19-4385; Rev 0; 3/09 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface The LM75 temperature sensor includes a delta-sigma analog-to-digital converter, and a digital overtemperature detector. The host can query the LM75 through its I 2 C interface to read temperature at any time. The open-drain overtemperature output (OS) sinks current when the programmable temperature limit is exceeded. The OS output operates in either of two modes, comparator or interrupt. The host controls the temperature at which the alarm is asserted (TOS) and the hysteresis temperature below which the alarm condition is not valid (THYST). Also, the LM75’s TOS and THYST registers can be read by the host. The address of the LM75 is set with three pins to allow multiple devices to work on the same bus. Power-up is in comparator mode, with defaults of TOS = +80°C and THYST = +75°C. The 3.0V to 5.5V supply voltage range, low supply current, and I2C interface make the LM75 ideal for many applications in thermal management and protection. Applications Features o SO (SOP) and µMAX® (µSOP) Packages o I2C Bus Interface o Separate Open-Drain OS Output Operates as Interrupt or Comparator/Thermostat Input o Register Readback Capability o Power-Up Defaults Permit Stand-Alone Operation as a Thermostat o 3.0V to 5.5V Supply Voltage o Low Operating Supply Current 250µA (typ), 1mA (max) o 4µA (typ) Shutdown Mode Minimizes Power Consumption o Up to Eight LM75s Can Be Connected to a Single Bus o Pin- and/or Register-Compatible with ImprovedPerformance Maxim Sensors Including MAX7500, MAX6625, MAX6626, DS75LV, and DS7505 Thermal System Management Thermal Protection Test Equipment Computers and Office Electronics Functional Diagram +VS = 3.0V to 5.5V 8 3 16 SILICON BANDGAP TEMPERATURE SENSOR µMAX is a registered trademark of Maxim Integrated Products, Inc. 9-BIT DELTASIGMA ADC 16 TOS SET POINT REGISTER Pin Configuration TOP VIEW SDA 1 SCL 2 OS GND 3 4 POINTER REGISTER CONFIGURATION REGISTER + LM75 8 +VS 7 A0 6 5 8 A2 16 8 THYST SET POINT REGISTER 16 16 1 7 A0 6 A1 5 A2 A1 OS SET POINT COMPARATOR W/ HYSTERESIS SDA 2-WIRE INTERFACE 2 SCL 4 μMAX (μSOP), SO Ordering Information/Selector Guide PIN-PACKAGE PKG SUPPLY VOLTAGE (V) TOP MARK LM75BIM-3+ PART 8 SO (SOP) Bulk 3.3 LM75BIM-3 LM75BIMX-3+ 8 SO (SOP) T&R 3.3 LM75BIM-3 LM75BIMM-3+ 8 µMAX (µSOP) Bulk 3.3 T01B LM75BIMMX-3+ 8 µMAX (µSOP) T&R 3.3 T01B LM75BIM-5+ 8 SO (SOP) Bulk 5.0 LM75BIM-5 LM75BIMX-5+ 8 SO (SOP) T&R 5.0 LM75BIM-5 LM75BIMM-5+ 8 µMAX (µSOP) Bulk 5.0 T00B LM75BIMMX-5+ 8 µMAX (µSOP) T&R 5.0 T00B Note: Devices are specified over the -55°C to +125°C temperature range and include I2C noise filter. +Denotes a lead(Pb)-free/RoHS-compliant package. T&R = Tape and reel. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 LM75 General Description LM75 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface ABSOLUTE MAXIMUM RATINGS (Note 1) +VS to GND ...........................................................-0.3V to +6.0V OS, SDA, SCL to GND...........................................-0.3V to +6.0V All Other Pins to GND .................................-0.3V to (+VS + 0.3V) Input Current at Any Pin (Note 2)..........................................5mA Package Input Current (Note 2)..........................................20mA OS Output Sink Current ......................................................10mA Continuous Power Dissipation (TA = +70°C) (Note 3) 8-Pin µMAX (µSOP) (derate 4.5mW/°C above +70°C) ..................................362mW 8-Pin SO (SOP) (derate 5.9mW/°C above +70°C) ........471mW Junction-to-Case Thermal Resistance (θJC) (Note 3) 8-Pin µMAX (µSOP) .......................................................42°C/W 8-Pin SO (SOP)..............................................................40°C/W Junction-to-Ambient Thermal Resistance (θJA) (Note 3) 8-Pin µMAX (µSOP) .....................................................221°C/W 8-Pin SO (SOP)............................................................170°C/W ESD Protection Human Body Model (RD = 1.5kΩ, CS = 100pF) All Pins to GND .................................................................