ADT7485A Temperature Sensor and Voltage Monitor with Simple Serial Transport The ADT7485A is a digital temperature sensor and voltage monitor for use in PC applications with Simple Serial Transport (SST) interface. It can monitor its own temperature as well as the temperature of a remote sensor diode. It can also monitor four external voltage channels and its own supply voltage. The ADT7485A is controlled by a single SST 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. http://onsemi.com MSOP−10 CASE 846AC Features PIN ASSIGNMENT 1 On-Chip Temperature Sensor 1 Remote Temperature Sensor Monitors Up to 5.0 Voltages SST Interface This Device is Pb-Free, Halogen Free and is RoHS Compliant Applications Personal Computers Portable Personal Devices Industrial Sensor Nets VCC 1 GND 2 D1+ 3 D1− 12 V 10 SST 9 ADD 8 2.5 V 4 7 VCCP 5 6 5.0 V ADT7485A (Top View) MARKING DIAGRAM 10 ON-CHIP TEMPERATURE SENSOR T21 AYWG G OFFSET REGISTERS 1 12 V 5.0 V VCCP 2.5 V D1+ INPUT ATTENUATORS AND ANALOG MULTIPLEXER DIGITAL MUX TEMPERATURE VALUE REGISTERS VCC A/D CONVERTER VOLTAGE VALUE REGISTERS D1− T21 A Y W G SST INTERFACE = Specific Device Code = Assembly Location = Year = Work Week = Pb-Free Package (Note: Microdot may be in either location) ADDRESS SELECTION ORDERING INFORMATION ADT7485A GND ADD SST See detailed ordering and shipping information in the package dimensions section on page 11 of this data sheet. Figure 1. Functional Block Diagram Semiconductor Components Industries, LLC, 2012 July, 2012 − Rev. 4 1 Publication Order Number: ADT7485A/D ADT7485A Table 1. PIN ASSIGNMENT Pin No. Mnemonic Type Description 1 VCC Power Supply 2 GND Ground 3 D1+ Analog Input Positive Connection to Remote 1 Temperature Sensor 4 D1− Analog Input Negative Connection to Remote 1 Temperature Sensor 5 12 V Analog Input 12 V Supply Monitor 6 5.0 V Analog Input 5.0 V Supply Monitor 7 VCCP Analog Input Processor Core Voltage Monitor 8 2.5 V Analog Input 2.5 V Supply Monitor 9 ADD Digital Input SST Address Select 10 SST Digital Input/Output 3.3 V 10%. VCC is also Monitored through this Pin Ground Pin SST Bidirectional Data Line Table 2. ABSOLUTE MAXIMUM RATINGS Parameter Rating Unit 4.0 V Voltage on 12 V Pin 16 V Voltage on 5.0 V Pin 7.0 V Supply Voltage (VCC) Voltage on 2.5 V and VCCP Pins 3.6 V −0.3 to +3.6 V Input Current at Any Pin 5.0 mA Package Input Current 20 mA Maximum Junction Temperature (TJ MAX) 150 C −65 to +150 C Voltage on Any Other Pin (Including SST Pin) Storage Temperature Range Lead Temperature, Soldering IR Peak Re-flow Temperature Lead Temperature (10 sec) C 260 300 ESD Rating 1,500 V Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability. NOTE: This device is ESD sensitive. Use standard ESD precautions when handling. Table 3. THERMAL CHARACTERISTICS (Note 1) Package Type 10-lead MSOP qJA qJC Unit 206 44 C/W 1. qJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages. http://onsemi.com 2 ADT7485A Table 4. ELECTRICAL CHARACTERISTICS (TA = TMIN to TMAX, VCC = VMIN to VMAX, unless otherwise noted) Parameter Test Conditions/Comments Min Typ Max Unit 3.0 3.3 3.6 V − 2.8 − V Continuous Conversions − 3.8 5.0 mA Local Sensor Accuracy 40C TA 70C; VCC = 3.3 V 5% −40C TA +100C − − +1.0 − 1.75 4.0 C Remote Sensor Accuracy −40C TD +125C; TA = 25C; VCC = 3.3 V −40C TD +125C; −40 TA 70C; VCC = 3.3 V 5% −40C TD +125C; −40 TA +100C − − +1.0 1.0 1.75 C − − 4.0 Low Level Mid Level High Level − − − 12 80 204 − − − mA − 0.016 − C − 1.5 − kW 2.3 − − V − − 0.8 V −1.0 − − mA − − 1.0 mA − 5.0 − pF Power Supply Supply