ADT7461AARMZ-REEL [ADI]
【1∑C Temperature Monitor with Series Resistance Cancellation; ± 1°C的温度监测器，串联电阻抵消
| 型号： | ADT7461AARMZ-REEL |
| 厂家： | 
ADI |
| 描述： | 【1∑C Temperature Monitor with Series Resistance Cancellation |
| 文件： | 总24页 (文件大小：549K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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C
AD 746±A
FEATURES
APPLICATIONS
On-chip and remote temperature sensor
0.25°C resolution/1°C accuracy on remote channel
1°C resolution/1°C accuracy on local channel
Desktop and notebook computers
Industrial controllers
Smart batteries
Automatically cancels up to 1.5 kΩ (typical) of resistance in
series with remote diode to allow noise filtering
Extended, switchable temperature measurement range
0°C to +127°C (default) or –64°C to +191°C
Automotive
Embedded systems
Burn-in applications
Instrumentation
Pin- and register-compatible with ADM1032 and ADT7461
2-wire SMBus serial interface with SMBus alert support
Programmable over/under temperature limits
Offset registers for system calibration
THERM
Up to two overtemperature fail-safe
Small 8-lead MSOP
outputs
240 μA operating current, 5 μA standby current
FUNCTIONAL BLOCK DIAGRAM
ADDRESS POINTER
REGISTER
ON-CHIP
TEMPERATURE
SENSOR
CONVERSION RATE
REGISTER
LOCAL TEMPERATURE
LOW-LIMIT REGISTER
LOCAL TEMPERATURE
VALUE REGISTER
LOCAL TEMPERATURE
HIGH-LIMIT REGISTER
REMOTE TEMPERATURE
LOW-LIMIT REGISTER
2
3
D+
D–
A-TO-D
CONVERTER
ANALOG
MUX
REMOTE TEMPERATURE
HIGH-LIMIT REGISTER
BUSY RUN/STANDBY
REMOTE TEMPERATURE
VALUE REGISTER
LOCAL THERM LIMIT
REGISTERS
EXTERNAL THERM LIMIT
REGISTERS
REMOTE OFFSET
REGISTER
CONFIGURATION
REGISTERS
EXTERNAL DIODE OPEN-CIRCUIT
INTERRUPT
MASKING
6
4
ALERT/THERM2
THERM
STATUS REGISTER
ADT7461A
SMBus INTERFACE
1
5
7
8
V
GND
SDATA
SCLK
DD
Figure 1.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registeredtrademarks arethe property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.461.3113
www.analog.com
©2006 Analog Devices, Inc. All rights reserved.
AD 746±AC
ABLECOFC°ON EN ꢄC
Features .............................................................................................. 1
Temperature Measurement Results ......................................... 11
Temperature Measurement Range........................................... 11
Temperature Data Format......................................................... 11
ADT7461A Registers ................................................................. 12
Serial Bus Interface..................................................................... 15
Addressing the Device............................................................... 15
Applications....................................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
General Description......................................................................... 3
Differences between the ADT7461A and the ADT7461 ........ 3
Specifications..................................................................................... 4
SMBus Timing Specifications..................................................... 5
Timing Diagram ........................................................................... 5
Absolute Maximum Ratings............................................................ 6
Thermal Resistance ...................................................................... 6
ESD Caution.................................................................................. 6
Pin Configuration and Function Descriptions............................. 7
Typical Performance Characteristics ............................................. 8
Theory of Operation ...................................................................... 10
Series Resistance Cancellation.................................................. 10
Temperature Measurement Method ........................................ 10
ALERT
Output............................................................................ 17
Low Power Standby Mode......................................................... 17
Sensor Fault Detection .............................................................. 18
The ADT7461A Interrupt System............................................ 18
Application Information ........................................................... 19
Thermal Inertia and Self-Heating............................................ 20
Layout Considerations............................................................... 20
Application Circuit..................................................................... 21
Outline Dimensions....................................................................... 22
Ordering Guide .......................................................................... 22
REVISION HISTORY
5/06—Rev. 0 to Rev. A
Changes to Features.......................................................................... 1
Changes to General Description .................................................... 3
Added Differences Between the ADT7461A and
the ADT7461 Section and Inserted Table 1 .................................. 3
Changes to Table 2............................................................................ 4
Changes to Table 4............................................................................ 6
Changes to Figure 14........................................................................ 9
Changes to Theory of Operation Section.................................... 10
Changes to Temperature Measurement Range Section............. 11
Changes to ADT7461A Registers Section................................... 12
Changes to Limit Registers Section.............................................. 13
Changes to Serial Bus Interface Section and Table 14............... 15
Changes to Low Power Standby Mode Section .......................... 17
4/06—Revision 0: Initial Version
Rev. A | Page 2 of 24
C
AD 746±A
GENERALCDEꢄ°RIP IONC
The ADT7461A1 is a dual-channel digital thermometer and
undertemperature/overtemperature alarm, intended for use in
PCs and thermal management systems. It is pin- and register-
compatible with the ADM1032 and the ADT7461. A feature of the
ADT7461A is series resistance cancellation, where up to 1.5 kΩ
(typical) of resistance in series with the temperature monitoring
diode can be automatically cancelled from the temperature result,
allowing noise filtering. The ADT7461A has a configurable
DIFFERENCES BETWEEN THE ADT7461A AND THE
ADT7461
Although the ADT7461A is pin- and register-compatible with
the ADT7461, there are some specification differences between
the two devices. A summary of these differences is shown in
Table 1.