±2kV Operating Temperature Range .........................-55°C to +125°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: When the input voltage (VI) at any pin exceeds the Absolute Maximum limits (VI < GND, VI > 6V or VI > +VS), the current at that pin should be limited to 5mA. The 20mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 5mA to four. Note 3: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a singlelayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (+VS = +3.0V to +5.5V, unless otherwise noted. Temperature accuracy specifications apply for +VS = 3.3V for versions with “-3” in the suffix and for +VS = 5V for versions with “-5” in the suffix. TA = -55°C to +125°C, unless otherwise noted. Typical values are at +VS = +5V, TA = +25°C.) (Notes 4, 5) PARAMETER SYMBOL Accuracy (6 σ) Accuracy (3 σ) (Note 6) CONDITIONS MIN MAX -2.0 +2.0 -55°C ≤ TA ≤ +125°C -3.0 +3.0 -25°C ≤ TA ≤ +100°C -1.5 +1.5 -55°C ≤ TA ≤ +125°C -2.0 +2.0 Resolution 9 Temperature Conversion Time Quiescent Supply Current Bits ms I2C inactive 0.25 0.5 mA Shutdown mode, +VS = 3V 4 Shutdown mode, +VS = 5V 6 (Note 9) tOF °C 300 µA 5.5 IOUT = 4.0mA (Note 8) OS Delay °C 100 3.0 OS Output Saturation Voltage UNITS (Note 7) +VS Supply Voltage Range OS Output Fall Time TYP -25°C ≤ TA ≤ +100°C 1 CL = 400pF, IO = 3mA (Note 10) V 0.8 V 6 Conversions 250 ns TOS Default Temperature (Note 11) 80 °C THYST Default Temperature (Note 11) 75 °C 2 _______________________________________________________________________________________ Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface (+VS = +3.0V to +5.5V, unless otherwise noted. Temperature accuracy specifications apply for +VS = 3.3V for versions with “-3” in the suffix and for +VS = 5V for versions with “-5” in the suffix. TA = -55°C to +125°C, unless otherwise noted. Typical values are at +VS = +5V, TA = +25°C.) (Notes 4, 5) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS LOGIC (SDA, SCL, A0, A1, A2) Input High Voltage VIH +VS x 0.7 +VS + 0.5 V Input Low Voltage VIL -0.3 +VS x 0.3 V Input High Current IIH VIN = 5V 1.0 µA Input Low Current IIL VIN = 0V Input Capacitance CIN 0.005 -1.0 All digital inputs -0.005 µA 20 pF Output High Current VOH = 5V 10 µA Output Low Voltage IOL = 3mA 0.4 V I2C-COMPATIBLE TIMING (Notes 12, 13) (Clock) SCL Period Bus timeout inactive 2.5 µs Data In Setup Time to SCL High tSU:DAT 10% of SDA to 10% of SCL 100 ns Data Out Stable After SCL Low tHD:DAT 10% of SCL to 10% of SDA 0 µs Start Condition Setup Time (SDA Low to SCL Low) tSU:STA 90% of SCL to 90% of SDA 100 ns STOP Condition Hold Time tHD:STO 100 ns SDA Time Low for Reset of Serial Interface tTIMEOUT Note 4: Note 5: Note 6: Note 7: Note 8: Note 9: Note 10: Note 11: Note 12: Note 13: Note 14: tSCL (Note 14) 75 325 ms All parts operate properly over the 3V to 5.5V supply voltage range. The devices are tested and specified for rated accuracy at their nominal supply voltage. All parameters are measured at TA = +25°C. Values over the temperature range are guaranteed by design. There is no industry-wide standard for temperature accuracy specifications. Maxim’s standard is 6-σ. The 3-σ specification is included to allow easier comparison to products built by manufacturers who use different standards. This specification indicates how often temperature data is updated. The devices can be read at any time without regard to conversion state, while yielding the last conversion result. For best accuracy, minimize output loading. Higher sink currents can affect sensor accuracy due to internal heating. OS delay is user programmable up to 6 over-limit conversions before OS is set to minimize false tripping in noisy environments. Guaranteed by design. Default values set at power-up. All timing specifications are guaranteed by design. Unless otherwise noted, these specifications apply for +VS = +5VDC for LIM75BIM-5 and LIM75BIMM-5 and +VS = +3.3VDC for LIM75BIM-3 and LIM75BIMM-3. CL (load capacitance) on