SST Digital Temperature Sensor and Voltage Monitor ADT7488A FEATURES GENERAL DESCRIPTION 1 on-chip temperature sensor 2 remote temperature sensors Monitors 3 voltage inputs, including VCC Simple Serial Transport™ (SST™) interface The ADT7488A is a simple digital temperature sensor for use in PC applications with Simple Serial Transport (SST) interface. It can monitor its own temperature as well as the temperature of two remote sensor diodes. It also measures the processor core voltage, VCCP; a 2.5 V supply voltage; and its own supply voltage, VCC. The ADT7488A is controlled by an SST single bidirectional data line. This device is a fixed-address SST client where the target address is chosen by the state of the address pin, ADD. APPLICATIONS Personal computers Portable personal devices Industrial sensor nets FUNCTIONAL BLOCK DIAGRAM ON-CHIP TEMPERATURE SENSOR ADT7488A OFFSET REGISTERS D1– D2+ D2– VCCP 2.5V INPUT ATTENUATORS AND ANALOG MULTIPLEXER A/D CONVERTER VOLTAGE VALUE REGISTERS GND SST INTERFACE ADDRESS SELECTION SST ADD 06351-001 D1+ DIGITAL MUX TEMPERATURE VALUE REGISTERS VCC Figure 1. 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: 781.461.3113 ©2006 Analog Devices, Inc. All rights reserved. ADT7488A TABLE OF CONTENTS Features .............................................................................................. 1 Voltage Measurement .................................................................... 12 Applications....................................................................................... 1 Analog-to-Digital Converter .................................................... 12 General Description ......................................................................... 1 Temperature Measurement ........................................................... 13 Functional Block Diagram .............................................................. 1 Temperature Measurement Method ........................................ 13 Revision History ............................................................................... 2 Reading Temperature Measurements...................................... 13 Specifications..................................................................................... 3 SST Temperature Sensor Data Format .................................... 14 Absolute Maximum Ratings............................................................ 5 Using Discrete Transistors ........................................................ 14 Thermal Resistance ...................................................................... 5 Layout Considerations............................................................... 14 ESD Caution.................................................................................. 5 Temperature Offset .................................................................... 15 Pin Configuration and Functional Descriptions.......................... 6 Application Schematic ............................................................... 15 Typical Performance Characteristics ............................................. 7 Outline Dimensions ....................................................................... 16 Product Description......................................................................... 9 Ordering Guide .......................................................................... 16 SST Interface ................................................................................. 