±1°C Temperature Monitor with Series Resistance Cancellation ADT7461* FEATURES PRODUCT DESCRIPTION On-chip and remote temperature sensor 0.25°C resolution/1°C accuracy on remote channel 1°C resolution/3°C accuracy on local channel Automatically cancels up to 3 kΩ (typ) of resistance in series with remote diode to allow noise filtering Extended, switchable temperature measurement range 0°C to +127°C (default) or –55°C to +150°C Pin and register compatible with ADM1032 2-wire SMBus serial interface with SMBus alert support Programmable over/under temperature limits Offset registers for system calibration Up to 2 overtemperature fail-safe THERM outputs Small 8-lead SOIC or MSOP package 170 µA operating current, 5.5 µA standby current The ADT7461 is a dual-channel digital thermometer and under/over temperature alarm, intended for use in PCs and thermal management systems. It is pin and register compatible with the ADM1032. The ADT7461 has three additional features: series resistance cancellation, where up to 3 kΩ (typical) of resistance in series with the temperature monitoring diode may be automatically cancelled from the temperature result, allowing noise filtering; configurable ALERT output; and an extended, switchable temperature measurement range. The ADT7461 can measure the temperature of a remote thermal diode accurate to ±1°C, and the ambient temperature accurate to ±3°C. The temperature measurement range defaults to 0°C to +127°C, compatible with ADM1032, but can be switched to a wider measurement range, from −55°C to +150°C. The ADT7461 communicates over a 2-wire serial interface compatible with system management bus (SMBus) standards. 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. APPLICATIONS Desktop and notebook computers Industrial controllers Smart batteries Automotive Enbedded systems Burn-in applications Instrumentation *Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239; 6,133,753; 6,169,442; other patents pending. 3 LOCAL TEMPERATURE LOW LIMIT REGISTER RUN/STANDBY REMOTE TEMPERATURE VALUE REGISTER SRC BLOCK LOCAL TEMPERATURE HIGH LIMIT REGISTER DIGITAL MUX 2 D– LOCAL TEMPERATURE VALUE REGISTER ADC BUSY D+ ADDRESS POINTER REGISTER LIMIT COMPARATOR ANALOG MUX CONVERSION RATE REGISTER DIGITAL MUX ON-CHIP TEMPERATURE SENSOR REMOTE TEMPERATURE LOW LIMIT REGISTER REMOTE TEMPERATURE HIGH LIMIT REGISTER LOCAL THERM LIMIT REGISTER REMOTE OFFSET REGISTER EXTERNAL THERM LIMIT REGISTER CONFIGURATION REGISTER EXTERNAL DIODE OPEN-CIRCUIT INTERRUPT MASKING STATUS REGISTER ADT7461 1 5 7 8 4 6 VDD GND SDATA SCLK THERM ALERT/ THERM2 04110-0-012 SMBus INTERFACE Figure 1. Functional Block Diagram Rev. 0 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: 781.326.8703 © 2003 Analog Devices, Inc. All rights reserved. ADT7461 TABLE OF CONTENTS ADT7461–Specifications................................................................. 3 Serial Bus Interface..................................................................... 14 SMBus Timing Specifications ......................................................... 4 Addressing the Device ............................................................... 14 Absolute Maximum Ratings............................................................ 5 Alert Output................................................................................ 16 Thermal Characteristics .............................................................. 5 Low Power Standby Mode......................................................... 16 Pin Configuration and Pin Function Descriptions...................... 6 Sensor Fault Detection .............................................................. 16 Typical Performance Characteristics ............................................. 7 The ADT7461 Interrupt System............................................... 17 Functional Description .................................................................... 9 Application Information ........................................................... 18 Series Resistance Cancellation.................................................... 9 Factors Affecting Diode Accuracy ........................................... 18 Temperature Measurement Method .......................................... 9 Thermal Inertia and Self-Heating ............................................ 19 Temperature Measurement Results.......................................... 10 Layout Considerations............................................................... 19 Temperature Measurement Range ........................................... 10 Application Circuit..................................................................... 20 Temperature Data Format ......................................................... 10 Outline Dimensions ....................................................................... 21 ADT7461 Registers .................................................................... 11 Ordering Guide .......................................................................... 21 REVISION HISTORY Revision 0: Initial Version Rev. 0 | Page 2 of 24 ADT7461 ADT7461–SPECIFICATIONS Table 1. ADT7461 Specifications at TA = −40°C to +120°C , VDD = 3 V to 5.5 V, unless otherwise noted. 