LM91 Diode Input Digital Temperature Sensor with Two-Wire Interface General Description The LM91 is a remote diode temperature sensor, Delta-Sigma analog-to-digital converter, and digital over-temperature detector with an SMBus™ interface. The LM91 senses its own temperature as well as the temperature of a target IC with a diode junction, such as a Pentium ® II processor or a diode connected 2N3904. A diode junction (semiconductor junction) is required on the target IC’s die. A host can query the LM91 at any time to read the temperature of this diode as well as the temperature state of the LM91 itself. A T_CRIT_A interrupt output becomes active when the temperature is greater than a programmable comparator limit, T_CRIT. The host can program as well as read back the state of the T_CRIT register. Power up default values for T_CRIT are as shown in the ordering information table. Three state logic inputs allow two pins (ADD0, ADD1) to select up to 9 SMBus address locations for the LM91. Features n Directly senses die temperature of remote ICs n Senses temperature of remote diodes n SMBus compatible interface, supports SMBus Timeout n Register readback capability n 7 bit plus sign temperature data format n 2 address select lines enable 9 LM91s to be connected to a single bus Key Specifications j Supply Voltage 3.0V - 3.6V j Supply Current 1 mA (max) j Local Temperature Accuracy ± 1.0˚C (typ) j Remote Diode Temperature Accuracy ± 4˚C (max) ± 6˚C (max) +60˚C to +100˚C 0˚C to +125˚C Applications n n n n n System Thermal Management Personal Computers Electronic Test Equipment Office Electronics HVAC Simplified Block Diagram 20034301 # Indicates Active Low (”NOT“) SMBus™ is a trademark of the Intel Corporation. Pentium ® II processor is a registered trademark of the Intel Corporation. I2C ® is a registered trademark of the Philips Corporation. © 2001 National Semiconductor Corporation DS200343 www.national.com LM91 Diode Input Digital Temperature Sensor with Two-Wire Interface November 2001 LM91 Connection Diagram QSOP-16 20034302 TOP VIEW Ordering Information SMBus Revision Level Noise Filter on SMBCLK Remote Diode Accuracy Local T_CRIT Default Remote T_CRIT Default 95 Units in Rail 1.1 20MHz ± 4˚C 127˚C 95˚C 2500 Units on Tape and Reel 1.1 20MHz ± 4˚C 127˚C 95˚C NS Package Number Transport Media LM91DIMQA MQA16A (QSOP-16) LM91DIMQAX MQA16A (QSOP-16) Order Number www.national.com 2 LM91 Typical Application 20034303 Pin Descriptions Label Pin # Function Typical Connection Manufacturing test pins. NC 1, 5, 9, 13, 16 VCC 2 D+ 3 D− 4 ADD0–ADD1 10, 6 GND 7, 8 Positive Supply Voltage Input Left floating. PC board traces may be routed through the pads for these pins. Although, the components that drive these traces should share the same supply as the LM91 so that the Absolute Maximum Voltage at any Pin rating is not violated. DC Voltage from 3.0V to 3.6V Diode Current Source To Diode Anode. Connected to remote discrete diode or to the diode on the external IC whose die temperature is being sensed. Diode Return Current Sink To Diode Cathode. Must be grounded when not used. User-Set SMBus (I2C) Address Inputs Ground (Low, “0”), VCC (High, “1”) or open (“TRI-LEVEL”) Power Supply Ground Ground 11 Critical Temperature Alarm, open-drain output Pull Up Resistor, Controller Interrupt Line or System Shutdown From and to Controller, Pull Up Resistor SMBData 12 SMBus (I2C) Serial Bi-Directional Data Line, open-drain output SMBCLK 14 SMBus (I2C) Clock Input From Controller No Connection Left floating. PC board traces may be routed through the pads for this pin. T_CRIT_A NC 15 3 www.national.com LM91 Absolute Maximum Ratings Supply Voltage (Note 1) Soldering Information, Lead Temperature QSOP Package (Note 3) −0.3V to 6.0V Voltage at