9 REVISION HISTORY 9/06—Revision 0: Initial Version Rev. 0 | Page 2 of 16 ADT7488A SPECIFICATIONS TA = TMIN to TMAX, VCC = VMIN to VMAX, unless otherwise noted. Table 1. Parameter POWER SUPPLY Supply Voltage, VCC Undervoltage Lockout Threshold Average Operating Supply Current, IDD TEMPERATURE-TO-DIGITAL CONVERTER Local Sensor Accuracy Min Typ Max Unit Test Conditions/Comments 3.0 3.3 2.8 3.8 3.6 5 V V mA Continuous conversions ±1.75 ±4 ±1 ±1.75 °C °C °C °C ±4 °C μA μA μA °C kΩ +1 Remote Sensor Accuracy +1 Remote Sensor Source Current 12 80 204 0.016 1.5 Resolution Series Resistance Cancellation DIGITAL INPUT (ADD) Input High Voltage, VIH Input Low Voltage, VIL Input High Current, IIH Input Low Current, IIL Pin Capacitance ANALOG-TO-DIGITAL CONVERTER (Including Multiplexer and Attenuators) Total Unadjusted Error (TUE) Differential Nonlinearity (DNL) Power Supply Sensitivity Conversion Time (Voltage Input) 1 Conversion Time (Local Temperature)1 Conversion Time (Remote Temperature)1 Total Monitoring Cycle Time1 Input Resistances VCCP and 2.5V Channels DIGITAL I/O (SST Pin) Input High Voltage , VIH Input Low Voltage, VIL Hysteresis1 Output High Voltage, VOH High Impedance State Leakage, ILEAK 2.3 0.8 −1 1 5 ±1.5 ±1 ±0.1 11 12 38 121 80 140 1.1 ±10 μA 300 mV p-p Rev. 0 | Page 3 of 16 The ADT7488A cancels 1.5 kΩ in series with the remote thermal diode VIN = VCC VIN = 0 10 bits Averaging enabled Averaging enabled Averaging enabled Averaging enabled kΩ 1.9 ±1 150 1.1 % LSB %/V ms ms ms ms V V mV V μA 0.4 High Impedance State Leakage, ILEAK Signal Noise Immunity, VNOISE 110 V V μA μA pF 40°C ≤ TA ≤ 70°C; VCC = 3.3 V ±5% −40°C ≤ TA ≤ +100°C −40°C ≤ TD ≤ +125°C; TA = 25°C; VCC = 3.3 V −40°C ≤ TD ≤ +125°C; −40 ≤ TA ≤ 70°C; VCC = 3.3 V ±5% −40°C ≤ TD ≤ +125°C; −40 ≤ TA ≤ +100°C Low level Mid level High level Between input switching levels ISOURCE = 6 mA (maximum) Device powered on SST bus; VSST = 1.1 V, VCC = 3.3 V Device unpowered on SST bus; VSST = 1.1 V, VCC = 0 V Noise glitches from 10 MHz to 100 MHz; width up to 50 ns ADT7488A Parameter SST TIMING Bitwise Period, tBIT High Level Time for Logic 1, tH1 2 High Level Time for Logic 0, tH02 Time to Assert SST High for Logic 1, tSU, HIGH Hold Time, tHOLD 3 Stop Time, tSTOP Time to Respond After a Reset, tRESET Response Time to Speed Negotiation After Power-Up Min Typ 0.495 0.6 × tBIT 0.2 × tBIT 0.75 × tBIT 0.25 × tBIT 1.25 × tBIT 2 × tBIT Max Unit 500 0.8 × tBIT 0.4 × tBIT 0.2 × tBIT 0.5 × tBIT-M 2 × tBIT μs μs μs μs μs μs 0.4 ms μs 500 1 Guaranteed by design, not production tested. Minimum and maximum bit times are relative to tBIT defined in the timing negotiation pulse. 3 Device is compatible with hold time specification as driven by SST originator. 2 Rev. 0 | Page 4 of 16 Test Conditions/Comments tBIT defined in speed negotiation See SST Specification Rev 1.0 Device responding to a constant low level driven by originator Time after power-up when device can participate in speed negotiation ADT7488A ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Table 2. Parameter Supply Voltage (VCC) Voltage on 2.5V and VCCP Pins Voltage on Any Other Pin (Including SST Pin) Input Current at Any Pin Package Input Current Maximum Junction Temperature (TJ max) Storage Temperature Range Lead Temperature, Soldering IR Peak Reflow Temperature Lead Temperature (10 sec) ESD Rating Rating 4V 3.6 V −0.3 V to +3.6 V ±5 mA ±20 mA 150°C −65°C to +150°C θJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages. Table 3. Thermal Resistance Package Type 10-Lead MSOP ESD CAUTION 260°C 300°C 1500 V 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 16 θJA 206 θJC 44 Unit °C/W ADT7488A VCC 1 10 SST GND 2 9 ADD 8 2.5V 7 VCCP 6 D2– D1+ 3 D1– 4 D2+ 5 ADT7488A TOP VIEW (Not to Scale) 06351-002 PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS Figure 2. 