Table 1. Differences Between the ADT7461A and the ADT7461
ALERT
output and an extended, switchable temperature meas-
Specification
ADT7461A ADT7461 Unit
urement range.
Supply Voltage
3 to 3.6
1
3 to 5.5
3
V
Maximum Local Sensor
Accuracy
°C
The ADT7461A can measure the temperature of a remote
thermal diode accurate to 1ꢀC and the ambient temperature
accurate to 3ꢀC. The temperature measurement range defaults
to 0ꢀC to +127ꢀC, compatible with the ADM1032, but it can be
switched to a wider measurement range of −64ꢀC to +191ꢀC.
Maximum Series Resistance
Cancellation
1.5
3
kΩ
Average Operating Supply
Current
16 Conversions/sec
Standby Mode
240
5
170
5.5
μA
μA
The ADT7461A communicates over a 2-wire serial interface,
compatible with system management bus (SMBus) standards.
The default SMBus address of the ADT7461A is 0x4C. An
ADT7461A-2 is available with an SMBus address of 0x4D. This
is useful if more than one ADT7461A is used on the same SMBus.
Maximum Conversion Time
One Shot, Averaging On
One Shot, Averaging Off
Remote Sensor Current Levels
High
52
8
114.6
12.56
ms
ms
ALERT
An
ture is out of range. The
that allows on/off control of a cooling fan. The
THERM
output signals when the on-chip or remote tempera-
220
82
13.5
96
36
6
μA
μA
μA
THERM
output is a comparator output
ALERT
Mid
Low
output
output, if required.
can be reconfigured as a second
1 Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239;
6,133,753; 6,169,442; 7,010,440 ; other patents pending.
Rev. A | Page 3 of 24
AD 746±AC
ꢄPE°IFI°A IONꢄC
TA = −40ꢀC to +125ꢀC, VDD = 3 V to 3.6 V, unless otherwise noted.
Table 2.
Parameter
Min Typ
Max Unit Test Conditions
POWER SUPPLY
Supply Voltage, VDD
Average Operating Supply Current, IDD
3.0
3.30 3.6
V
240
5
350
30
μA
μA
V
0.0625 conversions/sec rate1
Standby mode
VDD input, disables ADC, rising edge
Undervoltage Lockout Threshold
Power-On-Reset Threshold
2.55
1
2.5
V
TEMPERATURE-TO-DIGITAL CONVERTER
Local Sensor Accuracy
1
°C
0°C ≤ TA ≤ +70°C
0°C ≤ TA ≤ +85°C
−40°C ≤ TA ≤ +100°C
1.5 °C
2.5 °C
°C
Resolution
1
2
Remote Diode Sensor Accuracy
1
°C
0°C ≤ TA ≤ +70°C, −55°C ≤ TD ≤ +150°C
1.5 °C
2.5 °C
°C
0°C ≤ TA ≤ +85°C, −55°C ≤ TD2 ≤ +150°C
−40°C ≤ TA ≤ +100°C, −55°C ≤ TD2 ≤ +150°C
Resolution
Remote Sensor Source Current
0.25
220
82
13.5
40
μA
μA
μA
ms
High level3
Middle level3
Low level3
Conversion Time
52
From stop bit to conversion complete, one-shot mode with
averaging switched on
6
8
ms
One-shot mode with averaging off
(that is, conversion rate = 16-, 32-, or 64-conversions per
second)
Maximum Series Resistance Cancelled
OPEN-DRAIN DIGITAL OUTPUTS
1. 5
kΩ
Resistance split evenly on both the D+ and D– inputs
THERM ALERT THERM2
(
,
/
)
Output Low Voltage, VOL
High Level Output Leakage Current, IOH
0.4
1
V
μA
IOUT = −6.0 mA
VOUT = VDD
0.1
SMBus INTERFACE3, 4
Logic Input High Voltage, VIH
SCLK, SDATA
Logic Input Low Voltage, VIL
SCLK, SDATA
2.1
−1
V
V
3 V ≤ VDD ≤ 3.6 V
3 V ≤ VDD ≤ 3.6 V
0.8
Hysteresis
500
mV
V
SDA Output Low Voltage, VOL
Logic Input Current, IIH, IIL
SMBus Input Capacitance, SCLK, SDATA
SMBus Clock Frequency
SMBus Timeout5
0.4
+1
IOUT = −6.0 mA
μA
pF
kHz
ms
μs
5
400
64
1
25
User programmable
Master clocking in data
SCLK Falling Edge to SDATA Valid Time
1 See Table 10 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. Detailed procedures to enable it are in the Serial Bus Interface section of this data sheet.
