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1C Temperature Monitor with Series Resistance Cancellation ADT7461A
FEATURES
On-chip and remote temperature sensor 0.25C resolution/1C accuracy on remote channel 1C resolution/1C accuracy on local channel Automatically cancels up to 1.5 k (typical) of resistance in series with remote diode to allow noise filtering Extended, switchable temperature measurement range 0C to +127C (default) or -64C to +191C Pin- and register-compatible with ADM1032 and ADT7461 2-wire SMBus serial interface with SMBus alert support Programmable over/under temperature limits Offset registers for system calibration Up to two overtemperature fail-safe THERM outputs Small 8-lead MSOP 240 A operating current, 5 A standby current
APPLICATIONS
Desktop and notebook computers Industrial controllers Smart batteries Automotive Embedded systems Burn-in applications Instrumentation
FUNCTIONAL BLOCK DIAGRAM
ADDRESS POINTER REGISTER ON-CHIP TEMPERATURE SENSOR CONVERSION RATE REGISTER LOCAL TEMPERATURE VALUE REGISTER LOCAL TEMPERATURE LOW-LIMIT REGISTER LOCAL TEMPERATURE HIGH-LIMIT REGISTER REMOTE TEMPERATURE LOW-LIMIT REGISTER REMOTE TEMPERATURE HIGH-LIMIT REGISTER LOCAL THERM LIMIT REGISTERS EXTERNAL THERM LIMIT REGISTERS CONFIGURATION REGISTERS INTERRUPT MASKING
6 4
DIGITAL MUX
LIMIT COMPARATOR
D+ 2 D- 3
ANALOG MUX
A-TO-D CONVERTER BUSY RUN/STANDBY REMOTE TEMPERATURE VALUE REGISTER REMOTE OFFSET REGISTER
EXTERNAL DIODE OPEN-CIRCUIT STATUS REGISTER
DIGITAL MUX
ALERT/THERM2 THERM
ADT7461A
1 5 7
SMBus INTERFACE
8
VDD
GND
SDATA
SCLK
Figure 1.
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. Specifications subject to change without notice. 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 owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2006 Analog Devices, Inc. All rights reserved.
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ADT7461A TABLE OF CONTENTS
Features .............................................................................................. 1 Applications....................................................................................... 1 Functional Block Diagram .............................................................. 1 Revision History ............................................................................... 2 General Description ......................................................................... 3 Differences between the ADT7461A and the ADT7461 ........ 3 Specifications..................................................................................... 4 SMBus Timing Specifications ..................................................... 5 Timing Diagram ........................................................................... 5 Absolute Maximum Ratings............................................................ 6 Thermal Resistance ...................................................................... 6 ESD Caution.................................................................................. 6 Pin Configuration and Function Descriptions............................. 7 Typical Performance Characteristics ............................................. 8 Theory of Operation ...................................................................... 10 Series Resistance Cancellation.................................................. 10 Temperature Measurement Method ........................................ 10 Temperature Measurement Results ......................................... 11 Temperature Measurement Range ........................................... 11 Temperature Data Format......................................................... 11 ADT7461A Registers ................................................................. 12 Serial Bus Interface..................................................................... 15 Addressing the Device ............................................................... 15 ALERT Output............................................................................ 17 Low Power Standby Mode......................................................... 17 Sensor Fault Detection .............................................................. 18 The ADT7461A Interrupt System............................................ 18 Application Information ........................................................... 19 Thermal Inertia and Self-Heating............................................ 20 Layout Considerations............................................................... 20 Application Circuit..................................................................... 21 Outline Dimensions ....................................................................... 22 Ordering Guide .......................................................................... 22
REVISION HISTORY
5/06--Rev. 0 to Rev. A Changes to Features.......................................................................... 1 Changes to General Description .................................................... 3 Added Differences Between the ADT7461A and the ADT7461 Section and Inserted Table 1 .................................. 3 Changes to Table 2............................................................................ 4 Changes to Table 4............................................................................ 6 Changes to Figure 14........................................................................ 9 Changes to Theory of Operation Section.................................... 10 Changes to Temperature Measurement Range Section............. 11 Changes to ADT7461A Registers Section................................... 12 Changes to Limit Registers Section.............................................. 13 Changes to Serial Bus Interface Section and Table 14 ............... 15 Changes to Low Power Standby Mode Section .......................... 17 4/06--Revision 0: Initial Version
Rev. A | Page 2 of 24
ADT7461A GENERAL DESCRIPTION
The ADT7461A1 is a dual-channel digital thermometer and undertemperature/overtemperature alarm, intended for use in PCs and thermal management systems. It is pin- and registercompatible with the ADM1032 and the ADT7461. A feature of the ADT7461A is series resistance cancellation, where up to 1.5 k (typical) of resistance in series with the temperature monitoring diode can be automatically cancelled from the temperature result, allowing noise filtering. The ADT7461A has a configurable ALERT output and an extended, switchable temperature measurement range. The ADT7461A can measure the temperature of a remote thermal diode accurate to 1C and the ambient temperature accurate to 3C. The temperature measurement range defaults to 0C to +127C, compatible with the ADM1032, but it can be switched to a wider measurement range of -64C to +191C. The ADT7461A communicates over a 2-wire serial interface, compatible with system management bus (SMBus) standards. The default SMBus address of the ADT7461A is 0x4C. An ADT7461A-2 is available with an SMBus address of 0x4D. This is useful if more than one ADT7461A is used on the same SMBus. An ALERT output signals when the on-chip or remote temperature is out of range. The THERM output is a comparator output that allows on/off control of a cooling fan. The ALERT output can be reconfigured as a second THERM output, if required.
1
DIFFERENCES BETWEEN THE ADT7461A AND THE ADT7461
Although the ADT7461A is pin- and register-compatible with the ADT7461, there are some specification differences between the two devices. A summary of these differences is shown in Table 1. Table 1. Differences Between the ADT7461A and the ADT7461
Specification Supply Voltage Maximum Local Sensor Accuracy Maximum Series Resistance Cancellation Average Operating Supply Current 16 Conversions/sec Standby Mode Maximum Conversion Time One Shot, Averaging On One Shot, Averaging Off Remote Sensor Current Levels High Mid Low ADT7461A 3 to 3.6 1 1.5 ADT7461 3 to 5.5 3 3 Unit V C k
240 5 52 8 220 82 13.5
170 5.5 114.6 12.56 96 36 6
A A ms ms A A A
Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239; 6,133,753; 6,169,442; 7,010,440 ; other patents pending.
Rev. A | Page 3 of 24
ADT7461A SPECIFICATIONS
TA = -40C to +125C, VDD = 3 V to 3.6 V, unless otherwise noted. Table 2.