10-Lead MSOP Table 4. Pin Function Descriptions Pin No. 1 2 3 4 5 6 7 8 9 10 Mnemonic VCC GND D1+ D1− D2+ D2− VCCP 2.5V ADD SST Type Power supply Ground Analog input Analog input Analog input Analog input Analog input Analog input Digital input Digital input/output Description 3.3 V ± 10%. VCC is also monitored through this pin. Ground Pin. Positive Connection to Remote 1 Temperature Sensor. Negative Connection to Remote 1 Temperature Sensor. Positive Connection to Remote 2 Temperature Sensor. Negative Connection to Remote 2 Temperature Sensor. Processor Core Voltage Monitor. 2.5 V Supply Monitor. SST Address Select. SST Bidirectional Data Line. Rev. 0 | Page 6 of 16 ADT7488A TYPICAL PERFORMANCE CHARACTERISTICS 1.55 1.55 1.50 750Ω (~2mA) 1.50 750Ω (~2mA) 1.45 SST O/P (V) 1.40 270Ω (~5.2mA) 1.35 1.30 270Ω (~5.2mA) 1.40 1.35 1.30 120Ω (~10.6mA) 120Ω (~10.6mA) 1.25 1.25 2.8 3.0 3.2 3.4 3.6 1.20 –50 06351-007 1.20 2.6 VCC (V) 0 Figure 3. SST O/P Level vs. Supply Voltage 150 100 Figure 6. SST O/P Level vs. Temperature 3.9 3.56 3.55 DEV3 3.54 3.7 3.53 DEV2 DEV2 IDD (mA) 3.52 IDD (mA) 50 TEMPERATURE (°C) 06351-010 SST O/P (V) 1.45 3.51 3.50 3.49 3.48 DEV1 DEV3 3.5 DEV1 3.3 3.1 3.47 –5 15 35 55 75 95 115 2.9 2.65 TEMPERATURE (°C) 2.85 6 TEMPERATURE ERROR (°C) 6 5 4 HI SPEC (VCC = 3V) MEAN (VCC = 3.3V) 1 0 3.65 140 5 4 3 2 HI SPEC (VCC = 3V) 1 0 MEAN (VCC = 3.3V) –1 LO SPEC (VCC = 3.6V) LO SPEC (VCC = 3.6V) –40 –20 0 20 40 60 80 TEMPERATURE (°C) 100 120 140 06351-019 TEMPERATURE ERROR (°C) 7 –1 –60 3.45 Figure 7. Supply Current vs. Voltage 7 2 3.25 VCC (V) Figure 4. Supply Current vs. Temperature 3 3.05 06351-011 –25 06351-008 3.45 –45 06351-020 3.46 Figure 5. Local Temperature Error –2 –60 –40 –20 0 20 40 60 80 100 TEMPERATURE (°C) Figure 8. Remote Temperature Error Rev. 0 | Page 7 of 16 120 ADT7488A 15 0 10 D+ TO GND 5 DEV1_EXT1 DEV1_EXT2 DEV2_EXT1 DEV2_EXT2 DEV3_EXT1 DEV3_EXT2 –10 –20 0 –30 –10 D+ TO VCC –15 –20 DEV1_EXT1 DEV1_EXT2 DEV2_EXT1 ERROR (°C) ERROR (°C) –5 DEV2_EXT2 DEV3_EXT1 DEV3_EXT2 EXT2 –40 EXT1 –50 –60 –25 –70 –30 0 20 40 60 80 100 RESISTANCE (MΩ) Figure 9. Remote Temperature Error vs. PCB Resistance 7 25 6 100mV 15 60mV 10 5 10 20 30 50 40 Figure 12. Remote Temperature Error vs. Capacitance Between D1+ and D1− 30 20 0 CAPACITANCE (nF) TEMPERATURE ERROR (°C) TEMPERATURE ERROR (°C) –90 06351-021 –40 06351-016 –80 –35 40mV 0 40mV 5 4 3 20mV 2 1 1M 10M 1G 100M NOISE FREQUENCY (Hz) 0 10k 06351-014 100k 10M 1G 100M Figure 13. Temperature Error vs. Differential-Mode Noise Frequency 20 5 4 TEMPERATURE ERROR (°C) 15 10 5 125mV 0 50mV –5 3 2 1 125mV 0 50mV –1 100k 1M 10M 100M 1G POWER SUPPLY NOISE FREQUENCY (Hz) –3 10k 100k 1M 10M 100M 1G POWER SUPPLY NOISE FREQUENCY (Hz) Figure 11. Local Temperature Error vs. Power Supply Noise Figure 14. Remote Temperature Error vs. Power Supply Noise Rev. 0 | Page 8 of 16 06351-018 –2 06351-015 TEMPERATURE ERROR (°C) 1M NOISE FREQUENCY (Hz) Figure 10. Temperature Error vs. Common-Mode Noise Frequency –10 10k 100k 06351-017 10mV –5 10k ADT7488A PRODUCT DESCRIPTION The ADT7488A is a temperature- and voltage-monitoring device. The ADT7488A can monitor the temperature of two remote sensor diodes, plus its own internal temperature. It can also monitor up to three voltage channels, including its own supply voltage. SST INTERFACE Simple Serial Transport (SST) is a one-wire serial bus and a communications protocol between components intended for use in personal computers, personal hand-held devices, or other industrial sensor nets. The ADT7488A supports SST Rev 