Analog Devices ADM1031 a Datasheet

Intelligent Temperature Monitor

FEATURES Optimized for Pentium® III: Allows Reduced Guardbanding Software and Automatic Fan Speed Control Automatic Fan Speed Control Allows Control Indepen-

dent of CPU Intervention after Initial Setup

Control Loop Minimizes Acoustic Noise and Battery

Consumption

Remote Temperature Measurement Accurate to 1ⴗC

Using Remote Diode (Two Channels)

0.125ⴗC Resolution on External Temperature Channels Local Temperature Sensor with 0.25ⴗC Resolution Pulsewidth Modulation Fan Control (PWM) for Two Fans Programmable PWM Frequency Programmable PWM Duty Cycle Tach Fan Speed Measurement (Two Channels) Analog Input To Measure Fan Speed of 2-Wire Fans

(Using Sense Resistor)

2-Wire System Management Bus (SMBus) with ARA

Support

Overtemperature THERM Output Pin for CPU Throttling Programmable INT Output Pin Configurable Offsets for Temperature Channels 3 V to 5.5 V Supply Range Shutdown Mode to Minimize Power Consumption Limit Comparison of All Monitored Values

APPLICATIONS Notebook PCs, Network Servers and Personal Computers Telecommunications Equipment

and Dual PWM Fan Controller

ADM1031

PRODUCT DESCRIPTION

The ADM1031 is an ACPI-compliant three-channel digital thermometer and under/over temperature alarm, for use in personal computers and thermal management systems. Optimized for the Pentium III, the higher 1°C accuracy offered allows systems designers to safely reduce temperature guardbanding and increase system performance. Two Pulsewidth Modulated (PWM) Fan Control outputs control the speed of two cooling fans by varying output duty cycle. Duty cycle values between 33%–100% allow smooth control of the fans. The speed of each fan can be monitored via TACH inputs. The TACH inputs may be reprogrammed as analog inputs, allowing fan speeds for 2-wire fans to be measured via sense resistors. The device will also detect a stalled fan. A dedicated Fan Speed Control Loop provides control even without the intervention of CPU software. It also ensures that if the CPU or system locks up, each fan can still be controlled based on temperature measurements, and the fan speed adjusted to correct any changes in system temperature. Fan speed may also be controlled using existing ACPI software. Two inputs (four pins) are dedicated to remote temperature-sensing diodes with an accuracy of ±1°C, and an on-chip temperature sensor allows ambient temperature to be monitored. The device has a programmable INT output to indicate error conditions. There is a dedicated FAN_FAULT output to signal fan failure. The THERM pin is a fail-safe output for overtemperature conditions that can be used to throttle a CPU clock.

FUNCTIONAL BLOCK DIAGRAM

SLAVE

ADDRESS

FAN FILTER

FA N

CHARACTERISTICS

REGISTERS

FAN SPEED

CONFIG

FAN SPEED

COUNTER

ANALOG

MULTIPLEXER

*Patents pending.

PWM_OUT1

PWM_OUT2

TACH2 /AIN2

TACH1 /AIN1

D1+

D1–

D2+

D2–

ADM1031

PWM

CONTROLLERS

TACH SIGNAL

CONDITIONING

BANDGAP

TEMPERATURE

SENSOR

REV. A

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective companies.

SERIAL BUS

INTERFACE

ADDRESS

POINTER

INTERRUPT

STATUS

REGISTERS

LIMIT

COMPARATOR

VA LUE AND LIMIT

REGISTERS

OFFSET

ADC

2.5V

BANDGAP

REFERENCE

GND

REGISTERS

CONFIGURATION

REGISTERS

ADD

SDA

SCL

INT (SMBALERT)

THERM

FAN_FAULT

ADM1031–SPECIFICATIONS

(TA = T

MIN

to T

, VCC = V

MAX

MIN

to V

, unless otherwise noted.)

