 1,000,000 * VL) / RL) + e] / [1 - (c * 1,000,000 / RL)]
Vout = [-0.0075608961627175765 * Temperature_In_Fahrenheit + 0.0009416100793021476 * VDD - ((-0.000018778567101973768 * 1,000,000 * VL) / RL) + 2.87419156362495] / [1 - (-0.000018778567101973768 * 1,000,000 / RL)]
Example 2c:
If the ambient temperature is 77 degrees Fahrenheit, VDD = 4.1 Volts, Iout = -25uA, VL = 4.1, then
RL = ((2.29633262459838475886 - (4.1)) / (-25)) * 1,000,000 = 72,146.6950160646096456 Ohms.
Vout = (-0.0075608961627175765 * (77) + 0.0009416100793021476 * (4.1) - ((-0.000018778567101973768 * 1,000,000 * (4.1)) / 72,146.6950160646096456) + 2.87419156362495) / (1 - (-0.000018778567101973768 * 1,000,000 / 72,146.6950160646096456)) = 2.29633262459838475886 Volts
*Compare to result of Example 2a. They are the same.
Besides the limitations listed in the manufacturer's datasheet, below are the ranges used in my tests to derive the characteristic equations shown above. I anticipate adding wider temperature ranges to my tests as time and ambient temperatures permit which in turn will produce tweaks to the characteristic equations. However, I expect the characteristic equations above to be good for any situation in or "near" the domain criterion listed below.
-49.6992481203007uA <= Iout <= 47.840462905559uA
AND
75.6 degrees F <= Temperature_In_Fahrenheit <= 91.6 degrees F
AND
2.703V <= Vout <= 5.5V
I used 21 LMT87LPGs (TO-92S) to acquire the data. 570 different data points were used to determine the characteristic equations.
LMT87LPG from DigiKey: https://www.digikey.com/en/products/detail/texas-instruments/LMT87LPG/7427018
LMT87LPG from Mouser: https://www.mouser.com/ProductDetail/Texas-Instruments/LMT87LPG?qs=5aG0NVq1C4x9nVDnkGXLCQ%3D%3D
LMT87LPG from Arrow: https://www.arrow.com/en/products/lmt87lpg/texas-instruments
LMT87LPG from Newark: https://www.newark.com/texas-instruments/lmt87lpg/temperature-sensor-0-4deg-c-to/dp/52AH7150
Please note that the value of VDD used to create "Table 3. LMT87 Transfer Table" in LMT87.pdf is not given in LMT87.pdf.
The two LMT87LPG characteristic equations above get closest to the values in Table 3 of LMT87.pdf when VDD is as large as possible (VDD = +5.5V max.). I.e., the larger VDD is the better.
https://github.com/Joe0x7F/MCP9701A
The TMP35/TMP36/TMP37 is an unpredictable/unstable product. I recommend against ever using it.