
T
=
ERR
°
[273.15+T( C)]
h - 1.008
1.008
(
)
T
=
ERR
°
(273.15+100 C)
1.004
1.008
-
(
)
T
=1.48 C
ERR
°
SBOS441C
– SEPTEMBER 2009 – REVISED FEBRUARY 2011
Remote Sensing
3. Base resistance
< 100.
4. Tight control of VBE characteristics indicated by
The TMP431/32 are designed to be used with either
small variations in hFE (that is, 50 to 150).
discrete transistors or substrate transistors built into
processor chips and ASICs. Either NPN- or PNP-type
Based
on
these
criteria,
two
recommended
transistors can be used, as long as the base-emitter
small-signal transistors are the 2N3904 (NPN) or
junction is used as the remote temperature sense.
2N3906 (PNP).
NPN transistors must be diode-connected. PNP
transistors
can
either
be
transistor-
or
diode-
Measurement Accuracy and Thermal
Considerations
Errors in remote temperature sensor readings are
The temperature measurement accuracy of the
typically the consequence of the ideality factor and
TMP431/32 depends on the remote and/or local
current excitation used by the TMP431/32 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 TMP431/32 use 6
μA for
there will be a delay in the response of the sensor to
ILOW and 120μA for IHIGH. The TMP431/32 allow for
a temperature change in the system. For remote
different
temperature sensing applications that use a substrate
transistor (or a small, SOT23 transistor) placed close
to the device being monitored, this delay is usually
The ideality factor (
η) is a measured characteristic of
not a concern.
a remote temperature sensor diode as compared to
an ideal diode. The ideality factor for the TMP431/32
The local temperature sensor inside the TMP431/32
is trimmed to be 1.008. For transistors whose ideality
monitors the ambient air around the device. The
thermal
time
constant
for
the
TMP431/32
is
can be used to calculate the temperature error. Note
approximately 2 seconds. This constant implies that if
that for the equation to be used correctly, actual
the ambient air changes quickly by 100
°C, it would
temperature (
°C) must be converted to Kelvin (K).
take the TMP431/32 about 10 seconds (that is, five
thermal time constants) to settle to within 1
°C of the
final value. In most applications, the TMP431/32
package is in thermal contact with the printed circuit
Where:
board (PCB), as well as subjected to forced airflow.
η = Ideality factor of remote temperature sensor
The accuracy of the measured temperature directly
depends on how accurately the PCB and forced
T(
°C) = actual temperature
airflow temperatures represent the temperature that
TERR = Error in TMP431/32 reading due to η ≠
the
TMP431/32
is
measuring.
Additionally,
the
1.008
internal power dissipation of the TMP431/32 can
Degree delta is the same for
°C and K
(6)
cause the temperature to rise above the ambient or
For n = 1.004 and T(
°C) = 100°C:
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 TMP431/32 dissipate
(7)
1.82mW (PDIQ = 5.5V × 330μA). If both the
ALERT/THERM2 and THERM pins are each sinking
If a discrete transistor is used as the remote
1mA, an additional 0.8mW is dissipated (PDOUT =
temperature sensor with the TMP431/32, the best
1mA
× 0.4V + 1mA × 0.4V = 0.8mW). Total power
accuracy can be achieved by selecting the transistor
dissipation is then 2.62mW (PDIQ + PDOUT) and, with
according to the following criteria:
an
θJA of 150°C/W, causes the junction temperature
1. Base-emitter voltage
> 0.25V at 6μA, at the
to rise approximately 0.393
°C above the ambient.
highest sensed temperature.
2. Base-emitter voltage
< 0.95V at 120μA, at the
lowest sensed temperature.
2009–2011, Texas Instruments Incorporated
31