Parameter POWER SUPPLY Supply Voltage, VDD Average Operating Supply Current, IDD Undervoltage Lockout Threshold Power-On-Reset Threshold TEMPERATURE-TO-DIGITAL CONVERTER Local Sensor Accuracy Min 3.0 Typ 3.30 240 5 2.55 Max 3.6 350 30 2.5 1 1.5 2.5 1 1 1.5 2.5 0.25 220 82 13.5 40 6 Unit V A A V V C C C C C C C C A A A ms ms Test Conditions
0.0625 conversions/sec rate 1 Standby mode VDD input, disables ADC, rising edge
1
0C TA +70C 0C TA +85C -40C TA +100C 0C TA +70C, -55C TD 2 +150C 0C TA +85C, -55C TD2 +150C -40C TA +100C, -55C TD2 +150C High level 3 Middle level3 Low level3 From stop bit to conversion complete, one-shot mode with averaging switched on One-shot mode with averaging off (that is, conversion rate = 16-, 32-, or 64-conversions per second) Resistance split evenly on both the D+ and D- inputs
Resolution Remote Diode Sensor Accuracy
Resolution Remote Sensor Source Current
Conversion Time
52 8
Maximum Series Resistance Cancelled OPEN-DRAIN DIGITAL OUTPUTS (THERM, ALERT/THERM2) Output Low Voltage, VOL High Level Output Leakage Current, IOH SMBus INTERFACE3, 4 Logic Input High Voltage, VIH SCLK, SDATA Logic Input Low Voltage, VIL SCLK, SDATA Hysteresis SDA Output Low Voltage, VOL Logic Input Current, IIH, IIL SMBus Input Capacitance, SCLK, SDATA SMBus Clock Frequency SMBus Timeout 5 SCLK Falling Edge to SDATA Valid Time
1 2
1. 5
k
0.1 2.1
0.4 1
V A V
IOUT = -6.0 mA VOUT = VDD 3 V VDD 3.6 V 3 V VDD 3.6 V
0.8 500 -1 5 25 400 64 1 0.4 +1
V mV V A pF kHz ms s
IOUT = -6.0 mA
User programmable Master clocking in data
See Table 10 for information on other conversion rates. Guaranteed by characterization, but not production tested. 3 Guaranteed by design, but not production tested. 4 See SMBus Timing Specifications section for more information. 5 Disabled by default. Detailed procedures to enable it are in the Serial Bus Interface section of this data sheet.
Rev. A | Page 4 of 24
ADT7461A
SMBUS TIMING SPECIFICATIONS
Table 3.
Parameter 1 fSCLK tLOW tHIGH tR tF tSU; STA tHD; STA 2 tSU; DATT 3 tSU; STO 4 tBUF
1 2
Limit at TMIN and TMAX 400 1.3 0.6 300 300 600 600 100 600 1.3
Unit kHz max s min s min ns max ns max ns min ns min ns min ns min s min
Description Clock low period, between 10% points Clock high period, between 90% points Clock/data rise time Clock/data fall time Start condition setup time Start condition hold time Data setup time Stop condition setup time Bus free time between stop and start conditions
Guaranteed by design, but not production tested. Time from 10% of SDATA to 90% of SCLK. 3 Time for 10% or 90% of SDATA to 10% of SCLK. 4 Time for 90% of SCLK to 10% of SDATA.
TIMING DIAGRAM
tLOW
SCLK
tR
tF
tHD;STA
tHD;STA tHD;DAT
SDATA
tHIGH tSU;DAT
tSU;STA
tSU;STO
STOP START
START
STOP
Figure 2. Serial Bus Timing
Rev. A | Page 5 of 24
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tBUF
ADT7461A ABSOLUTE MAXIMUM RATINGS
Table 4.
Parameter Positive Supply Voltage (VDD) to GND D+ D- to GND SCLK, SDATA, ALERT, THERM Input Current, SDATA, THERM Input Current, D- ESD Rating, All Pins (Human Body Model) Maximum Junction Temperature (TJ Max) Storage Temperature Range Rating -0.3 V, +3.6 V -0.3 V to VDD + 0.3 V -0.3 V to +0.6 V -0.3 V to +3.6 V -1 mA, +50 mA 1 mA 1500 V 150C -65C to +150C
Stresses above those listed under Absolute Maximum Ratings may cause permanent 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 RESISTANCE
Table 5. Thermal Resistance
Package Type 8-Lead MSOP JA 142 JC 43.74 Unit C/W
ESD 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 this product 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.
Rev. A | Page 6 of 24
ADT7461A PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
VDD D+ D- THERM
1 2 3 4 8
ADT7461A
TOP VIEW (Not to Scale)
SCLK SDATA GND
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7 6 5
ALERT/THERM2
Figure 3. Pin Configuration
Table 6. Pin Function Descriptions
Pin No. 1 2 3 4 5 6 7 8 Mnemonic VDD D+ D- THERM GND ALERT/THERM2 SDATA SCLK Description Positive Supply, 3 V to 3.6 V. Positive Connection to Remote Temperature Sensor. Negative Connection to Remote Temperature Sensor. Open-Drain Output. Can be used to turn a fan on/off or throttle a CPU clock in the event of an overtemperature condition. Requires pull-up resistor. Supply Ground Connection. Open-Drain Logic Output Used as Interrupt or SMBus ALERT. This can also be configured as a second THERM output. Requires pull-up resistor. Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pull-up resistor. Logic Input, SMBus Serial Clock. Requires pull-up resistor.
