ADT7311 Data Sheet
Rev. 0 | Page 22 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.
SUPPLY DECOUPLING
The ADT7311 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 ADT7311.
If possible, the ADT7311 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.
0.1µF
ADT7311
TTL/CMOS
LOGIC
CIRCUITS
POWER
SUPPLY
09050-022
Figure 21. Use of Separate Traces to Reduce Power Supply Noise
POWERING FROM A SWITCHING REGULATOR
Precision analog devices such as the ADT7311 require a well-
filtered power source. If the ADT7311 is powered from a
switching regulator, noise may be generated in the 50 kHz to
3 MHz range that may affect the temperature accuracy specifica-
tion. To prevent this, it is recommended to connect an RC filter
between the power supply and V
DD
. The value of components
used should be carefully considered to ensure that the
ADT7311 operates within specification. The RC filter should be
mounted as far away as possible from the ADT7311 to ensure
that the thermal mass is kept as low as possible. For additional
information, contact Analog Devices.
TEMPERATURE MONITORING
The ADT7311 is ideal for monitoring the thermal environment
within hazardous automotive applications. The die accurately
reflects the thermal conditions that affect nearby integrated
circuits.
The ADT7311 measures and converts the temperature at the
surface of its own semiconductor chip. When the ADT7311 is
used to measure the temperature of a nearby heat source, the
thermal impedance between the heat source and the ADT7311
must be considered.
When the thermal impedance is determined, the temperature
of the heat source can be inferred from the ADT7311 output.
As much as 60% of the heat transferred from the heat source to
the thermal sensor on the ADT7311 die is discharged via the
copper tracks and the bond pads. Of the pads on the ADT7311,
the GND pad transfers most of the heat. Therefore, to measure
the temperature of a heat source, it is recommended that the
thermal resistance between the ADT7311 GND pad and the
GND of the heat source be reduced as much as possible.
QUICK GUIDE TO MEASURING TEMPERATURE
1. After power-up, reset the serial interface (load 32 consecutive
1s on DIN). This ensures all internal circuitary 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, proceed to read the configuration register (0x01),
T
CRIT
(0x04), T
HIGH
(0x06), and T
LOW
(0x07). Compare to
the specified defaults in Table 6 . If all the readings match,
the interface is operational.
4. Write to the configuration register to set theADT7311 to
the desired configuration. Read the temperature value
register. It should produce a valid temperature measurement.