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 Resolution Remote Diode Sensor Accuracy Min Typ Max Unit Test Conditions 3.0 3.30 170 5.5 5.5 2.55 5.5 215 10 20 2.8 2.5 V µA µA µA V V 0.0625 Conversions/Sec Rate1 Standby Mode , –40°C ≤ TA ≤ +85°C Standby Mode, +85°C ≤ TA ≤ +120°C VDD Input, Disables ADC, Rising Edge ±1 1 ±3 2.2 1 32.13 114.6 °C °C °C °C °C µA µA µA ms 3.2 12.56 ms ±1 ±3 Resolution Remote Sensor Source Current Conversion Time Maximum Series Resistance Cancelled OPEN-DRAIN DIGITAL OUTPUTS (THERM, ALERT/THERM2) Output Low Voltage, VOL High Level Output Leakage Current, IOH ALERT Output Low Sink Current SMBus INTERFACE3, 4 Logic Input High Voltage, VIH SCLK, SDATA Logic Input Low Voltage, VIL SCLK, SDATA Hysteresis SMBus Output Low Sink Current Logic Input Current, IIH, IIL SMBus Input Capacitance, SCLK, SDATA SMBus Clock Frequency SMBus Timeout5 SCLK Falling Edge to SDATA Valid Time 0.25 96 36 6 −40°C ≤ TA ≤ +100°C, 3 V ≤ VDD ≤ 3.6 V +60°C ≤ TA ≤ +100°C, −55°C ≤ TD 2 ≤ +150°C, 3 V ≤ VDD ≤ 3.6 V −40°C ≤ TA ≤ +120°C, −55°C ≤ TD 2 ≤ +150°C, 3 V ≤ VDD ≤ 5.5 V kΩ High Level3 Middle Level3 Low Level3 From Stop Bit to Conversion Complete (Both Channels) OneShot Mode with Averaging Switched On One-Shot Mode with Averaging Off (i.e., Conversion Rate = 16, 32, or 64 Conversions per Second) Resistance Split Evenly on Both the D+ and D– Inputs 1 V µA mA IOUT = −6.0 mA3 VOUT = VDD 3 ALERT forced to 0.4 V 2.1 V 3 V ≤ VDD ≤ 3.6 V V 3 V ≤ VDD ≤ 3.6 V 3 0.1 0.4 1 0.8 500 6 −1 +1 5 25 400 64 1 mV mA µA pF kHz ms µs SDATA Forced to 0.6 V User Programmable. Master Clocking in Data 1 See Table 8 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. Details on how to enable it are in the SMBus section of this data sheet. 2 Rev. 0 | Page 3 of 24 ADT7461 SMBus TIMING SPECIFICATIONS Table 2. SMBus Timing Specifications1 Parameter fSCLK tLOW tHIGH tR tF tSU; STA tHD; STA2 tSU; DAT3 tHD; DAT tSU; STO4 tBUF Limit at TMIN, TMAX 400 4.7 4 1 300 4.7 4 250 300 4 4.7 Unit kHz max µs min µs min µs max ns max µs min µs min ns min µs min µs min µs 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. Data Hold Time. Stop Condition Setup Time. Bus Free Time between Stop and Start Conditions. 1 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. 2 tR tLOW tF tHD;STA SCLK tHD;STA tHD;DAT tHIGH tSU;STA tSU;STO tSU;DAT tBUF STOP START START Figure 2. Serial Bus Timing Rev. 0 | Page 4 of 24 STOP 04110-0-001 SDATA ADT7461 ABSOLUTE MAXIMUM RATINGS Table 3. ADT7461 Absolute Maximum Ratings* 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 IR Reflow Peak Temperature Lead Temperature (Soldering 10 sec) THERMAL CHARACTERISTICS Rating −0.3 V, +5.5 V −0.3 V to VDD + 0.3 V −0.3 V to +0.6 V −0.3 V to +5.5 V −0.3 V to VDD + 0.3 V −1 mA, +50 mA ±1 mA 2000 V 150°C −65°C to +150°C 220°C 300°C 8-Lead SOIC Package θJA = 121°C/W 8-Lead MSOP Package θJA = 142°C/W *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. Rev. 0 | Page 5 of 24 ADT7461 VDD 1 8 SCLK D+ 2 ADT7461 7 SDATA D– 3 6 ALERT/THERM2 THERM TOP VIEW (Not to Scale) 4 5 GND 04110-0-013 PIN CONFIGURATION AND PIN FUNCTION DESCRIPTIONS Figure 3. Pin Configuration Table 4. Pin Function Descriptions Pin No. 1 2 3 Mnemonic VDD D+ D− 4 THERM 5 GND 6 ALERT/THERM2 7 8 SDATA SCLK Description Positive Supply, 3 V to 5.5 V. Positive Connection to Remote Temperature Sensor. Negative Connection to Remote Temperature Sensor. Open-Drain Output that can be used to turn a fan on/off or throttle a CPU clock in the event of an overtemperature condition. Requires pull-up to VDD. Supply Ground Connection. Open-Drain Logic Output used as interrupt or SMBus alert. This may 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. 0 | Page 6 of 24 ADT7461 TYPICAL PERFORMANCE CHARACTERISTICS 60 20 40 15 D+ TO GND TEMPERATURE ERROR (°C) 20 0 –20 D+ TO VCC –40 10 100mV INTERNAL 5 0 –5 100mV EXTERNAL 250mV INTERNAL –80 0 20 40 60 80 04110-0-015 –10 04110-0-017 –60 –15 100 0 20 LEAKAGE REISITANVE (MΩ) Figure 7. Temperature Error vs. Power Supply Noise Frequency 0 –0.1 –10 TEMPERATURE ERROR (°C) 0 –0.2 –0.3 –0.4 –0.5 –0.6 –10 10 30 50 70 90 110 130 –30 –40 –50 04110-0-018 –0.7 –20 –60 04110-0-022 TEMPERATURE ERROR (°C) Figure 4. Temperature Error vs. Leakage Resistance –0.8 –3 –70 0 150 5 10 15 20 25 CAPACITANCE (nF) TEMPERATURE (°C) Figure 5. Temperature Error vs. Actual Temperature Using 2N3906 Figure 8. Temperature Error vs. Capacitance between D+ and D− 180 800 160 700 140 100mV 600 5.5V 120 500 IDD (µA) 100 80 60 400 300 40 200 20 60mV 0 40mV –20 0 100 200 300 400 3V 04110-0-014 TEMPERATURE ERROR (°C) 40 FREQUENCY (MHz) 500 04110-0-019 TEMPERATURE ERROR (°C) 250mV EXTERNAL 100 0 0.01 600 FREQUENCY (MHz) 0.1 1 10 CONVERSION RATE (Hz) Figure 6. Temperature Error vs. Differential Mode Noise Frequency Figure 9. Operating Supply Current vs. Conversion Rate Rev. 0 | Page 7 of 24 100 ADT7461 60 7 6 100mV 5 IDD (µA) 40 30 4 3 20 2 60mV 40mV 0 0 100 200 300 1 400 500 0 3.0 3.2 600 04110-0-021 10 04110-0-016 TEMPERATURE ERROR (°C) 50 3.4 3.6 3.8 FREQUENCY (MHz) 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 VDD (V) Figure 10. Temperature Error vs. Common-Mode Noise Frequency Figure 12. Standby Current vs. Supply Voltage 40 50 45 35 TEMPERATURE ERROR (°C) 40 30 5.5V 20 15 10 3V 0 0 50 100 150 200 250 300 350 3.3V T = –30 30 3.3V T = +25 25 3.3V T = +120 20 5.5V T = –30 15 5.5V T = +25 10 5.5V T = +120 04110-0-023 5 35 5 04110-0-020 IDD (µA) 25 0 –5 400 0 SCL CLOCK FREQUENCY (kHz) 2 10 200 1k 2k 3k SERIES RESISTANCE (Ω) Figure 11. Standby Supply Current vs. Clock Frequency Figure 13. Temperature Error vs. Series Resistance Rev. 0 | Page 8 of 24 4k ADT7461 