Rev. A | Page 7 of 24
ADT7461A TYPICAL PERFORMANCE CHARACTERISTICS
3.5 3.0 DEV 1 DEV 2 DEV 3 DEV 4 DEV 5 DEV 6 DEV 7 DEV 8 DEV 9 DEV 10 DEV 11 DEV 12 DEV 13 DEV 14 DEV 15 DEV 16 MEAN HIGH 4 LOW 4 0 -2
TEMPERATURE ERROR (C)
2.0 1.5 1.0 0.5 0 -0.5
TEMPERATURE ERROR (C)
2.5
-4 -6 -8 -10 -12 -14 -16 DEV 4 DEV 3 DEV 2
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0
50 TEMPERATURE (C)
100
150
0
5
10
15
20
25
CAPACITANCE (nF)
Figure 4. Local Temperature Error vs. Temperature
3.5 3.0 DEV 1 DEV 2 DEV 3 DEV 4 DEV 5 DEV 6 DEV 7 DEV 8 DEV 9 DEV 10 DEV 11 DEV 12 DEV 13 DEV 14 DEV 15 DEV 16 HIGH 4 LOW 4 1000
Figure 7. Temperature Error vs. D+/D- Capacitance
DEV 2BC 900 800 700
TEMPERATURE ERROR (C)
2.5 2.0 1.5 1.0 0.5 0 -0.5
IDD (A)
600 500 400 300 200 100 DEV 4BC
DEV 3BC
05571-015
0
50 TEMPERATURE (C)
100
150
0.1
1 CONVERSION RATE (Hz)
10
100
Figure 5. Remote Temperature Error vs. Actual Temperature
Figure 8. Operating Supply Current vs. Conversion Rate
10 5
422 420
D+ TO GND
TEMPERATURE ERROR (C)
DEV 2BC 418 416 414 412 410 408 3.0
0 -5 D+ TO VCC -10 -15 -20 -25
IDD (A)
DEV 3BC DEV 4BC
05571-016
LEAKAGE RESISTANCE (M)
VDD (V)
Figure 6. Temperature Error vs. D+/D- Leakage Resistance
Figure 9. Operating Supply Current vs. Voltage
Rev. A | Page 8 of 24
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1
10
100
3.1
3.2
3.3
3.4
3.5
3.6
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-1.0 -50
0 0.01
05571-017
-1.0 -50
-18
ADT7461A
4.4 DEV 2 4.2
TEMPERATURE ERROR (C)
80 70 60 50 40 30 20 10 20mV 0 50mV 100mV
4.0 DEV 3 3.8 DEV 4 3.6 3.4 3.2 3.0 3.0
IDD (A)
VDD (V)
05571-020
3.1
3.2
3.3
3.4
3.5
3.6
0
100
200 300 400 NOISE FREQUENCY (MHz)
500
600
Figure 10. Standby Supply Current vs. Voltage
35 30 25
Figure 13. Temperature Error vs. Differential-Mode Noise Frequency
60
DEV 2BC DEV 3BC DEV 4BC
50
TEMPERATURE ERROR (C)
40
ISTBY (A)
20 15 10 5 0
30
20
10
05571-021
1
10 FSCL (kHz)
100
1000
0
500
1000 SERIES RESISTANCE ()
1500
2000
Figure 11. Standby Supply Current vs. Clock Frequency
Figure 14. Temperature Error vs. Series Resistance
25
TEMPERATURE ERROR (C)
20 100mV
15
10
50mV
5
20mV
0
100
200 300 400 NOISE FREQUENCY (MHz)
500
600
Figure 12. Temperature Error vs. Common-Mode Noise Frequency
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0
Rev. A | Page 9 of 24
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0
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-10
ADT7461A THEORY OF OPERATION
The ADT7461A is a local and remote temperature sensor and over/under temperature alarm, with the added ability to automatically cancel the effect of 1.5 k (typical) of resistance in series with the temperature monitoring diode. When the ADT7461A is operating normally, the on-board ADC operates in a free running mode. The analog input multiplexer alternately selects either the on-chip temperature sensor to measure its local temperature or the remote temperature sensor. The ADC digitizes these signals and the results are stored in the local and remote temperature value registers. The local and remote measurement results are compared with the corresponding high, low, and THERM temperature limits, stored in eight on-chip registers. Out-of-limit comparisons generate flags that are stored in the status register. A result that exceeds the high temperature limit or the low temperature limit causes the ALERT output to assert. The ALERT output also asserts if an external diode fault is detected. Exceeding the THERM temperature limits causes the THERM output to assert low. The ALERT output can be reprogrammed as a second THERM output. The limit registers are programmed and the device controlled and configured via the serial SMBus. The contents of any register are also read back via the SMBus. Control and configuration functions consist of switching the device between normal operation and standby mode, selecting the temperature measurement range, masking or enabling the ALERT output, switching Pin 6 between ALERT and THERM2, and selecting the conversion rate.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode, measuring the base emitter voltage (VBE) of a transistor operated at constant current. However, this technique requires calibration to null the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADT7461A measures the change in VBE when the device operates at three different currents. Previous devices used only two operating currents, but it is the use of a third current that allows automatic cancellation of resistances in series with the external temperature sensor. Figure 15 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, but it can equally be a discrete transistor. If a discrete transistor is used, the collector is not grounded but is linked to the base. To prevent ground noise interfering with the measurement, the more negative terminal of the sensor is not referenced to ground, but is biased above ground by an internal diode at the D- input. C1 may be added as a noise filter (a recommended maximum value of 1000 pF). However, a better option in noisy environments is to add a filter, as described in the Noise Filtering section. See the Layout Considerations section for more information on C1. To measure VBE, the operating current through the sensor is switched among three related currents. As shown in Figure 15, N1 x I and N2 x I are different multiples of the current, I. The currents through the temperature diode are switched between I and N1 x I, giving VBE1; and then between I and N2 x I, giving VBE2. The temperature is then calculated using the two VBE measurements. This method also cancels the effect of any series resistance on the temperature measurement. The resulting VBE waveforms are passed through a 65 kHz low-pass filter to remove noise and then to a chopper-stabilized amplifier. This amplifies and rectifies the waveform to produce a dc voltage proportional to VBE. The ADC digitizes this voltage producing a temperature measurement. To reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles for low conversion rates. At rates of 16-, 32-, and 64-conversions/second, no digital averaging occurs. Signal conditioning and measurement of the internal temperature sensor are performed in the same manner.
SERIES RESISTANCE CANCELLATION
Parasitic resistance to the D+ and D- inputs to the ADT7461A, seen in series with the remote diode, is caused by a variety of factors, including PCB track resistance and track length. This series resistance appears as a temperature offset in the remote sensor's temperature measurement. This error typically causes a 0.5C offset per ohm of parasitic resistance in series with the remote diode. The ADT7461A automatically cancels the effect of this series resistance on the temperature reading, giving a more accurate result, without the need for user characterization of this resistance. The ADT7461A is designed to automatically cancel typically up to 1.5 k of resistance. By using an advanced temperature measurement method, this process is transparent to the user. This feature permits resistances to be added to the sensor path to produce a filter, allowing the part to be used in noisy environments. See the section on Noise Filtering for more details.
Rev. A | Page 10 of 24
ADT7461A
VDD I N1 x I N2 x I IBIAS
D+ REMOTE SENSING TRANSISTOR C11 D-
VOUT+
LPF
TO ADC VOUT-
fC = 65kHz
BIAS DIODE
1CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.
Figure 15. Input Signal Conditioning
TEMPERATURE MEASUREMENT RESULTS
The results of the local and remote temperature measurements are stored in the local and remote temperature value registers and compared with limits programmed into the local and remote high and low limit registers. The local temperature value is in Register 0x00 and has a resolution of 1C. The external temperature value is stored in two registers, with the upper byte in Register 0x01 and the lower byte in Register 0x10. Only the two MSBs in the external temperature low byte are used giving the external temperature measurement a resolution of 0.25C. Table 7 lists the data format for the external temperature low byte. Table 7. Extended Temperature Resolution (Remote Temperature Low Byte)
Extended Resolution 0.00C 0.25C 0.50C 0.75C Remote Temperature Low Byte 0 000 0000 0 100 0000 1 000 0000 1 100 0000
The extended temperature range is selected by setting Bit 2 of the configuration register to 1. The temperature range is 0C to 127C when Bit 2 equals 0. A valid result is available in the next measurement cycle after changing the temperature range. In extended temperature mode, the upper and lower temperature that can be measured by the ADT7461A is limited by the remote diode selection. The temperature registers can have values from -64C to +191C. However, most temperature sensing diodes have a maximum temperature range of -55C to +150C. Above +150C, they may lose their semiconductor characteristics and approximate conductors instead. This results in a diode short. In this case, a read of the temperature result register gives the last good temperature measurement. Therefore, the temperature measurement on the external channel may not be accurate for temperatures that are outside the operating range of the remote sensor. It should be noted that although both local and remote temperature measurements can be made while the part is in extended temperature mode, the ADT7461A itself should not be exposed to temperatures greater than those specified in the absolute maximum ratings section. Further, the device is only guaranteed to operate as specified at ambient temperatures from -40C to +120C.
