Rainbow Electronics LM77 User Manual

LM77 9-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire Interface

July 2001

LM77 9-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire

Interface

General Description

The LM77 is a digital temperature sensor and thermal window comparator with an I window-comparator architecture of the LM77 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 upwith default thresholds of 2˚C T T_CRIT.

The LM77’s 3.0V to 5.5V supply voltage range, Serial Bus interface, 9-bit + sign output, and full-scale range of over 128˚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, automotive, and HVAC applications.

HYST

, 10˚C T

C™Serial Bus interface. The

LOW

, 64˚C T

, and 80˚C

HIGH

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 LM77s can be connected to a single bus n 9-bit + sign output; full-scale reading of over 128˚C n SOP and MSOP 8-lead packages

Key Specifications

Supply Voltage 3.0V to 5.5V

Supply Current operating 250 µA (typ)

500 µA (max)

shutdown 5 µA (typ)

Temperature

Accuracy

−10˚C to 65˚C

−25˚C to 100˚C

−55˚C to 125˚C

1.5˚C(max)

2˚C(max)

3˚C(max)

Applications

n System Thermal Management n Personal Computers n Office Electronics n Electronic Test Equipment n Automotive n HVAC

I2C®is a registered trademark of Philips Corporation.

Simplified Block Diagram

LM77

Connection Diagram

DS100136-1

SOP8 or MSOP

DS100136-2

LM77 See NS Package Number M08A or MM08A

Ordering Information

Order Number Supply Voltage Package Supplied In

LM77CIM-3 3.3V SOP8, M08A Rail LM77CIMX-3 3.3V SOP8, M08A 2500 Units on Tape and Reel LM77CIM-5 5V SOP8, M08A Rail LM77CIMX-5 5V SOP8, M08A 2500 Units on Tape and Reel LM77CIMM-3 3.3V MSOP8,

MUA08A

LM77CIMMX-3 3.3V MSOP8,

MUA08A

LM77CIMM-5 5V MSOP8,

MUA08A

LM77CIMMX-5 5V MSOP8,

MUA08A

Rail

3500 Units on Tape and Reel

Rail

3500 Units on Tape and Reel

Pin Description

Label Pin

SDA 1 Serial Bi-Directional Data Line. Open Drain Output From Controller SCL 2 Serial Bus Clock Input From Controller T_CRIT_A 3 Critical Temperature Alarm Open Drain Output Pull Up Resistor, Controller Interrupt Line

GND 4 Power Supply Ground Ground INT 5 Interrupt Open Drain Output Pull Up Resistor, Controller Interrupt Line +V

A0–A1 7,6 User-Set Address Inputs Ground (Low, “0”) or +V

8 Positive Supply Voltage Input DC Voltage from 3V to 5.5V

Function Typical Connection

or System Hardware Shutdown

(High, “1”)

www.national.com 2

Pin Description (Continued)

LM77

DS100136-3

FIGURE 1. Typical Application

www.national.com3

Absolute Maximum Ratings (Note 1)

LM77

Supply Voltage −0.3V to 6.5V Voltage at any Pin −0.3V to (+V Input Current at any Pin 5 mA Package Input Current (Note 2) 20 mA T_CRIT_A and INT Output Sink

Current 10 mA T_CRIT_A and INT Output

+ 0.3V )

Soldering Information, Lead Temperature

SOP and MSOP Package (Note 3)

Vapor Phase (60 seconds) 215˚C Infrared (15 seconds) 220˚C

ESD Susceptibility (Note 4)

Human Body Model 2500V Machine Model 250V

Voltage 6.5V Storage Temperature −65˚C to +125˚C

Operating Ratings (Notes 1, 5)

Specified Temperature Range T (Note 6) −55˚C to +125˚C Supply Voltage Range (+V

)(Note 7) +3.0V to +5.5V

Temperature-to-Digital Converter Characteristics

Unless otherwise noted, these specifications apply for +VS=+5 Vdc±10% for LM77CIM-5, LM77CIMM-5 and +VS=+3.3 Vdc

10% for LM77CIM-3, LM77CIMM-3 (Note 7). Boldface limits apply for TA=TJ=T

=+25˚C, unless otherwise noted.

