ADT7320 Data Sheet
Rev. 0 | Page 22 of 24
APPLICATIONS INFORMATION
THERMAL RESPONSE TIME POWERING FROM A SWITCHING REGULATOR
Thermal response is a function of the thermal mass of the
temperature sensor, but it is also heavily influenced by the
mass of the object that the IC is mounted to. For example, a large
PCB containing large amounts of copper tracing can act as a
large heat sink and slow the thermal response. For a faster thermal
response, it is recommended that the sensor be mounted on a
PCB that is as small as possible.
Precision analog devices such as the ADT7320 require a well-
filtered power source. If the ADT7320 is powered from a switching
regulator, noise may be generated above 50 kHz that may affect
the temperature accuracy specifications. To prevent this, an RC
filter should be used between the power supply and ADT7320
V
DD
. The value of the components used should be carefully
considered to ensure that the peak value of the supply noise is
less than 1 mV. The RC filter should be mounted as far away as
possible from the ADT7320 to ensure that the thermal mass is
kept as low as possible.
Figure 10 shows the typical response time of less than 2 seconds to
reach 63.2% of the temperature span of the DUT. The temperature
values are the digital bytes read back through the digital interface.
The response time includes all delays incurred on chip during
signal processing.
TEMPERATURE MEASUREMENT
The ADT7320 accurately measures and converts the temperature
at the surface of its own semiconductor chip. Thermal paths run
through the leads, the exposed pad, as well as the plastic package.
When the ADT7320 is used to measure the temperature of a
nearby heat source, the thermal impedance between the heat
source and the ADT7320 must be considered because this
impedance affects the accuracy and thermal response of the
measurement.
SUPPLY DECOUPLING
The ADT7320 must have a decoupling capacitor connected
between V
DD
and GND; otherwise, incorrect temperature
readings will be obtained. A 0.1 μF decoupling capacitor, such
as a high frequency ceramic type, must be used and mounted as
close as possible to the V
DD
pin of the ADT7320.
If possible, the ADT7320 should be powered directly from the
system power supply. This arrangement, shown in Figure 21,
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
critical for the temperature accuracy specifications to be
achieved.
For air or surface temperature measurements, take care to isolate
the package, leads, and exposed pad from ambient air temperature.
Use of a thermally conductive adhesive can help to achieve a
more accurate surface temperature measurement.
QUICK GUIDE TO MEASURING TEMPERATURE
The following is a quick guide for measuring temperature in
continuous conversion mode (default power-up mode). Execute
each step sequentially.
0.1µF
ADT7320
TTL/CMOS
LOGIC
CIRCUITS
POWER
SUPPLY
09012-022
1. After power-up, reset the serial interface (load 32
consecutive 1s on DIN). This ensures all internal circuitry
is properly initialized.
2. Verify the setup by reading the device ID
(Register Address 0x03). It should read 0xC3.
3. After consistent consecutive readings are obtained from
Step 2, read the configuration register (Register Address 0x01),
T
CRIT
(Register Address 0x04), T
HIGH
(Register Address 0x06),
and T
LOW
(Register Address 0x07). Compare these values
with the specified defaults in Table 6. If all the readings
match, the interface is operational.
Figure 21. Use of Separate Traces to Reduce Power Supply Noise
4. Write to the configuration register to set the ADT7320 to
the desired configuration. Read the temperature value register.
It should produce a valid temperature measurement.