Any Pin: + NC (Pins 1,5,9), ADD0, ADD1, D All other pins (except D−) −0.3V to (VCC + 0.3V) 20 mA SMBData, T_CRIT_A Output Sink Current 10 mA 250V Operating Ratings (Note 1) and (Note 5) TMIN to TMAX Specified Temperature Range Output Voltage LM91D 6.0V Storage Temperature 2500V Machine Model 5 mA Package Input Current (Note 2) 220˚C Human Body Model ± 1 mA Input Current at All Other Pins (Note 2) 215˚C Infrared (15 seconds) ESD Susceptibility (Note 4) −0.3V to 6.0V D− Input Current Vapor Phase (60 seconds) 0˚C to +125˚C Supply Voltage Range (VCC) −65˚C to +150˚C +3.0V to +3.6V Temperature-to-Digital Converter Characteristics Unless otherwise noted, these specifications apply for VCC =+3.0 Vdc to +3.6 Vdc. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ =+25˚C, unless otherwise noted. Parameter Conditions Remote Temperature Error using Diode Connected 2N3904 (Note 8) and (Note 9) LM91D Limits Units (Note 6) (Note 7) (Limit) ±1 Local Temperature Error (Note 8) Remote Temperature Error using Pentium Diode (Note 8) and (Note 9) Typical ˚C +60˚C ≤TA ≤ +100˚C, VCC = 3.3 Vdc ±4 0˚C ≤ TA ≤ +125˚C, VCC = 3.3 Vdc ±6 ˚C (max) +60˚C ≤TA ≤ +100˚C, VCC = 3.3 Vdc +2, −6 ˚C (max) 0˚C ≤ TA ≤ +125˚C, VCC = 3.3 Vdc +4, −8 ˚C (max) Resolution 8 Bits 1 Temperature Conversion Time (Note 11) Quiescent Current (Note 10) SMBus (I2C Inactive) D− Source Voltage Diode Source Current ˚C (max) ˚C 120 145 ms 0.500 1 mA (max) 0.7 V (D+ − D−)=+ 0.65V; high level 160 50 µA (min) Low level 16 µA (max) 5 µA (min) T_CRIT_A Output Saturation Voltage IOUT = 3.0 mA 0.4 V (max) Power-On Reset Threshold On VCC input, falling edge 2.2 1.2 V (max) V (min) www.national.com 4 µA (max) LM91 Logic Electrical Characteristics DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for VCC =+3.0 to 3.6 Vdc. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ =+25˚C, unless otherwise noted. Symbol Parameter Conditions Typical LM91D Units (Note 6) Limits (Note 7) (Limit) SMBData, SMBCLK VIN(1) Logical “1” Input Voltage 2.1 V (min) VIN(0) Logical “0”Input Voltage 0.8 V (max) IIN(1) Logical “1” Input Current VIN = VCC 0.005 1.0 µA (max) IIN(0) Logical “0” Input Current VIN = 0V −0.005 −1.0 µA (max) VCC 1.6 V (min) ADD0, ADD1 VIN(1) Logical “1” Input Voltage VIN(0) Logical “0”Input Voltage GND 0.5 V (max) IIN(1) Logical “1” Input Current VIN = VCC 50 600 µA (max) IIN(0) Logical “0” Input Current VIN = 0V 50 600 µA (max) ALL DIGITAL INPUTS CIN Input Capacitance 20 pF ALL DIGITAL OUTPUTS IOH High Level Output Current VOH = VCC 100 µA (max) VOL SMBus Low Level Output Voltage IOL = 3 mA IOL = 6 mA 0.4 0.6 V (max) SMBus DIGITAL SWITCHING CHARACTERISTICS Unless otherwise noted, these specifications apply for VCC =+3.0 Vdc to +3.6 Vdc, CL (load capacitance) on output lines = 80 pF. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25˚C, unless otherwise noted. The switching characteristics of the LM91 fully meet or exceed the published specifications of the SMBus or I2C bus. The following parameters are the timing relationships between SMBCLK and SMBData signals related to the LM91. They are not necessarily the I2C or SMBus bus specifications. Symbol Parameter fSMB SMBus Clock Frequency tLOW SMBus Clock Low Time Conditions Typical Limits Units (Note 6) (Note 7) (Limit) 400 10 kHz (max) kHz (min) 1.3 25 µs (min) ms (max) 25 ms (max) 0.6 µs (min) 10% to 10% tLOWSEXT Cumulative Clock Low Extend Time tHIGH SMBus Clock High Time 90% to 90% tR;SMB SMBus Rise Time 10% to 90% 1 µs tF;SMB SMBus Fall Time 90% to 10% 0.3 µs tOF Output Fall Time CL = 400 pF IO = 3 mA tTIMEOUT SMBData and SMBCLK Time Low for Reset of Serial Interface (Note 12) 250 ns (max) 25 40 ms (min) ms (max) t1 SMBCLK (Clock) Period 2.5 µs (min) t2, tSU;DAT Data In Setup Time to SMBCLK High 100 ns (min) t 3, tHD;DAT Data Out Stable after SMBCLK Low 0 0.9 ns (min) µs (max) t4, tHD;STA SMBData Low Setup Time to SMBCLK Low 100 ns (min) t 5, tSU;STO SMBData High Delay Time after SMBCLK High (Stop Condition Setup) 100 ns (min) t 6, tSU;STA SMBus Start-Condition Setup Time 0.6 µs (min) 5 www.national.com LM91 SMBus DIGITAL SWITCHING CHARACTERISTICS Unless otherwise noted, these specifications apply for VCC =+3.0 Vdc to +3.6 Vdc, CL (load capacitance) on output lines = 80 pF. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25˚C, unless otherwise noted. The switching characteristics of the LM91 fully meet or exceed the published specifications of the SMBus or I2C bus. The following parameters are the timing relationships between SMBCLK and SMBData signals related to the LM91. They are not necessarily the I2C or SMBus bus specifications. Symbol tBUF Parameter Conditions SMBus Free Time Typical Limits Units (Note 6) (Note 7) (Limit) 1.3 µs (min) SMBus Communication 20034304 SMBus TIMEOUT 20034313 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 power supplies (VI < GND or VI > VCC), the current at that pin should be limited to 5 mA. The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 5 mA to four. Parasitics and or ESD protection circuitry are shown in the figure below for the LM91’s pins. The nominal breakdown voltage of the zener D3 is 6.5V. Care should be taken not to forward bias the parasitic diode, D1, present on pins: NC pins 1,5 and 9, D+, ADD1 and ADD0. Doing so by more than 50 mV may corrupt a temperature or voltage measurement. www.national.com 6 NC (pins 1, 5, 9) D1 D2 x x x D− ADD0, ADD1 x D4 Pin Name D1 D2 D3 x T_CRIT_A x x SMBData x x x x NC (pin 13) x x x x x x VCC D+ D3 x SMBCLK D4 x NC (pin 16) x Note: An x indicates that the diode exists. 20034308 FIGURE 1. ESD Protection Input Structure Note 3: See AN-450 “Surface Mounting Methods and Their Effect on Product Reliability” or the section titled “Surface Mount” found in a current National Semiconductor Linear Data Book for other methods of soldering surface mount devices. Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Machine model, 200 pF discharged directly into each pin. Note 5: Thermal resistance of the QSOP-16 package is 130˚C/W, junction-to-ambient when attached to a printed circuit board with 2 oz. foil. Note 6: Typicals are at TA = 25˚C and represent most likely parametric norm. Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 8: The Temperature Error specification does not include an additional error of ± 1˚C, caused by the quantization error. Note 9: The Temperature Error will vary less than ± 1.0˚C for a variation in VCC of 3V to 3.6V from the nominal of 3.3V. Note 10: Quiescent current will not increase substantially with an active SMBus. Note 11: This specification is provided only to indicate how often temperature data is updated. The LM91 can be read at any time without regard to conversion state (and will yield last conversion result). Note 12: Holding the SMBData and/or SMBCLK lines Low for a time interval greater than tTIMEOUT will cause the LM91 to reset SMBData and SMBCLK to the IDLE state of an SMBus communication (SMBCLK and SMBData set High). 20034305 FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity) 7 www.national.com LM91 Pin Name LM91 address select pins ADD1 and ADD0, and are set by connecting these pins to ground for a low, (0) , to VCC for a high, (1), or left floating (TRI-LEVEL). 