1.0. SST is a licensable bus technology from Analog Devices, Inc., and Intel Corporation. To inquire about obtaining a copy of the Simple Serial Transport Specification or an SST technology license, please email Analog Devices at [email protected] or write to Analog Devices, 3550 North First Street, San Jose, CA 95134, Attention: SST Licensing, M/S B7-24. ADT7488A Client Address The client address for the ADT7488A is selected using the address pin. The address pin is connected to a float detection circuit, which allows the ADT7488A to distinguish between three input states: high, low (GND), and floating. The address range for the fixed address, discoverable device is 0x48 to 0x4A. Table 5. ADT7488A Selectable Addresses ADD Low (GND) Float High Rev. 0 | Page 9 of 16 Address Selected 0x48 0x49 0x4A ADT7488A Command Summary Table 6 summarizes the commands supported by the ADT7488A device when directed at the target address selected by the fixed address pin. It contains the command name, command code (CC), write data length (WL), read data length (RL), and a brief description. Table 6. Command Code Summary Command Ping() Command Code, CC 0x00 Write Length, WL 0x00 Read Length, RL 0x00 Description Shows a nonzero FCS over the header if present. GetIntTemp() 0x00 0x01 0x02 GetExt1Temp() 0x01 0x01 0x02 Shows the temperature of the device’s internal thermal diode. Shows the temperature of External Thermal Diode 1. GetExt2Temp() 0x02 0x01 0x02 Shows the temperature of External Thermal Diode 2. GetAllTemps() 0x00 0x01 0x06 GetVoltVCC() GetVolt2.5V() GetVoltVCCP() 0x12 0x13 0x14 0x01 0x01 0x01 0x02 0x02 0x02 Returns a 6-byte block of data (GetIntTemp, GetExt1Temp, GetExt2Temp). Shows the voltage attached to VCC input. Shows the voltage attached to 2.5V input. Shows the voltage attached to VCCP input. GetAllVolts() 0x12 0x01 0x06 SetExt1Offset() 0xe0 0x03 0x00 GetExt1Offset() 0xe0 0x01 0x02 SetExt2Offset() 0xe1 0x03 0x00 GetExt2Offset () ResetDevice() 0xe1 0x01 0x02 0xf6 0x01 0x00 0xf7 0xf7 0x01 0x01 0x08 0x10 GetDIB() Rev. 0 | Page 10 of 16 Shows all voltage measurements in a 6-byte block of data (GetVoltVcc, GetVolt2.5, GetVoltVccp). Sets the offset used to correct errors in External Diode 1. Shows the offset that the device is using to correct errors in External Diode 1. Sets the offset used to correct errors in External Diode 2. Returns the offset the device is using to correct errors in External Diode 2. Functional reset. The ADT7488A also responds to this command when directed to the Target Address 0x00. Shows information used by SW to identify the device’s capabilities. Can be in 8- or 16-byte format. ADT7488A Command Code Details ADT7488A Device Identifier Block Table 9. ResetDevice() Command The GetDIB() command retrieves the device identifier block (DIB), which provides information to identify the capabilities of the ADT7488A. The data returned can be in 8- or 16-byte format. The full 16 bytes of DIB is detailed in Table 7. The 8-byte format involves the first eight bytes described in this table. Byte-sized data is returned in the respective fields as it appears in Table 7. Word-sized data, including vendor ID, device ID, and data values use little endian format, that is, the LSB is returned first, followed by the MSB. Table 7. 