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TMP431, TMP432

SBOS441G – SEPTEMBER 2009 – REVISED DECEMBER 2015

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Typical Application (continued)

If a discrete transistor is used as the remote temperature sensor with the TMP43x, 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 VBE characteristics indicated by small variations in hFE (that is, 50 to 150)

Based on these criteria, two recommended small-signal transistors are the 2N3904 (NPN) or 2N3906 (PNP).

9.2.2 Detailed Design Procedure

The temperature measurement accuracy of the TMP43x depends on the remote and/or local temperature sensor

being at the same temperature as the system point being monitored. Clearly, if the temperature sensor is not in

good thermal contact with the part of the system being monitored, then there will be a delay in the response of

the sensor to a temperature change in the system. For remote temperature sensing applications that use a

substrate transistor (or a small, SOT23 transistor) placed close to the device being monitored, this delay is

usually not a concern.

The local temperature sensor inside the TMP43x monitors the ambient air around the device. The thermal time

constant for the TMP43x is approximately 2 s. This constant implies that if the ambient air changes quickly by

100°C, it would take the TMP43x about 10 seconds (that is, five thermal time constants) to settle to within 1°C of

the final value. In most applications, the TMP43x package is in 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 TMP43x is

measuring. Additionally, the internal power dissipation of the TMP43x can cause the temperature to rise above

the ambient or PCB temperature. The internal power dissipated as a result of exciting the remote temperature

sensor is negligible because of the small currents used. For a 5.5-V supply and maximum conversion rate of

eight conversions per second, the TMP43x dissipate 1.82 mW (PDIQ = 5.5 V × 330 μA). If both the

ALERT/THERM2 and THERM pins are each sinking 1 mA, an additional 0.8 mW is dissipated (PDOUT = 1 mA ×

0.4 V + 1 mA × 0.4 V = 0.8 mW). Total power dissipation is then 2.62 mW (PDIQ + PDOUT) and, with an θJA of

150°C/W, causes the junction temperature to rise approximately 0.393°C above the ambient.

9.2.3 Application Curve

Figure 22 shows the typical step response to a submerging of a sensor in an oil bath with temperature of 100ºC.

Figure 22. Temperature Step Response

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