1.0 Functional Description The LM91 temperature sensor incorporates a band-gap type temperature sensor using a Local or Remote diode and an 8-bit ADC (Delta-Sigma Analog-to-Digital Converter). The LM91 is compatible with the serial SMBus and I2C interfaces. Digital comparators compare Local and Remote readings to user-programmable setpoints (LT_CRIT and RT_CRIT). Activation of the T_CRIT_A output indicates that a temperature reading is greater than the limit preset in a T_CRIT register. Therefore, the complete slave address is: A6 A5 A4 A3 A2 A1 A0 MSB LSB and is selected as follows: Address Select Pin State 1.1 T_CRIT_A OUTPUT, T_CRIT LIMITS ADD0 LM91 SMBus Slave Address ADD1 A6:A0 binary 0 0 001 1000 0 TRI-LEVEL 001 1001 0 1 001 1010 TRI-LEVEL 0 010 1001 TRI-LEVEL TRI-LEVEL 010 1010 The Status Register can be read to determine which event caused the alarm. A bit in the Status Register is set high to indicate T_CRIT temperature alarm, see Section 1.8.3. TRI-LEVEL 1 010 1011 1 0 100 1100 1 TRI-LEVEL 100 1101 Local and remote temperature diodes are sampled alternately by the A/D converter. The T_CRIT_A output and the Status Register flags are updated at the completion of a conversion, which takes approximately 60 ms. T_CRIT_A and the Status Register flags are reset only after the Status Register is read and if the temperature is below the setpoint. 1 1 100 1110 T_CRIT_A is activated when the Local temperature reading is greater than the limit preset in the local critical temperature setpoint register (LT_CRIT) or when the Remote temperature reading is greater than the limit preset in the remote critical temperature setpoint register (RT_CRIT), as shown in Figure 3. The T_CRIT_A mask bit (bit 7 of the Configuration Register) when set will disable the T_CRIT_A output. The LM91 latches the state of the address select pins during the first read or write on the SMBus. Changing the state of the address select pins after the first read or write to any device on the SMBus will not change the slave address of the LM91. 1.4 TEMPERATURE DATA FORMAT Temperature data can be read from the Local Temperature, Remote Temperature, and T_CRIT setpoint registers. Temperature data can only be written to the T_CRIT setpoint registers. Temperature data is represented by an 8-bit, two’s complement byte with an LSB (Least Significant Bit) equal to 1˚C: Temperature 20034306 Digital Output Binary Hex +125˚C 0111 1101 7Dh +25˚C 0001 1001 19h FIGURE 3. T_CRIT_A Temperature Response Diagram +1˚C 0000 0001 01h 1.2 POWER-ON RESET DEFAULT STATES LM91 always powers up to these known default states: 1. Local Temperature set to 0˚C 2. Remote Temperature set to 0˚C until the LM91 senses a diode present or open circuit on the D+ and D− input pins. 3. Status Register set to 00h. 4. Command Register set to 00h; T_CRIT_A enabled. 0˚C 0000 0000 00h −1˚C 1111 1111 FFh 5. 1110 0111 E7h −55˚C 1100 1001 C9h 1.5 OPEN-DRAIN OUTPUTS SMBData and T_CRIT_A outputs are open-drain and do not have internal pull-ups. A “high” level will not be observed on these pins until pull-up current is provided from some external source, typically a pull-up resistor. Choice of resistor value depends on many system factors but, in general, the pull-up resistor should be as large as possible. This will minimize any local temperature reading errors due to self heating of the LM91. The maximum resistance of the pull-up, based on LM91 specification for High Level Output Current, to provide a 2V high level, is 30 kΩ. Local T_CRIT set to 127˚C and Remote T_CRIT set to 95˚C. 1.3 SMBus INTERFACE The LM91 operates as a slave on the SMBus, so the SMBCLK line is an input (no clock is generated by the LM91) and the SMBData line is bi-directional. According to SMBus specifications, the LM91 has a 7-bit slave address. Bit 4 (A3) of the slave address is hard wired inside the LM91 to a 1. The remainder of the address bits are controlled by the www.national.com −25˚C 1.6 DIODE FAULT DETECTION Before each remote conversion the LM91 goes through an external diode fault detection sequence. If the D+ input