MAX

Parameter Min Typ Max Unit Test Conditions/Comments

POWER SUPPLY

Supply Voltage, V Supply Current, I

3.0 3.30 5.5 V

1.4 3 mA Interface Inactive, ADC Active 32 50 µA Standby Mode

TEMPERATURE-TO-DIGITAL CONVERTER

Local Sensor Accuracy ± 1 ± 3 °C Resolution 0.25 °C Remote Diode1 Sensor Accuracy ± 0.5 ± 1 °C60°C ≤ T Remote Diode2 Sensor Accuracy ± 0.5 ± 1.75 °C60°C ≤ T

≤ 100°C

Resolution 0.125 °C Remote Sensor Source Current 180 µA High Level

11 µA Low Level

OPEN-DRAIN DIGITAL OUTPUTS (THERM, INT, FAN_FAULT, PWM_OUT)

Output Low Voltage, V High-Level Output Leakage Current, I

0.4 V I

0.1 1 µAV

= –6.0 mA; V

OUT

= VCC; V

OUT

= 3 V

OPEN-DRAIN SERIAL DATA BUS OUTPUT (SDA)

Output Low Voltage, V High-Level Output Leakage Current, I

0.4 V I

0.1 1 µAV

= –6.0 mA; V

OUT

= V

OUT

= 3 V

SERIAL BUS DIGITAL INPUTS (SCL, SDA)

Input High Voltage, V Input Low Voltage, V

2.1 V

0.8 V

Hysteresis 500 mV

DIGITAL INPUT LOGIC LEVELS

(ADD, THERM, TACH1/2)

Input High Voltage, V Input Low Voltage, V

2.1 V

0.8 V

DIGITAL INPUT LEAKAGE CURRENT

Input High Current, I Input Low Current, I Input Capacitance, C

–1 µAV

1 µAV

5pF

= V = 0

FAN RPM-TO-DIGITAL CONVERTER

Accuracy ± 6% 60°C ≤ T

≤ 100°C

Full-Scale Count 255 TACH Nominal Input RPM 4400 RPM Divisor N = 1, Fan Count = 153

2200 RPM Divisor N = 2, Fan Count = 153 1100 RPM Divisor N = 4, Fan Count = 153 550 RPM Divisor N = 8, Fan Count = 153

Conversion Cycle Time 637 ms

SCLK

BUF

SU;STA

HD;STA

LOW

HIGH

SU;DAT

HD;DAT

10 100 kHz See Figure 1

50 ns See Figure 1

4.7 µs See Figure 1

4.7 µs See Figure 1 4 µs See Figure 1

SU;STO

4 µs See Figure 1

1.3 µs See Figure 1 450µs See Figure 1

1000 ns See Figure 1

300 ns See Figure 1 250 ns See Figure 1 300 ns See Figure 1

= 0.8 V for a falling edge and V

= 2.2 V for a rising edge.

SERIAL BUS TIMING

Clock Frequency, f Glitch Immunity, t Bus Free Time, t Start Setup Time, t Start Hold Time, t Stop Condition Setup Time t SCL Low Time, t SCL High Time, t SCL, SDA Rise Time, t SCL, SDA Fall Time, t Data Setup Time, t Data Hold Time, t

NOTES

Typicals are at TA = 25°C and represent most likely parametric norm. Shutdown current typ is measured with VCC = 3.3 V.

ADD is a three-state input that may be pulled high, low or left open-circuit.

Timing specifications are tested at logic levels of V

Specifications subject to change without notice.

–2–

REV. A

ADM1031

ABSOLUTE MAXIMUM RATINGS*

Positive Supply Voltage (VCC) . . . . . . . . . . . . . . . . . . . . .6.5 V

Voltage on Any Input or Output Pin . . . . . . . . –0.3 V to +6.5 V

Input Current at Any Pin . . . . . . . . . . . . . . . . . . . . . . . ± 5 mA

Package Input Current . . . . . . . . . . . . . . . . . . . . . . . ± 20 mA

Maximum Junction Temperature (T

) . . . . . . . . . . 150°C

JMAX

Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C

Lead Temperature, Soldering

Vapor Phase 60 sec . . . . . . . . . . . . . . . . . . . . . . . . . . 215°C

Infrared 15 sec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200°C

ESD Rating All Pins . . . . . . . . . . . . . . . . . . . . . . . . . . 2000 V

*Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

THERMAL CHARACTERISTICS

16-Lead QSOP Package

= 105°C/W, θ

SCL

= 39°C/W

HD:STA

LOW

HD:DAT

HIGH

SU:DAT

ORDERING GUIDE

Temperature Package Package

Model Range Description Option

ADM1031ARQ 0°C to 100°C 16-Lead QSOP RQ-16

HD:STA

SU:STA

SU:STO

SDA

BUF

Figure 1. Diagram for Serial Bus Timing

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the ADM1031 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.

