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LM76 ±0.5˚C, ±1˚C, 12-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire Interface PRELIMINARY November 1999 LM76 ±0.5˚C, ±1˚C, 12-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire Interface General Description The LM76 is a digital temperature sensor and thermal window comparator with an I2C™ Serial Bus interface with an accuracy of ±1˚C for the temperature ranges of 70˚C to 100˚C and −10˚C to +45˚C. The LM76CHM is specified with an accuracy ±0.5˚C at 25˚C. The window-comparator architecture of the LM76 eases the design of temperature control systems conforming to the ACPI (Advanced Configuration and Power Interface) specification for personal computers. The open-drain Interrupt (INT) output becomes active whenever temperature goes outside a programmable window, while a separate Critical Temperature Alarm (T_CRIT_A) output becomes active when the temperature exceeds a programmable critical limit. The INT output can operate in either a comparator or event mode, while the T_CRIT_A output operates in comparator mode only. The host can program both the upper and lower limits of the window as well as the critical temperature limit. Programmable hysterisis as well as a fault queue are available to minimize false tripping. Two pins (A0, A1) are available for address selection. The sensor powers up with default thresholds of 2˚C THYST, 10˚C TLOW, 64˚C THIGH, and 80˚C T_CRIT. The LM76’s 3.3V and 5.0V supply voltage, Serial Bus interface, 12-bit + sign output, and full-scale range of over 127˚C make it ideal for a wide range of applications. These include thermal management and protection applications in personal computers, electronic test equipment, office electronics and bio-medical applications. Features n Window comparison simplifies design of ACPI compatible temperature monitoring and control. n Serial Bus interface n Separate open-drain outputs for Interrupt and Critical Temperature shutdown n Shutdown mode to minimize power consumption n Up to 4 LM76s can be connected to a single bus n 12-bit + sign output; full-scale reading of over 127˚C Key Specifications n Supply Voltage n Supply Current n Temperature Accuracy n Resolution operating shutdown +25˚C −10˚C to +45˚C 70˚C to 100˚C 3.3V or 5.0V 250 µA (typ) 450 µA (max) 8 µA (max) ±0.5˚C(max) ±1.0˚C(max) ±1.0˚C(max) 0.0625˚C Applications n System Thermal Management n Personal Computers n Office Electronics n HVAC Simplified Block Diagram I2C® is a registered trademark of Philips Corporation. © 1999 National Semiconductor Corporation DS101015 DS101015-1 www.national.com LM76 Connection Diagram SO-8 Ordering Information DS101015-2 LM76 See NS Package Number M08A Order Number LM76CHM-5 LM76CHMX-5 LM76CNM-3 LM76CNMX-3 Supply Voltage 5.0V 5.0V 3.3V 3.3V Acurracy ±0.5˚C ±1.0˚C ±0.5˚C ±1.0˚C ±1˚C ±1˚C Temperature Range for Accuracy 25˚C −10˚C to 45˚C 25˚C −10˚C to 45˚C 70˚C to 100˚C 70˚C to 100˚C Transport Media 95 units in Rail 2500 Units on Tape and Reel 95 units in Rail 2500 Units on Tape and Reel Pin Description Label SDA SCL T_CRIT_A GND INT +VS A0–A1 Pin # 1 2 3 4 5 8 7,6 Function Serial Bi-Directional Data Line, Open Drain Output, CMOS Logic Level Serial Bus Clock Input, CMOS Logic Level Critical Temperature Alarm, Open Drain Output Power Supply Ground Interrupt, Open Drain Output Positive Supply Voltage Input User-Set Address Inputs, TTL Logic Level Typical Connection Pull Up Resistor, Controller I2C Data Line From Controller I2C Clock Line Pull Up Resistor, Controller Interrupt Line or System Hardware Shutdown Ground Pull Up Resistor, Controller Interrupt Line DC Voltage from 3.3V power supply or 5V. Ground (Low, “0”) or +VS (High, “1”) www.national.com FIGURE 1. Typical Application DS101015-3 2 LM76 Absolute Maximum Ratings (Note 1) Supply Voltage