FUNCTIONAL DESCRIPTION The ADT7461 is a local and remote temperature sensor and over/under temperature alarm, with the added ability to automatically cancel the effect of 3 kΩ (typical) of resistance in series with the temperature monitoring diode. When the ADT7461 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, the low temperature limit, or an external diode fault will cause the ALERT output to assert low. Exceeding THERM temperature limits causes the THERM output to assert low. The ALERT output can be reprogrammed as a second THERM output. The limit registers can be programmed, and the device controlled and configured, via the serial SMBus. The contents of any register can also be read back via the SMBus. Control and configuration functions consist of: switching the device between normal operation and standby mode, selecting the temperature measurement scale, masking or enabling the ALERT output, switching Pin 6 between ALERT and THERM2, and selecting the conversion rate. SERIES RESISTANCE CANCELLATION Parasitic resistance, seen in series with the remote diode, to the D+ and D− inputs to the ADT7461, 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 0.5°C offset per ohm of parasitic resistance in series with the remote diode. The ADT7461 automatically cancels out 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 ADT7461 is designed to automatically cancel typically up to 3 kΩ of resistance. By using an advanced temperature measurement method, this is transparent to the user. This feature allows 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. 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. Unfortunately, this technique requires calibration to null out the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADT7461 is to measure the change in VBE when the device is operated at three different currents. Previous devices have 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 14 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 could equally be a discrete transistor. If a discrete transistor is used, the collector will not be grounded and should be 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 optionally be added as a noise filter (recommended maximum value 1000 pF). However, a better option in noisy environments is to add a filter, as described in the section on Noise Filtering. See the section on Layout Considerations for more information on C1. To measure ∆VBE, the operating current through the sensor is switched among three related currents. Shown in Figure 14, N1 × I and N2 × I are different multiples of the current I. The currents through the temperature diode are switched between I and N1 × I, giving ∆VBE1, and then between I and N2 × I, giving ∆VBE2. The temperature may then be calculated using the two ∆VBE measurements. This method can also be shown to cancel 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 and a temperature measurement is produced. 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 takes place. Signal conditioning and measurement of the internal temperature sensor is performed in the same manner. Rev. 0 | Page 9 of 24 ADT7461 VDD N1×I N2×I IBIAS D+ REMOTE SENSING TRANSISTOR VOUT+ C1* TO ADC BIAS DIODE D– LOW-PASS FILTER fC = 65kHz VOUT– *CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS. 04110-0-002 I Figure 14. Input Signal Conditioning TEMPERATURE MEASUREMENT RESULTS TEMPERATURE MEASUREMENT RANGE The results of the local and remote temperature measurements are stored in the local and remote temperature value registers and are compared with limits programmed into the local and remote high and low limit registers. The temperature measurement range for both internal and external measurements is, by default, 0°C to +127°C. However, the ADT7461 can be operated using an extended temperature range. It can measure the full temperature range of an external diode, from −55°C to +150°C. The user can switch between these two temperature ranges by setting or clearing Bit 2 in the configuration register. A valid result is available in the next measurement cycle after changing the temperature range. The local temperature value is in Register 0×00 and has a resolution of 1°C. The external temperature value is stored in two registers, with the upper byte in Register 0×01 and the lower byte in Register 0×10. Only the two MSBs in the external temperature low byte are used. This gives the external temperature measurement a resolution of 0.25°C. Table 5 shows the data format for the external temperature low byte. Table 5. Extended Temperature Resolution (Remote Temperature Low Byte) Extended Resolution 0.00°C 0.25°C 0.50°C 0.75°C Remote Temperature Low Byte 0 000 0000 0 100 0000 1 000 0000 1 100 0000 In extended temperature mode, the upper and lower temperature that can be measured by the ADT7461 is limited by the remote diode selection. The temperature registers themselves can have values from −64°C to +191°C. However, most temperature sensing diodes have a maximum temperature range of −55°C to +150°C. It should be noted that while both local and remote temperature measurements can be made while the part is in extended temperature mode, the ADT7461 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 −40°C to +120°C. When reading the full external temperature value, both the high and low byte, the two registers should be read in succession. Reading one register does not lock the