Datasheet

Data Sheet ADT7420
Rev. 0 | Page 23 of 24
APPLICATIONS INFORMATION
THERMAL RESPONSE TIME
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 the IC is mounted to. For example, a large PCB
containing large amounts of copper tracking can act as a large
heat sink and slow the thermal response. For a faster thermal
response, it is recommended to mount the sensor on as small a
PCB as possible.
Figure 10 shows the typical response time of less than two
seconds to reach 63.2% of its temperature span. The tempera-
ture value is read back as a 16-bit value through the digital
interface. The response time includes all delays incurred on
chip during signal processing.
SUPPLY DECOUPLING
The ADT7420 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 ADT7420.
If possible, the ADT7420 should be powered directly from the
system power supply. This arrangement, shown in Figure 20,
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.
0.1µF
ADT7420
TTL/CMOS
LOGIC
CIRCUITS
POWER
SUPP
LY
09013-022
Figure 20. Use of Separate Traces to Reduce Power Supply Noise
POWERING FROM A SWITCHING REGULATOR
Precision analog devices, such as the ADT7420 require a well-
filtered power source. If the ADT7420 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
ADT7420 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 ADT7420 to ensure that the thermal
mass is kept as low as possible.
TEMPERATURE MEASUREMENT
The ADT7420 accurately measures and converts the tempera-
ture 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 ADT7420 is used to measure the
temperature of a nearby heat source, the thermal impedance
between the heat source and the ADT7420 must be considered
because this impacts the accuracy and thermal response of the
measurement.
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.
1. After powering up the ADT7420, verify the setup by
reading the device ID (Register Address 0x0B). It should
read 0xCB.
2. After consistent consecutive readings are obtained from
Step 1, proceed to read the configuration register (0x03),
T
CRIT
(0x08, 0x09), T
HIGH
(0x04, 0x05), and T
LOW
(0x06,
0x07) registers. Compare to the specified defaults in Table 6.
If all the readings match, the interface is operational.
3. Write to the configuration register to set the ADT7420 to
the desired configuration.
4. Read the temperature value MSB register, followed by
the temperature value LSB register. Both registers should
produce a valid temperature measurement.