REMOTE SENSING

MEASUREMENT ACCURACY AND THERMAL

h - 1.008

1.008

T

=

err

() ´ (273.15+T( C))

°

(6)

T

ERR

+

1.004 * 1.008

1.008

273.15 ) 100°C

T

ERR

+ 1.48°C

(7)

TMP441

TMP442

www.ti.com .............................................................................................................................................. SBOS425A – DECEMBER 2008 – REVISED MARCH 2009

lowest sensed temperature.

3. Base resistance < 100

Ω.

The TMP441/42 are designed to be used with either

discrete transistors or substrate transistors built into

4. Tight control of VBE characteristics indicated by

processor chips and ASICs. Either NPN- or PNP-type

small variations in hFE (that is, 50 to 150).

transistors can be used, as long as the base-emitter

Based

on

these

criteria,

two

recommended

junction is used as the remote temperature sense.

small-signal transistors are the 2N3904 (NPN) or

NPN transistors must be diode-connected. PNP

2N3906 (PNP).

transistors

can

either

be

transistor-

or

diode-connected (see Figure 12).

Errors in remote temperature sensor readings are

CONSIDERATIONS

typically the consequence of the ideality factor and

The temperature measurement accuracy of the

current excitation used by the TMP441/42 versus the

TMP441/42 depends on the remote and/or local

manufacturer-specified operating current for a given

temperature sensor being at the same temperature

transistor. Some manufacturers specify a high-level

as the system point being monitored. Clearly, if the

and low-level current for the temperature-sensing

temperature sensor is not in good thermal contact

substrate transistors. The TMP441/42 use 6

µA for

with the part of the system being monitored, then

ILOW and 120µA for IHIGH. The TMP441/42 allow for

there will be a delay in the response of the sensor to

different

η-factor values; see the η-Factor Correction

a temperature change in the system. For remote

Register section. The ideality factor (

η) is a measured

temperature-sensing applications that use a substrate

characteristic of a remote temperature sensor diode

transistor (or a small, SOT23 transistor) placed close

as compared to an ideal diode.

to the device being monitored, this delay is usually

The ideality factor for the TMP441/42 is trimmed to

not a concern.

be 1.008. For transistors that have an ideality factor

The local temperature sensor inside the TMP441/42

that does not match the TMP441/42, Equation 6 can

monitors the ambient air around the device. The

be used to calculate the temperature error. Note that

thermal

time

constant

for

the

TMP441/42

is

for

the

equation

to

be

used

correctly,

actual

approximately two seconds. This constant implies

temperature (°C) must be converted to kelvins (K).

that if the ambient air changes quickly by 100°C, it

would take the TMP441/42 approximately 10 seconds

(that is, five thermal time constants) to settle to within

1°C of the final value. In most applications, the

Where:

TMP441/42 package is in electrical, and therefore

η = ideality factor of remote temperature sensor

thermal, contact with the printed circuit board (PCB),

as well as subjected to forced airflow. The accuracy

T(°C) = actual temperature

of the measured temperature directly depends on

TERR = error in TMP441/42 due to n ≠ 1.008

how

accurately

the

PCB

and

forced

airflow

Degree delta is the same for °C and K

temperatures represent the temperature that the

TMP441/42 is measuring. Additionally, the internal

For

η = 1.004 and T(°C) = 100°C:

power dissipation of the TMP441/42 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

5.5V supply and maximum conversion rate of eight

If a discrete transistor is used as the remote

conversions per second, the TMP441/42 dissipate

temperature sensor with the TMP441/42, the best

5.2mW (PDIQ = 5.5V × 950µA). A θJA of 100°C/W

accuracy can be achieved by selecting the transistor

causes the junction temperature to rise approximately

according to the following criteria:

+0.23°C above the ambient.

1. Base-emitter voltage > 0.25V at 6

µA, at the

highest sensed temperature.

2. Base-emitter voltage < 0.95V at 120

µA, at the

Copyright © 2008–2009, Texas Instruments Incorporated

Submit Documentation Feedback

21

Product Folder Link(s): TMP441 TMP442