other, so both should be read before the next conversion finishes. In practice, there is more than enough time to read both registers, as transactions over the SMBus are significantly faster than a conversion time. TEMPERATURE DATA FORMAT The ADT7461 has two temperature data formats. When the temperature measurement range is from 0°C to +127°C (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 in the offset binary data format are offset by +64. Examples of temperatures in both data formats are shown in Table 6. Rev. 0 | Page 10 of 24 ADT7461 Table 6. Temperature Data Format (Local and Remote Temperature High Byte) Temperature –55°C 0°C +1°C +10°C +25°C +50°C +75°C +100°C +125°C +127°C +150°C 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 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 1 Offset binary scale temperature values are offset by +64. Binary scale temperature measurement returns 0 for all temperatures < 0°C. 3 Binary scale temperature measurement returns 127 for all temperature > 127°C. 2 The user may switch between measurement ranges at any time. Switching the range will also switch the data format. The next temperature result following the switching will be reported back to the register in the new format. However, the contents of the limit registers will 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 can be found in the Limit Registers section. ADT7461 REGISTERS The ADT7461 contains 22 8-bit registers in total. These registers are used to store the results of remote and local temperature measurements and high and low temperature limits and to configure and control the device. A description of these registers follows, and further details are given in Table 7 through Table 11. Address Pointer Register The address pointer register itself does not have, or require, an address, as 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 ADT7461, which is stored in the address pointer register. It is to this register address that the second byte of a write operation is written to or to which a subsequent read operation is performed. The power-on default value of the address pointer register is 0×00, so if a read operation is performed immediately after power-on, without first writing to the address pointer, the value of the local temperature will be returned, since its register address is 0×00. Temperature Value Registers The ADT7461 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 0×00. The external temperature value high byte register is at Address 0×01, with the low byte register at Address 0×10. The power-on default for all three registers is 0×00. Configuration Register The configuration register is Address 0×03 at read and Address 0×09 at write. Its power-on default is 0×00. Only four bits of the configuration register are used. Bits 0, 1, 3, and 4 are reserved and should not be written to by the user. Bit 7 of the configuration register is used to mask 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 only applies 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 0, power-on default, the device is in operating mode with the 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, so values can be read from or written to the ADT7461 via the SMBus in this mode. 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 will cause these signals to be updated. Bit 5 determines the configuration of Pin 6 on the ADT7461. 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 setup 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 0°C to +127°C. Setting Bit 2 to 1 means that the measurement range is set to the extended temperature range. Rev. 0 | Page 11 of 24 ADT7461 Limit Registers Table 7. Configuration Register Bit Assignments Bit Name 7 MASK1 6 RUN/STOP 5 ALERT/THERM2 4– 3 Reserved 2 Temperature Range Select 1– 0 Reserved Function 0 = ALERT Enabled 1 = ALERT Masked 0 = Run 1 = Standby 0 = ALERT 1 = THERM2 Power-On Default The ADT7461 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 12 for details of the limit registers’ addresses and their power-on default values. 0 0 0 0 0 = 0°C to 127°C 1 = Extended Range 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 80°C, then measuring 81°C will result in an out-of-limit condition, setting a flag in the status register. If the low limit register is programmed with 0°C, measuring 0°C or lower will result in an out-of-limit condition. 0 0 Conversion Rate Register The conversion rate register is Address 0×04 at read and Address 0×0A 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 0×0A) to 16 seconds (Code 0×00). For example, a conversion rate of 8 conversions/second means that beginning at 125 ms intervals the device performs a conversion on the internal and the external temperature channels. This 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 0×08, giving a rate of 16 conversions per second. Use of slower conversion times greatly reduces the device power consumption, as shown in Table 8. Table 8. Conversion Rate Register Codes Code Conversion/Second 0×00 0×01 0×02 0×03 0×04 0×05 0×06 0×07 0×08 0×09 0×0A 0×0B to 0×FF 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64 Reserved Average Supply Current µA Typ at VDD = 5.5 V 121.33 128.54 131.59 146.15 169.14 233.12 347.42 638.07 252.44 417.58 816.87 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 10°C is provided that applies to both THERM channels. This hysteresis value may be reprogrammed to any value after power up (Register Address 0×21). It is important to remember that the temperature limits data format is the same as the temperature measurement data format. So 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 switch automatically. 