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

excitation used by the TMP461-EP device 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 TMP461-EP device uses 7.5μA for ILOW and 120μA for IHIGH.

The ideality factor (η) is a measured characteristic of a remote temperature sensor diode as compared to an

ideal diode. The TMP461-EP allows for different η-factor values; see the η-Factor Correction Register section.

The ideality factor for the TMP461-EP device is trimmed to be 1.008. For transistors that have an ideality factor

that does not match the TMP461-EP, Equation 4 can be used to calculate the temperature error.

Note

For Equation 4 to be used correctly, the actual temperature (°C) must be converted to Kelvin (K).

T

=

ERR

h

1.008

-

1.008

(273.15+T( C))

°

´

(4)

where

• TERR = error in the TMP461-EP device because η ≠ 1.008,

• η = ideality factor of the remote temperature sensor,

• T(°C) = actual temperature, and

In Equation 4, the degree of delta is the same for °C and K.

For η = 1.004 and T(°C) = 100°C:

.

.

.

.

.

ERR

ERR

1 004

1 008

T

=

273 15

100 C

1 008

T

1 48 C

§

·

u

q

¨

¸

©

¹

q

(5)

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

achieved by selecting the transistor according to the following criteria:

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

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

3. Base resistance is < 100Ω.

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-EP device monitors the ambient air around the device. The

thermal time constant for the TMP461-EP device is approximately 1.1 seconds. This constant implies that if

the ambient air changes quickly by 100°C, then the TMP461-EP device takes approximately 5.5 seconds (that

is, five thermal time constants) to settle to within 1°C of the final value. In most applications, the TMP461-EP

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-EP is measuring. Additionally, the

internal power dissipation of the TMP461-EP 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 the temperature

sensor channel enabled. Equation 7 shows an example with local and remote sensor channels enabled and 16

TMP461-EP

SNIS241 – SEPTEMBER 2025

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