PSP

REV. A

–3–

ADM1031

PIN FUNCTION DESCRIPTIONS

Pin No. Mnemonic Description

1 PWM_OUT1 Digital Output (Open-Drain). Pulsewidth modulated output to control fan speed. Requires pull-

up resistor (10 kΩ typical).

2 TACH1/AIN1 Digital/Analog Input. Fan tachometer input to measure FAN1 fan speed. May be reprogrammed as

an analog input to measure speed of a 2-wire fan via a sense resistor (2 Ω typical).

3 PWM_OUT2 Digital Output (Open-Drain). Pulsewidth Modulated output to control FAN2 fan speed.

Requires pull-up resistor (10 kΩ typical).

4 TACH2/AIN2 Digital/Analog Input. Fan tachometer input to measure FAN2 fan speed. May be repro-

grammed as an analog input to measure speed of a 2-wire fan via a sense resistor (2 Ω typical). 5GND System Ground. 6V

7 THERM Digital I/O (Open-Drain). An active low thermal overload output that indicates a violation of a

8 FAN_FAULT Digital Output (Open-Drain). Can be used to signal a fan fault. Drives second fan to full speed

9 D1– Analog Input. Connected to cathode of first remote temperature-sensing diode. The temperature-

10 D1+ Analog Input. Connected to anode of first remote temperature-sensing diode. 11 D2– Analog Input. Connected to cathode of second remote temperature-sensing diode. 12 D2+ Analog Input. Connected to anode of second remote temperature-sensing diode. 13 ADD Three-State Logic Input. Sets two lower bits of device SMBus address. 14 INT (SMBALERT)Digital Output (Open-Drain). Can be programmed as an interrupt (SMBus ALERT) output for

15 SDA Digital I/O. Serial Bus Bidirectional Data. Open-drain output. Requires pull-up resistor

16 SCL Digital Input. Serial Bus Clock. Requires pull-up resistor (2.2 kΩ typical).

Power. Can be powered by 3.3 V Standby power if monitoring in low power states is required.

temperature set point (overtemperature). Also acts as an input to provide external fan control.

When this pin is pulled low by an external signal, a status bit is set, and the fan speed is set to

full-on. Requires pull-up resistor (10 kΩ).

if one fan fails. Requires pull-up resistor (typically 10 kΩ).

sensing element is either a Pentium III substrate transistor or a general-purpose 2N3904.

temperature/fan speed interrupts. Requires pull-up resistor (10 kΩ typical).

(2.2 kΩ typical).

PIN CONFIGURATION

GND

THERM

ADM1031

TOP VIEW

(Not to Scale)

PWM_OUT1

TACH1/AIN1

PWM_OUT2

TACH2/AIN2

FAN_FAULT

SCL

SDA

INT(SMBALERT)

ADD

D2+

D2–

D1+

D1–

–4–

REV. A

Typical Performance Characteristics–ADM1031

DXP – DXN CAPACITANCE – nF

–2

–10

1472.2

REMOTE TEMPERATURE ERROR – ⴗC

3.3

4.7

10 22

–3

–5

–7

–9

–6

–11 –12 –13 –14 –15 –16

–1

–4

–8

–5

–10

–15

REMOTE TEMPERATURE ERROR – ⴗC

–20

3.3 LEAKAGE RESISTANCE – M⍀

DXP TO GND

DXP TO VCC (3.3V)