When reading the full external temperature value, read the LSB first. This causes the MSB to be locked (that is, the ADC does not write to it) until it is read. This feature ensures that the results read back from the two registers come from the same measurement.
TEMPERATURE DATA FORMAT
The ADT7461A has two temperature data formats. When the temperature measurement range is from 0C to 127C (default), the temperature data format for both internal and external temperature results is binary. When the measurement range is in extended mode, an offset binary data format is used for both internal and external results. Temperature values are offset by 64C in the offset binary data format. Examples of temperatures in both data formats are shown in Table 8.
TEMPERATURE MEASUREMENT RANGE
The temperature measurement range for both internal and external measurements is, by default, 0C to +127C. However, the ADT7461A can be operated using an extended temperature range. The extended measurement range is -64C to +191C. Therefore, the ADT7461A can be used to measure the full temperature range of an external diode, from -55C to +150C.
Rev. A | Page 11 of 24
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ADT7461A
Table 8. Temperature Data Format (Temperature High Byte)
Temperature -55C 0C +1C +10C +25C +50C +75C +100C +125C +127C +150C
1 2
Binary 0 000 00002 0 000 0000 0 000 0001 0 000 1010 0 001 1001 0 011 0010 0 100 1011 0 110 0100 0 111 1101 0 111 1111 0 111 11113
Offset Binary1 0 000 1001 0 100 0000 0 100 0001 0 100 1010 0 101 1001 0 111 0010 1 000 1011 1 010 0100 1 011 1101 1 011 1111 1 101 0110
The external temperature value high byte register is at Address 0x01, with the low byte register at Address 0x10. The power-on default for all three registers is 0x00.
Configuration Register
The configuration register is Address 0x03 at read and Address 0x09 at write. Its power-on default is 0x00. Only four bits of the configuration register are used. Bit 0, Bit 1, Bit 3, and Bit 4 are reserved; the user does not write to them. Bit 7 of the configuration register masks the ALERT output. If Bit 7 is 0, the ALERT output is enabled. This is the power-on default. If Bit 7 is set to 1, the ALERT output is disabled. This applies only if Pin 6 is configured as ALERT. If Pin 6 is configured as THERM2, then the value of Bit 7 has no effect. If Bit 6 is set to 0, which is power-on default, the device is in operating mode with ADC converting. If Bit 6 is set to 1, the device is in standby mode and the ADC does not convert. The SMBus does, however, remain active in standby mode; therefore, values can be read from or written to the ADT7461A via the SMBus. The ALERT and THERM outputs are also active in standby mode. Changes made to the registers in standby mode that affect the THERM or ALERT outputs cause these signals to be updated. Bit 5 determines the configuration of Pin 6 on the ADT7461A. If Bit 5 is 0 (default), then Pin 6 is configured as an ALERT output. If Bit 5 is 1, then Pin 6 is configured as a THERM2 output. Bit 7, the ALERT mask bit, is only active when Pin 6 is configured as an ALERT output. If Pin 6 is set up as a THERM2 output, then Bit 7 has no effect. Bit 2 sets the temperature measurement range. If Bit 2 is 0 (default value), the temperature measurement range is set between 0C to +127C. Setting Bit 2 to 1 sets the measurement range to the extended temperature range (-64C to +191C). Table 9. Configuration Register Bit Assignments
Bit 7 6 5 4, 3 2 1, 0 Name MASK1 RUN/STOP ALERT/THERM2 Reserved Temperature Range Select Reserved Function 0 = ALERT Enabled 1 = ALERT Masked 0 = Run 1 = Standby 0 = ALERT 1 = THERM2 0 = 0C to 127C 1 = Extended Range Power-On Default 0 0 0 0 0 0
Offset binary scale temperature values are offset by 64C. Binary scale temperature measurement returns 0C for all temperatures <0C. 3 Binary scale temperature measurement returns 127C for all temperatures >127C.
The user can switch between measurement ranges at any time. Switching the range likewise switches the data format. The next temperature result following the switching is reported back to the register in the new format. However, the contents of the limit registers do not change. It is up to the user to ensure that when the data format changes, the limit registers are reprogrammed as necessary. More information on this is found in the Limit Registers section.
ADT7461A REGISTERS
The ADT7461A contains 22, 8-bit registers in total. These registers store the results of remote and local temperature measurements, high and low temperature limits, and configure and control the device. See the Address Pointer Register section through the Consecutive ALERT Register section of this data sheet for more information on the ADT7461A registers. Additional details are shown in Table 9 through Table 13. The entire register map is available in Table 14.
Address Pointer Register
The address pointer register itself does not have, nor does it require, an address because the first byte of every write operation is automatically written to this register. The data in this first byte always contains the address of another register on the ADT7461A that is stored in the address pointer register. It is to this register address that the second byte of a write operation is written, or to which a subsequent read operation is performed. The power-on default value of the address pointer register is 0x00. Therefore, if a read operation is performed immediately after power-on, without first writing to the address pointer, the value of the local temperature is returned because its register address is 0x00.
Temperature Value Registers
The ADT7461A has three registers to store the results of local and remote temperature measurements. These registers can only be written to by the ADC and can be read by the user over the SMBus. The local temperature value register is at Address 0x00.
Rev. A | Page 12 of 24
ADT7461A
Conversion Rate Register
The conversion rate register is Address 0x04 at read and Address 0x0A at write. The lowest four bits of this register are used to program the conversion rate by dividing the internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, or 1024 to give conversion times from 15.5 ms (Code 0x0A) to 16 seconds (Code 0x00). For example, a conversion rate of eight conversions per second means that beginning at 125 ms intervals, the device performs a conversion on the internal and the external temperature channels. The conversion rate register can be written to and read back over the SMBus. The higher four bits of this register are unused and must be set to 0. The default value of this register is 0x08, giving a rate of 16 conversions per second. Use of slower conversion times greatly reduces the device power consumption. Table 10. Conversion Rate Register Codes
Code 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0A 0x0B to 0xFF Conversion/Second 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64 Reserved Time (Seconds) 16 8 4 2 1 500 m 250 m 125 m 62.5 m 31.25 m 15.5 m
Exceeding either the local or remote THERM limit asserts THERM low. When Pin 6 is configured as THERM2, exceeding either the local or remote high limit asserts THERM2 low. A default hysteresis value of 10C is provided that applies to both THERM channels. This hysteresis value can be reprogrammed to any value after power-up (Register Address 0x21). It is important to remember that the temperature limits data format is the same as the temperature measurement data format. Therefore, if the temperature measurement uses default binary, then the temperature limits also use the binary scale. If the temperature measurement scale is switched, however, the temperature limits do not automatically switch. The user must reprogram the limit registers to the desired value in the correct data format. For example, if the remote low limit is set at 10C with the default binary scale, the limit register value is 0000 1010b. If the scale is switched to offset binary, the value in the low temperature limit register needs to be reprogrammed to 0100 1010b.