Voltage, VCC Undervoltage Lockout Threshold Average Operating Supply Current, IDD Temperature-to-Digital Converter Remote Sensor Source Current Resolution Series Resistance Cancellation The ADT7485A Cancels 1.5 kW in Series with the Remote Thermal Diode Digital Input (ADD) Input High Voltage, VIH Input Low Voltage, VIL Input High Current, IIH VIN = VCC Input Low Current, IIL VIN = 0 Pin Capacitance Analog-to-Digital Converter (Including Multiplexer and Attenuators) Total Unadjusted Error (TUE) 12 V and 5.0 V Channels For All Other Channels − − − − 2.0 1.5 % Differential Non-linearity (DNL) 10 Bits − − 1.0 LSB Power Supply Sensitivity − 0.1 − %/V Conversion Time (Voltage Input) (Note 1) Averaging Enabled − − 11 ms Conversion Time (Local Temperature) (Note 1) Averaging Enabled − − 12 ms Conversion Time (Remote Temperature) (Note 1) Averaging Enabled − − 38 ms Total Monitoring Cycle Time (Note 1) Averaging Enabled − 145 − ms 80 95 180 110 120 230 140 150 280 kW Input Resistances VCCP and 2.5 V Channels 5.0 V Channel 12 V Channel http://onsemi.com 3 ADT7485A Table 4. ELECTRICAL CHARACTERISTICS (continued) (TA = TMIN to TMAX, VCC = VMIN to VMAX, unless otherwise noted) Parameter Test Conditions/Comments Min Typ Max Unit 1.1 − − V − − 0.4 V − 150 − mV Digital I/O (SST Pin) Input High Voltage , VIH Input Low Voltage, VIL Hysteresis (Note 1) Between Input Switching Levels Output High Voltage, VOH ISOURCE = 6 mA (maximum) 1.1 − 1.9 V High Impedance State Leakage, ILEAK Device Powered On SST Bus; VSST = 1.1 V, VCC = 3.3 V − − 1.0 mA High Impedance State Leakage, ILEAK Device Non-powered On SST Bus; VSST = 1.1 V, VCC = 0 V − − 10 mA Signal Noise Immunity, VNOISE Noise Glitches from 10 MHz to 100 MHz; Width Up to 50 ns 300 − − mV p-p SST Timing Bitwise Period, tBIT High Level Time for Logic 1, tH1 (Note 2) tBIT Defined in Speed Negotiation High Level Time for Logic 0, tH0 (Note 2) Time to Assert SST High for Logic 1, tSU, HIGH Hold Time, tHOLD (Note 3) See SST Specification Rev 1.0 Stop Time, tSTOP Device Responding to a Constant Low Level Driven by Originator Time to Respond After a Reset, tRESET Response Time to Speed Negotiation After Powerup Time after Powerup when Device Can Participate in Speed Negotiation 0.495 − 500 ms 0.6 tBIT 0.75 tBIT 0.8 tBIT ms 0.2 tBIT 0.25 tBIT 0.4 tBIT ms − − 0.2 tBIT ms − − 0.5 tBIT−M ms 1.25 tBIT 2 tBIT 2 tBIT ms − − 0.4 ms − 500 − ms 1. Guaranteed by design, not production tested. 2. 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. http://onsemi.com 4 ADT7485A TYPICAL PERFORMANCE CHARACTERISTICS 1.55 3.56 3.54 750 W (~2 mA) 1.45 3.53 1.40 IDD (mA) SST O/P (V) DEV 3 3.55 1.50 270 W (~5.2 mA) 1.35 1.30 DEV 2 3.52 3.51 3.50 3.49 3.48 120 W (~10.6 mA) DEV 1 3.47 1.25 3.46 1.20 2.6 2.8 3.0 3.2 3.4 3.45 −45 −25 3.6 −5 VCC (V) 7 1.55 6 1.50 5 HI SPEC (VCC = 3.0 V) 2 MEAN (VCC = 3.3 V) 0 0 20 95 115 1.35 120 W (~10.6 mA) 1.25 40 60 1.20 −50 80 100 120 140 0 50 100 150 TEMPERATURE (C) TEMPERATURE (C) Figure 4. Local Temperature Error Figure 5. SST O/P Level vs. Temperature 3.9 TEMPERATURE ERROR (C) 7 3.7 DEV2 DEV3 IDD (mA) 75 270 W (~5.2 mA) 1.40 1.30 LO SPEC (VCC = 3.6 V) −1 −60 −40 −20 55 750 W (~2 mA) 1.45 4 3 35 Figure 3. Supply Current vs. Temperature SST O/P (V) TEMPERATURE ERROR (C) Figure 2. SST O/P Level vs. Supply Voltage 1 15 TEMPERATURE (C) 3.5 DEV1 3.3 3.1 2.9 2.65 2.85 3.05 3.25 3.45 6 5 4 3 2 1 0 −1 HI SPEC (VCC = 3.0 V) MEAN (VCC = 3.3 V) LO SPEC (VCC = 3.6 V) −2 −60 −40 −20 3.65 VCC (V) 0 20 40 60 80 100 120 140 TEMPERATURE (C) Figure 6. Supply Current vs. Voltage