10 30

100

TPC 1. Temperature Error vs. PCB Track Resistance

ⴗ

REMOTE TEMPERATURE ERROR –

–1

VIN = 200mV p-p

500k 2M

VIN = 100mV p-p

4M 6M 10M 100M 400M

FREQUENCY – Hz

110

100

READING – ⴗC

06010

40 50

PIII TEMPERATURE – ⴗC

70 80 90 100 110

TPC 4. Pentium III Temperature Measurement vs. ADM1031 Reading

TPC 2. Temperature Error vs. Power Supply Noise Frequency

ⴗ

REMOTE TEMPERATURE ERROR –

–1

0 400M100k 1M

VIN = 40mV p-p

200M 300M

100M

FREQUENCY – Hz

VIN = 20mV p-p

500M

TPC 3. Temperature Error vs. Common-Mode Noise Frequency

REV. A

TPC 5. Temperature Error vs. Capacitance between D+ and D–

110

100

␮

SUPPLY CURRENT –

0751

25 50

SCLK FREQUENCY – kHz

VCC = 5V

VCC = 3.3V

100 250 500 750 1000

TPC 6. Standby Current vs. Clock Frequency

–5–

ADM1031

REMOTE TEMPERATURE ERROR – ⴗC

–1

0 400M100k

VIN = 30mV p-p

VIN = 20mV p-p

100M

FREQUENCY – Hz

200M 300M

500M

TPC 7. Temperature Error vs. Differential-Mode Noise Frequency

200

180

160

140

120

100

SUPPLY CURRENT – ␮A

–20

0 1.1 1.3 1.5 1.7 1.9 2.1

ADD = V

SUPPLY VOLTAGE – V

ADD = GND

ADD = Hi-Z

2.9 4.5

2.5

0.08

–0.08

–0.16

–0.24

ⴗ

–0.32

–0.40

ERROR –

–0.48

–0.56

–0.64

–0.72

–0.80

20 40 60 80 85 100 105 120

TEMPERATURE – ⴗC

TPC 10. Remote Temperature Sensor Error

1.30

1.25

1.20

1.15

1.10

1.05

1.00

0.95

SUPPLY CURRENT – mA

0.90

0.85

0.80

2.0

2.8 3.2 3.6 4.0 4.4 4.82.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0

2.4 SUPPLY VOLTAGE – V

TPC 8. Standby Supply Current vs. Supply Voltage

0.08

–0.08

–0.16

–0.24

ⴗ

–0.32

–0.40

ERROR –

–0.48

–0.56

–0.64

–0.72

–0.80

20 40 60 80 85 100 105 120

TEMPERATURE – ⴗC

TPC 9. Local Sensor Temperature Error

TPC 11. Supply Current vs. Supply Voltage

120

110

100

TEMPERATURE – ⴗC

4681013579

TIME – Sec

TPC 12. Response to Thermal Shock

–6–

REV. A

ADM1031

GENERAL DESCRIPTION

The ADM1031 is a temperature monitor and dual PWM fan controller for microprocessor-based systems. The device communicates with the system via a serial System Management Bus. The serial bus controller has a hardwired address pin for device selection (Pin 13), a serial data line for reading and writing addresses and data (Pin 15), and an input line for the serial clock (Pin 16). All control and programming functions of the ADM1031 are performed over the serial bus. The device also supports Alert Response Address (ARA).

INTERNAL REGISTERS OF THE ADM1031

A brief description of the ADM1031’s principal internal registers is given below. More detailed information on the function of each register is given in Table XII to Table XXIX.

Configuration Register

Provides control and configuration of various functions on the device.

Address Pointer Register

This register contains the address that selects one of the other internal registers. When writing to the ADM1031, the first byte of data is always a register address, which is written to the Address Pointer Register.

Status Registers

These registers provide status of each limit comparison.

Value and Limit Registers

The results of temperature and fan speed measurements are stored in these registers, along with their limit values.

Fan Speed Config Register

This register is used to program the PWM duty cycle for each fan.

Offset Registers

Allows the temperature channel readings to be offset by a 5-bit two’s complement value written to these registers. These values will automatically be added to the temperature values (or subtracted from if negative). This allows the systems designer to optimize the system if required, by adding or subtracting up to 15°C from a temperature reading.

Fan Characteristics Registers

These registers are used to select the spin-up time, PWM frequency, and speed range for the fans used.

THERM Limit Registers

These registers contain the temperature values at which THERM will be asserted.

MIN/TRANGE

Registers

These registers are read/write registers that hold the minimum temperature value below which the fan will not run when the device is in Automatic Fan Speed Control Mode. These registers also hold the temperature range value that defines the range over which auto fan control will be provided, and hence determines the temperature at which the fan will run at full speed.

SERIAL BUS INTERFACE

Control of the ADM1031 is carried out via the SMBus. The ADM1031 is connected to this bus as a slave device, under the control of a master device, e.g., the 810 chipset.

The ADM1031 has a 7-bit serial bus address. When the device is powered up, it will do so with a default serial bus address. The five MSBs of the address are set to 01011, the two LSBs are determined by the logical state of Pin 13 (ADD). This is a

three-state input that can be grounded, connected to V

CC,

or left open-circuit to give three different addresses. The state of the ADD pin is only sampled at power-up, so changing ADD with power on will have no effect until the device is powered off, then on again.

