adt6402 Analog Devices, Inc., adt6402 Datasheet - Page 11

• Manufacturer Part Number: adt6402
• Description: Low Cost, 2.7 V To 5.5 V, Pin-selectable Temperature Switches In Sot-23
• Manufacturer: Analog Devices, Inc.

APPLICATIONS INFORMATION
THERMAL RESPONSE TIME
The time required for a temperature sensor to settle to a specified
accuracy is a function of the thermal mass of the sensor and
the thermal conductivity between the sensor and the object
being sensed. Thermal mass is often considered equivalent to
capacitance. Thermal conductivity is commonly specified using
the symbol Q and can be thought of as thermal resistance. It is
commonly specified in units of degrees per watt of power
transferred across the thermal joint. Thus, the time required for
the ADT6401/ADT6402 to settle to the desired accuracy is
dependent on the characteristics of the SOT-23 package, the
thermal contact established in that particular application, and
the equivalent power of the heat source. In most applications,
the settling time is best determined empirically.
SELF-HEATING EFFECTS
The temperature measurement accuracy of the ADT6401/
ADT6402 can be degraded in some applications due to self-
heating. Errors can be introduced from the quiescent dissipation
and power dissipated when converting. The magnitude of these
temperature errors depends on the thermal conductivity of the
ADT6401/ADT6402 package, the mounting technique, and the
effects of airflow. At 25°C, static dissipation in the ADT6401/
ADT6402 is typically 99 μW operating at 3.3 V. In the 6-lead
SOT-23 package mounted in free air, this accounts for a tempera-
ture increase due to self-heating of
It is recommended that current dissipated through the device be
kept to a minimum because it has a proportional effect on the
temperature error.
SUPPLY DECOUPLING
The ADT6401/ADT6402 should be decoupled with a 0.1 μF
ceramic capacitor between V
important when the ADT6401/ADT6402 are mounted remotely
from the power supply. Precision analog products such as the
ADT6401/ADT6402 require well-filtered power sources.
Because the ADT6401/ADT6402 operate from a single supply,
it may seem convenient to tap into the digital logic power supply.
Unfortunately, the logic supply is often a switch-mode design,
which generates noise in the 20 kHz to 1 MHz range. In addition,
fast logic gates can generate glitches that are hundreds of millivolts
in amplitude due to wiring resistance and inductance.
ΔT = P
DISS
× θ
JA
= 99 μW × 240°C/W = 0.024°C
CC
and GND. This is particularly
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If possible, the ADT6401/ADT6402 should be powered directly
from the system power supply. This arrangement, shown in
Figure 22, isolates the analog section from the logic-switching
transients. Even if a separate power supply trace is not available,
generous supply bypassing reduces supply line induced errors.
Local supply bypassing consisting of a 0.1 μF ceramic capacitor
is advisable to achieve the temperature accuracy specifications.
This decoupling capacitor must be placed as close as possible to
the ADT6401/ADT6402 V
TEMPERATURE MONITORING
The ADT6401/ADT6402 are ideal for monitoring the thermal
environment within electronic equipment. For example, the
surface-mount package accurately reflects the exact thermal
conditions that affect nearby integrated circuits.
The ADT6401/ADT6402 measure and convert the temperature
at the surface of its own semiconductor chip. When the ADT6401/
ADT6402 are used to measure the temperature of a nearby heat
source, the thermal impedance between the heat source and the
ADT6401/ADT6402 must be as low as possible.
As much as 60% of the heat transferred from the heat source to
the thermal sensor on the ADT6401/ADT6402 die is discharged
via the copper tracks, package pins, and bond pads. Of the pins
on the ADT6401/ADT6402, the GND pin transfers most of the
heat. Therefore, to monitor the temperature of a heat source, it is
recommended that the thermal resistance between the ADT6401/
ADT6402 GND pin and the GND of the heat source be reduced
as much as possible.
For example, the unique properties of the ADT6401/ADT6402
can be used to monitor a high power dissipation microproces-
sor. The ADT6401/ADT6402 device in its SOT-23 package is
mounted directly beneath the pin grid array (PGA) package of
the microprocessor. The ADT6401/ADT6402 require no external
characterization.
TTL/CMOS
CIRCUITS
LOGIC
Figure 22. Separate Traces Used to Reduce Power Supply Noise
SUPPLY
POWER
CC
pin.
ADT6401/ADT6402
0.1 µ F
ADT6401/
ADT6402