16-Byte DIB Details Byte 0 1 Name Device Capabilities Version/Revision Value 0xc0 0x10 2, 3 Vendor ID 00x11d4 4, 5 Device ID 0x7488 6 7 Device Interface Function Interface Reserved Reserved Reserved Reserved Reserved Reserved Revision ID Client Device Address 0x01 0x00 Description Fixed address device Meets Version 1 of the SST specification Contains company ID number in little endian format Contains device ID number in little endian format SST device Reserved 0x00 0x00 0x00 0x00 0x00 0x00 0x05 0x48 to 0x4a Reserved Reserved Reserved Reserved Reserved Reserved Contains revision ID Dependent on the state of the address pin 8 9 10 11 12 13 14 15 Ping() The Ping() command verifies if a device is responding at a particular address. The ADT7488A shows a valid nonzero FCS in response to the Ping() command when correctly addressed. Table 8. Ping() Command Target Address (Not necessary) Write Length 0x00 Read Length 0x00 FCS ResetDevice() This command resets the register map and conversion controller. The reset command can be global or directed at the client address of the ADT7488A. Target Address Device Address Write Length 0x01 Read Length 0x00 Reset command 0xf6 FCS GetIntTemp() The ADT7488A shows the local temperature of the device in response to the GetIntTemp() command. The data has a little endian, 16-bit, twos complement format. GetExt1Temp() Prompted by the GetExt1Temp() command, the ADT7488A shows the temperature of Remote Diode 1 in little endian, 16-bit, twos complement format. The ADT7488A shows 0x8000 in response to this command if the external diode is an open or short circuit. GetExt2Temp() Prompted by the GetExt2Temp() command, ADT7488A shows the temperature of Remote Diode 2 in little endian, 16-bit, twos complement format. The ADT7488A shows 0x8000 in response to this command if the external diode is an open or short circuit. GetAllTemps() The ADT7488A shows the local and remote temperatures in a 6-byte block of data (internal temperature first, followed by External 1 temperature, followed by External 2 temperature) in response to a GetAllTemps() command. SetExtOffset() This command sets the offset that the ADT7488A will use to correct errors in the external diode. The offset is set in little endian, 16-bit, twos complement format. The maximum offset is ±128°C with +0.25°C resolution. GetExtOffset() This command causes the ADT7488A to show the offset that it is using to correct errors in the external diode. The offset value is returned in little endian format, that is, LSB before MSB. ADT7488A Response to Unsupported Commands A full list of command codes supported by the ADT7488A is given in Table 6. The offset registers (Command Code 0xe0) are the only registers that the user can write to. The other defined registers are read only. Writing to Register Addresses 0x02, 0x09, and 0x15 to 0xdf shows a valid FSC, but no action is taken by the ADT7488A. The ADT7488A shows an invalid FSC if the user attempts to write to the device between Command Codes 0xe2 to 0xee. These registers are reserved for the manufacturer’s use only, and no data can be written to the device via these addresses. Rev. 0 | Page 11 of 16 ADT7488A VOLTAGE MEASUREMENT The ADT7488A has two external voltage measurement channels. It can also measure its own supply voltage, VCC. Pins 7 and 8 measure the supplies of the processor core voltage (VCCP), and 2.5V pins, respectively. The VCC supply voltage measurement is carried out through the VCC pin (Pin 1). The 2.5V pin can be used to monitor a chipset supply voltage in a computer system. ANALOG-TO-DIGITAL CONVERTER To allow for the tolerance of these supply voltages, the ADC produces a specific output for each nominal input voltage and therefore has adequate headroom to cope with overvoltages. The full-scale voltage that can be recorded for each channel is shown in Table 10. Table 10. Maximum Reported Input Voltages Full-Scale Voltage 4V 4V 4V The internal structure for the analog inputs