Voltage at any Pin Input Current at any Pin Package Input Current (Note 2) T_CRIT_A and INT Output Sink Current T_CRIT_A and INT Output Voltage Storage Temperature −0.3V to 6.5V −0.3V to (+VS + 0.3V) 5mA 20mA 10mA 6.5V −65˚C to +125˚C Soldering Information, Lead Temperature SOP Package (Note 3) Vapor Phase (60 seconds) Infrared (15 seconds) ESD Susceptibility (Note 4) Human Body Model Machine Model Operating Ratings(Notes 1, 5) 215˚C 220˚C 3000V 250V Operating Temperature Range Specified Temperature Range (Note 6) LM76CHM-5 LM76CNM-3 Supply Voltage Range (+VS)(Note 7) −55˚C to +150˚C TMIN to TMAX −20˚C to +85˚C −55˚C to +125˚C +3.0V to +5.5V Temperature-to-Digital Converter Characteristics Unless for the otherwise noted, these LM76CHM-5. (Note 7). specifications apply for Boldface limits apply f+oVr ST=A+=3.3TJV=dcT±M5IN%tofoTr MthAeX;LaMll7o6tChNerMli-m3itasnTdAf=or T+JV=S+=2+55˚C.0, Vdc ±10% unless oth- erwise noted. Parameter Accuracy (Note 7) Resolution Conditions TA = −25˚C to +125˚C for LM76CNM-3 TA = +70˚C to +100˚C TA = −20˚C to +85˚C for LM76CHM-5 TA = −10˚C to +45˚C TA = +25˚C (Note 10) Typical (Note 8) ±2.5 ±1.5 13 0.0625 LM76CNM-3 Limits (Note 9) LM76CHM-5 Limits (Note 9) ±1.0 ±1.0 ±0.5 Units (Limit) ˚C (max) Bits ˚C Temperature Conversion Time Quiescent Current (Note 11) I2C Inactive I2C Active 400 500 1000 ms 0.25 0.25 0.5 mA 0.45 mA (max) THYST Default Temperature TLOW Default Temperature THIGH Default Temperature TCRIT Default Temperature Shutdown Mode: TA=+85˚C TA=+25˚C (Notes 13, 14) (Note 14) (Note 14) (Note 14) 5 12 8 2 10 64 80 µA 18 µA (max) µA (max) 12 µA (max) ˚C ˚C ˚C ˚C Logic Electrical Characteristics LoDMtIhG7eI6rTClAimNLiMtDs-C3TAaC=nHdTAJfoR=rA++2CV5TS˚EC=R,+uI5Sn.0TleIVCssdScoUt±hne1ler0ws%sisefootrhneothrtewediLs. eM7n6oCteHd,Mt-h5e.s.eBsopledcfiaficceatlioimnsitsapapplyplfyorfoforrT+AV=S =T+J3.=3 Vdc TMIN ±5% for the to TMAX; all Symbol Parameter Conditions Typical (Note 8) Limits (Note 9) Units (Limit) VIN(1) SDA and SCL Logical “1” Input Voltage +VS x 0.7 V (min) +VS+0.3 V (max) 3 www.national.com LM76 Logic Electrical Characteristics (Continued) DIGITAL DC LM76CNM-3 CHARACTERISTICS Unless and for +VS=+5.0 Vdc ±10% footrhethrwe iLseM7n6oCteHd,Mt-h5e.s.eBsopledcfiaficceatlioimnsitsapapplyplfyorfoforrT+AV=S=T+J3.=3 Vdc TMIN ±5% for the to TMAX; all other limits TA= TJ=+25˚C, unless otherwise noted. Symbol Parameter Conditions Typical (Note 8) Limits (Note 9) Units (Limit) VIN(0) SDA and SCL Logical “0” Input Voltage −0.3 V (min) VIN(HYST) SDA and SCL Digital Input Hysteresis +VS x 0.3 V (max) 500 250 mV (min) VIN(1) A0 and A1 Logical “1” Input Voltage 2.0 V (min) VIN(0) A0 and A1 Logical “0” Input Voltage +VS+0.3 −0.3 V (max) V (min) 0.8 V (max) IIN(1) IIN(0) CIN IOH VOL Logical “1” Input Current Logical “0” Input Current Capacitance of All Digital Inputs High Level Output Current Low Level Output Voltage T_CRIT_A Output Saturation Voltage VIN = + VS VIN = 0V VOH = + VS IOL = 3 mA IOUT = 4.0 mA (Note 12) 0.005 −0.005 20 1.0 µA (max) −1.0 µA (max) pF 10 µA (max) 0.4 V (max) 0.8 V (max) T_CRIT_A Delay 1 Conversions (max) tOF Output Fall Time CL = 400 pF 250 ns (max) IO = 3 mA SERIAL BUS DIGITAL SWITCHING CHARACTERISTICS Unless Vdc ±5% for the LM76CNM-3 and for +VS=+5.0 Vdc ±10% for the otherwise noted, these LM76CHM-5, CL (load specifications capacitance) apply for on output +liVneSs=+=3.830 pF unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25˚C, unless oth- erwise noted. The switching characteristics of the LM76 fully meet or exceed the published specifications of the I2C bus. The following pa- rameters are the timing relationship between SCL and SDA signal related to the LM76. They are not the I2C bus specifica- tions. Symbol Parameter Conditions Typical (Note 8) Limits (Note 9) Units (Limit) t1 SCL (Clock) Period t2 Data in Set-Up Time to SCL High t3 Data Out Stable after SCL Low t4 SDA Low Set-Up Time to SCL Low (Start Condition) t5 SDA High Hold Time after SCL High (Stop Condition) 2.5 µs(min) 100 ns(min) 0 ns(min) 100 ns(min) 100 ns(min) www.national.com DS101015-4 4 LM76 Logic Electrical Characteristics (Continued) 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 > +VS) 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. 