Status Register
The status register is a read-only register at Address 0x02. It contains status information for the ADT7461A. When Bit 7 of the status register is high, it indicates that the ADC is busy converting. The other bits in this register flag the out-of-limit temperature measurements (Bit 6 to Bit 3, and Bit 1 to Bit 0) and the remote sensor open circuit (Bit 2). If Pin 6 is configured as an ALERT output, the following applies: If the local temperature measurement exceeds its limits, Bit 6 (high limit) or Bit 5 (low limit) of the status register asserts to flag this condition. If the remote temperature measurement exceeds its limits, then Bit 4 (high limit) or Bit 3 (low limit) asserts. Bit 2 asserts to flag an open circuit condition on the remote sensor. These five flags are NOR'ed together, so if any of them is high, the ALERT interrupt latch is set and the ALERT output goes low. Reading the status register clears the five flags, Bit 6 to Bit 2, provided the error conditions causing the flags to be set have gone away. A flag bit can be reset only if the corresponding value register contains an in-limit measurement or if the sensor is good. The ALERT interrupt latch is not reset by reading the status register. It resets when the ALERT output has been serviced by the master reading the device address, provided the error condition has gone away and the status register flag bits are reset.
Limit Registers
The ADT7461A has eight limit registers: high, low, and THERM temperature limits for both local and remote temperature measurements. The remote temperature high and low limits span two registers each, to contain an upper and lower byte for each limit. There is also a THERM hysteresis register. All limit registers can be written to, and read back over, the SMBus. See Table 14 for details of the limit register addresses and their poweron default values. When Pin 6 is configured as an ALERT output, the high limit registers perform a > comparison, while the low limit registers perform a comparison. For example, if the high limit register is programmed with 80C, then measuring 81C results in an out-of-limit condition, setting a flag in the status register. If the low limit register is programmed with 0C, measuring 0C or lower results in an out-of-limit condition.
Rev. A | Page 13 of 24
ADT7461A
When Flag 1 and/or Flag 0 are set, the THERM output goes low to indicate that the temperature measurements are outside the programmed limits. The THERM output does not need to be reset, unlike the ALERT output. Once the measurements are within the limits, the corresponding status register bits are automatically reset and the THERM output goes high. The user may add hysteresis by programming Register 0x21. The THERM output is reset only when the temperature falls to limit value minus the hysteresis value. When Pin 6 is configured as THERM2, only the high temperature limits are relevant. If Flag 6 and/or Flag 4 are set, the THERM2 output goes low to indicate that the temperature measurements are outside the programmed limits. Flag 5 and Flag 3 have no effect on THERM2. The behavior of THERM2 is otherwise the same as THERM. Table 11. Status Register Bit Assignments
Bit 7 6 5 4 3 2 1 0
1
Table 12. Sample Offset Register Codes
Offset Value -128C -4C -1C -0.25C 0C +0.25C +1C +4C +127.75C 0x11 1000 0000 1111 1100 1111 1111 1111 1111 0000 0000 0000 0000 0000 0001 0000 0100 0111 1111 0x12 00 00 0000 00 00 0000 00 00 0000 11 00 0000 00 00 0000 01 00 0000 00 00 0000 00 00 0000 11 00 0000
One-Shot Register
The one-shot register is used to initiate a conversion and comparison cycle when the ADT7461A is in standby mode, after which the device returns to standby. Writing to the oneshot register address (0x0F) causes the ADT7461A to perform a conversion and comparison on both the internal and the external temperature channels. This is not a data register as such, and it is the write operation to Address 0x0F that causes the one-shot conversion. The data written to this address is irrelevant and is not stored.
Name BUSY LHIGH1 LLOW1 RHIGH1 RLOW1 OPEN1 RTHRM LTHRM
Function 1 when ADC converting 1 when local high temperature limit tripped 1 when local low temperature limit tripped 1 when remote high temperature limit tripped 1 when remote low temperature limit tripped 1 when remote sensor open circuit 1 when remote THERM limit tripped 1 when local THERM limit tripped
Consecutive ALERT Register
The value written to this register determines how many out-oflimit measurements must occur before an ALERT is generated. The default value is that one out-of-limit measurement generates an ALERT. The maximum value that can be chosen is 4. The purpose of this register is to allow the user to perform some filtering of the output. This is particularly useful at the fastest three conversion rates, where no averaging takes place. This register is at Address 0x22. Table 13. Consecutive ALERT Register Bit
Register Value1 yxxx 000x yxxx 001x yxxx 011x yxxx 111x
1
These flags stay high until the status register is read or they are reset by POR unless Pin 6 is configured as THERM2. Then, only Bit 2 remains high until the status register is read or is reset by POR.
Offset Register
Offset errors can be introduced into the remote temperature measurement by clock noise or when the thermal diode is located away from the hot spot. To achieve the specified accuracy on this channel, these offsets must be removed. The offset value is stored as a 10-bit, twos complement value in Register 0x11 (high byte) and Register 0x12 (low byte, left justified). Only the upper two bits of Register 0x12 are used. The MSB of Register 0x11 is the sign bit. The minimum, programmable offset is -128C, and the maximum is +127.75C. The value in the offset register is added to, or subtracted from, the measured value of the remote temperature. The offset register powers up with a default value of 0C and has no effect unless the user writes a different value to it.
Number of Out-of-Limit Measurements Required 1 2 3 4
x = don't care bit. y = SMBus timeout bit. Default = 0. See the Serial Bus Interface section.
