u

u

u

u

u

Q

Q

Q

Average Conversion Current

Local ADC Conversion Time

Conversions per Second

Local Active I

Remote ADC Conversion Time

Conversions per Second

Remote Active I

Standby Mode I

1

Local ADC Conversion Ti

ª

º

u

¬

¼

me

Remote ADC Conversion Time

Conversions per Second

.

.

.

.

.

ERR

ERR

1 004

1 008

T

=

273 15

100 C

1 008

T

1 48 C

§

·

u

q

¨

¸

©

¹

q

T

=

ERR

h

1.008

-

1.008

(273.15+T( C))

°

´

25

TMP461-SP

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SBOS876B – SEPTEMBER 2017 – REVISED FEBRUARY 2020

Product Folder Links: TMP461-SP

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Copyright © 2017–2020, Texas Instruments Incorporated

Typical Application (continued)

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

that does not match the TMP461-SP, 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).

where

•

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

•

η = ideality factor of the remote temperature sensor,

•

T(°C) = actual temperature, and

(4)

In Equation 4, the degree of 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 TMP461-SP, 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).

8.2.2 Detailed Design Procedure

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

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

ambient air changes quickly by 100°C, then the TMP461-SP 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-SP 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-SP is measuring. Additionally, the internal

power dissipation of the TMP461-SP 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-SP device dissipates 0.531 mW (PDIQ = 3.3 V × 161

μA) when both the remote and local channels are enabled.

(6)