is shown in Figure 15. The input circuit consists of an input protection diode and an attenuator, plus a capacitor that forms a firstorder, low-pass filter to provide input immunity to high frequency noise. Returned Data LSB, MSB 2.5V 0x13 LSB, MSB VCCP 0x14 LSB, MSB Voltage Data Format The returned voltage value is in twos complement, 16-bit, binary format. The format is structured so that voltages in the range of ±32 V can be reported. In this way, the reported value represents the number of 1/1024 V in the actual reading, allowing a resolution of approximately 1 mV. Voltage 3.3 3 2.5 1 0 30pF 45kΩ 94kΩ 30pF MUX 17.5kΩ 52.5kΩ 35pF 06351-006 VCCP Command Code 0x12 68kΩ 71kΩ 2.5VIN Voltage Channel VCC Table 12. Analog-to-Digital Output Code vs. VIN Input Circuitry 3.3VIN The voltage measurement command codes are detailed in Table 11. Each voltage measurement has a read length of two bytes in little endian format (LSB followed by MSB). All voltages can be read together by addressing Command Code 0x12 with a read length of 0x06. The data is retrieved in the order listed in Table 11. Table 11. Voltage Measurement Command Codes All analog inputs are multiplexed into the on-chip, successive approximation, analog-to-digital converter (ADC). This has a resolution of 10 bits. The basic input range is 0 V to 2.25 V, but the inputs have built-in attenuators to allow measurement of 2.5 V, 3.3 V, 5 V, 12 V, and the processor core voltage (VCCP) without any external components. Voltage Channel VCC 2.5V VCCP Voltage Measurement Command Codes Figure 15. Internal Structure of Analog Inputs Rev. 0 | Page 12 of 16 Twos Complement MSB LSB 0000 1101 0011 0011 0000 1100 0000 0000 0000 1010 0000 0000 0000 0100 0000 0000 0000 0000 0000 0000 ADT7488A TEMPERATURE MEASUREMENT The ADT7488A monitors one local and two remote temperature channels. Monitoring of each of the channels is done in a round-robin sequence. The monitoring sequence is in the order shown in Table 13. Table 13. Temperature Monitoring Sequence Channel Number 0 Measurement Local temperature Conversion Time (ms) 12 1 Remote 1 temperature 38 2 Remote 2 temperature 38 To measure ΔVBE, the operating current through the sensor is switched between three related currents. Figure 16 shows N1 × I and N2 × I as different multiples of the current I. The currents through the temperature diode are switched between I and N1 × I, giving ΔVBE1, and then between I and N2 × I, giving ΔVBE2. The temperature can then be calculated using the two ΔVBE measurements. This method can also cancel the effect of series resistance on the temperature measurement. The resulting ΔVBE waveforms are passed through a 65 kHz low-pass filter to remove noise and then through a chopper-stabilized amplifier to amplify and rectify the waveform, producing a dc voltage proportional to ΔVBE. 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. Signal conditioning and measurement of the internal temperature sensor is performed in the same manner. N1 × I N2 × I I TEMPERATURE MEASUREMENT METHOD A simple method for measuring temperature is to exploit the negative temperature coefficient of a diode by measuring the base-emitter voltage (VBE) of a transistor operated at constant current. Unfortunately, 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 ADT7488A measures the change in VBE when the device is operated at three different currents. Figure 16 shows the input signal conditioning used to measure the output of a remote temperature sensor. This figure shows the remote sensor as a substrate transistor, which is provided for temperature monitoring on some