Figure 7. Remote Temperature Error http://onsemi.com 5 ADT7485A TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d) 30 15 D+ TO GND 5 DEV2_EXT2 DEV3_EXT1 DEV3_EXT2 ERROR (C) 0 −5 −10 −15 DEV1_EXT1 DEV1_EXT2 DEV2_EXT1 D+ TO VCC −20 DEV2_EXT2 DEV3_EXT1 DEV3_EXT2 −25 −30 20 15 0 20 40 60 80 60 mV 10 5 40 mV 0 −35 −40 100 mV 25 TEMPERATURE ERROR (C) 10 DEV1_EXT1 DEV1_EXT2 DEV2_EXT1 −5 10k 100 100k Figure 8. Remote Temperature Error vs. PCB Resistance −10 15 −20 10 5 125 mV 0 −30 ERROR (C) TEMPERATURE ERROR (C) 1G 0 EXT2 −40 EXT1 −50 −60 50 mV −70 −5 −80 100k 1M 10M 100M −90 1G 0 10 POWER SUPPLY NOISE FREQUENCY (Hz) 20 30 40 50 CAPACITANCE (nF) Figure 10. Local Temperature Error vs. Power Supply Noise Figure 11. Remote Temperature Error vs. Capacitance Between D1+ and D1− 7 5 40 mV 6 TEMPERATURE ERROR (C) TEMPERATURE ERROR (C) 100M Figure 9. Temperature Error vs. Common-Mode Noise Frequency 20 5 4 3 20 mV 2 1 0 10k 10M NOISE FREQUENCY (Hz) RESISTANCE (MW) −10 10k 1M 10 mV 100k 1M 10M 100M 4 3 2 1 0 50 mV −1 −2 −3 10k 1G 125 mV 100k 1M 10M 100M 1G POWER SUPPLY NOISE FREQUENCY (Hz) NOISE FREQUENCY (Hz) Figure 12. Temperature Error vs. Differential-Mode Noise Frequency Figure 13. Remote Temperature Error vs. Power Supply Noise http://onsemi.com 6 ADT7485A Product Description ADT7485A Client Address The ADT7485A is a temperature- and voltage-monitoring device. The ADT7485A can monitor the temperature of one remote sensor diode, plus its own internal temperature. It can also monitor up to five voltage channels, including its own supply voltage. The client address for the ADT7485A is selected using the address pin. The address pin is connected to a float detection circuit, which allows the ADT7485A to distinguish between three input states: high, low (GND), and floating. The address range for the fixed address, discoverable device is 0x48 to 0x4A. SST Interface 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 ADT7485A 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. Table 5. ADT7485A SELECTABLE ADDRESSES ADD Address Selected Low (GND) 0x48 Float 0x49 High 0x4A Command Summary Table 6 summarizes the commands supported by the ADT7485A 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 Code, CC Write Length, WL Read Length, RL Ping() 0x00 0x00 0x00 Shows a nonzero FCS over the header if present. GetIntTemp() 0x00 0x01 0x02 Shows the temperature of the device’s internal thermal diode. GetExtTemp() 0x01 0x01 0x02 Shows the temperature of External Thermal Diode. GetAllTemps() 0x00 0x01 0x04 Returns a 4-byte block of data (GetIntTemp, GetExt1Temp). GetVolt12V() 0x10 0x01 0x02 Shows the voltage attached to 12 V input. GetVolt5V() 0x11 0x01 0x02 Shows the voltage attached to 5.0 V input. GetVoltVCC() 0x12 0x01 0x02 Shows the voltage attached to VCC input. GetVolt2.5V() 0x13 0x01 0x02 Shows the voltage attached to 2.5 V input. GetVoltVCCP() 0x14 0x01 0x02 Shows the voltage attached to VCCP input. GetAllVolts() 0x10 0x01 0x10 Shows all voltage measurement values. SetExtOffset() 0xe0 0x02 0x00 Sets the offset used to correct errors in External Diode. GetExtOffset() 0xe0 0x01 0x01 Shows the offset that the device is using to correct errors in External Diode. ResetDevice() 0xf6 0x01 0x00 Functional reset. The ADT7485A also responds to this command when directed to the Target Address 0x00. GetDIB() 0xf7 0xf7 0x01 0x01 0x08 0x10 Shows information used by SW to identify the device’s capabilities. Can be in 8- or 16-byte format. Command Command Code Details Description 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. ADT7485A Device Identifier Block The GetDIB() command retrieves the device identifier block (DIB), which provides information to identify the capabilities of the ADT7485A. The data returned can be in 8- or 16-byte format. The full 16 bytes of DIB is detailed in http://onsemi.com 7 ADT7485A 16-bit, twos complement format. The ADT7485A shows 0x8000 in response to this command if the external diode is an open or short circuit. Table 7. 