output lines = 80pF, unless otherwise specified. The switching characteristics of the LM75 fully meet or exceed the published specifications of the I2C bus. These parameters are the timing relationships between SCL and SDA signals related to the LM75. They are not I2C bus specifications. Holding the SDA line low for a time greater than tTIMEOUT causes the device to reset SDA to the IDLE state of the serial bus communication (SDA set high). _______________________________________________________________________________________ 3 LM75 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) +VS = +5V 270 260 +VS = +3V 250 240 230 5 1.0 +VS = +5V 4 3 +VS = +3V 2 -25 5 35 65 95 TEMPERATURE (°C) 125 0.5 0 -0.5 -1.0 1 -1.5 0 -55 4 TYPICAL PARTS 1.5 ACCURACY (°C) 280 ACCURACY vs. TEMPERATURE 2.0 LM75 toc02 290 6 SHUTDOWN SUPPLY CURRENT (μA) LM75 toc01 300 SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE LM75 toc03 QUIESCENT SUPPLY CURRENT vs. TEMPERATURE QUIESCENT SUPPLY CURRENT (μA) LM75 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface -2.0 -55 -25 5 35 65 95 125 -55 -25 5 TEMPERATURE (°C) 35 65 95 125 TEMPERATURE (°C) Pin Description 4 PIN NAME 1 SDA FUNCTION 2 SCL Serial-Data Clock Input. Open drain. Connect SCL to a pullup resistor. 3 OS Overtemperature Shutdown Output. Open drain. Connect OS to a pullup resistor. 4 GND 5 A2 2-Wire Interface Address Input. Connect A2 to GND or +VS to set the desired I2C bus address. Do not leave unconnected (see Table 1). 6 A1 2-Wire Interface Address Input. Connect A1 to GND or +VS to set the desired I2C bus address. Do not leave unconnected (see Table 1). 7 A0 2-Wire Interface Address Input. Connect A0 to GND or +VS to set the desired I2C bus address. Do not leave unconnected (see Table 1). 8 +VS Positive Supply Voltage Input. Bypass to GND with a 0.1µF bypass capacitor. Serial-Data Input/Output Line. Open drain. Connect SDA to a pullup resistor. Ground _______________________________________________________________________________________ Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface The LM75 temperature sensor measures temperature and converts the data into digital form using a bandgap type temperature sensor and a 9-bit sigma-delta ADC. An I2C-compatible 2-wire serial interface allows access to conversion results. The LM75 accepts standard I2C commands to read the data, set the overtemperature alarm (OS) trip thresholds, and configure other characteristics. While reading the Temperature register, any changes in temperature are ignored until the read is completed. The Temperature register is updated for the new temperature measurement upon completion of the read operation. OS Output, TOS, and THYST Limits In comparator mode (see Figure 1), the open-drain OS output asserts when the temperature rises above the limit programmed into the TOS register, and becomes high impedance when the temperature falls below the limit set in the THYST register. In this mode the LM75 operates as a thermostat, and the OS output can be used to take action to reduce the temperature (e.g., turn on a cooling fan, reduce clock speed, or shut down the system). In interrupt mode, exceeding TOS also asserts OS. OS remains asserted until a read operation is performed on any of the registers. Once OS has asserted due to crossing above TOS and is then reset, it is asserted again only when the temperature drops below THYST. The output then remains asserted until it is reset by a read. It is then asserted again if the temperature rises above TOS, and so on. Putting the LM75 into shutdown mode also resets OS. Power-Up and Power-Down The LM75 powers up to a known state, as indicated in Table 2. Some of these settings are summarized as following: • Comparator mode • TOS = +80°C • THYST = +75°C • OS active low • Command byte pointer = 0x00 I2C-Compatible Bus Interface From a software perspective, the LM75 appears as a set of byte-wide registers that contain temperature data, alarm threshold values, and control bits. A standard I2Ccompatible, 2-wire serial interface