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: LM76 θJA (thermal resistance, junction-to-ambient) when attached to a printed circuit board with 2 oz. foil is 200˚C/W. Note 6: While the LM76 has a full-scale-range in excess of 128˚C, prolonged operation at temperatures above 125˚C is not recommended. Note 7: The LM76 will operate properly over the +VS supply voltage range of 3V to 5.5V for the LM76CNM-3 and the LM76CHM-5. The LM76CNM-3 is tested and specified for rated accuracy at the nominal supply voltage of 3.3V. Accuracy of the LM76CNM-3 will degrade 0.2˚C for a ±1% variation in +VS from the nominal value. The LM76CHM-5 is tested and specified for a rated accuracy at the nominal supply voltage of 5.0V. Accuracy of the LM76CHM-5 will degrade 0.08˚C for a ±1% variation in +VS from the nominal value. Note 8: Typicals are at TA = 25˚C and represent most likely parametric norm. Note 9: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 10: 12 bits + sign, two’s complement Note 11: This specification is provided only to indicate how often temperature data is updated. The LM76 can be read at any time without regard to conversion state (and will yield last conversion result). If a conversion is in process it will be interrupted and restarted after the end of the read. Note 12: For best accuracy, minimize output loading. Higher sink currents can affect sensor accuracy with internal heating. This can cause an error of 0.64˚C at full rated sink current and saturation voltage based on junction-to-ambient thermal resistance. Note 13: Hysteresis value adds to the TLOW setpoint value (e.g.: if TLOW setpoint = 10˚C, and hysteresis = 2˚C, then actual hysteresis point is 10+2 = 12˚C); and subtracts from the THIGH and T_CRIT setpoints (e.g.: if THIGH setpoint = 64˚C, and hysteresis = 2˚C, then actual hysteresis point is 64−2 = 62˚C). For a detailed discussion of the function of hysteresis refer to Section 1.1, TEMPERATURE COMPARISON, and Figure 3. Note 14: Default values set at power up. Electrical Characteristics Continued DS101015-5 FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity) 1.0 Functional Description The LM76 temperature sensor incorporates a band-gap type temperature sensor, 13-bit ADC, and a digital comparator with user-programmable upper and lower limit values. The comparator activates either the INT line for temperatures outside the TLOW and THIGH window, or the T_CRIT_A line for temperatures which exceed T_CRIT. The lines are programmable for mode and polarity. 1.1 TEMPERATURE COMPARISON LM76 provides a window comparison against a lower (TLOW) and upper (THIGH) trip point. A second upper trip point (T_CRIT) functions as a critical alarm shutdown. Figure 3 depicts the comparison function as well as the modes of operation. 1.1.1 STATUS BITS The internal Status bits operate as follows: 5 www.national.com LM76 1.0 Functional Description (Continued) “True”: Temperature above a THIGH or T_CRIT is “true” for those respective bits. A “true” for TLOW is temperature below TLOW. “False”: Assuming temperature has previously crossed above THIGH or T_CRIT, then the temperature must drop below the points corresponding THYST (THIGH − THYST or T_CRIT − THYST) in order for the condition to be false. For TLOW, assuming temperature has previously crossed below TLOW, a “false” occurs when temperature goes above TLOW + THYST. The Status bits are not affected by reads or any other actions, and always represent the state of temperature vs. setpoints. 