16-BYTE DIB DETAILS Byte Name Value Description 0 Device Capabilities 0xc0 Fixed Address Device 1 Version/Revision 0x10 Meets Version 1 of SST Specification GetAllTemps() 2, 3 Vendor ID 00x11d4 Contains Company ID Number in Little Endian Format 4, 5 Device ID 0x7485 Contains Device ID Number in Little Endian Format The ADT7485A shows the local and remote temperatures in a 4-byte block of data (internal temperature first, followed by external temperature) in response to a GetAllTemps() command. SetExtOffset() 6 Device Interface 0x01 SST Device 7 Function Interface 0x00 Reserved This command sets the offset that the ADT7485A 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 128C with +0.25C resolution. 8 Reserved 0x00 Reserved GetExtOffset() 9 Reserved 0x00 Reserved 10 Reserved 0x00 Reserved 11 Reserved 0x00 Reserved 12 Reserved 0x00 Reserved 13 Reserved 0x00 Reserved 14 Revision ID 0x05 Contains Revision ID 15 Client Device Address 0x48 to 0x4a This command causes the ADT7485A 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. ADT7485A Response to Unsupported Commands A full list of command codes supported by the ADT7485A 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 ADT7485A. The ADT7485A 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. Dependent on the State of Address Pin Ping() The Ping() command verifies if a device is responding at a particular address. The ADT7485A shows a valid non-zero FCS in response to the Ping() command when correctly addressed. Table 8. PING() COMMAND Voltage Measurement Target Address Write Length Read Length (Not Necessary) 0x00 0x00 FCS The ADT7485A has four external voltage measurement channels. It can also measure its own supply voltage, VCC. Pins 5 and 8 measure the supplies of the 12 V, 5.0 V, processor core voltage (VCCP), and 2.5 V pins, respectively. The VCC supply voltage measurement is carried out through the VCC pin (Pin 1). The 2.5 V pin can be used to monitor a chip-set supply voltage in a computer system. ResetDevice() This command resets the register map and conversion controller. The reset command can be global or directed at the client address of the ADT7485A. Analog-to-Digital Converter Table 9. RESETDEVICE() COMMAND Target Address Write Length Read Length Reset Command Device Address 0x01 0x00 0xf6 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, 5.0 V, 12 V, and the processor core voltage (VCCP) without any external components. 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 overvoltage. The full-scale voltage that can be recorded for each channel is shown in Table 10. FCS GetIntTemp() The ADT7485A shows the local temperature of the device in response to the GetIntTemp() command. The data has a little endian, 16-bit, twos complement format. GetExtTemp() Prompted by the GetExtTemp() command, the ADT7485A shows the temperature of the remote diode in little endian, http://onsemi.com 8 ADT7485A Voltage Data Format Table 10. MAXIMUM REPORTED INPUT VOLTAGES Voltage Channel Full-scale Voltage 12 V 16 V 5.0 V 8.0 V VCC 4.0 V 2.5 V 4.0 V VCCP 4.0 V 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. Table 12. ANALOG-TO-DIGITAL OUTPUT VS. VIN Twos Complement Voltage Input Circuitry The internal structure for the analog inputs is shown in Figure 14. The input circuit consists of an input protection diode and an attenuator, plus a capacitor that forms a first-order, low-pass filter to provide input immunity to high frequency noise. 