Rev. A | Page 14 of 24
ADT7461A
Table 14. List of Registers
Read Address (Hex) Not Applicable 00 01 02 03 04 05 06 07 08 Not Applicable 10 11 12 13 14 19 20 21 22 FE FF
1
Write Address (Hex) Not Applicable Not Applicable Not Applicable Not Applicable 09 0A 0B 0C 0D 0E 0F 1 Not Applicable 11 12 13 14 19 20 21 22 Not Applicable Not Applicable
Name Address Pointer Local Temperature Value External Temperature Value High Byte Status Configuration Conversion Rate Local Temperature High Limit Local Temperature Low Limit External Temperature High Limit High Byte External Temperature Low Limit High Byte One-Shot External Temperature Value Low Byte External Temperature Offset High Byte External Temperature Offset Low Byte External Temperature High Limit Low Byte External Temperature Low Limit Low Byte External THERM Limit Local THERM Limit THERM Hysteresis Consecutive ALERT Manufacturer ID Die Revision Code
Power-On Default Undefined 0000 0000 (0x00) 0000 0000 (0x00) Undefined 0000 0000 (0x00) 0000 1000 (0x08) 0101 0101 (0x55) (85C) 0000 0000 (0x00) (0C) 0101 0101 (0x55) (85C) 0000 0000 (0x00) (0C) 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0101 0101 (0x55) (85C) 0101 0101 (0x55) (85C) 0000 1010 (0x0A) (10C) 0000 0001 (0x01) 0100 0001 (0x41) 0101 0110 (0x56)
Writing to Address 0x0F causes the ADT7461A to perform a single measurement. It is not a data register, and it does not matter what data is written to it.
SERIAL BUS INTERFACE
Control of the ADT7461A is carried out via the serial bus. The ADT7461A is connected to this bus as a slave device, under the control of a master device. The ADT7461A has an SMBus timeout feature. When this is enabled, the SMBus times out after typically 25 ms of no activity. However, this feature is not enabled by default. Bit 7 of the consecutive alert register (Address = 0x22) should be set to enable it.
1.
ADDRESSING THE DEVICE
In general, every SMBus device has a 7-bit device address, except for some devices that have extended 10-bit addresses. When the master device sends a device address over the bus, the slave device with that address responds. The ADT7461A is available with one device address, 0x4C (1001 100b). An ADT7461A-2 is also available. The ADT7461A-2 has an SMBus address of 0x4D (1001 101b). This is to allow two ADT7461A devices on the same bus, or if the default address conflicts with an existing device on the SMBus. The serial bus protocol operates as follows: 2.
The master initiates a data transfer by establishing a start condition, defined as a high-to-low transition on SDATA, the serial data line, while SCLK, the serial clock line, remains high. This indicates that an address/data stream follows. All slave peripherals connected to the serial bus respond to the start condition and shift in the next eight bits, consisting of a 7-bit address (MSB first) plus an R/W bit, which determines the direction of the data transfer, that is, whether data is 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 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 writes to the slave device. If the R/W bit is a 1, the master reads from the slave device. Data is sent over the serial bus in a sequence 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, since a low-to-high transition when the clock is high can 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.
Rev. A | Page 15 of 24
ADT7461A
3. When all data bytes have been read or written, stop conditions are established. In write mode, the master pulls the data line high during the tenth clock pulse to assert a stop condition. In read mode, the master device overrides 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 takes 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. To write data to one of the device data registers, or to read data from it, the address pointer register must be set so that the correct data register is addressed. The first byte of a write operation always contains a valid 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 procedure is illustrated in Figure 16. 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.
Any number of bytes of data are transferable 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. For the ADT7461A, write operations contain either one or two bytes, while read operations contain one byte.
1 SCLK 9
1
9
SDATA START BY MASTER
A6
A5
A4
A3
A2
A1
A0
R/W ACK. BY ADT7461A
D7
D6
D5
D4
D3
D2
D1
D0 ACK. BY ADT7461A
FRAME 1 SERIAL BUS ADDRESS BYTE 1 SCLK (CONTINUED)
FRAME 2 ADDRESS POINTER REGISTER BYTE 9
SDATA (CONTINUED)
D7
D6
D5
D4
D3
D2
D1
D0 ACK. BY ADT7461A STOP BY MASTER
FRAME 3 DATA BYTE
Figure 16. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
1 SCLK 9 1 9
SDATA START BY MASTER
A6
A5
A4
A3
A2
A1
A0
R/W ACK. BY ADT7461A
D7
D6
D5
D4
D3
D2
D1
D0 ACK. BY STOP BY ADT7461A MASTER
FRAME 1 SERIAL BUS ADDRESS BYTE
FRAME 2 ADDRESS POINTER REGISTER BYTE
Figure 17. Writing to the Address Pointer Register Only
1 SCLK 9 1 9
SDATA START BY MASTER
A6
A5
A4
A3
A2
A1
A0
R/W ACK. BY ADT7461A
D7
D6
D5
D4
D3
D2
D1
D0 ACK. BY STOP BY ADT7461A MASTER
FRAME 1 SERIAL BUS ADDRESS BYTE
FRAME 2 ADDRESS POINTER REGISTER BYTE
Figure 18. Reading from a Previously Selected Register
Rev. A | Page 16 of 24
05571-007
05571-006
05571-005
ADT7461A
When reading data from a register there are two possibilities. * If the address pointer register value of the ADT7461A 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 writing to the ADT7461A as before, but only the data byte containing the register read address is sent, because data is not to be written to the register (see Figure 17). 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 (see Figure 18). * If the address pointer register is known to be at the desired address, data can be read from the corresponding data register without first writing to the address pointer register and the bus transaction shown in Figure 17 can be omitted. 1. 2. One or more ALERT outputs can be connected to a common SMBALERT line that is connected to the master. When the SMBALERT line is pulled low by one of the devices, the following procedure occurs (see Figure 19):
MASTER RECEIVES SMBALERT START ALERT RESPONSE ADDRESS MASTER SENDS ARA AND READ COMMAND RD ACK DEVICE ADDRESS DEVICE SENDS ITS ADDRESS NO STOP ACK
05571-008
Figure 19. Use of SMBALERT
SMBALERT is pulled low. 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. The device whose ALERT output is low responds to the alert response address and the master reads its device address. As the device address is seven bits, an LSB of 1 is added. The address of the device is now known and it can be interrogated in the usual way. If more than one device's ALERT output is low, the one with the lowest device address takes priority, in accordance with normal SMBus arbitration.
Notes * It is possible to read a data byte from a data register without first writing to the address pointer register. However, 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. Some of the registers have different addresses for read and write operations. The write address of a register must be written to the address pointer if data is to be written to that register, but it may not be possible to read data from that address. The read address of a register must be written to the address pointer before data can be read from that register.
3.
4.
*
Once the ADT7461A has responded to the alert response address, it resets its ALERT output, provided that the error condition that caused the ALERT no longer exists. If the SMBALERT line remains low, the master sends the ARA again, and so on until all devices whose ALERT outputs were low have responded.
LOW POWER STANDBY MODE
The ADT7461A can be put into low power standby mode by setting Bit 6 of the configuration register. When Bit 6 is low, the ADT7461A operates normally. When Bit 6 is high, the ADC is inhibited, and any conversion in progress is terminated without writing the result to the corresponding value register. However, the SMBus is still enabled. Power consumption in the standby mode is reduced to 5 A if there is no SMBus activity, or 30 A if there are clock and data signals on the bus. When the device is in standby mode, it is possible to initiate a one-shot conversion of both channels by writing to the one-shot register (Address 0x0F), after which the device returns to standby. It does not matter what is written to the one-shot register, all data written to it is ignored. It is also possible to write new values to the limit register while in standby mode. If the values stored in the temperature value registers are outside the new limits, an ALERT is generated, even though the ADT7461A is still in standby.