Parameter Conditions

Accuracy T

= −10˚C to +65˚C

= −25˚C to +100˚C

= −55˚C to +125˚C

Typical

(Note 8)

Resolution (Note 10) 10

0.5 Temperature Conversion Time (Note 11) 70 125 ms Quiescent Current I

C Inactive 0.25 mA

C Active 0.25 0.5 mA (max)

Shutdown Mode 5 10 µA

Default Temperature (Notes 13, 14) 2 ˚C

HYST

Default Temperature (Note 14) 10 ˚C

LOW

Default Temperature (Note 14) 64 ˚C

HIGH

Default Temperature (Note 14) 80 ˚C

MIN

to T

MAX

Limits

(Note 9)

1.5

2.0

3.0

; all other limits TA=

MIN

Units

(Limit)

˚C (max)T

Bits

˚C

to T

MAX

Logic Electrical Characteristics

DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for +VS=+5 Vdc±10% for LM77CIM-5,

LM77CIMM-5 and +V other limits T

A=TJ

Symbol Parameter Conditions

IN(1)

IN(0)

IN(1)

IN(0)

IN(1)

IN(0)

www.national.com 4

=+3.3 Vdc±10% for LM77CIM-3, LM77CIMM-3. Boldface limits apply for TA=TJ=T

MIN

to T

=+25˚C, unless otherwise noted.

Typical

(Note 8)

SDA and SCL Logical “1” Input

Limits

(Note 9)

+VSx 0.7 V (min)

Voltage

+0.3 V (max)

SDA and SCL Logical “0” Input

−0.3 V (min)

Voltage

x 0.3 V (max)

A0 and A1 Logical “1” Input

2.0 V (min)

Voltage

+0.3 V (max)

A0 and A1 Logical “0” Input

−0.3 V (min)

Voltage

0.8 V (max)

Logical “1” Input Current VIN=+V

0.005 1.0 µA (max) Logical “0” Input Current VIN= 0V −0.005 −1.0 µA (max) Capacitance of All Digital Inputs 20 pF

MAX

Units

(Limit)

; all

Logic Electrical Characteristics (Continued)

DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for +VS=+5 Vdc±10% for LM77CIM-5,

LM77CIMM-5 and +V other limits T

A=TJ

Symbol Parameter Conditions

=+3.3 Vdc±10% for LM77CIM-3, LM77CIMM-3. Boldface limits apply for TA=TJ=T

MIN

to T

MAX

=+25˚C, unless otherwise noted.

High Level Output Current VOH=+V

Typical

(Note 8)

Limits

(Note 9)

(Limit)

10 µA (max)

Units

Low Level Output Voltage IOL=3mA 0.4 V (max) T_CRIT_A Output Saturation

Voltage

= 4.0 mA

OUT

(Note 12)

0.8 V (max)

T_CRIT_A Delay 1 Conversions

(max)

Output Fall Time CL= 400 pF 250 ns (max)

=3mA

LM77

; all

www.national.com5

Logic Electrical Characteristics (Continued)

LM77

SERIAL BUS DIGITAL SWITCHING CHARACTERISTICS Unless otherwise noted, these specifications apply for +VS=+5 Vdc

10% for LM77CIM-5 and LM77CIMM-5, +VS=+3.3 Vdc±10% for LM77CIM-3 and LM77CIMM-3, CL (load capacitance) on

output lines = 80 pF unless otherwise specified. Boldface limits apply for T

A=TJ=TMIN

+25˚C, unless otherwise noted. The switching characteristics of the LM77 fully meet or exceed the published specifications of the I rameters are the timing relationship between SCL and SDA signal related to the LM77. They are not the I

Symbol Parameter Conditions

SCL (Clock) Period 2.5 µs(min) Data in Set-Up Time to SCL High 100 ns(min) Data Out Stable after SCL Low 0 ns(min) SDA Low Set-Up Time to SCL Low (Start Condition) 100 ns(min) SDA High Hold Time after SCL High (Stop Condition) 100 ns(min)

to T

; all other limits TA=TJ=

MAX

C bus. The following pa-

Typical

(Note 8)

(Note 9)

C bus specifications.

Limits

Units

(Limit)

DS100136-4

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 (V 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: LM77 θ

250˚C/W for the MSOP-8 (MUA08A) package.

Note 6: While the LM77 has a full-scale-range in excess of 128˚C, prolonged operation at temperatures above 125˚C is not recommended. Note 7: Both part numbers of the LM77 will operate properly over the +V

accuracy at their nominal supply voltage. Accuracy will typically degrade 1˚C/V of variation in +V

Note 8: Typicals are at T Note 9: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 10: 9 bits + sign, two’s complement Note 11: This specification is provided only to indicate how often temperature data is updated. The LM77 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 T

subtracts from the T discussion of the function of hysteresis refer to

Note 14: Default values set at power up.

(thermal resistance, junction-to-ambient) when attached to a printed circuit board with 2 oz. foil is: 200˚C/W for the SOP-8 (M08A) package,

HIGH

) at any pin exceeds the power supplies (V

= 25˚C and represent most likely parametric norm.

setpoint value (e.g.: if T

and T_CRIT setpoints (e.g.: if T

LOW

Section 1.1

HIGH

LOW

setpoint = 64˚C, and hysteresis = 2˚C, then actual hysteresis point is 64−2 = 62˚C). For a detailed

, TEMPERATURE COMPARISON, and

GND or V

supply voltage range of 3V to 5.5V . The devices are tested and specified for rated

setpoint = 10˚C, and hysteresis = 2˚C, then actual hysteresis point is 10+2 = 12˚C); and

+VS) the current at that pin should be limited to 5 mA. The 20 mA

as it varies from the nominal value.