120 kW 12VIN 20 kW 30 pF 47 kW 71 kW 30 pF MUX 45 kW 2.5VIN 94 kW 30 pF 0011 0000 0000 0000 0001 0100 0000 0000 3.3 0000 1101 0011 0011 3.0 0000 1100 0000 0000 2.5 0000 1010 0000 0000 1.0 0000 0100 0000 0000 0 0000 0000 0000 0000 Table 13. TEMPERATURE MONITORING SEQUENCE Channel Number 17.5 kW VCCP 12 5.0 The ADT7485A has two dedicated temperature measurement channels: one for measuring the temperature of an on-chip band gap temperature sensor, and one for measuring the temperature of a remote diode, usually located in the CPU or GPU. The ADT7485A monitors one local and one remote temperature channel. Monitoring of each of the channels is done in a round-robin sequence. The monitoring sequence is in the order shown in Table 13. 30 pF 68 kW 3.3VIN LSB Temperature Measurement 93 kW 5VIN MSB 52.5 kW 35 pF 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 0x10 with a read length of 0x10. The data is retrieved in the order listed in Table 11. Table 11. VOLTAGE MEASUREMENT COMMAND CODE Command Code Returned Data 0x10 LSB, MSB 5.0 V 0x11 LSB, MSB VCC 0x12 LSB, MSB 2.5 V 0x13 LSB, MSB VCCP 0x14 LSB, MSB 0 Local Temperature 12 1 Remote 1 Temperature 38 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 ADT7485A measures the change in VBE when the device is operated at three different currents. Figure 15 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 Voltage Measurement Command Codes 12 V Conversion Time (ms) Temperature Measurement Method Figure 14. Internal Structure of Analog Inputs Voltage Channel Measurement http://onsemi.com 9 ADT7485A biased above ground by an internal diode at the D1− input. If the sensor is operating in an extremely noisy environment, I REMOTE SENSING TRANSISTOR N1 I N2 I C1 can be added as a noise filter. Its value should not exceed 1,000 pF. IBIAS VCC D1+ VOUT+ C1* To ADC D1− VOUT− BIAS DIODE LOW-PASS FILTER fC = 65 kHz *CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS. Figure 15. Signal Conditioning for Remote Diode Temperature Sensors To measure DVBE, the operating current through the sensor is switched between three related currents. Figure 15 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 DVBE1, and then between I and N2 I, giving DVBE2. The temperature can then be calculated using the two DVBE measurements. This method can also cancel the effect of series resistance on the temperature measurement. The resulting DVBE 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 DVBE. 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. 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 0.016C resolution. Table 15. SST TEMPERATURE DATA FORMAT Twos Complement Temperature (5C) MSB LSB −125 1110 0000 1100 0000 −80 1110 1100 0000 0000 −40 1111 0110 0000 0000 −20 1111 1011 0011 1110 −5 1111 1110 1100 0000 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 0x04. The command codes and returned data are described in Table 14. Command Code Internal 0x00 LSB, MSB External 0x01 LSB, MSB All Temps 0x00 Internal LSB, Internal MSB; External LSB, External MSB 1111 1111 1100 0000 0 0000 0000 0000 0000 +1 0000 0000 0100 0000 +5 0000 0001 0100 0000 +20 0000 0100 1100 0010 +40 0000 1010 