Datasheet

T =
ERR
´ °(273.15+100 C)
1.004 1.008
1.008
-
(
)
T =1.48 C
ERR
°
T =
ERR
´ °[273.15+T( C)]
h - 1.008
1.008
(
)
TMP431
TMP432
www.ti.com SBOS441F –SEPTEMBER 2009–REVISED AUGUST 2013
REMOTE SENSING 3. Base resistance < 100Ω.
4. Tight control of V
BE
characteristics indicated by
The TMP431/32 are designed to be used with either
small variations in h
FE
(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 small-
transistors can be used, as long as the base-emitter
signal transistors are the 2N3904 (NPN) or 2N3906
junction is used as the remote temperature sense.
(PNP).
NPN transistors must be diode-connected. PNP
transistors can either be transistor- or diode-
MEASUREMENT ACCURACY AND THERMAL
connected (see Figure 15).
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 6μA for
there will be a delay in the response of the sensor to
I
LOW
and 120μA for I
HIGH
. The TMP431/32 allow for
a temperature change in the system. For remote
different η-factor values; see the η- Factor Correction
temperature sensing applications that use a substrate
Register section.
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
factor does not match the TMP431/32, Equation 6
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
• T
ERR
= 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 (PD
IQ
= 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 (PD
OUT
=
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 (PD
IQ
+ PD
OUT
) and, with
according to the following criteria:
an θ
JA
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.
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