Table I. ADD Pin Truth Table

ADD Pin A1 A0 GND 0 0 No Connect 1 0 V

If ADD is left open-circuit, the default address will be 0101110.

The facility to make hardwired changes at the ADD pin allows the user to avoid conflicts with other devices sharing the same serial bus; for example, if more than one ADM1031 is used in a system.

The serial bus protocol operates as follows:

1. The master initiates data transfer by establishing a START

condition, defined as a high-to-low transition on the serial data line SDA while the serial clock line SCL remains high. This indicates that an address/data stream will follow. All slave peripherals connected to the serial bus respond to the START condition, and shift in the next 8 bits, consisting of a 7-bit address (MSB first) plus an R/W bit that determines the direction of the data transfer, i.e., whether data will be written to or read from the slave device.

The peripheral whose address corresponds to the transmitted address responds by pulling the data line low during the low period before the ninth clock pulse, known as the Acknowledge Bit. All other devices on the bus now remain idle while the selected device waits for data to be read from or written to it. If the R/W bit is a 0, the master will write to the slave device. If the R/W bit is a 1, the master will read from the slave device.

2. Data is sent over the serial bus in sequences of nine clock

pulses, eight bits of data followed by an Acknowledge Bit from the slave device. Transitions on the data line must occur during the low period of the clock signal and remain stable during the high period, as a low-to-high transition when the clock is high may be interpreted as a STOP signal. The number of data bytes that can be transmitted over the serial bus in a single READ or WRITE operation is limited only by what the master and slave devices can handle.

3. When all data bytes have been read or written, stop condi-

tions are established. In WRITE mode, the master will pull the data line high during the tenth clock pulse to assert a STOP condition. In READ mode, the master device will override the acknowledge bit by pulling the data line high during the low period before the ninth clock pulse. This is known as No Acknowledge. The master will then take the data line low during the low period before the tenth clock pulse, then high during the tenth clock pulse to assert a STOP condition.

Any number of bytes of data may be transferred over the serial bus in one operation, but it is not possible to mix read and write in one operation, because the type of operation is determined at the beginning and cannot subsequently be changed without starting a new operation.

REV. A

–7–

ADM1031

In the case of the ADM1031, write operations contain either one or two bytes, and read operations contain one byte, and perform the following functions.

To write data to one of the device data registers or read data from it, the Address Pointer Register must be set so that the correct data register is addressed; data can then be written into that register or read from it. The first byte of a write operation always contains an address that is stored in the Address Pointer Register. If data is to be written to the device, the write operation contains a second data byte that is written to the register selected by the address pointer register.

This is illustrated in Figure 2a. The device address is sent over the bus followed by R/W set to 0. This is followed by two data bytes. The first data byte is the address of the internal data register to be written to, which is stored in the Address Pointer Register. The second data byte is the data to be written to the internal data register.

When reading data from a register there are two possibilities:

1. If the ADM1031’s Address Pointer Register value is unknown or not the desired value, it is first necessary to set it to the correct value before data can be read from the desired data register. This is done by performing a write to the ADM1031

SCL

as before, but only the data byte containing the register address is sent, as data is not to be written to the register. This is shown in Figure 2b.

A read operation is then performed consisting of the serial bus address, R/W bit set to 1, followed by the data byte read from the data register. This is shown in Figure 2c.

2. If the Address Pointer Register is known to be already at the desired address, data can be read from the corresponding data register without first writing to the Address Pointer Register, so Figure 2b can be omitted.

NOTES

1. Although it is possible to read a data byte from a data register without first writing to the Address Pointer Register, if the Address Pointer Register is already at the correct value, it is not possible to write data to a register without writing to the Address Pointer Register, because the first data byte of a write is always written to the Address Pointer Register.

2. In Figures 2a to 2c, the serial bus address is shown as the default value 01011(A1)(A0), where A1 and A0 are set by the three-state ADD pin.