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 10°C and the default binary scale is being used, the limit register value should be 0000 1010b. If the scale is switched to offset binary, the value in the low temperature limit register should be reprogrammed to be 0100 1010b. Status Register The status register is a read-only register, at Address 0×02. It contains status information for the ADT7461. Bit 7 of the status register indicates that the ADC is busy converting when it is high. The other bits in this register flag the out-of-limit temperature measurements (Bits 6–3 and Bits 1–0) 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’d together, so if any of them is high, the ALERT interrupt latch will be set and the ALERT output will go low. Rev. 0 | Page 12 of 24 ADT7461 Reading the status register will clear the five flags, Bits 6–2, provided the error conditions that caused the flags to be set have gone away. A flag bit can only be reset 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 will be reset 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 have been reset. 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 reset automatically, and the THERM output goes high. The user may add hysteresis by programming Register 0×21. The THERM output will be reset only when the temperature falls to limit value–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 9. Status Register Bit Assignments Bit 7 6 5 4 3 2 1 0 Name BUSY LHIGH* LLOW* RHIGH* RLOW* OPEN* 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 *These flags stay high until the status register is read, or they are reset by POR. Offset Register Offset errors may be introduced into the remote temperature measurement by clock noise or by the thermal diode being 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 Registers 0×11 (high byte) and 0×12 (low byte, left justified). Only the upper 2 bits of register 0×12 are used. The MSB of Register 0×11 is the sign bit. The minimum offset that can be programmed is −128°C, and the maximum is +127.75°C. The value in the offset register is added or subtracted to the measured value of the remote temperature. The offset register powers up with a default value of 0°C and will have no effect unless the user writes a different value to it. Table 10. Sample Offset Register Codes Offset Value −128°C −4°C −1°C −0.25°C 0°C +0.25°C +1°C +4°C +127.75°C 0×11 1000 0000 1111 1100 1111 1111 1111 1111 0000 0000 0000 0000 0000 0001 0000 0100 0111 1111 0×12 00 00 0000 00 00 0000 00 000000 10 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 ADT7461 is in standby mode, after which the device returns to standby. Writing to the one-shot register address (0×0F) causes the ADT7461 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 0×0F that causes the one-shot conversion. The data written to this address is irrelevant and is not stored. 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 0×22. Table 11. Consecutive ALERT Register Bit Register Value y××× 000× y××× 001× y××× 011× y××× 111× Number of Out-of-Limit Measurements Required 1 2 3 4 × = Don’t care bit. y = SMBus timeout bit. Default = 0. See SMBus section for more information. Rev. 0 | Page 13 of 24 ADT7461 Table 12. List of ADT7461 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 Write Address (Hex) Not Applicable Not Applicable Not Applicable Not Applicable 09 0A 0B 0C 0D 0E 0F 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 (0×00) 0000 0000 (0×00) Undefined 0000 0000 (0×00) 0000 1000 (0×08) 0101 0101 (0×55) (85°C) 0000 0000 (0×00) (0°C) 0101 0101 (0×55) (85°C) 0000 0000 (0×00) (0°C) 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0110 1100 (0×55) (85°C) 0101 0101 (0×55) (85°C) 0000 1010 (0×0A) (10°C) 0000 0001 (0×01) 0100 0001 (0×41) 0101 0001 (0×51) *Writing to address 0F causes the ADT7461 to perform a single measurement. It is not a data register as such and it does not matter what data is written to it. SERIAL BUS INTERFACE The serial bus protocol operates as follows: Control of the ADT7461 is carried out via the serial bus. The ADT7461 is connected to this bus as a slave device, under the control of a master device. 1. The master initiates data transfer by establishing a START condition, defined as a high-to-low transition on the serial data line SDATA, while the serial clock line SCLK 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 eight bits, consisting of a 7-bit address (MSB first) plus an R/W bit, which 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 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 may be interpreted as a STOP signal. The number of data bytes that can be transmitted over the The ADT7461 has an SMBus timeout feature. When this is enabled, the SMBus will timeout after typically 25 ms of no activity. However, this feature is not enabled by default. Bit 7 of the consecutive alert register (Address = 0×22) should be set to enable it. The ADT7461 supports packet error checking (PEC) and its use is optional. It is triggered by supplying the extra clock for the PEC byte. The PEC byte is calculated using CRC-8. The frame check sequence (FCS) conforms to CRC-8 by the polynomial C(x ) = x 8 + x 2 + x1 + 1 Consult the SMBus 1.1 specification for more information (www.smbus.org). 