reads temperature data and writes control bits and alarm threshold data. Each device responds to its own I2C slave address, which is selected using A0, A1, and A2. See Table 1. TOS TEMPERATURE THYST OS OUTPUT (COMPARATOR MODE) OS SET ACTIVE LOW OS OUTPUT (INTERRUPT MODE) OS SET ACTIVE LOW READ OPERATION READ OPERATION READ OPERATION Figure 1. OS Output Temperature Response Diagram Table 1. Slave Address BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 1 0 0 1 A2 A1 A0 RD/W Table 2. Register Functions ADDRESS (hex) POR STATE (hex) POR STATE (binary) POR STATE (°C) READ/ WRITE Temperature 00 000X 0000 0000 0XXX XXXX — Read only Configuration 01 00 0000 0000 — R/W THYST 02 4B0X 0100 1011 0XXX XXXX 75 R/W TOS 03 500X 0101 0000 0XXX XXXX 80 R/W REGISTER NAME X = Don’t care. _______________________________________________________________________________________ 5 LM75 Detailed Description LM75 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface SDA tBUF tSU:DAT tSU:STA tSU:STO tHD:DAT tLOW tHD:STA SCL tHIGH tHD:STA tR tF START CONDITION (S) ACKNOWLEDGE (A) REPEATED START CONDITION (SR) STOP CONDITION (P) START CONDITION (S) PARAMETERS ARE MEASURED FROM 10% TO 90%. Figure 2. Serial Bus Timing Table 3. Temperature, THYST, and TOS Register Definition UPPER BYTE D15 Sign bit 1= Negative 0 = Positive LOWER BYTE D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 MSB 64°C 32°C 16°C 8°C 4°C 2°C 1°C LSB 0.5°C X X X X X X X X = Don’t care. Table 4. Temperature Data Output Format DIGITAL OUTPUT 0111 1101 0 BINARY HEX +125 0111 1101 0XXX XXXX 7D0X +25 0001 1001 0XXX XXXX 190X +0.5 0000 0000 1XXX XXXX 008X 0 0000 0000 0XXX XXXX 000X -0.5 1111 1111 1XXX XXXX FF8X -25 1110 0111 0XXX XXXX E70X 1100 1001 0XXX XXXX C90X -55 X = Don’t care. BINARY OUTPUT CODE (9MSB) TEMPERATURE (°C) 0001 1001 0 0000 0000 1 0000 0000 0 1111 1111 1 1110 0111 0 1100 1001 0 -55 -25 -0.5 0 +0.5 LOCAL TEMPERATURE ( C) +25 Figure 3. Temperature-to-Digital Transfer Function 6 _______________________________________________________________________________________ +125 START BY MASTER START BY MASTER START BY MASTER ADDRESS BYTE REPEAT START BY MASTER ADDRESS BYTE ADDRESS BYTE MOST SIGNIFICANT DATA BYTE ACK BY MASTER ACK BY LM75 POINTER BYTE MOST SIGNIFICANT DATA BYTE ACK BY LM75 ACK BY MASTER LEAST SIGNIFICANT DATA BYTE DATA BYTE NO ACK BY MASTER (c) TYPICAL 1-BYTE READ FROM CONFIGURATION REGISTER WITH PRESET POINTER. ACK BY LM75 STOP COND BY MASTER (b) TYPICAL POINTER SET FOLLOWED BY IMMEDIATE READ FOR 2-BYTE REGISTER SUCH AS TEMP, THIGH, TLOW. ADDRESS BYTE ACK BY MASTER (a) TYPICAL 2-BYTE READ FROM PRESET POINTER LOCATION SUCH AS TEMP, THIGH, TLOW. ACK BY LM75 LEAST SIGNIFICANT DATA BYTE NO ACK BY MASTER NO ACK BY MASTER STOP COND BY MASTER LM75 STOP COND BY MASTER Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface Figure 4. I2C-Compatible Timing Diagram (Read) _______________________________________________________________________________________ 7 8 START BY MASTER START BY MASTER START BY MASTER ADDRESS BYTE ADDRESS BYTE ADDRESS BYTE ACK BY LM75 ACK BY LM75 ACK BY LM75 ACK BY LM75 REPEAT START BY MASTER ADDRESS BYTE POINTER BYTE POINTER BYTE CONFIGURATION BYTE MOST SIGNIFICANT DATA BYTE (c) THIGH AND TLOW WRITE ACK BY LM75 (b) CONFIGURATION REGISTER WRITE ACK BY LM75 STOP COND BY MASTER ACK BY LM75 ACK BY LM75 ACK BY LM75 (a) TYPICAL POINTER SET FOLLOWED BY IMMEDIATE READ FROM CONFIGURATION REGISTER POINTER BYTE LEAST SIGNIFICANT DATA BYTE DATA BYTE ACK BY LM75 STOP COND BY MASTER NO ACK BY MASTER STOP COND BY MASTER LM75 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface Figure 5. I2C-Compatible Timing Diagram (Write) _______________________________________________________________________________________ Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface Shutdown Set bit D0 in the Configuration register to 1 to place the LM75 in shutdown mode and reduce supply current to 4µA. In interrupt mode, entering shutdown resets the OS output. While in shutdown, the I2C remains active and T OS and T