1.1.2 HARDWIRE OUTPUTS The T_CRIT_A hardwire output mirrors the T_CRIT_A flag, when the flag is true, the T_CRIT_A output is asserted at all times regardless of mode. Reading the LM76 has no effect on the T_CRIT_A output, although the internal conversion is restarted. The behavior of the INT hardwire output is as follows: Comparator Interrupt Mode (Default): User reading part resets output until next measurement completes. If condition is still true, output is set again at end of next conversion cycle. For example, if a user never reads the part, and temperature goes below TLOW then INT becomes active. It would stay that way until temperature goes above TLOW + THYST. However if the user reads the part, the output would be reset. At the end of the next conversion cycle, if the condition is true, it is set again. If not, it remains reset. Event Interrupt Mode: User reading part resets output until next condition ″event″ occurs (in other words, output is only set once for a true condition, if reset by a read, it remains reset until the next triggering threshold has been crossed). Conversely, if a user never read the part, the output would stay set indefinitely after the first event that set the output. An “event” for Event Interrupt Mode is defined as: 1. Transitioning upward across a setpoint, or 2. Transitioning downward across a setpoint’s corresponding hysteresis (after having exceeded that setpoint). For example, if a user never read the part, and temperature went below TLOW then INT would become active. It would stay that way forever if a user never read the part. However if the user read the part, the output would be reset. Even if the condition is true, it will remain reset. The temperature must cross above TLOW + THYST to set the output again. In either mode, reading any register in the LM76 restarts the conversion. This allows a designer to know exactly when the LM76 begins a comparison. This prevents unnecessary Interrupts just after reprogramming setpoints. Typically, system Interrupt inputs are masked prior to reprogramming trip points. By doing a read just after resetting trip points, but prior to unmasking, unexpected Interrupts are prevented. Avoid programming setpoints so close that their hysteresis values overlap. An example would be that with a THYST value of 2˚C then setting THIGH and TLOW to within 4˚C of each other will violate this restriction. To be more specific, with THYST set to 2˚C assume THIGH set to 64˚C. If TLOW is set equal to, or higher than 60˚C this restriction is violated. 1.2 DEFAULT SETTINGS The LM76 always powers up in a known state. LM76 power up default conditions are: 1. Comparator Interrupt Mode 2. TLOW set to 10˚C 3. THIGH set to 64˚C 4. T_CRIT set to 80˚C 5. THYST set to 2˚C 6. INT and T_CRIT_A active low 7. Pointer set to “00”; Temperature Register The LM76 registers will always reset to these default values when the power supply voltage is brought up from zero volts as the supply crosses the voltage level plotted in the following curve. The LM76 registers will reset again when the power supply drops below the voltage plotted in this curve. Average Power on Reset Voltage vs Temperature DS101015-18 1.3 SERIAL BUS INTERFACE The LM76 operates as a slave on the Serial Bus, so the SCL line is an input (no clock is generated by the LM76) and the SDA line is a bi-directional serial data line. According to Serial Bus specifications, the LM76 has a 7-bit slave address. The five most significant bits of the slave address are hard wired inside the LM76 and are “10010”. The two least significant bits of the address are assigned to pins A1–A0, and are set by connecting these pins to ground for a low, (0); or to +VS for a