Datasheet
REMOTE SENSING
MEASUREMENT ACCURACY AND THERMAL
h - 1.008
1.008
T =
err
()
´ (273.15+T( C))°
(6)
T
ERR
+
ǒ
1.004 * 1.008
1.008
Ǔ
ǒ
273.15 ) 100°C
Ǔ
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 V
BE
characteristics indicated by
processor chips and ASICs. Either NPN- or PNP-type small variations in h
FE
(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 µ A for
with the part of the system being monitored, then
I
LOW
and 120 µ A for I
HIGH
. 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
T
ERR
= 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 (PD
IQ
= 5.5V × 950 µ A). A θ
JA
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 µ A, at the
highest sensed temperature.
2. Base-emitter voltage < 0.95V at 120 µ A, at the
Copyright © 2008 – 2009, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Link(s): TMP441 TMP442