Figure 3

www.national.com 6

Logic Electrical Characteristics (Continued)

FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)

LM77

DS100136-5

1.0 Functional Description

The LM77 temperature sensor incorporates a band-gap type temperature sensor, 10-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 T for temperatures which exceed T_CRIT. The lines are programmable for mode and polarity.

1.1 TEMPERATURE COMPARISON

LM77 provides a window comparison against a lower (T and upper (T (T_CRIT) functions as a critical alarm shutdown. depicts the comparison function as well as the modes of operation.

1.1.1 STATUS BITS

The internal Status bits operate as follows: “True”: Temperature above a T

those respective bits. A “true” for T T

LOW

“False”: Assuming temperature has previously crossed above T

HIGH

below the points corresponding T T_CRIT − T T

, assuming temperature has previously crossed below

LOW

, a “false” occurs when temperature goes above T

LOW

HYST

The Status bits are not affected by reads or any other actions, and always represent the state of temperature vs. setpoints.

and T

LOW

) trip point. A second upper trip point

HIGH

window, or the T_CRIT_A line

HIGH

Figure 3

or T_CRIT is “true” for

HIGH

is temperature below

LOW

or T_CRIT, then the temperature must drop

) in order for the condition to be false. For

HYST

HYST(THIGH

−T

HYST

LOW

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 LM77 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 T would stay that way until temperature goes above T T

. However if the user reads the part, the output would

HYST

then INT becomes active. It

LOW

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 T

then INT would become active. It would

LOW

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 T

LOW+THYST

to set the output

again. In either mode, reading any register in the LM77 restarts the

conversion. This allows a designer to know exactly when the LM77 begins a comparison. This prevents unnecessary In-

www.national.com7

1.0 Functional Description (Continued)

LM77

terrupts 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 T of 2˚C then setting T

HIGH

and T

to within 4˚C of each

LOW

other will violate this restriction. To be more specific, with T

set to 2˚C assume T

HYST

equal to, or higher than 60˚C this restriction is violated.

set to 64˚C. If T

HIGH

1.2 DEFAULT SETTINGS

The LM77 always powers up in a known state. LM77 power up default conditions are:

1. Comparator Interrupt Mode

2. T

3. T

LOW HIGH

set to 10˚C

set to 64˚C

4. T_CRIT set to 80˚C

5. T

HYST

set to 2˚C

6. INT and T_CRIT_A active low

7. Pointer set to “00”; Temperature Register The LM77 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 LM77 registers will reset again when the power supply drops below the voltage plotted in this curve.

HYST

LOW

value

is set

Average Power on Reset Voltage vs Temperature

DS100136-18

1.3 SERIAL BUS INTERFACE

The LM77 operates as a slave on the Serial Bus, so the SCL line is an input (no clock is generated by the LM77) and the SDA line is a bi-directional serial data line. According to Serial Bus specifications, the LM77 has a 7-bit slave address. The five most significant bits of the slave address are hard wired inside the LM77 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 +V

for a high, (1).

Therefore, the complete slave address is:

10010A1A0

MSB LSB

www.national.com 8

1.0 Functional Description (Continued)

LM77

DS100136-6

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 LM77 is read.

FIGURE 3. Temperature Response Diagram

1.4 TEMPERATURE DATA FORMAT

Temperature data can be read from the Temperatureand 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 LM77 will prevent data from being updated. Temperature data is represented by a 10-bit, two’s complement word with an LSB (Least Significant Bit) equal to 0.5˚C:

Temperature Digital Output

Binary Hex

+130˚C 01 0000 0100 104h +125˚C 00 1111 1010 0FAh

+25˚C 00 0011 0010 032h

+0.5˚C 00 0000 0001 001h

0˚C 00 0000 0000 000h

−0.5˚C 11 1111 1111 3FFh

−25˚C 11 1100 1110 3CEh

−55˚C 11 1001 0010 392h

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 LM77 takes miliseconds to respond to the shutdown command.

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 LM77. The maximum resistance of the pull up, based on LM77 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 LM77 is used in noisy environments. The 4 faults must occur consecutively to set flags as well as INT and T_CRIT_A outputs. The fault queue is enabled by setting bit 4 of the Configuration Register high (see Section

1.11).

www.national.com9

1.0 Functional Description (Continued)

LM77

1.8 INTERNAL REGISTER STRUCTURE

There are four data registers in the LM77, 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 LM77 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 LM77 will always include the address byte and the Pointer byte. A write to the Configuration register requires one data byte, while the T

LOW,THIGH

, and T_CRIT

registers require two data bytes. Reading the LM77 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 LM77), then the read can simply consist of an address byte,

DS100136-7

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 LM77 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 LM77 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 LM77.

