T

ERR

+

1.004 * 1.008

1.008

273.15 ) 100°C

T

ERR

+ 1.48°C

TMP451

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SBOS686A – JUNE 2013 – REVISED DECEMBER 2014

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 TMP451, the best accuracy can be

achieved by selecting the transistor according to the following criteria:

1. Base-emitter voltage > 0.25 V at 7.5

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

8.2.2 Detailed Design Procedure

The local temperature sensor inside the TMP451 device monitors the ambient air around the device. The thermal

time constant for the TMP451 device is approximately two seconds. This constant implies that if the ambient air

changes quickly by 100°C, it would take the TMP451 device about 10 seconds (that is, five thermal time

constants) to settle to within 1°C of the final value. In most applications, the TMP451 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 TMP451 is measuring. Additionally, the internal power

dissipation of the TMP451 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 3.3-V supply and maximum conversion rate of 16 conversions per second, the TMP451

device dissipates 0.54 mW (PDIQ = 3.3 V × 165 μA). A θJA of 171.3°C/W causes the junction temperature to rise

approximately 0.09°C above the ambient.

The temperature measurement accuracy of the TMP451 device 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

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

is usually not a concern.

8.2.3 Application Curves

The following curves show the performance capabilities of the TMP451 device. Figure 43 shows the accuracy

performance in an oil-bath temperature drift of a population of 16 standrard 2N3906 transistors measured in a

diode-connected configuration. Figure 44 shows the typical step response to a submerging of a sensor in an oil

bath with temperature of 100°C.

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