high, (1). Therefore, the complete slave address is: 1 0 0 1 0 A1 A0 MSB LSB www.national.com 6 LM76 1.0 Functional Description (Continued) DS101015-6 Note: Event Interrupt mode is drawn as if the user is reading the part. If the user doesn’t read, the outputs would go low and stay that way until the LM76 is read. FIGURE 3. Temperature Response Diagram 1.4 TEMPERATURE DATA FORMAT Temperature data can be read from the Temperature and Set Point registers; and written to the Set Point registers. Temperature data can be read at any time, although reading faster than the conversion time of the LM76 will prevent data from being updated. Temperature data is represented by a 13-bit, two’s complement word with an LSB (Least Significant Bit) equal to 0.0625˚C: Temperature Digital Output Binary Hex 1.5 SHUTDOWN MODE Shutdown mode is enabled by setting the shutdown bit in the Configuration register via the Serial Bus. Shutdown mode reduces power supply current to 5 µA typical. T_CRIT_A is reset if previously set. Since conversions are stoped during shutdown, T_CRIT_A and INT will not be operational. The Serial Bus interface remains active. Activity on the clock and data lines of the Serial Bus may slightly increase shutdown mode quiescent current. Registers can be read from and written to in shutdown mode. The LM76 takes miliseconds to respond to the shutdown command. +130˚C +125˚C +80˚C +64˚C +25˚C +10˚C +2˚C +0.0625˚C 0˚C −0.0625˚C −25˚C −55˚C 0 1000 0 010 0000 0 0111 1101 0000 0 0101 1010 0000 0 0100 0000 0000 0 0001 1001 0000 0 0000 1010 0000 0 0000 0010 0000 0 0000 0000 0001 00 0000 0000 1 1111 1111 1111 1 1110 0111 0000 1 1100 1001 0000 08 20h 07 D0h 05 90h 04 00h 01 90h 00 A0h 00 20h 00 01h 00 00h 1F FFh 1E 70h 1C 90h 1.6 INT AND T_CRIT_A OUTPUT The INT and T_CRIT_A outputs are open-drain outputs 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 errors due to internal heating of the LM76. The maximum resistance of the pull up, based on LM76 specification for High Level Output Current, to provide a 2 volt high level, is 30K ohms. 1.7 FAULT QUEUE A fault queue of up to 4 faults is provided to prevent false tripping when the LM76 is used in noisy environments. The 4 faults must occur consecutively to set flags as well as INT 7 www.national.com LM76 1.0 Functional Description (Continued) and T_CRIT_A outputs. The fault queue is enabled by setting bit 4 of the Configuration Register high (see Section 1.11). 1.8 INTERNAL REGISTER STRUCTURE There are four data registers in the LM76, selected by the Pointer register. At power-up the Pointer is set to “00”; the location for the Temperature Register. The Pointer register latches the last location it was set to. In Interrupt Mode, a read from the LM76 resets the INT output. Placing the device in Shutdown mode resets the INT and T_CRIT_A outputs. All registers are read and write, except the Temperature register which is read only. A write to the LM76 will always include the address byte and the Pointer byte. A write to the Configuration register requires one data byte, while the TLOW, THIGH, and T_CRIT registers require two data bytes. Reading the LM76 can take place either of two ways: If the location latched in the Pointer is correct (most of the time it is expected that the Pointer will point to the Temperature register because it will be the data most frequently read from the LM76), then the read can simply consist of an address byte, DS101015-7 followed by retrieving the corresponding number of data bytes. If the Pointer needs to be set, then an address byte, pointer byte, repeat start, and another address byte plus required number of data bytes will accomplish a read. The first data byte is the most significant byte with most significant bit first, permitting only as much data as necessary to be read to determine the temperature