Datasheet

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SBOS383C − DECEMBER 2006 − REVISED MAY 2008
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22
REMOTE SENSING
The TMP411 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. Either a transistor or diode
connection can also be used; see Figure 11.
Errors in remote temperature sensor readings will be the
consequence of the ideality factor and current excitation
used by the TMP411 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
TMP411 uses 6µA for I
LOW
and 120µA for I
HIGH
. The
TMP411 allows for different n-factor values; see the
N-Factor Correction Register section.
The ideality factor (n) is a measured characteristic of a
remote temperature sensor diode as compared to an ideal
diode. The ideality factor for the TMP411 is trimmed to be
1.008. For transistors whose ideality factor does not match
the TMP411, 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
Kelvin (°K).
T
ERR
+
ǒ
n * 1.008
1.008
Ǔ
ǒ
273.15 ) T
ǒ
°C
Ǔ
Ǔ
Where:
n = Ideality factor of remote temperature sensor
T(°C) = actual temperature
T
ERR
= Error in TMP411 reading due to n ≠ 1.008
Degree delta is the same for °C and °K
For n = 1.004 and T(°C) = 100°C:
T
ERR
+
ǒ
1.004 * 1.008
1.008
Ǔ
ǒ
273.15 ) 100°C
Ǔ
T
ERR
+*1.48°C
If a discrete transistor is used as the remote temperature
sensor with the TMP411, the best accuracy can be
achieved by selecting the transistor according to the
following criteria:
1. Base-emitter voltage > 0.25V at 6µA, at the highest
sensed temperature.
2. Base-emitter voltage < 0.95V at 120µA, at the lowest
sensed temperature.
3. Base resistance < 100Ω.
4. Tight control of V
BE
characteristics indicated by small
variations in h
FE
(that is, 50 to 150).
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 TMP411
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 TMP411 monitors
the ambient air around the device. The thermal time
constant for the TMP411 is approximately two seconds.
This constant implies that if the ambient air changes
quickly by 100°C, it would take the TMP411 about 10
seconds (that is, five thermal time constants) to settle to
within 1°C of the final value. In most applications, the
TMP411 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 TMP411 is measuring. Additionally,
the internal power dissipation of the TMP411 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 conversions per
second, the TMP411 dissipates 1.82mW (PD
IQ
= 5.5V ×
330µA). If both the ALERT
/THERM2 and THERM pins are
each sinking 1mA, an additional power of 0.8mW is
dissipated (PD
OUT
= 1mA × 0.4V + 1mA × 0.4V = 0.8mW).
Total power dissipation is then 2.62mW (PD
IQ
+ PD
OUT
)
and, with an q
JA
of 150°C/W, causes the junction
temperature to rise approximately 0.393°C above the
ambient.
(4)
(5)