ALERT OUTPUT
This is applicable when Pin 6 is configured as an ALERT output. The ALERT output goes low whenever an out-of-limit measurement is detected, or if the remote temperature sensor is open circuit. It is an open-drain output and requires a pull-up resistor. Several ALERT outputs can be wire-OR'ed together, so that the common line goes low if one or more of the ALERT outputs goes low. The ALERT output can be used as an interrupt signal to a processor, or as an SMBALERT. Slave devices on the SMBus cannot normally signal to the bus master that they want to talk, but the SMBALERT function allows them to do so.
Rev. A | Page 17 of 24
ADT7461A
SENSOR FAULT DETECTION
At its D+ input, the ADT7461A contains internal sensor fault detection circuitry. This circuit can detect situations where an external remote diode is either not connected or incorrectly connected to the ADT7461A. A simple voltage comparator trips if the voltage at D+ exceeds VDD - 1 V (typical), signifying an open circuit between D+ and D-. The output of this comparator is checked when a conversion is initiated. Bit 2 of the status register (open flag) is set if a fault is detected. If the ALERT pin is enabled, setting this flag causes ALERT to assert low. If the user does not wish to use an external sensor with the ADT7461A, tie the D+ and D- inputs together to prevent continuous setting of the open flag. Table 15. THERM Hysteresis
THERM Hysteresis 0C 1C 10C Binary Representation 0 000 0000 0 000 0001 0 000 1010
Figure 20 shows how the THERM and ALERT outputs operate. The ALERT output can be used as a SMBALERT to signal to the host via the SMBus that the temperature has risen. The user can use the THERM output to turn on a fan to cool the system, if the temperature continues to increase. This method ensures that there is a fail-safe mechanism to cool the system, without the need for host intervention.
TEMPERATURE 100C 90C 80C 70C 60C 50C 40C RESET BY MASTER ALERT THERM 1 2 3 4
05571-009
THE ADT7461A INTERRUPT SYSTEM
The ADT7461A has two interrupt outputs, ALERT and THERM. Both have different functions and behavior. ALERT is maskable and responds to violations of software programmed temperature limits or an open-circuit fault on the external diode. THERM is intended as a fail-safe interrupt output that cannot be masked. If the external or local temperature exceeds the programmed high temperature limits, or equals or exceeds the low temperature limits, the ALERT output is asserted low. An open-circuit fault on the external diode also causes ALERT to assert. ALERT is reset when serviced by a master reading its device address, provided the error condition has gone away and the status register has been reset. The THERM output asserts low if the external or local temperature exceeds the programmed THERM limits. THERM temperature limits should normally be equal to or greater than the high temperature limits. THERM is reset automatically when the temperature falls back within the THERM limit. The external and local limits are set by default to 85C. A hysteresis value can be programmed; in which case, THERM resets when the temperature falls to the limit value minus the hysteresis value. This applies to both local and remote measurement channels. The power-on hysteresis default value is 10C, but this can be reprogrammed to any value after power-up. The hysteresis loop on the THERM outputs is useful when THERM is used, for example, as an on/off controller for a fan. The user's system can be set up so that when THERM asserts, a fan is switched on to cool the system. When THERM goes high again, the fan can be switched off. Programming a hysteresis value protects from fan jitter, where the temperature hovers around the THERM limit, and the fan is constantly switched.
THERM LIMIT THERM LIMIT-HYSTERESIS HIGH TEMP LIMIT
Figure 20. Operation of the ALERT and THERM Interrupts
* *
If the measured temperature exceeds the high temperature limit, the ALERT output asserts low. If the temperature continues to increase and exceeds the THERM limit, the THERM output asserts low. This can be used to throttle the CPU clock or switch on a fan. The THERM output deasserts (goes high) when the temperature falls to THERM limit minus hysteresis. In Figure 20, the default hysteresis value of 10C is shown. The ALERT output deasserts only when the temperature has fallen below the high temperature limit, and the master has read the device address and cleared the status register. Pin 6 on the ADT7461A can be configured as either an ALERT output or as an additional THERM output. THERM2 asserts low when the temperature exceeds the programmed local and/or remote high temperature limits. It is reset in the same manner as THERM and is not maskable. The programmed hysteresis value also applies to THERM2.
*
*
* *
*
Figure 21 shows how THERM and THERM2 operate together to implement two methods of cooling the system. In this example, the THERM2 limits are set lower than the THERM limits. The THERM2 output is used to turn on a fan. If the temperature continues to rise and exceeds the THERM limits, the THERM output provides additional cooling by throttling the CPU.
Rev. A | Page 18 of 24
ADT7461A
TEMPERATURE 90C 80C 70C 60C 50C 40C 30C 1 THERM2 THERM 2 3
05571-010
Remote Sensing Diode
THERM LIMIT
THERM2 LIMIT
4
The ADT7461A is designed to work with substrate transistors built into processors or with discrete transistors. Substrate transistors are generally PNP types with the collector connected to the substrate. Discrete types are either PNP or NPN transistors connected as diodes (base-shorted to collector). If an NPN transistor is used, the collector and base are connected to D+ and the emitter to D-. If a PNP transistor is used, the collector and base are connected to D- and the emitter to D+. To reduce the error due to variations in both substrate and discrete transistors, consider several factors: * The ideality factor, nF, of the transistor is a measure of the deviation of the thermal diode from ideal behavior. The ADT7461A is trimmed for an nF value of 1.008. The following equation may be used to calculate the error introduced at a temperature, T (C), when using a transistor whose nF does not equal 1.008. Consult the processor data sheet for the nF values. T = (nF - 1.008)/1.008 x (273.15 Kelvin + T) To factor this in, the user writes the T value to the offset register. It is then automatically added to, or subtracted from, the temperature measurement. Some CPU manufacturers specify the high and low current levels of the substrate transistors. The high current level of the ADT7461A, IHIGH, is 220 A and the low level current, ILOW, is 13.5 A. If the ADT7461A current levels do not match the current levels specified by the CPU manufacturer, it may become necessary to remove an offset. The CPU data sheet should advise whether this offset needs to be removed and how to calculate it. This offset is programmed to the offset register. It is important to note that if more than one offset must be considered, the algebraic sum of these offsets must be programmed to the offset register.
Figure 21. Operation of the THERM and THERM2 Interrupts
* * *
When the THERM2 limit is exceeded, the THERM2 signal asserts low. If the temperature continues to increase and exceeds the THERM limit, the THERM output asserts low. The THERM output deasserts (goes high) when the temperature falls to THERM limit minus hysteresis. In Figure 21, there is no hysteresis value shown. As the system cools further, and the temperature falls below the THERM2 limit, the THERM2 signal resets. Again, no hysteresis value is shown for THERM2. *
*
Both the external and internal temperature measurements cause THERM and THERM2 to operate as described.