is 8 bit 4 (RTCRIT) of the Status Register will be set which will activate the T_CRIT_A output, if enabled. If D+ is shorted to GND or D−, the temperature reading will be 0˚C and bit 2 of the Status Register will not be set. (Continued) shorted to VCC or floating then the temperature reading will be +127˚C, bit 2 (OPEN) of the Status Register will be set. If the Remote T_CRIT setpoint is set to less than +127˚C then 1.7 COMMUNICATING with the LM91 20034309 Acknowledge from the Master (No Acknowledge is typically used as a signal for the slave that the Master has read its last byte). There are 10 data registers in the LM91, selected by the Command Register. At power-up the Command Register is set to “00”, the location for the Read Local Temperature Register. The Command Register latches whatever the last location it was set to. Reading the Status Register resets T_CRIT_A. All registers are predefined as read only or write only. Read and write registers with the same function contain mirrored data. A Write to the LM91 will always include the address byte and the command byte. A write to any register requires one data byte. Reading the LM91 can take place either of two ways: 1. If the location latched in the Command Register is correct (most of the time it is expected that the Command Register will point to one of the Read Temperature Registers because that will be the data most frequently read from the LM91), the read can simply consist of an address byte, followed by retrieving the data byte. 2. If the Command Register needs to be set, then an address byte, command byte, repeat start, and another address byte will accomplish a read. The data byte has the most significant bit first. At the end of a read, the LM91 can accept either Acknowledge or No 1.7.1 SMBus TIMEOUT The LM91 SMBus interface circuitry will be reset to the SMBus idle state if the SMBData or SMBCLK lines are held low for more than 40 ms. The LM91 may or may not reset the state SMBData or SMBCLK if either of these lines are held low between 25 ms and 40 ms. Holding SMBData or SMBCLK low for less than or equal to 25 ms will not reset the interface circuitry. The LM91 has a built-in internal timer to guarantee that the interface is reset if the SMBData line were to get stuck low. This can commonly occur when the master is reset while the slave is transmitting low. This enhancement to the SMBus TIMEOUT specification ensures error free performance even in remote systems where complete power supply shutdown, for reset, is a nuisance. This would have to occur since many cost effective temperature sensors such as the LM91 do not have a pin dedicated for reset. 9 www.national.com LM91 1.0 Functional Description LM91 1.0 Functional Description (Continued) 1.8 LM91 REGISTERS 1.8.1 COMMAND REGISTER Selects which registers will be read from or written to. Data for this register should be transmitted during the Command Byte of the SMBus write communication. P7 P6 P5 P4 0 0 0 0 P3 P2 P1 P0 Command Select P0-P7: Command Select: Command Select Power On Default State Address Register Name Register Function < P7:P0 > hex < D7:D0 > binary < D7:D0 > decimal 00h 0000 0000 0 RLT Read Local Temperature 01h 0000 0000 0 RRT Read Remote Temperature 02h 0000 0000 0 RS Read Status 03h 0000 0000 0 RC Read Configuration 04h 0000 0000 0 RMID Manufacturers ID 05h 0111 1111 127 RLCS Read Local T_CRIT Setpoint Read Remote T_CRIT Setpoint 07h 0101 1111 95 RRCS 09h 0000 0000 0 WC 0Bh 0111 1111 127 WLCS Write Local T_CRIT Setpoint 0Dh 0101 1111 95 WRCS Write Remote T_CRIT Setpoint Write Configuration 1.8.2 LOCAL and REMOTE TEMPERATURE REGISTERS (Read Only Address 00h and 01h): D7 D6 D5 D4 D3 D2 D1 D0 MSB Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB D7–D0: Temperature Data. One LSB = 1˚C. Two’s complement format. 