HYST limit registers along with the Configuration register remain accessible to the master. Fault Queue The fault queue prevents OS false tripping in noisy environments. The number of faults set in the queue (up to 6) must occur to trip the OS output. Comparator/Interrupt Mode The events that trigger OS are identical between comparator and interrupt modes. In comparator mode, OS is asserted when the temperature rises above the TOS value. OS is deasserted when the temperature drops below the THYST value. In interrupt mode, OS is asserted when the temperature rises above the TOS value or falls below the THYST value. OS is deasserted only after performing a read operation. OS Output The OS output is an open-drain output without an internal pullup. Connect a pullup resistor from OS to +VS. Using larger resistance values reduces any temperature errors due to self heating from current entering OS. OS Polarity The OS polarity can be programmed for active-low or active-high operation. In active-low operation, OS goes low when triggered by a temperature event. Internal Registers The LM75’s Pointer register selects between four data registers (see Figure 6). At power-up, the pointer is set to read the Temperature register at address 0x00. The Pointer register latches the last location to which it was set. All registers are read and write, except the Temperature register, which is read only. Write to the Configuration register by writing an address byte, a data pointer byte, and a data byte. If 2 data bytes are written, the second data byte overrides the first. The TOS and THYST registers require 1 address byte, 1 pointer byte, and 2 data bytes. If only 1 data byte is written, it is saved in bits D15–D8 of the respective register. If more than 2 data bytes are written, only the first 2 bytes are recognized while the remaining bytes are ignored. Read from the LM75 in one of two ways. If the location latched in the Pointer register is set from the previous read, the new read consists of an address byte, followed by retrieving the corresponding number of data bytes. If the Pointer register needs to be set to a new address, perform a read operation by writing an address byte, pointer byte, repeat start, and another address byte. An inadvertent 8-bit read from a 16-bit register, with the D7 bit low, can cause the device to stop in a state where the SDA line is held low. Ordinarily, this would prevent any further bus communication until the master sends nine additional clock cycles or SDA goes high. At that time, a stop condition resets the device. If the additional clock cycles are not generated by the master, the LM75 bus resets and unlocks after the bus timeout period has elapsed. +VS A0 A1 A2/RESET LM75 SDA SMBus™ INTERFACE BLOCK SCL DATA OS ADDRESS POINTER REGISTER (SELECTS REGISTER FOR COMMUNICATION) REGISTER SELECT TEMPERATURE (READ ONLY) POINTER = 0000 0000 CONFIGURATION (READ/WRITE) POINTER = 0000 0001 TOS SET POINT (READ/WRITE) POINTER = 0000 0011 THYST SET POINT (READ/WRITE) POINTER = 0000 0010 GND Figure 6. Block Diagram SMBus is a trademark of Intel Corp. _______________________________________________________________________________________ 9 LM75 Temperature Data Format Temperature data is stored in the Temperature, TOS Set Point, and THYST Set Point registers. The temperature data format is 9 bits, two’s complement, and the register is read out in 2 bytes: an upper byte and a lower byte. Bits D15–D7 contain the temperature data, with the LSB representing 0.5°C and the MSB representing the sign bit (see Table 3). The MSB is transmitted first. The last 7 bits of the lower byte, bits D6–D0, are don’t cares. LM75 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface Table 5. Configuration Register Definition D7 D6 D5 D4 D3 D2 D1 D0 0 0 0 Fault Queue Fault Queue OS Polarity Comparator/ Interrupt Shutdown Configuration Register The 8-bit Configuration register sets the fault queue, OS polarity, shutdown control, and whether the OS output functions in comparator or interrupt mode. When