condition. For instance, if the first four bits of the temperature data indicates a critical condition, the host processor could immediately take action to remedy the excessive temperature. At the end of a read, the LM76 can accept either Acknowledge or No Acknowledge from the Master (No Acknowledge is typically used as a signal for the slave that the Master has read its last byte). An inadvertent 8-bit read from a 16-bit register, with the D7 bit low, can cause the LM76 to stop in a state where the SDA line is held low as shown in Figure 4. This can prevent any further bus communication until at least 9 additional clock cycles have occurred. Alternatively, the master can issue clock cycles until SDA goes high, at which time issuing a “Stop” condition will reset the LM76. www.national.com DS101015-8 FIGURE 4. Inadvertent 8-Bit Read from 16-Bit Register where D7 is Zero (“0”) 8 LM76 1.0 Functional Description (Continued) 1.9 POINTER REGISTER (Selects which registers will be read from or written to): P0–P2: Register Select: P3–P7: Must be kept zero. P7 P6 P5 P4 P3 P2 P1 P0 0 0 0 0 0 Register Select P2 P1 P0 Register 0 0 0 Temperature (Read only) (Power-up default) 0 0 1 Configuration (Read/Write) 0 1 0 THYST (Read/Write) 0 1 1 T_CRIT (Read/Write) 1 0 0 TLOW (Read/Write) 1 0 1 THIGH (Read/Write) 1.10 TEMPERATURE REGISTER (Read Only): D15 D14 D13 D12 D11 Sign MSB Bit Bit 9 Bit 8 10 D10 Bit 7 D9 Bit 6 D8 Bit 5 D7 Bit 4 D6 Bit 3 D0–D2: Status Bits D3–D15: Temperature Data. One LSB = 0.0625˚C. Two’s complement format. D5 Bit 2 D4 Bit 1 D3 Bit 0 D2 CRIT D1 HIGH D0 LOW Status Bits 1.11 CONFIGURATION REGISTER (Read/Write): D7 D6 D5 D4 D3 D2 D1 D0 0 0 0 Fault Queue INT Polarity T_CRIT_A INT Mode Shutdown Polarity D0: Shutdown - When set to 1 the LM76 goes to low power shutdown mode. Power up default of “0”. D1: Interrupt mode - 0 is Comparator Interrupt mode, 1 is Event Interrupt mode. Power up default of “0”. D2, D3: T_CRIT_A and INT Polarity - 0 is active low, 1 is active high. Outputs are open-drain. Power up default of “0” D4: Fault Queue - When set to 1 the Fault Queu is enabled, see Section 1.7. Power up default of “0”. D5–D7: These bits are used for production testing and must be kept zero for normal operation. 1.12 THYST, TLOW, THIGH AND T_CRIT_A REGISTERS (Read/Write): D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 Sign MSB Bit Bit 9 Bit 8 Bit7 Bit6 Bit5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 X X X 10 D0–D2: Undefined D3–D15: THYST, TLOW, THIGH or T_CRIT Trip Temperature Data. Power up default is TLOW = 10˚C, THIGH = 64˚C, T_CRIT = 80˚C, THYST = 2˚C. THYST is subtracted from THIGH, and T_CRIT, and added to TLOW. Avoid programming setpoints so close that their hysteresis values overlap. See Section 1.1. 9 www.national.com LM76 2.0 I2C Timing Diagrams DS101015-9 Typical 2-Byte Read From Preset Pointer Location Such as Temp or Comparison Registers DS101015-10 Typical Pointer Set Followed by Immediate Read for 2-Byte Register such as Temp or Comparison Registers DS101015-11 Typical 1-Byte Read from Configuration Register with Preset Pointer Typical Pointer Set Followed by Immediate Read from Configuration Register DS101015-12 Configuration Register Write Comparison Register Write FIGURE 6. Timing Diagrams DS101015-13 DS101015-14 www.national.com 10 LM76 3.0 Application Hints The temperature response graph in Figure 7 depicts a typical application designed to meet ACPI requirements. In this type of application, the temperature scale is given an arbitrary value of ″granularity″, or the window within which temperature notification events should occur. The LM76 can be programmed to the window size chosen by the designer, and will issue interrupts to the processor whenever the window limits have been crossed. The internal flags permit quick determination