T

ERR

+

1.004 * 1.008

1.008

273.15 ) 100°C

T

ERR

+ 1.48°C

T

=

ERR

h

1.008

-

1.008

(273.15+T( C))

°

´

TMP451

www.ti.com

SBOS686 – JUNE 2013

REMOTE SENSING

The TMP451 is designed to be used with either discrete transistors or substrate transistors built into processor

chips and ASICs. Either NPN or PNP transistors can be used, as long as the base-emitter junction is used as the

remote temperature sense. NPN transistors must be diode-connected. PNP transistors can either be transistor-

or diode-connected (see Figure 11).

Errors in remote temperature sensor readings are typically the consequence of the ideality factor and current

excitation used by the TMP451 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

TMP451 uses 7.5

The ideality factor (

η) is a measured characteristic of a remote temperature sensor diode as compared to an

ideal diode. The TMP451 allows for different

η-factor values; see the η-Factor Correction Register section.

The ideality factor for the TMP451 is trimmed to be 1.008. For transistors that have an ideality factor that does

not match the TMP451, 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).

(4)

Where:

η = ideality factor of remote temperature sensor

T(°C) = actual temperature

Degree delta is the same for °C and K.

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.

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

MEASUREMENT ACCURACY AND THERMAL CONSIDERATIONS

The temperature measurement accuracy of the TMP451 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.

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

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

quickly by 100°C, it would take the TMP451 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

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