TERR +ǒ1.004 *1.008
1.008 Ǔ ǒ273.15 )100°CǓ
TERR +1.48°C
TMP421
TMP422
TMP423
SBOS398C –JULY 2007–REVISED MAY 2012
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For η= 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 5.5V supply and maximum
conversion rate of eight conversions per second, the
TMP421/22/23 dissipate 2.3mW (PDIQ = 5.5V ×
(5) 415μA). A θJA of 100°C/W (for SOT23 package)
causes the junction temperature to rise approximately
If a discrete transistor is used as the remote +0.23°C above the ambient.
temperature sensor with the TMP421/22/23, the best
accuracy can be achieved by selecting the transistor LAYOUT CONSIDERATIONS
according to the following criteria: Remote temperature sensing on the TMP421/22/23
1. Base-emitter voltage > 0.25V at 6μA, at the measures very small voltages using very low
highest sensed temperature. currents; therefore, noise at the IC inputs must be
2. Base-emitter voltage < 0.95V at 120μA, at the minimized. Most applications using the
lowest sensed temperature. TMP421/22/23 will have high digital content, with
3. Base resistance < 100Ω.several clocks and logic level transitions creating a
4. Tight control of VBE characteristics indicated by noisy environment. Layout should adhere to the
small variations in hFE (that is, 50 to 150). following guidelines:
1. Place the TMP421/22/23 as close to the remote
Based on these criteria, two recommended small- junction sensor as possible.
signal transistors are the 2N3904 (NPN) or 2N3906
(PNP). 2. Route the DXP and DXN traces next to each
other and shield them from adjacent signals
MEASUREMENT ACCURACY AND THERMAL through the use of ground guard traces; see
CONSIDERATIONS Figure 20. If a multilayer PCB is used, bury these
traces between ground or V+ planes to shield
The temperature measurement accuracy of the them from extrinsic noise sources. 5 mil
TMP421/22/23 depends on the remote and/or local (0.127mm) PCB traces are recommended.
temperature sensor being at the same temperature 3. Minimize additional thermocouple junctions
as the system point being monitored. Clearly, if the caused by copper-to-solder connections. If these
temperature sensor is not in good thermal contact junctions are used, make the same number and
with the part of the system being monitored, then approximate locations of copper-to-solder
there will be a delay in the response of the sensor to connections in both the DXP and DXN
a temperature change in the system. For remote connections to cancel any thermocouple effects.
temperature-sensing applications using a substrate
transistor (or a small, SOT23 transistor) placed close 4. Use a 0.1μF local bypass capacitor directly
to the device being monitored, this delay is usually between the V+ and GND of the TMP421/22/23;
not a concern. see Figure 21. Minimize filter capacitance
between DXP and DXN to 1000pF or less for
The local temperature sensor inside the optimum measurement performance. This
TMP421/22/23 monitors the ambient air around the capacitance includes any cable capacitance
device. The thermal time constant for the between the remote temperature sensor and the
TMP421/22/23 is approximately two seconds. This TMP421/22/23.
constant implies that if the ambient air changes 5. If the connection between the remote
quickly by 100°C, it would take the TMP421/22/23 temperature sensor and the TMP421/22/23 is
about 10 seconds (that is, five thermal time less than 8 in (20.32 cm) long, use a twisted-wire
constants) to settle to within 1°C of the final value. In pair connection. Beyond 8 in, use a twisted,
most applications, the TMP421/22/23 package is in shielded pair with the shield grounded as close to
electrical, and therefore thermal, contact with the the TMP421/22/23 as possible. Leave the remote
printed circuit board (PCB), as well as subjected to sensor connection end of the shield wire open to
forced airflow. The accuracy of the measured avoid ground loops and 60Hz pickup.
temperature directly depends on how accurately the
PCB and forced airflow temperatures represent the 6. Thoroughly clean and remove all flux residue in
temperature that the TMP421/22/23 is measuring. and around the pins of the TMP421/22/23 to
Additionally, the internal power dissipation of the avoid temperature offset readings as a result of
TMP421/22/23 can cause the temperature to rise leakage paths between DXP or DXN and GND,
above the ambient or PCB temperature. The internal or between DXP or DXN and V+.
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