ADT6501/ADT6502/ADT6503/ADT6504 Data Sheet
Rev. B | Page 10 of 16
APPLICATION INFORMATION
THERMAL RESPONSE TIME
The time required for a temperature sensor to settle to a specified
accuracy is a function of the sensor’s thermal mass 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 ADT650x 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 ADT6501/
ADT6502/ADT6503/ADT6504 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 ADT650x package, the mounting
technique, and the effects of airflow. At 25°C, static dissipation
in the ADT650x is typically 99 µW operating at 3.3 V. In the
5-lead SOT-23 package mounted in free air, this accounts for a
temperature increase due to self-heating of
ΔT = P
DISS
× θ
JA
= 99 µW × 240°C/W = 0.024°C
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 ADT6501/ADT6502/ADT6503/ADT6504 should be
decoupled with a 0.1 µF ceramic capacitor between V
CC
and
GND. This is particularly important when the ADT650x are
mounted remotely from the power supply. Precision analog
products such as the ADT650x require well filtered power
sources. Because the ADT650x 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 mV in
amplitude due to wiring resistance and inductance.
If possible, the ADT650x should be powered directly from the
system power supply. This arrangement, shown in Figure 19,
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 ADT650x V
CC
pin.
Figure 19. Separate Traces Used to Reduce Power Supply Noise
TEMPERATURE MONITORING
The ADT6501/ADT6502/ADT6503/ADT6504 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 ADT650x measure and convert the temperature at the
surface of its own semiconductor chip. When the ADT650x are
used to measure the temperature of a nearby heat source, the
thermal impedance between the heat source and the ADT650x
must be as low as possible.
As much as 60% of the heat transferred from the heat source to
the thermal sensor on the ADT650x die is discharged via the
copper tracks, package pins, and bond pads. Of the pins on the
ADT650x, the GND pins transfer most of the heat. Therefore,
to monitor the temperature of a heat source, it is recommended
that the thermal resistance between the ADT650x GND pins
and the GND of the heat source be reduced as much as possible.
For example, the unique properties of the ADT650x can be used
to monitor a high power dissipation microprocessor. The
ADT650x device in its SOT-23 package is mounted directly
beneath the microprocessor’s pin grid array (PGA) package.
The ADT650x requires no external characterization.
0.1
µ
F
ADT650x
TTL/CMOS
LOGIC
CIRCUITS
POWER
SUPPLY
06096-010