of whether the temperature is rising or falling. The T_CRIT limit would typically use its separate output to activate hardware shutdown circuitry separate from the processor. This is done because it is expected that if temperature has gotten this high that the processor may not be responding. The separate circuitry can then shut down the system, usually by shutting down the power supply. Note that the INT and T_CRIT_A outputs are separate, but can be wire-or’d together. Alternatively the T_CRIT_A can be diode or’d to the INT line in such a way that a T_CRIT_A event activates the INT line, but an INT event does not activate the T_CRIT_A line. This may be useful in the event that it is desirable to notify both the processor and separate T_CRIT_A shutdown circuitry of a critical temperature alarm at the same time (maybe the processor is still working and can coordinate a graceful shutdown with the separate shutdown circuit). To implement ACPI compatible sensing it is necessary to sense whenever the temperature goes outside the window, issue an interrupt, service the interrupt, and reprogram the window according to the desired granularity of the temperature scale. The reprogrammed window will now have the current temperature inside it, ready to issue an interrupt whenever the temperature deviates from the current window. To understand this graph, assume that at the left hand side the system is at some nominal temperature. For the 1st event temperature rises above the upper window limit, THIGH, causing INT to go active. The system responds to the interrupt by querying the LM76’s status bits and determines that THIGH was exceeded, indicating that temperature is rising. The system then reprograms the temperature limits to a value higher by an amount equal to the desired granularity. Note that in Event Interrupt Mode, reprogramming the limits has caused a second, known, interrupt to be issued since temperature has been returned within the window. In Comparator Interrupt Mode, the LM76 simply stops issuing interrupts. The 2nd event is another identical rise in temperature. The 3rd event is typical of a drop in temperature. This is one of the conditions that demonstrates the power of the LM76, as the user receives notification that a lower limit is exceeded in such a way that temperature is dropping. The Critical Alarm Event activates the separate T_CRIT_A output. Typically, this would feed circuitry separate from the processor on the assumption that if the system reached this temperature, the processor might not be responding. 11 www.national.com LM76 3.0 Application Hints (Continued) DS101015-15 Note: Event Interrupt mode is drawn as if the user is reading the part. If the user doesn’t read, the outputs would go low and stay that way until the LM76 is read. FIGURE 7. Temperature Response Diagram for ACPI Implementation 4.0 Typical Applications www.national.com FIGURE 8. Typical Application DS101015-16 12 LM76 4.0 Typical Applications (Continued) DS101015-19 FIGURE 9. ACPI Compatible Terminal Alarm Shutdown. By powering the LM76 from auxilary output of the power supply, a non-functioning overheated computer can be powered down to preserve as much of the system as possible. 13 www.national.com LM76 ±0.5˚C, ±1˚C, 12-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire Interface Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead (0.150" Wide) Molded Small Outline Package (SOP), JEDEC Order Number LM76CNM-3, LM76CNMX-3, LM76CHM-5 or LM76CHM-5X NS Package Number M08A 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. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com www.national.com National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: sea.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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