ADT7461 AD | Alldatasheet
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±1°C Temperature Monitor with Series Resistance Cancellation ADT7461*
FEATURES
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
APPLICATIONS
Desktop and notebook computers Industrial controllers Smart batteries Automotive Enbedded systems Burn-in applications Instrumentation PRODUCT DESCRIPTION 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 ther- mal diode accurate to ±1°C, and the ambient temperature accu- rate 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 compati- ble 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. *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. 04110-0-012 DIGITAL MUX DIGITAL MUX SCLKSDATAGNDVDD ADDRESS POINTER REGISTER LOCAL TEMPERATURE LOW LIMIT REGISTER LOCAL TEMPERATURE HIGH LIMIT REGISTER REMOTE TEMPERATURE LOW LIMIT REGISTER REMOTE TEMPERATURE HIGH LIMIT REGISTER LIMIT COMPARATOR LOCAL TEMPERATURE VALUE REGISTER REMOTE TEMPERATURE VALUE REGISTER ADCANALOG MUX ON-CHIP TEMPERATURE SENSOR RUN/STANDBYBUSY REMOTE OFFSET REGISTER EXTERNAL DIODE OPEN-CIRCUIT ADT7461 STATUS REGISTER SMBus INTERFACE LOCAL THERM LIMIT REGISTER EXTERNAL THERM LIMIT REGISTER CONFIGURATION REGISTER INTERRUPT MASKING 751 8 THERM ALERT/ THERM2 SRC BLOCK CONVERSION RATE REGISTER Figure 1. Functional Block Diagram Information furn ished by An alog D evices is believed to be accurate and reliable. infringements of patents or other rights of third parties that may result from its use. registered trademarks are the property of their respective owners. Fax: 781.326.8703 © 2003 Analog Devices, Inc. All rights reserved.
REVISION HISTORY
Revision 0: Initial Version Rev. 0 | Page 2 of 24
Table 1. ADT7461 Specifications at TA = −40°C to +120°C , VDD = 3 V to 5.5 V, unless otherwise noted. SMBus Timeout5 25 64 ms User Programmable. 1 See Table 8 for information on other conversion rates. 2 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.
Table 2. SMBus Timing Specifications1 tLOW 4.7 µs min Clock Low Period, between 10% Points. tHIGH 4 µs min Clock High Period, between 90% Points. tR 1 µs max Clock/Data Rise Time. tF 300 ns max Clock/Data Fall Time. tSU; STA 4.7 µs min Start Condition Setup Time. tHD; STA2 4 µs min Start Condition Hold Time. tSU; DAT3 250 ns min Data Setup Time. tHD; DAT 300 µs min Data Hold Time. tSU; STO4 4 µs min Stop Condition Setup Time. tBUF 4.7 µs min Bus Free Time between Stop and Start Conditions. 1 Guaranteed by design but not production tested. 2 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. Figure 2. Serial Bus Timing
Table 3. ADT7461 Absolute Maximum Ratings*
Figure 3. Pin Configuration Table 4. Pin Function Descriptions 1 VDD Positive Supply, 3 V to 5.5 V. 2 D+ Positive Connection to Remote Temperature Sensor. 3 D− Negative Connection to Remote Temperature Sensor. overtemperature condition. Requires pull-up to VDD. 5 GND Supply Ground Connection. output. Requires pull-up resistor. 7 SDATA Logic Input/Output, SMBus Serial Data. Open-drain output. Requires pull-up resistor. 8 SCLK Logic Input, SMBus Serial Clock. Requires pull-up resistor.
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 gen- erate 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 con- trolled 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 resis- tance. 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 fea- ture 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 for more details. Noise Filtering Noise Filtering 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 collec- tor 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 (recom- mended maximum value 1000 pF). However, a better option in noisy environments is to add a filter, as described in the section on . See the section on for more information on C1. Layout Considerations 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 volt- age 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 tempera- ture sensor is performed in the same manner. Rev. 0 | Page 9 of 24
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS. Figure 14. Input Signal Conditioning remote high and low limit registers. format for the external temperature low byte. Table 5. Extended Temperature Resolution (Remote and low byte, the two registers should be read in succession. over the SMBus are significantly faster than a conversion time. measurement cycle after changing the temperature range. specified at ambient temperatures from −40°C to +120°C.
