Datasheet ADT7420 (Analog Devices) - 23

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Data Sheet ADT7420 APPLICATIONS INFORMATION THERMAL RESPONSE TIME POWERING FROM A SWITCHING REGULATOR
Thermal response is a function of the thermal mass of the Precision analog devices, such as the ADT7420 require a well- temperature sensor, but it is also heavily influenced by the mass filtered power source. If the ADT7420 is powered from a of the object the IC is mounted to. For example, a large PCB switching regulator, noise may be generated above 50 kHz that containing large amounts of copper tracking can act as a large may affect the temperature accuracy specifications. To prevent heat sink and slow the thermal response. For a faster thermal this, an RC filter should be used between the power supply and response, it is recommended to mount the sensor on as small a ADT7420 VDD. The value of the components used should be PCB as possible. carefully considered to ensure that the peak value of the supply Figure 10 shows the typical response time of less than two noise is less than 1 mV. The RC filter should be mounted as far seconds to reach 63.2% of its temperature span. The tempera- away as possible from the ADT7420 to ensure that the thermal ture value is read back as a 16-bit value through the digital mass is kept as low as possible. interface. The response time includes all delays incurred on
TEMPERATURE MEASUREMENT
chip during signal processing. The ADT7420 accurately measures and converts the tempera-
SUPPLY DECOUPLING
ture at the surface of its own semiconductor chip. Thermal The ADT7420 must have a decoupling capacitor connected paths run through the leads, the exposed pad, as well as the between V plastic package. When the ADT7420 is used to measure the DD and GND; otherwise, incorrect temperature readings will be obtained. A 0.1 μF decoupling capacitor such as temperature of a nearby heat source, the thermal impedance a high frequency ceramic type must be used and mounted as between the heat source and the ADT7420 must be considered close as possible to the V because this impacts the accuracy and thermal response of the DD pin of the ADT7420. measurement. If possible, the ADT7420 should be powered directly from the system power supply. This arrangement, shown in Figure 20, For air or surface temperature measurements, take care to isolates the analog section from the logic-switching transients. isolate the package, leads, and exposed pad from ambient air Even if a separate power supply trace is not available, generous temperature. Use of a thermally conductive adhesive can help supply bypassing reduces supply-line induced errors. Local to achieve a more accurate surface temperature measurement. supply bypassing consisting of a 0.1 μF ceramic capacitor is
QUICK GUIDE TO MEASURING TEMPERATURE
critical for the temperature accuracy specifications to be The following is a quick guide for measuring temperature in achieved. continuous conversion mode (default power-up mode). Execute each step sequentially.
TTL/CMOS LOGIC CIRCUITS 0.1µF ADT7420
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
POWER
Step 1, proceed to read the configuration register (0x03), 22 0
SUPPLY
3- 01 TCRIT (0x08, 0x09), THIGH (0x04, 0x05), and TLOW (0x06, 09 Figure 20. Use of Separate Traces to Reduce Power Supply Noise 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. Rev. A | Page 23 of 24 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS I2C TIMING SPECIFICATIONS Timing Diagram ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER DETAILS NORMAL MODE ONE-SHOT MODE CT and INT Operation in One-Shot Mode 1 SPS MODE SHUTDOWN FAULT QUEUE TEMPERATURE DATA FORMAT TEMPERATURE CONVERSION FORMULAS 16-Bit Temperature Data Format 13-Bit Temperature Data Format 10-Bit Temperature Data Format 9-Bit Temperature Data Format REGISTERS ADDRESS POINTER REGISTER TEMPERATURE VALUE REGISTERS STATUS REGISTER CONFIGURATION REGISTER THIGH SETPOINT REGISTERS TLOW SETPOINT REGISTERS TCRIT SETPOINT REGISTERS THYST SETPOINT REGISTER ID REGISTER SERIAL INTERFACE SERIAL BUS ADDRESS WRITING DATA READING DATA RESET GENERAL CALL INT AND CT OUTPUTS UNDERTEMPERATURE AND OVERTEMPERATURE DETECTION Comparator Mode Interrupt Mode APPLICATIONS INFORMATION THERMAL RESPONSE TIME SUPPLY DECOUPLING POWERING FROM A SWITCHING REGULATOR TEMPERATURE MEASUREMENT QUICK GUIDE TO MEASURING TEMPERATURE OUTLINE DIMENSIONS ORDERING GUIDE