3. The ADM1031 also supports the Read Byte protocol, as described in the System Management Bus specification.

SDA

START BY

MASTER

1011

SERIAL BUS ADDRESS BYTE

FRAME 1

SCL (CONTINUED)

SDA (CONTINUED)

R/W

ACK. BY

ADM1031

ADDRESS POINTER REGISTER BYTE

FRAME 2

FRAME 3

DATA BYTE

ACK. BY

ADM1031

ACK. BY

ADM1031

STOP BY

MASTER

Figure 2a. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

ACK. BY ADM1031

STOP BY MASTER

SCL

SDA

START BY

MASTER

1011

FRAME 1

SERIAL BUS ADDRESS BYTE

R/W

ACK. BY

ADM1031

ADDRESS POINTER REGISTER BYTE

FRAME 2

Figure 2b. Writing to the Address Pointer Register Only

SCL

SDA

START BY

MASTER

1011

SERIAL BUS ADDRESS BYTE

FRAME 1

R/W

ACK. BY

ADM1031

FRAME 2

DATA BYTE FROM ADM1031

Figure 2c. Reading Data from a Previously Selected Register

–8–

NO ACK.

BY MASTER

STOP BY MASTER

REV. A

ADM1031

ALERT RESPONSE ADDRESS

Alert Response Address (ARA) is a feature of SMBus devices that allows an interrupting device to identify itself to the host when multiple devices exist on the same bus.

The INT output can be used as an interrupt output or can be used as an SMBALERT. One or more INT outputs can be connected to a common SMBALERT line connected to the master. If a device’s INT line goes low, the following procedure occurs:

1. SMBALERT pulled low.

2. Master initiates a read operation and sends the Alert Response Address (ARA = 0001 100). This is a general call address that must not be used as a specific device address.

3. The device whose INT output is low responds to the Alert Response Address, and the master reads its device address. The address of the device is now known and can be interrogated in the usual way.

4. If more than one device’s INT output is low, the one with the lowest device address will have priority, in accordance with normal SMBus arbitration.

5. Once the ADM1031 has responded to the Alert Response Address, it will reset its INT output; however, if the error condition that caused the interrupt persists, INT will be reasserted on the next monitoring cycle.

TEMPERATURE MEASUREMENT SYSTEM Internal Temperature Measurement

The ADM1031 contains an on-chip bandgap temperature sensor. The on-chip ADC performs conversions on the output of this sensor and outputs the temperature data in 10-bit two’s complement format. The resolution of the local temperature sensor is 0.25°C. The format of the temperature data is shown in Table II.

External Temperature Measurement

The ADM1031 can measure the temperatures of two external diode sensors or diode-connected transistors, connected to Pins 9 and 10 and Pins 11 and 12.

These pins are dedicated temperature input channels. The function of Pin 7 is as a THERM input/output and is used to flag overtemperature conditions.

The forward voltage of a diode or diode-connected transistor, operated at a constant current, exhibits a negative temperature coefficient of about –2 mV/°C. Unfortunately, the absolute value of V

, varies from device to device, and individual

calibration is required to null this out, so the technique is unsuitable for mass production.

The technique used in the ADM1031 is to measure the change in V

when the device is operated at two different currents.

This is given by:

∆V

= KT/q × ln (N)

where:

K is Boltzmann’s constant. q is charge on the carrier. T is absolute temperature in Kelvins. N is ratio of the two currents.

Figure 3 shows the input signal conditioning used to measure the output of an external temperature sensor. This figure shows the external sensor as a substrate transistor, provided for temperature monitoring on some microprocessors, but it could equally well be a discrete transistor.

REMOTE

SENSING

TRANSISTOR

IN ⴛ II

D+

D–

BIAS

DIODE

BIAS

LOW-PASS

FILTER

= 65kHz

OUT+

OUT–

ADC

Figure 3. Signal Conditioning

If a discrete transistor is used, the collector will not be grounded, and should be linked to the base. If a PNP transistor is used, the base is connected to the D– input and the emitter to the D+ input. If an NPN transistor is used, the emitter is connected to the D– input and the base to the D+ input.

One LSB of the ADC corresponds to 0.125°C, so the ADM1031 can theoretically measure temperatures from –127°C to +127.75°C, although –127°C is outside the operating range for the device. The extended temperature resolution data format is shown in Tables III and IV.