Table 6. Temperature Data Format (Local and Remote Temperature High Byte) 1 Offset binary scale temperature values are offset by +64. 2 Binary scale temperature measurement returns 0 for all temperatures < 0°C. 3 Binary scale temperature measurement returns 127 for all temperature > 127°C. The user may switch between measurement ranges at any time. follows, and further details are given in Table 7 through Table 11. to which a subsequent read operation is performed. SMBus. The local temperature value register is at Address 0×00. default for all three registers is 0×00. and should not be written to by the user. is configured as THERM2, then the value of Bit 7 has no effect. from or written to the ADT7461 via the SMBus in this mode. Bit 5 is 0, (default) then Pin 6 is configured as an ALERT output. urement range is set to the extended temperature range.
Table 7. Configuration Register Bit Assignments
7 MASK1 0 = ALERT Enabled
6 RUN/STOP 0 = Run
5 ALERT/THERM2 0 = ALERT
3 Reserved 0
2 Temperature
0 Reserved 0
This register can be written to and read back over the SMBus. Table 8. Conversion Rate Register Codes lower will result in an out-of-limit condition. to any value after power up (Register Address 0×21). reprogrammed to be 0100 1010b. contains status information for the ADT7461. and the remote sensor open circuit (Bit 2). latch will be set and the ALERT output will go low.
otherwise the same as THERM. Table 9. Status Register Bit Assignments
7 BUSY 1 When ADC Converting
6 LHIGH* 1 When Local High Temperature Limit Tripped
5 LLOW* 1 When Local Low Temperature Limit Tripped
4 RHIGH* 1 When Remote High Temperature Limit Tripped
3 RLOW* 1 When Remote Low Temperature Limit Tripped
2 OPEN* 1 When Remote Sensor Open Circuit
1 RTHRM 1 When Remote THERM Limit Tripped
0 LTHRM 1 When Local THERM Limit Tripped
*These flags stay high until the status register is read, or they are reset by POR. racy on this channel, these offsets must be removed. Registers 0×11 (high byte) and 0×12 (low byte, left justified). ured value of the remote temperature. will have no effect unless the user writes a different value to it. Table 10. Sample Offset Register Codes limit measurements must occur before an ALERT is generated. ates an ALERT. The maximum value that can be chosen is 4. fastest three conversion rates, where no averaging takes place. This register is at Address 0×22. Table 11. Consecutive ALERT Register Bit y = SMBus timeout bit. Default = 0. See SMBus section for more information.
Table 12. List of ADT7461 Registers
00 Not Applicable Local Temperature Value 0000 0000 (0×00)
01 Not Applicable External Temperature Value High Byte 0000 0000 (0×00)
02 Not Applicable Status Undefined
10 Not Applicable External Temperature Value Low Byte 0000 0000
*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. except for some devices that have extended, 10-bit addresses. available with one device address, 0×4C (1001 100b).
- The master initiates data transfer by establishing a START
high. This indicates that an address/data stream will follow. ter will read from the slave device.
- Data is sent over the serial bus in a sequence of nine clock
when the clock is high may be interpreted as a STOP signal.
- 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. Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23 packages, are suitable devices to use. THERMAL INERTIA AND SELF-HEATING Accuracy depends on the temperature of the remote sensing diode and/or the internal temperature sensor being at the same temperature as that being measured. 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. 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 self- heating is negligible. In the case of the ADT7461, the worst-case condition occurs when the device is converting at 64 conver- sions 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 resis- tance, θJA, of the SOIC-8 package is about 121°C/W. 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 sens- ing 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. 04110-0-010 5MIL 5MIL 5MIL 5MIL 5MIL 5MIL 5MIL GND GND
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
Figure 24. 8-Lead Standard Small Outline Package
0.65 BSC
1.10 MAX
Figure 25. 8-Lead Micro Small Outline Package
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Rev. 0 | Page 24 of 24 NOTES © 2003 A nalog Devices, Inc. All rig hts reserved. Trademarks and regis- tered trade marks are the property of their respective owners . C04110-0-10/03(0)