1.8.3 STATUS REGISTER (Read Only Address 02h): D7 D6 D5 D4 D3 D2 D1 D0 0 LTCRIT 0 RTCRIT 0 OPEN 0 0 Power up default is with all bits “0” (zero). D2: OPEN: When set to 1 indicates a Remote Diode disconnect. D4: RTCRIT: When set to 1 indicates an RT_CRIT alarm. D6: LTCRIT: When set to 1 indicates an LT_CRIT alarm. D7, D5, D3, D1–D0: These bits are always set to 0. 1.8.4 Manufacturers ID Register (Read Address 04h) Default value 00h. 1.8.5 CONFIGURATION REGISTER (Read Address 03h /Write Address 09h): D7 D6 D5 D4 D3 D2 D1 D0 T_CRIT_A mask 0 0 0 0 0 0 0 Power up default is with all bits “0” (zero). D7: T_CRIT_A mask: When set to 1 T_CRIT_A interrupts are masked. D6–D0: These bits are always set to 0. A write of 1 will return a 0 when read. www.national.com 10 LM91 1.0 Functional Description (Continued) 1.8.6 LOCAL AND REMOTE T_CRIT REGISTERS (Read/Write): D7 D6 D5 D4 D3 D2 D1 D0 MSB Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB D7–D0: RT_CRIT and LT_CRIT setpoint temperature data. Power up default is LT_CRIT =127˚C and RT_CRIT = 95˚C. 2.0 SMBus Timing Diagrams 20034310 (a) Serial Bus Write to the internal Command Register followed by a the Data Byte 20034311 (b) Serial Bus Write to the internal Command Register 20034312 (c) Serial Bus Read from a Register with the internal Command Register preset to desired value. FIGURE 4. Serial Bus Timing Diagrams path of best thermal conductivity is between the die and the pins, its temperature will effectively be that of the printed circuit board lands and traces soldered to the LM91’s pins. This presumes that the ambient air temperature is almost the same as the surface temperature of the printed circuit board; if the air temperature is much higher or lower than the 3.0 Application Hints The LM91 can be applied easily in the same way as other integrated-circuit temperature sensors, and its remote diode sensing capability allows it to be used in new ways as well. It can be soldered to a printed circuit board, and because the 11 www.national.com LM91 3.0 Application Hints where: (Continued) • surface temperature, the actual temperature of the of the LM91 die will be at an intermediate temperature between the surface and air temperatures. Again, the primary thermal conduction path is through the leads, so the circuit board temperature will contribute to the die temperature much more strongly than will the air temperature. To measure temperature external to the LM91’s die, use a remote diode. This diode can be located on the die of a target IC, allowing measurement of the IC’s temperature, independent of the LM91’s temperature. The LM91 has been optimized to measure the remote diode of a Pentium II processor as shown in Figure 5. A discrete diode can also be used to sense the temperature of external objects or ambient air. Remember that a discrete diode’s temperature will be affected, and often dominated, by the temperature of its leads. • q is the electron charge, • k is the Boltzmann’s constant, • N is the current ratio, • T is the absolute temperature in ˚K. The temperature sensor then measures ∆VBE and converts to digital data. In this equation, k and q are well defined universal constants, and N is a parameter controlled by the temperature sensor. The only other parameter is η, which depends on the diode that is used for measurement. Since ∆VBE is proportional to both η and T, the variations in η cannot be distinguished from variations in temperature. Since the non-ideality factor is not controlled by the temperature sensor, it will directly add to the inaccuracy of the sensor. For the Pentium II Intel specifies a ± 1% variation in η from part to part. As an example, assume a temperature sensor has an accuracy specification