Datasheet

1.004 1.008
( ) (273.15 100 )
1.008
1.48
ERR
ERR
T C
T C
-
= ´ + °
= - °
1.008
( ) (273.15 ( ))
1.008
ERR
n
T T C
-
= ´ + °
TMP411-Q1
www.ti.com SBOS527F –DECEMBER 2010–REVISED NOVEMBER 2013
REMOTE SENSING
4. Tight control of V
BE
characteristics indicated by
small variations in h
FE
(that is, 50 to 150).
The TMP411-Q1 is designed to be used with either
discrete transistors or substrate transistors built into
Based on these criteria, two recommended small-
processor chips and ASICs. Either NPN or PNP
signal transistors are the 2N3904 (NPN) or 2N3906
transistors can be used, as long as the base-emitter
(PNP).
junction is used as the remote temperature sense.
Either a transistor or diode connection can also be
MEASUREMENT ACCURACY AND THERMAL
used; see Figure 11. CONSIDERATIONS
Errors in remote temperature-sensor readings are the The temperature measurement accuracy of the
consequence of the ideality factor and current TMP411-Q1 depends on the remote and/or local
excitation used by the TMP411-Q1 versus the temperature sensor being at the same temperature
manufacturer-specified operating current for a given as the system point being monitored. Clearly, if the
transistor. Some manufacturers specify a high-level temperature sensor is not in good thermal contact
and low-level current for the temperature-sensing with the part of the system being monitored, then
substrate transistors. The TMP411-Q1 uses 6 μA for there is a delay in the response of the sensor to a
I
LOW
and 120 μA for I
HIGH
. The TMP411-Q1 allows for temperature change in the system. For remote
different n-factor values; see the N-Factor Correction temperature sensing applications using a substrate
Register section. transistor (or a small, SOT23 transistor) placed close
to the device being monitored, this delay is usually
The ideality factor (n) is a measured characteristic of
not a concern.
a remote temperature sensor diode as compared to
an ideal diode. The ideality factor for the TMP411-Q1 The local temperature sensor inside the TMP411-Q1
is trimmed to be 1.008. For transistors whose ideality monitors the ambient air around the device. The
factor does not match the TMP411-Q1, Equation 4 thermal time constant for the TMP411-Q1 is
can be used to calculate the temperature error. Note approximately two seconds. This constant implies
that for the equation to be used correctly, actual that if the ambient air changes quickly by 100°C, it
temperature (°C) must be converted to Kelvin (K). would take the TMP411-Q1 about 10 seconds (that
is, five thermal time constants) to settle to within 1°C
of the final value. In most applications, the TMP411-
(4)
Q1 package is in electrical and therefore thermal
contact with the printed circuit board (PCB), as well
where:
as subjected to forced airflow. The accuracy of the
n = Ideality factor of remote temperature sensor
measured temperature directly depends on how
T(°C) = actual temperature
accurately the PCB and forced airflow temperatures
represent the temperature that the TMP411-Q1 is
T
ERR
= Error in TMP411-Q1 reading due to n ≠
measuring. Additionally, the internal power dissipation
1.008
of the TMP411-Q1 can cause the temperature to rise
Degree delta is the same for °C and °K
above the ambient or PCB temperature. The internal
For n = 1.004 and T(°C) = 100 ° C:
power dissipated as a result of exciting the remote
temperature sensor is negligible because of the small
currents used. For a 5.5-V supply and maximum
conversion rate of eight conversions per second, the
TMP411-Q1 dissipates 1.82 mW (PD
IQ
= 5.5 V × 330
(5)
μA). If both the ALERT/THERM2 and THERM pins
If a discrete transistor is used as the remote
are each sinking 1 mA, an additional power of 0.8
temperature sensor with the TMP411-Q1, the best
mW is dissipated (PD
OUT
= 1 mA × 0.4 V + 1 mA ×
accuracy can be achieved by selecting the transistor
0.4 V = 0.8 mW). Total power dissipation is then 2.62
according to the following criteria:
mW (PD
IQ
+ PD
OUT
) and, with a θ
JA
of 150°C/W,
1. Base-emitter voltage > 0.25 V at 6 μA, at the
causes the junction temperature to rise approximately
highest sensed temperature.
0.393°C above the ambient.
2. Base-emitter voltage < 0.95 V at 120 μA, at the
lowest sensed temperature.
3. Base resistance < 100 Ω.
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