Table II. Temperature Data Format (Local Temperature and Remote Temperature High Bytes)

Temperature (ⴗC) Digital Output

–128°C 1000 0000 –125°C 1000 0011 –100°C 1001 1100 –75°C 1011 0101 –50°C 1100 1110 –25°C 1110 0111 –1°C 1111 1111 0°C 0000 0000 +1°C 0000 0001 +10°C 0000 1010 +25°C 0001 1001 +50°C 0011 0010 +75°C 0100 1011 +100°C 0110 0100 +125°C 0111 1101 +127°C 0111 1111

REV. A

–9–

ADM1031

Table III. Remote Sensor Extended Temperature Resolution

Extended Remote Temperature Resolution (ⴗC) Low Bits

0.000 000

0.125 001

0.250 010

0.375 011

0.500 100

0.625 101

0.750 110

0.875 111

The extended temperature resolution for the local and remote channels is stored in the Extended Temperature Resolution Register (Register 0x06), and is outlined in Table XVIII.

Table IV. Local Sensor Extended Temperature Resolution

Extended Local Temperature Resolution (ⴗC) Low Bits

0.00 00

0.25 01

0.50 10

0.75 11

To prevent ground noise interfering with the measurement, the more negative terminal of the sensor is not referenced to ground, but biased above ground by an internal diode at the D– input. If the sensor is used in a very noisy environment, a capacitor of value up to 1000 pF may be placed between the D+ and D– inputs to filter the noise.

To measure ∆V

, the sensor is switched between operating

ΒΕ

currents of I and N × I. The resulting waveform is passed through a 65 kHz low-pass filter to remove noise, then to a chopperstabilized amplifier that performs the functions of amplification and rectification of the waveform to produce a dc voltage proportional to ∆V

. This voltage is measured by the ADC to give

a temperature output in 11-bit two’s complement format. To further reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles. An external temperature measurement nominally takes 9.6 ms.

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments and care must be taken to protect the analog inputs from noise, particularly when measuring the very small voltages from a remote diode sensor. The following precautions should be taken:

1. Place the ADM1031 as close as possible to the remote sensing diode. Provided that the worst noise sources such as clock generators, data/address buses, and CRTs are avoided, this distance can be 4 to 8 inches.

2. Route the D+ and D– tracks close together, in parallel, with grounded guard tracks on each side. Provide a ground plane under the tracks if possible.

3. Use wide tracks to minimize inductance and reduce noise pickup. 10 mil track minimum width and spacing is recommended.

GND

D+

D–

GND

10MIL

Figure 4. Arrangement of Signal Tracks

4. Try to minimize the number of copper/solder joints, which can cause thermocouple effects. Where copper/solder joints are used, make sure that they are in both the D+ and D– path and at the same temperature.

Thermocouple effects should not be a major problem as 1°C corresponds to about 200 µV, and thermocouple voltages are about 3 µV/°C of temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200 µV.

5. Place a 0.1 µF bypass capacitor close to the ADM1031.

6. If the distance to the remote sensor is more than 8 inches, the use of twisted pair cable is recommended. This will work up to about 6 to 12 feet.

7. For really long distances (up to 100 feet) use shielded twisted pair such as Belden #8451 microphone cable. Connect the twisted pair to D+ and D– and the shield to GND close to the ADM1031. Leave the remote end of the shield unconnected to avoid ground loops.

Because the measurement technique uses switched current sources, excessive cable and/or filter capacitance can affect the measurement. When using long cables, the filter capacitor C1 may be reduced or removed. In any case the total shunt capacitance should not exceed 1000 pF.

Cable resistance can also introduce errors. 1 Ω series resistance introduces about 0.5°C error.

ADDRESSING THE DEVICE

ADD (Pin 13) is a three-state input. It is sampled, on power-up to set the lowest two bits of the serial bus address. Up to three addresses are available to the systems designer via this address pin. This reduces the likelihood of conflicts with other devices attached to the System Management Bus.

THE ADM1031 INTERRUPT SYSTEM

The ADM1031 has two interrupt outputs, INT and THERM. These have different functions. INT responds to violations of software programmed temperature limits and is maskable (described in more detail later).

THERM is intended as a “fail-safe” interrupt output that cannot be masked. If the temperature is below the low temperature limit, the INT pin will be asserted low to indicate an out-of-limit condition. If the temperature exceeds the high temperature limit, the INT pin will also be asserted low. A third limit; THERM limit, may be programmed into the device to set the temperature limit above which the overtemperature THERM pin will be

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