of ± 4˚C at room temperature of 25˚C and the process used to manufacture the diode has a non-ideality variation of ± 1%. The resulting accuracy of the temperature sensor at room temperature will be: TACC = ± 4˚C + ( ± 1% of 298˚K) = ± 7˚C . The additional inaccuracy in the temperature measurement caused by η, can be eliminated if each temperature sensor is calibrated with the remote diode that it will be paired with. 3.2 PCB LAYOUT for MINIMIZING NOISE In a noisy environment, such as a processor mother board, layout considerations are very critical. Noise induced on traces running between the remote temperature diode sensor and the LM91 can cause temperature conversion errors. The following guidelines should be followed: 1. Place a 0.1 µF power supply bypass capacitor as close as possible to the VCC pin and the recommended 2.2 nF capacitor as close as possible to the D+ and D− pins. Make sure the traces to the 2.2 nF capacitor are matched. 2. Ideally, the LM91 should be placed within 10 cm of the Processor diode pins with the traces being as straight, short and identical as possible. 3. Diode traces should be surrounded by a GND guard ring to either side, above and below if possible. This GND guard should not be between the D+ and D− lines. In the event that noise does couple to the diode lines it would be ideal if it is coupled common mode. That is equally to the D+ and D− lines.(See Figure 6) 4. Avoid routing diode traces in close proximity to power supply switching or filtering inductors. 5. Avoid running diode traces close to or parallel to high speed digital and bus lines. Diode traces should be kept at least 2 cm. apart from the high speed digital traces. 6. If it is necessary to cross high speed digital traces, the diode traces and the high speed digital traces should cross at a 90 degree angle. 7. The ideal place to connect the LM91’s GND pin is as close as possible to the Processors GND associated with the sense diode. For the Pentium II this would be pin A14. 20034316 Pentium Temperature vs LM91 Temperature Reading FIGURE 5. Most silicon diodes do not lend themselves well to this application. It is recommended that a 2N3904 transistor base emitter junction be used with the collector tied to the base. A diode connected 2N3904 approximates the junction available on a Pentium microprocessor for temperature measurement. Therefore, the LM91 can sense the temperature of this diode effectively. 3.1 ACCURACY EFFECTS OF DIODE NON-IDEALITY FACTOR The technique used in today’s remote temperature sensors is to measure the change in VBE at two different operating points of a diode. For a bias current ratio of N:1, this difference is given as: www.national.com η is the non-ideality factor of the process the diode is manufactured on, 12 LM91 3.0 Application Hints (Continued) 20034315 FIGURE 6. Recommended Diode Trace Layout munication is rather low (400 kHz max) care still needs to be taken to ensure proper termination within a system with multiple parts on the bus and long printed circuit board traces. Noise on the digital lines, overshoot greater than VCC and undershoot less than GND, may prevent successful SMBus communication with the LM91. SMBus no acknowledge is the most common symptom, causing unnecessary traffic on the bus. Although, the SMBus maximum frequency of com- 4.0 Typical Applications 20034317 Using a Diode Connected 2N3904 as a Remote Temperture Sensing Element 13 www.national.com LM91 Diode Input Digital Temperature Sensor with Two-Wire Interface Physical Dimensions inches (millimeters) unless otherwise noted 16-Lead QSOP Package Order Number LM91DIMQA, or LM91DIMQAX NS Package Number MQA16 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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