writing to the Configuration register, set bits D7, D6, and D5 to zero (see Table 5). Bits D4 and D3, the fault queue bits, determine the number of faults necessary to trigger an OS condition (see Table 6). The number of faults set in the queue must occur consecutively to trip the OS output. The fault queue prevents OS false tripping in noisy environments. Set bit D2, the OS polarity bit, to zero to force the OS output polarity to active low. Set bit D2 to 1 to set the OS output polarity to active high. OS is an open-drain output under all conditions and requires a pullup resistor to output a high voltage (see Figure 1). Set bit D1, the comparator/interrupt bit to zero to operate OS in comparator mode. In comparator mode, OS is asserted when the temperature rises above the TOS value. OS is deasserted when the temperature drops below the THYST value (see Figure 1). Set bit D1 to 1 to operate OS in interrupt mode. OS is asserted in interrupt mode when the temperature rises above the TOS value or falls below the THYST value. OS is deasserted only after performing a read operation. Set bit D0, the shutdown bit, to zero for normal operation. Set bit D0 to 1 to shutdown the LM75’s internal blocks. The I2C interface remains active as long as the shutdown bit is set. The TOS, THYST, and Configuration registers can still be written to and read from while in shutdown. Applications Information LM75 measures the temperature of its own die. The thermal path between the die and the outside world determines the accuracy of temperature measurements. Most of the heat flows in to or out of the die 10 Table 6. Configuration Register Fault Queue Bits D4 D3 NUMBER OF FAULTS 0 0 1 (POR state) 0 1 2 1 0 4 1 1 6 through the leads. Because of this, the LM75 most easily measures the PCB temperature. For ambient temperature measurements, mount the LM75 on a separate PCB away from high power sources. Temperature errors due to self heating of the LM75 die is minimal due to the low supply current. Digital Noise Issues The lowpass filters in the SCL and SDA digital lines mitigate the effects of bus noise, and make communications in noisy environments more robust. Good layout practices also help. Keep switching power supplies away from digital lines, and arrange for high-speed digital traces to cross SCL and SDA at right angles. Properly terminate long PCB traces and bus traces connected to multiple slaves. Serial bus no-acknowledge (which causes unnecessary bus traffic) is the most common symptom of excessive noise coupling into the SDA and SCL lines. Noise with amplitude greater than the LM75’s hysteresis (400mVP-P, typ), overshoot greater than 300mV above +VS, and undershoot more than 300mV below GND may prevent successful serial communication. Resistance can be added in series with the SDA and SCL lines to help filter noise and ringing. A 5kΩ resistor placed in series with the SCL line and as close as possible to the SCL pin, with the 5pF to 10pF stray capacitance of the device, provides a 6MHz to 12MHz lowpass filter, which is sufficient filtering in many cases. ______________________________________________________________________________________ Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface LM75 3V to 5.5V 3V to 5.5V 12V R3 10kΩ R1 10kΩ R2 10kΩ R3 10kΩ +VS GND or +V SCL A0 A1 A2 OS N +Vs LM75 SCL SDA SDA 12V 300mA FAN MOTOR R2 10kΩ OS LM75 GND TO Microcontroller GND OS Figure 7. I2C Controlled Temperature Sensor 3V to 5.5V C1 0.1μF Figure 8. Fan Control SHDN OUT- BIAS GND MAX4364 IN+ R1 10kΩ OUT+ INC2 0.1μF +Vs R5 200kΩ C4 6.8nF C3 6.8nF LM75 GND VCC R2 10kΩ R3 10kΩ C5 6.8nF R4 10kΩ Figure 9. Temperature Sensor with Audible Alarm ______________________________________________________________________________________ 11 LM75 Digital Temperature Sensor and Thermal Watchdog with 2-Wire Interface Package Information Chip Information PROCESS: CMOS For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 8 SO (SOP) S8-2 21-0041 8 µMAX (µSOP) U8-1 21-0036 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.