microprocessors, but it could also be a discrete transistor. If a discrete transistor is used, the collector is not grounded and should be linked to the base. To prevent ground noise from 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 D1− input. If the sensor is operating in an extremely noisy environment, C1 can be added as a noise filter. Its value should not exceed 1000 pF. REMOTE SENSING TRANSISTOR VDD IBIAS VOUT+ D1+ C1* TO ADC D1– BIAS DIODE LOW-PASS FILTER fC = 65kHz VOUT– *CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS. 06351-005 The ADT7488A has three dedicated temperature measurement channels: one for measuring the temperature of an on-chip band gap temperature sensor, and two for measuring the temperature of a remote diode, usually located in the CPU or GPU. Figure 16. Signal Conditioning for Remote Diode Temperature Sensors READING TEMPERATURE MEASUREMENTS The temperature data returned is two bytes in little endian format, that is, LSB before MSB. All temperatures can be read together by using Command Code 0x00 with a read length of 0x06. The command codes and returned data are described in Table 14. Table 14. Temperature Channel Command Codes Temp Channel Internal Command Code 0x00 Returned data LSB, MSB External 1 External 2 All Temps 0x01 0x02 0x00 LSB, MSB LSB, MSB Internal LSB, internal MSB; External 1 LSB, External 1 MSB, External 2 LSB, External 2 MSB Rev. 0 | Page 13 of 16 ADT7488A SST TEMPERATURE SENSOR DATA FORMAT LAYOUT CONSIDERATIONS The data for temperature is structured to allow values in the range of ±512°C to be reported. Thus, the temperature sensor format uses a twos complement, 16-bit binary value to represent values in this range. This format allows temperatures to be represented with approximately a 0.016°C resolution. Digital boards can be electrically noisy environments. Take the following precautions to protect the analog inputs from noise, particularly when measuring the very small voltages from a remote diode sensor: • Table 15. SST Temperature Data Format Twos Complement MSB LSB 1110 0000 1100 0000 1110 1100 0000 0000 1111 0110 0000 0000 1111 1011 0011 1110 1111 1110 1100 0000 1111 1111 1100 0000 0000 0000 0000 0000 0000 0000 0100 0000 0000 0001 0100 0000 0000 0100 1100 0010 0000 1010 0000 0000 0001 0100 0000 0000 0001 1111 0100 0000 • • 5mil GND 5mil D1+ 5mil 5mil 5mil D1– 5mil GND USING DISCRETE TRANSISTORS Figure 18. Arrangements of Signal Tracks If a discrete transistor is used, the collector is not grounded and should be linked to the base. If a PNP transistor is used, the base is connected to the D− input and the emitter is connected to the D+ input. If an NPN transistor is used, the emitter is connected to the D− input and the base is connected to the D+ input. Figure 17 shows how to connect the ADT7488A to an NPN or PNP transistor for temperature measurement. To prevent ground noise from 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. ADT7488A D1+ D1+ D1– 2N3906 PNP ADT7488A D1– 06351-003 2N3904 NPN 5mil 06351-009 Temperature (°C) −125 −80 −40 −20 −5 −1 0 +1 +5 +20 +40 +80 +125 Place the ADT7488A as close as possible to the remote sensing diode. Provided that the worst noise sources, such as clock generators, data/address buses, and CRTs, are avoided, this distance can be four to eight inches. Route the D1+ and D1− tracks close together in parallel with grounded guard tracks on each side. Provide a ground plane under the tracks if possible. Use wide tracks to minimize inductance and reduce noise pickup. A 5 mil track minimum width and spacing is recommended. Figure 17. Connections for NPN and PNP Transistors