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 will respond. The ADT7461 is available with one device address, 0×4C (1001 100b). Rev. 0 | Page 14 of 24 ADT7461 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. serial bus in a single read or write operation is limited only by what the master and slave devices can handle. When all data bytes have been read or written, stop conditions 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. This is illustrated in Figure 15. 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 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. In the case of the ADT7461, write operations contain either one or two bytes, while read operations contain one byte. 1 9 1 9 SCLK A6 SDATA A5 A4 A3 A2 A1 A0 R/W START BY MASTER D6 D7 D5 D4 D3 D2 D1 D0 ACK. BY ADT7461 ACK. BY ADT7461 FRAME 1 SERIAL BUS ADDRESS BYTE FRAME 2 ADDRESS POINTER REGISTER BYTE 1 9 SCLK (CONTINUED) D7 SDATA (CONTINUED) D6 D5 D4 D2 D3 D1 D0 ACK. BY ADT7461 STOP BY MASTER FRAME 3 DATA BYTE 04110-0-003 Figure 15. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register 1 9 1 9 SCLK A6 A5 A4 A3 A2 A1 A0 R/W START BY MASTER D7 D6 D5 D4 D3 D2 D1 D0 ACK. BY ADT7461 ACK. BY ADT7461 FRAME 1 SERIAL BUS ADDRESS BYTE STOP BY MASTER FRAME 2 ADDRESS POINTER REGISTER BYTE 04110-0-004 SDATA Figure 16. Writing to the Address Pointer Register Only 1 9 1 9 SCLK SDATA A6 A5 A4 A3 A2 A1 START BY MASTER A0 R/W D7 D6 D5 D4 D3 D2 D1 FRAME 1 SERIAL BUS ADDRESS BYTE FRAME 2 DATA BYTE FROM ADT7461 Figure 17. Reading from a Previously Selected Register Rev. 0 | Page 15 of 24 D0 ACK. BY ADT7461 ACK. BY ADT7461 STOP BY MASTER 04110-0-005 3. ADT7461 MASTER RECEIVES SMBALERT • If the ADT7461’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 ADT7461 as before, but only the data byte containing the register read address is sent, as data is not to be written to the register. This is shown in Figure 16. 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 17. • 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 and the bus transaction shown in Figure 16 can be omitted. Notes 1. 2. 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. Do not forget that some of the ADT7461 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. 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 to VDD. Several ALERT outputs can be wire-ORed together, so that the common line will go 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 it may be used 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. One or more ALERT outputs can be connected to a common SMBALERT line connected to the master. When the SMBALERT line is pulled low by one of the devices, the following procedure occurs as illustrated in Figure 18. START ALERT RESPONSE ADDRESS DEVICE ADDRESS RD ACK MASTER SENDS ARA AND READ COMMAND DEVICE SENDS ITS ADDRESS NO STOP ACK 04110-0-006 When reading data from a register there are two possibilities: Figure 18. Use of SMBALERT 1. SMBALERT is 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 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. 4. If more than one device’s ALERT output is low, the one with the lowest device address will have priority, in accordance with normal SMBus arbitration. 5. Once the ADT7461 has responded to the alert response address, it will reset its ALERT output, provided that the error condition that caused the ALERT no longer exists. If the SMBALERT line remains low, the master will send the ARA again, and so on until all devices whose ALERT outputs were low have responded. LOW POWER STANDBY MODE The ADT7461 can be put into low power standby mode by setting Bit 6 of the configuration register. When Bit 6 is low, the ADT7461 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. The SMBus is still enabled. Power consumption in the standby mode is reduced to less than 10 µA if there is no SMBus activity or 100 µA if there are clock and data signals on the bus. When the device is in standby mode, it is still possible to initiate a one-shot conversion of both channels by writing to the oneshot register (Address 0×0F), after which the device will return 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 now outside the new limits, an ALERT is generated, even though the ADT7461 is still in standby. SENSOR FAULT DETECTION The ADT7461 has sensor fault detection circuitry internally at its D+ input. This circuit can detect situations where an external remote diode is not connected, or is incorrectly connected, to Rev. 0 | Page 16 of 24 ADT7461 the ADT7461. 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 will cause ALERT to assert low. 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 being switched. THERM Hysteresis Binary Representation If the user does not wish to use an external sensor with the ADT7461, then in order to prevent the OPEN flag being set continuously, the user should tie the D+ and D− inputs of the ADT7461 together. 