0000 0000 +80 0001 0100 0000 0000 +125 0001 1111 0100 0000 Using Discrete Transistors Table 14. TEMPERATURE CHANNEL COMMAND CODES Temp Channel −1 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 16 shows how to connect the ADT7485A 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 D1− input. Returned Data SST Temperature Sensor Data Format The data for temperature is structured to allow values in the range of 512C to be reported. Thus, the temperature http://onsemi.com 10 ADT7485A 5. Thermocouple effects should not be a major problem because 1C corresponds to about 240 mV, and thermocouple voltages are about 3 mV/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. 6. Place a 0.1 mF bypass capacitor close to the ADT7485A. 7. 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. 8. 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 ADT7485A. Leave the remote end of the shield unconnected to avoid ground loops. ADT7485A 2N3904 NPN 2N3906 PNP D1+ D1− ADT7485A D1+ D1− Figure 16. Connections for NPN and PNP Transistors The ADT7485A shows an external temperature value of 0x8000 if the external diode is an open or short circuit. Layout Considerations 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: 1. Place the ADT7485A 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. 2. 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. 3. Use wide tracks to minimize inductance and reduce noise pickup. A 5 mil track minimum width and spacing is recommended. GND 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 W series resistance introduces about 0.5C error. 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 ADT7485A has temperature offset command code of 0xe0 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 ADT7485A. The offset is automatically added to every temperature measurement. The maximum offset is 128C with 0.25C 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. 5 MIL 5 MIL D1+ 5 MIL 5 MIL 5 MIL D1− 5 MIL GND 5 MIL Figure 17. Arrangement of Signal Tracks 4. Try to minimize the number of copper/solder joints, which can cause thermocouple effects. Where copper/solder joints are used, make sure that they are in both the D1+ and D1− paths and are at the same temperature. Table 16. ORDERING INFORMATION Device Order Number* ADT7485AARMZ−R Package Type Package Option Shipping† 10-lead MSOP RM−10 3,000 Tape & Reel †For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D. *This is Pb-Free package. http://onsemi.com 11 ADT7485A PACKAGE DIMENSIONS MSOP−10 CASE 846AC−01 ISSUE O NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: MILLIMETER. 3. DIMENSION “A” DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.15 (0.006) PER SIDE. 4. DIMENSION “B” DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSION. INTERLEAD FLASH OR PROTRUSION SHALL NOT EXCEED 0.25 (0.010) PER SIDE. 5. 846B−01 OBSOLETE. NEW STANDARD 846B−02 −A− −B− K D 8 PL 0.08 (0.003) PIN 1 ID G 0.038 (0.0015) −T− SEATING PLANE M T B S A S C H L J MILLIMETERS MIN MAX 2.90 3.10 2.90 3.10 0.95 1.10 0.20 0.30 0.50 BSC 0.05 0.15 0.10 0.21 4.75 5.05 0.40 0.70 DIM A B C D G H J K L INCHES MIN MAX 0.114 0.122 0.114 0.122 0.037 0.043 0.008 0.012 0.020 BSC 0.002 0.006 0.004 0.008 0.187 0.199 0.016 0.028 SOLDERING FOOTPRINT* 10X 1.04 0.041 0.32 0.0126 3.20 0.126 8X 10X 4.24 0.167 0.50 0.0196 SCALE 8:1 5.28 0.208 mm Ǔ ǒinches *For additional information on our Pb-Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting Techniques Reference Manual, SOLDERRM/D. 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