The ADT7488A shows an external temperature value of 0x8000 if the external diode is an open or short circuit. • • • • • Try to minimize the number of copper/solder joints, which can cause thermocouple effects. Where copper/solder joints are used, make sure that they are in both the D1+ and D1− paths and are at the same temperature. Thermocouple effects should not be a major problem because 1°C corresponds to about 240 μV, and thermocouple voltages are about 3 μV/°C of the 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 ADT7488A. If the distance to the remote sensor is more than eight inches, the use of a twisted pair cable is recommended. This works for distances of about 6 feet to 12 feet. For very long distances (up to 100 feet), use shielded twisted pair cables, such as Belden #8451 microphone cables. Connect the twisted pair cable to D+ and D− and the shield to GND, close to the ADT7488A. Leave the remote end of the shield unconnected to avoid ground loops. Because the measurement technique uses switched current sources, excessive cable and/or filter capacitance can affect the measurement. When using long cables, the filter capacitor can be reduced or removed. Cable resistance can also introduce errors. A 1 Ω series resistance introduces about 0.5°C error. Rev. 0 | Page 14 of 16 ADT7488A offset is automatically added to every temperature measurement. The maximum offset is ±128°C with 0.25°C resolution. The offset format is the same as the temperature data format—16-bit, twos complement notation, as shown in Table 15. The offset should be programmed in little endian format, that is, LSB before MSB. The offset value is also returned in little endian format when read. TEMPERATURE OFFSET As CPUs run faster, it is more difficult to avoid high frequency clocks when routing the D+ and D− tracks around a system board. Even when the recommended layout guidelines are followed, there may still be temperature errors, attributed to noise being coupled onto the D+ and D− lines. High frequency noise generally has the effect of producing temperature measurements that are consistently too high by a specific amount. The ADT7488A has temperature offset command codes of 0xe0 and 0xe1 through which a desired offset can be set. By doing a one-time calibration of the system, the offset caused by system board noise can be calculated and nulled by specifying it in the ADT7488A. The VCC A typical application circuit for the ADT7488A is shown in Figure 19. The ADT7488A can be used in conjunction with the ADP3192 controller to monitor the CPU power. The result is a complete thermal and power monitor for the CPU. ADT7488A 1 VCC SST 10 2 GND ADD 9 3 D1+ 2.5V 8 4 D1– VCCP 7 5 D2+ D2– 6 SST POWER MONITORING R = 1kΩ POWER ADP3192 CPU CONTROLLER IMON C = 10nF VCCP CPU DRIVERS ADP3120A THERMAL DIODE Figure 19. ADT7488A Application Schematic: CPU Thermal- and Power-Supply Monitoring Rev. 0 | Page 15 of 16 06351-004 2N3905 NPN APPLICATION SCHEMATIC ADT7488A OUTLINE DIMENSIONS 3.10 3.00 2.90 10 3.10 3.00 2.90 1 6 5 5.15 4.90 4.65 PIN 1 0.50 BSC 0.95 0.85 0.75 0.15 0.05 1.10 MAX 0.33 0.17 SEATING PLANE 0.23 0.08 8° 0° 0.80 0.60 0.40 COPLANARITY 0.10 COMPLIANT TO JEDEC STANDARDS MO-187-BA Figure 20. 10-Lead Mini Small Outline Package [MSOP] (RM-10) Dimensions shown in millimeters ORDERING GUIDE Model ADT7488AARMZ-REEL 1 ADT7488AARMZ-REEL71 1 Temperature Range –40°C to +125°C –40°C to +125°C Package Description 10-Lead MSOP 10-Lead MSOP Z = Pb-free part. ©2006 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D06351-0-9/06(0) Rev. 0 | Page 16 of 16 Package Option RM-10 RM-10 Branding T24 T24