0°C 1°C 10°C 0 000 0000 0 000 0001 0 000 1010 Most temperature sensing diodes have an operating temperature range of −55°C to +150°C. Above 150°C, they lose their semiconductor characteristics and approximate conductors instead. This results in a diode short, setting the OPEN flag. The external diode in this case will no longer give an accurate temperature measurement. A read of the temperature result register will give the last good temperature measurement. The user should be aware that, while the diode fault is triggered, the temperature measurement on the external channel may not be accurate. Figure 19 shows how the THERM and ALERT outputs operate. A user may wish to use the ALERT output as a SMBALERT to signal to the host via the SMBus that the temperature has risen. The user could use the THERM output to turn on a fan to cool the system, if the temperature continues to increase. This method would ensure that there is a fail-safe mechanism to cool the system, without the need for host intervention. Table 13. THERM Hysteresis TEMPERATURE 100°C 90°C THE ADT7461 INTERRUPT SYSTEM 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. The 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 THERM limit is set by default to 85°C, as is the local THERM limit. A hysteresis value can be programmed, in which case, THERM will reset 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 10°C, but this may be reprogrammed to any value after power-up. The hysteresis loop on the THERM outputs is useful when THERM is used for on/off control of a fan. The user’s system can be set up so that when THERM asserts a fan can be 80°C THERM LIMIT 70°C THERM LIMIT-HYSTERESIS 60°C HIGH TEMP LIMIT 50°C 40°C RESET BY MASTER ALERT THERM 1 4 2 3 04110-0-007 The ADT7461 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. Figure 19. Operation of the ALERT and THERM Interrupts 1. If the measured temperature exceeds the high temperature limit, the ALERT output will assert low. 2. 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. 3. The THERM output deasserts (goes high) when the temperature falls to THERM limit minus hysteresis. In Figure 19, the default hysteresis value of 10°C is shown. 4. 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 ADT7461 can be configured as either an ALERT output or as an additional THERM output. THERM2 will assert low when the temperature exceeds the programmed local and/or remote high temperature limits. It is reset in the same manner as THERM, and it is not maskable. The programmed hysteresis value applies to THERM2 also. Rev. 0 | Page 17 of 24 ADT7461 The construction of a filter allows the ADT7461 and the remote temperature sensor to operate in noisy environments. Figure 21 shows a low-pass R-C-R filter, with the following values: R = 100 Ω and C = 1 nF. This filtering reduces both common-mode noise and differential noise. 100Ω REMOTE TEMPERATURE SENSOR TEMPERATURE 100Ω 90°C 80°C D+ 1nF D– 04110-0-009 Figure 20 shows how THERM and THERM2 might 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 could be used to turn on a fan. If the temperature continues to rise and exceeds the THERM limits, the THERM output could provide additional cooling by throttling the CPU. Figure 21. Filter Between Remote Sensor and ADT7461 THERM LIMIT 70°C FACTORS AFFECTING DIODE ACCURACY 60°C THERM2 LIMIT 50°C 40°C 30°C 1 4 2 THERM 04110-0-008 THERM2 3 Figure 20. Operation of the THERM and THERM2 Interrupts 1. When the THERM2 limit is exceeded, the THERM2 signal asserts low. 2. If the temperature continues to increase and exceeds the THERM limit, the THERM output asserts low. 3. The THERM output deasserts (goes high) when the temperature falls to THERM limit minus hysteresis. In Figure 20, there is no hysteresis value shown. 4. 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. Remote Sensing Diode The ADT7461 is designed to work with substrate transistors built into processors or with discrete transistors. Substrate transistors will generally be PNP types with the collector connected to the substrate. Discrete types can be either PNP or NPN transistor connected as a diode (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, a number of factors should be taken into consideration: • ( ) ∆T = n f – 1.008 /1.008 × (273.15 Kelvin + T ) The temperature measurement could be either the local or the external temperature measurement. To factor this in, the user can write the ∆T value to the offset register. It will then be automatically added to or subtracted from the temperature measurement by the ADT7461. APPLICATION INFORMATION Noise Filtering For temperature sensors operating in noisy environments, previous 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 1000 pF. While this capacitor will reduce the noise, it will not eliminate it, making it difficult to use the sensor in a very noisy environment. The ideality factor, nf, of the transistor is a measure of the deviation of the thermal diode from ideal behavior. The ADT7461 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. • The ADT7461 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. Rev. 0 | Page 18 of 24 Some CPU manufacturers specify the high and low current levels of the substrate transistors. The high current level of the ADT7461, IHIGH, is 96 µA and the low level current, ILOW, is 6 µA. If the ADT7461 current levels do not match the current levels specified by the CPU manufacturer, it may become necessary to remove an offset. The CPUs data sheet will advise whether this offset needs to be removed and how to calculate it. This offset may be 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. ADT7461 If a discrete transistor is being used with the ADT7461, the best accuracy will be obtained by choosing devices according to the following criteria: • Base-emitter voltage greater than 0.25 V at 6 µA, at the highest operating temperature. • Base-emitter voltage less than 0.95 V at 100 µA, at the lowest operating temperature. • Base resistance less than 100 Ω. • Small variation in hFE (say 50 to 150) that indicates tight control of VBE characteristics. LAYOUT CONSIDERATIONS Digital boards can be electrically noisy environments, and the ADT7461 is measuring very small voltages from the remote sensor, so care must be taken to minimize noise induced at the sensor inputs. The following precautions should be taken: • Place the ADT7461 as close as possible to the remote sensing diode. Provided that the worst noise sources, i.e., 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 pick-up, a 5 mil track width and spacing is recommended. Provide a ground plane under the tracks if possible. Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23 packages, are suitable devices to use. THERMAL INERTIA AND SELF-HEATING The on-chip sensor, however, will often be remote from the processor and will only be monitoring the general ambient temperature around the package. The thermal time constant of the SOIC-8 package in still air is about 140 seconds, and if the ambient air temperature quickly changed by 100 degrees, it would take about 12 minutes (5 time constants) for the junction temperature of the ADT7461 to settle within 1 degree of this. In practice, the ADT7461 package will be in electrical, and hence thermal, contact with a PCB and may also be in a forced airflow. How accurately the temperature of the board and/or the forced airflow reflects the temperature to be measured will also affect the accuracy. Self-heating due to the power dissipated in the ADT7461 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 selfheating is negligible. In the case of the ADT7461, 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, θJA, of the SOIC-8 package is about 121°C/W. 5MIL GND 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. A number of factors can affect this. Ideally, the sensor should be 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 will cause 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 will either be a substrate transistor in the processor or can be a small package device, such as SOT-23, placed in close proximity to it. 5MIL D+ 5MIL 5MIL D– 5MIL GND 5MIL 04110-0-010 5MIL Figure 22. 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 1°C 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 µF bypass capacitor close to the VDD pin. In extremely noisy environments, an input filter capacitor may be placed across D+ and D− close to the ADT7461. This capacitance can effect the temperature measurement, so care must be taken to ensure that any capacitance seen at D+ and D− is a maximum of 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. This will work up to about 6 feet to 12 feet. Rev. 0 | Page 19 of 24 ADT7461 For really long distances (up to 100 feet), use shielded twisted pair, such as Belden No. 8451 microphone cable. Connect the twisted pair to D+ and D− and the shield to GND close to the ADT7461. Leave the remote end of the shield unconnected to avoid ground loops. APPLICATION CIRCUIT Figure 23 shows a typical application circuit for the ADT7461, 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 already provided elsewhere in the system. Because the measurement technique uses switched current sources, excessive cable or filter capacitance can affect the measurement. When using long cables, the filter capacitance may be reduced or removed. The SCLK and SDATA pins of the ADT7461 can be interfaced directly to the SMBus of an I/O controller, such as the Intel 820 chipset. ADT7461 VDD 3V TO 3.6V 0.1µF D+ TYP 10kΩ SCLK D– 2N3906 OR CPU THERMAL DIODE SHIELD SMBUS CONTROLLER SDATA ALERT/ THERM2 VDD THERM 5V OR 12V TYP 10kΩ GND FAN ENABLE Figure 23. Typical Application Circuit Rev. 0 | Page 20 of 24 FAN CONTROL CIRCUIT 04110-0-011 • ADT7461 OUTLINE DIMENSIONS 5.00 (0.1968) 4.80 (0.1890) 8 5 4.00 (0.1574) 3.80 (0.1497) 1 6.20 (0.2440) 5.80 (0.2284) 4 1.27 (0.0500) BSC 0.50 (0.0196) × 45° 0.25 (0.0099) 1.75 (0.0688) 1.35 (0.0532) 0.25 (0.0098) 0.10 (0.0040) 0.51 (0.0201) COPLANARITY SEATING 0.31 (0.0122) 0.10 PLANE 8° 0.25 (0.0098) 0° 1.27 (0.0500) 0.40 (0.0157) 0.17 (0.0067) COMPLIANT TO JEDEC STANDARDS MS-012AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN Figure 24. 8-Lead Standard Small Outline Package [SOIC] (R-8) Dimensions shown in millimeters and (inches) 3.00 BSC 8 5 4.90 BSC 3.00 BSC 4 PIN 1 0.65 BSC 1.10 MAX 0.15 0.00 0.38 0.22 COPLANARITY 0.10 0.23 0.08 0.80 0.60 0.40 8° 0° SEATING PLANE COMPLIANT TO JEDEC STANDARDS MO-187AA Figure 25. 8-Lead Micro Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters ORDERING GUIDE Model ADT7461AR ADT7461AR-REEL ADT7461AR-REEL7 ADT7461ARM ADT7461ARM-REEL ADT7461ARM-REEL7 EVAL-ADT7461EB Operating Temperature Range −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C Package Description 8-Lead SOIC Package 8-Lead SOIC Package 8-Lead SOIC Package 8-Lead MSOP Package 8-Lead MSOP Package 8-Lead MSOP Package ADT7461 Evaluation Board Rev. 0 | Page 21 of 24 Package Option R-8 R-8 R-8 RM-8 RM-8 RM-8 Branding Information ADT7461AR ADT7461AR ADT7461AR T1B T1B T1B SMBus Address 4C 4C 4C 4C 4C 4C ADT7461 NOTES Rev. 0 | Page 22 of 24 ADT7461 NOTES Rev. 0 | Page 23 of 24 ADT7461 NOTES © 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. C04110-0-10/03(0) Rev. 0 | Page 24 of 24