FIGURE 4. Inadvertent 8-Bit Read from 16-Bit Register where D7 is Zero (“0”)

www.national.com 10

DS100136-8

1.0 Functional Description (Continued)

1.9 POINTER REGISTER

(Selects which registers will be read from or written to):

P7 P6 P5 P4 P3 P2 P1 P0

00000 Register Select

P0–P2: Register Select:

P2 P1 P0 Register

0 0 0 Temperature (Read only) (Power-up

default) 0 0 1 Configuration (Read/Write) 010T 0 1 1 T_CRIT (Read/Write) 100T 101T

P3–P7: Must be kept zero.

1.10 TEMPERATURE REGISTER

(Read Only):

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Sign Sign Sign Sign MSB Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

D0–D2: Status Bits D3–D15: Temperature Data. One LSB = 0.5˚C. Two’s complement format.

(Read/Write)

HYST

(Read/Write)

LOW

(Read/Write)

HIGH

CRIT HIGH LOW

Status Bits

LM77

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 LM77 goes to low power shutdown mode. Power up default of “0”. D1: Interrupt mode-0isComparator Interrupt mode, 1 is Event Interrupt mode. Power up default of “0”. D2, D3: T_CRIT_A and INT Polarity-0isactive 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 T

HYST,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 Sign Sign Sign MSB Bit7 Bit6 Bit5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 X X X D0–D2: Undefined D3–D15: T

HYST

HYST,TLOW,THIGH

= 2˚C. is subtracted from T

Avoid programming setpoints so close that their hysteresis values overlap. See

or T_CRIT Trip Temperature Data. Power up default is T

, and T_CRIT, and added to T

HIGH

LOW

= 10˚C, T

LOW

Section 1.1

= 64˚C, T_CRIT = 80˚C,

HIGH

www.national.com11

2.0 I2C Timing Diagrams

LM77

Typical 2-Byte Read From Preset Pointer Location Such as Temp or Comparison Registers

Typical Pointer Set Followed by Immediate Read for 2-Byte Register such as Temp or Comparison Registers

DS100136-9

DS100136-10

DS100136-11

Typical 1-Byte Read from Configuration Register with Preset Pointer

Typical Pointer Set Followed by Immediate Read from Configuration Register

Configuration Register Write

Comparison Register Write

FIGURE 6. Timing Diagrams

DS100136-12

DS100136-13

DS100136-14

www.national.com 12

3.0 Application Hints

The temperature response graph in cal 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 LM77 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_Acan 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).

Figure 7

depicts a typi-

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, T

, causing INT to go active. The system responds to the

HIGH

interrupt by querying the LM77’s status bits and determines that T 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 LM77 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 LM77, 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.

was exceeded, indicating that temperature is

HIGH

LM77

www.national.com13

3.0 Application Hints (Continued)

LM77

DS100136-15

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 LM77 is read.

FIGURE 7. Temperature Response Diagram for ACPI Implementation

4.0 Typical Applications

DS100136-16

FIGURE 8. Typical Application

www.national.com 14

4.0 Typical Applications (Continued)

LM77

DS100136-17

FIGURE 9. Remote HVAC temperature sensor communicates via 3 wires, including thermostat signals.

DS100136-19

FIGURE 10. ACPI Compatible Terminal Alarm Shutdown. By powering the LM77 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.

www.national.com15

Physical Dimensions inches (millimeters) unless otherwise noted

LM77

8-Lead (0.150" Wide) Molded Small Outline Package (SOP),

JEDEC Registration Number MS-012 (variation AA),

Order Number LM77CIM-3, LM77CIMX-3, LM77CIM-5 or LM77CIMX-5

NS Package Number M08A

www.national.com 16

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

LM77 9-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire

Interface

8-Lead (0.150" Wide) Molded Mini Small Outline Package (MSOP),

JEDEC Registration Number MO-187 (variation AA),

Order Number LM77CIMM-3, LM77CIMMX-3, LM77CIMM-5 or LM77CIMMX-5

NS Package Number MUA08A

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

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.

labeling, can be reasonably expected to result in a significant injury to the user.

National Semiconductor Corporation

Americas Email: support@nsc.com

www.national.com

National Semiconductor Europe

Fax: +49 (0) 180-530 85 86

Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790

National Semiconductor Asia Pacific Customer Response Group

Tel: 65-2544466 Fax: 65-2504466 Email: ap.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.

Rainbow Electronics LM77 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual