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TMP461

www.ti.com

SBOS722B – JUNE 2015 – REVISED AUGUST 2016

Product Folder Links: TMP461

Submit Documentation Feedback

Copyright © 2015–2016, Texas Instruments Incorporated

Typical Application (continued)

For η = 1.004 and T(°C) = 100°C:

(5)

If a discrete transistor is used as the remote temperature sensor with the TMP461, the best accuracy can be

achieved by selecting the transistor according to the following criteria:

1. Base-emitter voltage is > 0.25 V at 7.5 μA, at the highest-sensed temperature.

2. Base-emitter voltage is < 0.95 V at 120 μA, at the lowest-sensed temperature.

3. Base resistance is < 100

Ω.

4. Tight control of VBE characteristics indicated by small variations in hFE (that is, 50 to 150).

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

9.2.2 Detailed Design Procedure

The local temperature sensor inside the TMP461 device monitors the ambient air around the device. The thermal

time constant for the TMP461 device is approximately two seconds. This constant implies that if the ambient air

changes quickly by 100°C, then the TMP461 device takes approximately 10 seconds (that is, five thermal time

constants) to settle to within 1°C of the final value. In most applications, the TMP461 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 TMP461 is measuring. Additionally, the internal power

dissipation of the TMP461 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. Equation 6 can be used to calculate the average conversion current for power dissipation and

self-heating based on the number of conversions per second and temperature sensor channel enabled.

Equation 7 shows an example with local and remote sensor channels enabled and 16 conversions per second;

see the Electrical Characteristics table for typical values required for these calculations. For a 3.3-V supply and a

conversion rate of 16 conversions per second, the TMP461 device dissipates 0.531 mW (PDIQ = 3.3 V × 161 μA)

when both the remote and local channels are enabled.

(6)

(7)

The temperature measurement accuracy of the TMP461 device depends on the remote and local temperature

sensor being at the same temperature as the system point being monitored. If the temperature sensor is not in

good thermal contact with the part of the system being monitored, then there is a delay between the sensor

response and the system changing temperature. This delay is usually not a concern for remote temperature-

sensing applications that use a substrate transistor (or a small, SOT23 transistor) placed close to the device

being monitored.