AN-876 One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com AN-876 Replacing National’s LM75 with the Analog Devices, Inc. ADT75 by Donal McNamara INTRODUCTION The ADT75 has two grades, the ADT75A and the ADT75B. The ADT75B is a higher temperature accurate part, and the ADT75A is designed as a second source for the LM75. The ADT75 is a functional and pin- and package-compatible replacement for the LM75. No software or hardware change is required when replacing the LM75 with the ADT75. TEMPERATURE ERROR PERFORMANCE OVER TEMPERATURE The performance capabilities of the ADT75A grade and LM75 over temperature are completely compatible. Both parts have similar performance graphs (see Figure 3). Note that if more accuracy is required, the ADT75B grade is recommended. 1.0 OS/ALERT 3 TOP VIEW GND 4 (Not to Scale) 8 VDD 0.8 7 A0 0.6 6 A1 5 A2 Figure 1. ADT75 Pin Configuration 8 +VS SCL 2 7 A0 O.S. 3 6 A1 GND 4 5 A2 VDD = 3.3V 0.2 VDD = 5V 0 –0.2 –0.4 –0.6 06439-002 SDA 1 0.4 –0.8 –1.0 –55 Figure 2. LM75 Pin Configuration –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 06439-003 ADT75 TEMPERATURE ERROR (°C) SCL 2 06439-001 SDA 1 Figure 3. ADT75 and LM75—Typical Temperature Accuracy COMPATIBILITY Table 1. Feature Comparison of the ADT75 and the LM75 Feature I2C Interface I²C Bus Address 8-Lead MSOP and 8-Lead SOIC Identical Register Addresses1 Identical Register Content Identical Temperature Accuracy 9-Bit Resolution 3.3 V and 5 V Operation Similar AC/DC Parameters Identical OS and Configuration Functions Twos Complement Coding 1 ADT75A Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes LM75 Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes The ADT75 and the LM75 have identical register content. The ADT75 contains additional register bits for one shot mode, SMBus alert enable, and extra temperature resolution bits. The contents of all these additional registers can be ignored without having to modify LM75 software. Rev. 0 | Page 1 of 4 AN-876 OVERTEMPERATURE MODES • The ADT75 powers up in default as an OS pin. The LM75 also powers up in default as an OS pin. • The comparator mode is the default overtemperature power-up mode for both the ADT75 and the LM75. • Both parts can be programmed to have the OS pin be either active low or active high. Both parts follow the interrupt response shown in Figure 4. Note that there is an additional feature on the ADT75 that is not available on the LM75. This is the SMBus alert function. If the user chooses to employ this feature on the ADT75, Bit 7 of the configuration register must be set to enable. TEMPERATURE 82°C 81°C TOS 80°C 79°C 78°C 77°C 76°C THYST 75°C 74°C 73°C OS/ALERT PIN (COMPARATOR MODE) POLARITY = ACTIVE LOW OS/ALERT PIN (INTERRUPT MODE) POLARITY = ACTIVE LOW OS/ALERT PIN (COMPARATOR MODE) POLARITY = ACTIVE HIGH OS/ALERT PIN (INTERRUPT MODE) POLARITY = ACTIVE HIGH TIME WRITE TO 0x04 REG. 2 READ1 WRITE TO 0x04 REG. 2 READ1 WRITE TO 0x04 REG. 2 1READ FROM ANY REGISTER. 2THERE IS A 60ms DELAY BETWEEN WRITING TO THE ONE-SHOT REGISTER AND THE OS/ALERT PIN GOING ACTIVE. THIS IS DUE TO THE CONVERSION TIME. Figure 4. OS/ALERT Output Temperature Response Diagram Rev. 0 | Page 2 of 4 06439-004 READ1 AN-876 DIFFERENCES BETWEEN THE ADT75 AND THE LM75 The ADT75 has two grades, the ADT75A and the ADT75B. The ADT75B is a higher temperature-accurate part. The ADT75A is designed as a second source for the LM75. There are small differences between the ADT75A and the LM75 that do not affect the capability of either part being used in the same socket. The ADT75A differences are an enhancement on the LM75 and are listed in Table 2. Table 2. Differences Between the ADT75A and the LM75 Feature VDD Data Sheet Specifications at VDD Temperature Range Temperature Accuracy Resolution (Bits) Temperature Conversion Time (Maximum) Supply Current (I2C Inactive) LM75 Conditions 3.3 V only and 5 V only 3.3 V and 5 V −55°C to +125°C TA = −55°C to +125°C ±3°C maximum Comments ADT75A Conditions 3.0 V to 5.5 V Comments TA = +100°C to +125°C 8, 9 100 ms LM75B @ 3.3 V and 5 V LM75C @ 3.3 V and 5 V Shutdown Current (3.3 V) Shutdown Current (5 V) One-Shot Register Not present on LM75 500 μA 1 mA 4 μA typical 6 μA typical N/A SMBus Alert Not present on LM75 N/A VDD = 3.3 V VDD = 5 V Typical = 3 μA Typical = 5.5 μA Present on ADT75; 1 current consumption @ 3.3 V Setting Bit D7 in the configuration register enables SMBus alert function, if required 3 V to 5.5 V −55°C to +125°C 4.5 V to 5.5 V, ±2°C typical 8, 9, 10, 11, 12 60 ms 500 μA 525 μA 8 μA maximum 12 μA maximum 78.6 μW typical 1 1 Specially designed to reduce power consumption. The temperature accuracy specifications of the ADT75B are far better than those of the LM75, as shown in Table 3. Table 3. Comparison of Temperature Accuracy Specifications for the LM75 and the ADT75B LM75 Temperature Range TA = −25°C to +100°C TA = −55°C to +120°C ADT75B Accuracy ±2°C maximum ±3°C maximum Temperature Range TA = 0°C to 70°C TA = −25°C to +100°C TA = −55°C to +100°C TA = 100°C to 125°C @ 3 V to 3.6 V VDD Rev. 0 | Page 3 of 4 Accuracy ±1°C maximum ±2°C maximum ±3°C maximum ±3°C maximum AN-876 ONE-SHOT REGISTER One-shot mode is a feature of the ADT75. It is not available in the LM75. LM75 software has no effect on this register and, therefore, does not need to be modified. This register is specifically designed to minimize power consumption. See the ADT75 data sheet for more information. When one-shot mode is active, the following sequence of events occurs: 1. 2. 3. 4. The ADT75 is powered down. 0x04 is written to the address pointer register. A conversion is performed The part returns to power-down. Table 4. ADT75 Registers Pointer Address 0x00 0x01 0x02 0x03 0x04 Register Name Temperature Value Configuration THYST TOS Setpoint One Shot ©2006 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN06439-0-11/06(0) Rev. 0 | Page 4 of 4 Power-On Default 0x00 0x00 0x4B00 (75°C) 0x5000 (80°C) 0xXX