APPLICATION INFORMATION
Noise Filtering
For temperature sensors operating in noisy environments, the industry standard practice was to place a capacitor across the D+ and D- pins to help combat the effects of noise. However, large capacitances affect the accuracy of the temperature measurement, leading to a recommended maximum capacitor value of 1,000 pF. Although this capacitor reduces the noise, it does not eliminate it, making it difficult to use the sensor in a very noisy environment. The ADT7461A has a major advantage over other devices when it comes to eliminating the effects of noise on the external sensor. The series resistance cancellation feature allows a filter to be constructed between the external temperature sensor and the part. The effect of any filter resistance seen in series with the remote sensor is automatically cancelled from the temperature result. The construction of a filter allows the ADT7461A and the remote temperature sensor to operate in noisy environments. Figure 22 shows a low-pass R-C-R filter, where R = 100 and C = 1 nF. This filtering reduces both common-mode and differential noise.
100 REMOTE TEMPERATURE SENSOR 1nF 100 D- D+
05571-012
If a discrete transistor is used with the ADT7461A, the best accuracy is obtained by choosing devices according to the following criteria: * * * * Base-emitter voltage greater than 0.25 V at 6 A, at the highest operating temperature Base-emitter voltage less than 0.95 V at 100 A, at the lowest operating temperature Base resistance less than 100 Small variation in hFE (50 to 150) that indicates tight control of VBE characteristics
Transistors, such as the 2N3904, 2N3906, or equivalents in SOT-23 packages are suitable devices to use.
Figure 22. Filter Between Remote Sensor and ADT7461A Factors Affecting Diode Accuracy
Rev. A | Page 19 of 24
ADT7461A
THERMAL INERTIA AND SELF-HEATING
Accuracy depends on the temperature of the remote sensing diode and/or the internal temperature sensor being at the same temperature as that being measured. Many factors can affect this. Ideally, place the sensor in good thermal contact with the part of the system being measured. If it is not, the thermal inertia caused by the sensor's mass causes a lag in the response of the sensor to a temperature change. In the case of the remote sensor, this should not be a problem since it is either a substrate transistor in the processor or a small package device, such as the SOT-23, placed in close proximity to it. The on-chip sensor, however, is often remote from the processor and only monitors the general ambient temperature around the package. How accurately the temperature of the board and/or the forced airflow reflects the temperature to be measured dictates the accuracy of the measurement. Self-heating due to the power dissipated in the ADT7461A or the remote sensor causes the chip temperature of the device or remote sensor to rise above ambient. However, the current forced through the remote sensor is so small that self-heating is negligible. In the case of the ADT7461A, the worst-case condition occurs when the device is converting at 64 conversions per second while sinking the maximum current of 1 mA at the ALERT and THERM output. In this case, the total power dissipation in the device is about 4.5 mW. The thermal resistance, JA, of the 8-lead MSOP is approximately 142C/W.
GND
5MIL 5MIL
D+
5MIL 5MIL
D-
5MIL
05571-011
5MIL
GND
5MIL
Figure 23. Typical Arrangement of Signal Tracks
*
Try to minimize the number of copper/solder joints that 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 1C corresponds to about 200 mV, and thermocouple voltages are about 3 mV/C of temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200 mV. Place a 0.1 F bypass capacitor close to the VDD pin. In extremely noisy environments, place an input filter capacitor across D+ and D- close to the ADT7461A. This capacitance can effect the temperature measurement, so ensure that any capacitance seen at D+ and D- is, at maximum, 1,000 pF. This maximum value includes the filter capacitance, plus any cable or stray capacitance between the pins and the sensor diode. If the distance to the remote sensor is more than 8 inches, the use of twisted pair cable is recommended. A total of 6 feet to 12 feet is needed. For really long distances (up to 100 feet), use a shielded twisted pair, such as the Belden No. 8451 microphone cable. Connect the twisted pair to D+ and D- and the shield to GND close to the ADT7461A. Leave the remote end of the shield unconnected to avoid ground loops.
*
*
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and the ADT7461A is measuring very small voltages from the remote sensor, so care must be taken to minimize noise induced at the sensor inputs. Take the following precautions: * Place the ADT7461A as close as possible to the remote sensing diode. Provided that the worst noise sources, that is, clock generators, data/address buses, and CRTs are avoided, this distance can be 4 inches to 8 inches. Route the D+ and D- tracks close together, in parallel, with grounded guard tracks on each side. To minimize inductance and reduce noise pickup, a 5 mil track width and spacing is recommended. Provide a ground plane under the tracks, if possible. *
*
Because the measurement technique uses switched current sources, excessive cable or filter capacitance can affect the measurement. When using long cables, the filter capacitance can be reduced or removed.
Rev. A | Page 20 of 24
ADT7461A
APPLICATION CIRCUIT
Figure 24 shows a typical application circuit for the ADT7461A, using a discrete sensor transistor connected via a shielded, twisted pair cable. The pull-ups on SCLK, SDATA, and ALERT are required only if they are not provided elsewhere in the system.
VDD
The SCLK pin and the SDATA pin of the ADT7461A can be interfaced directly to the SMBus of an I/O controller, such as the Intel(R) 820 chipset.
3V TO 3.6V 0.1F TYP 10k SMBUS CONTROLLER 5V OR 12V
ADT7461A
D+ D- SHIELD 2N3906 OR CPU THERMAL DIODE SCLK SDATA ALERT/ THERM2 THERM GND
VDD
TYP 10k FAN ENABLE
Figure 24. Typical Application Circuit
Rev. A | Page 21 of 24
05571-013
FAN CONTROL CIRCUIT
ADT7461A OUTLINE DIMENSIONS
3.20 3.00 2.80
3.20 3.00 2.80 PIN 1
8
5
1
5.15 4.90 4.65
4
0.65 BSC 0.95 0.85 0.75 0.15 0.00 0.38 0.22 SEATING PLANE 1.10 MAX 8 0 0.80 0.60 0.40
0.23 0.08
COPLANARITY 0.10
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 25. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
ORDERING GUIDE
Model ADT7461AARMZ 1 ADT7461AARMZ-REEL1 ADT7461AARMZ-REEL71 ADT7461AARMZ-21 ADT7461AARMZ-2REEL1 ADT7461AARMZ-2RL71
1
Temperature Range -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C
Package Description 8-Lead MSOP 8-Lead MSOP 8-Lead MSOP 8-Lead MSOP 8-Lead MSOP 8-Lead MSOP
Package Option RM-8 RM-8 RM-8 RM-8 RM-8 RM-8
Branding T1K T1K T1K T1L T1L T1L
SMBus Address 4C 4C 4C 4D 4D 4D
Z = Pb-free part.
Rev. A | Page 22 of 24
ADT7461A NOTES
Rev. A | Page 23 of 24
ADT7461A NOTES
(c)2006 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D05571-0-5/06(A)
Rev. A | Page 24 of 24

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