TMP421-Q1 Datasheet, PDF (26/38 Pages) Texas Instruments – Remote and Local Temperature Sensor

TMP421-Q1, TMP422-Q1, TMP423-Q1
SBOS821 – NOVEMBER 2016
www.ti.com
Errors in remote temperature sensor readings are typically the consequence of the ideality factor and current
excitation used by the TMP421-Q1, TMP422-Q1, and TMP423-Q1 versus the manufacturer-specified operating
current for a given transistor. Some manufacturers specify a high-level and low-level current for the temperature-
sensing substrate transistors. The TMP421-Q1, TMP422-Q1, and TMP423-Q1 use 6 μA for I_LOW and 120 μA for
I_HIGH.
The ideality factor (η-factor) is a measured characteristic of a remote temperature sensor diode as compared to
an ideal diode. The TMP421-Q1, TMP422-Q1, and TMP423-Q1 allow for different η-factor values; see the η-
Factor Correction Register section.
The ideality factor for the TMP421-Q1, TMP422-Q1, and TMP423-Q1 is trimmed to be 1.008. For transistors that
have an ideality factor that does not match the TMP421-Q1, TMP422-Q1, and TMP423-Q1, Equation 4 can be
used to calculate the temperature error. Note that for the equation to be used correctly, actual temperature (°C)
must be converted to kelvins (K).
h - 1.008
T_ERR = _1.008 ´ (273.15 + T(°C))
where
• η = ideality factor of remote temperature sensor
• T(°C) = actual temperature
• T_ERR = error in TMP421-Q1, TMP422-Q1, and TMP423-Q1 because η ≠ 1.008
• Degree delta is the same for °C and K
(4)
For η = 1.004 and T (°C) = 100°C:
T_ERR
=
æ 1.004 - 1.008 ö
çè _1.008 ÷ø
´
273.15 + 100°C
T_ERR = 1.48°C
(5)
If a discrete transistor is used as the remote temperature sensor with the TMP421-Q1, TMP422-Q1, and
TMP423-Q1, the best accuracy can be achieved by selecting the transistor according to the following criteria:
1. Base-emitter voltage > 0.25 V at 6 μA, at the highest sensed temperature.
2. Base-emitter voltage < 0.95 V at 120 μA, at the lowest sensed temperature.
3. Base resistance < 100 Ω.
4. Tight control of V_BE characteristics indicated by small variations in h_FE (that is, 50 to 150).
Based on these criteria, two recommended small-signal transistors are the 2N3904 (NPN) or 2N3906 (PNP).
9.2.1.2 Detailed Design Procedure
The local temperature sensor inside the TMP421-Q1, TMP422-Q1, and TMP423-Q1 is influenced by the ambient
air around the device but mainly monitors the PCB temperature that it is mounted to. In most applications, the
TMP421-Q1, TMP422-Q1, and TMP423-Q1 package is in electrical, and therefore thermal, contact with the
printed-circuit board (PCB), as well as subjected to forced airflow. The accuracy of the measured temperature
directly depends on how accurately the PCB and forced airflow temperatures represent the temperature that the
TMP421-Q1, TMP422-Q1, and TMP423-Q1 is measuring. Additionally, the internal power dissipation of the
TMP421-Q1, TMP422-Q1, and TMP423-Q1 can cause the temperature to rise above the ambient or PCB
temperature. The internal power is negligible because of the small current drawn by TMP421-Q1, TMP422-Q1,
and TMP423-Q1 (see the Measurement Accuracy and Thermal Considerations section for more details).
For this design example a diode connected 2N3906 transistor was used thus the default setting of the n-factor
and offset can be used.
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