Rev. A | Page 4 of 24
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AD 746±A
SMBUS TIMING SPECIFICATIONS
Table 3.
Parameter1
Limit at TMIN and TMAX
Unit
Description
fSCLK
tLOW
tHIGH
tR
tF
tSU; STA
tHD; STA
tSU; DAT
tSU; STO
tBUF
400
1.3
0.6
300
300
600
600
100
600
1.3
kHz max
μs min
μs min
ns max
ns max
ns min
ns min
ns min
ns min
μs min
Clock low period, between 10% points
Clock high period, between 90% points
Clock/data rise time
Clock/data fall time
Start condition setup time
Start condition hold time
Data setup time
Stop condition setup time
Bus free time between stop and start conditions
2
3
4
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.
TIMING DIAGRAM
tR
tF
tHD;STA
tLOW
SCLK
tHIGH
tSU;STA
tHD;STA
tSU;STO
tHD;DAT
tSU;DAT
SDATA
tBUF
STOP START
START
STOP
Figure 2. Serial Bus Timing
Rev. A | Page 5 of 24
AD 746±AC
ABꢄOLU ECꢁAXIꢁUꢁCRA INGꢄC
Table 4.
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.
Parameter
Rating
Positive Supply Voltage (VDD) to GND
D+
D− to GND
SCLK, SDATA, ALERT, THERM
Input Current, SDATA, THERM
Input Current, D−
−0.3 V, +3.6 V
−0.3 V to VDD + 0.3 V
−0.3 V to +0.6 V
−0.3 V to +3.6 V
−1 mA, +50 mA
1 mA
THERMAL RESISTANCE
Table 5. Thermal Resistance
ESD Rating, All Pins (Human Body Model)
Maximum Junction Temperature (TJ Max)
Storage Temperature Range
1500 V
150°C
−65°C to +150°C
Package Type
θJA
θJC
Unit
8-Lead MSOP
142
43.74
°C/W
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
Rev. A | Page 6 of 24
C
AD 746±A
PINC°ONFIGURA IONCANDCFUN° IONCDEꢄ°RIP IONꢄC
V
1
2
3
4
8
7
6
5
SCLK
DD
ADT7461A
D+
D–
SDATA
TOP VIEW
ALERT/THERM2
GND
(Not to Scale)
THERM
Figure 3. Pin Configuration
Table 6. Pin Function Descriptions
Pin No.
Mnemonic
Description
1
2
3
4
VDD
D+
D−
THERM
Positive Supply, 3 V to 3.6 V.
Positive Connection to Remote Temperature Sensor.
Negative Connection to Remote Temperature Sensor.
Open-Drain Output. Can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pull-up resistor.
5
6
GND
Supply Ground Connection.
ALERT/THERM2 Open-Drain Logic Output Used as Interrupt or SMBus ALERT. This can also be configured as a second
THERM output. Requires pull-up resistor.
7
8
SDATA
SCLK
Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pull-up resistor.
Logic Input, SMBus Serial Clock. Requires pull-up resistor.
Rev. A | Page 7 of 24
AD 746±AC
YPI°ALCPERFORꢁAN°EC°HARA° ERIꢄ I°ꢄC
3.5
0
–2
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 15
DEV 16
MEAN
HIGH 4Σ
LOW 4Σ
3.0
2.5
2.0
1.5
1.0
0.5
0
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
–4
–6
–8
–10
–12
–14
–16
–18
DEV 3
DEV 2
DEV 4
–0.5
–1.0
–50
0
50
100
150
0
5
10
15
20
25
TEMPERATURE (°C)
CAPACITANCE (nF)
Figure 4. Local Temperature Error vs. Temperature
Figure 7. Temperature Error vs. D+/D− Capacitance
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1000
900
800
700
600
500
400
300
200
100
0
DEV 2BC
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 15
DEV 16
HIGH 4Σ
LOW 4Σ
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 4BC
DEV 3BC
–0.5
–1.0
–50
0
50
TEMPERATURE (°C)
100
150
0.01
0.1
1
10
100
CONVERSION RATE (Hz)
Figure 5. Remote Temperature Error vs. Actual Temperature
Figure 8. Operating Supply Current vs. Conversion Rate
10
422
420
418
416
414
412
410
408
5
DEV 2BC
D+ TO GND
0
–5
D+ TO V
CC
–10
–15
–20
–25
DEV 3BC
DEV 4BC
1
10
LEAKAGE RESISTANCE (MΩ)
100
3.0
3.1
3.2
3.3
(V)
3.4
3.5
3.6
V
DD
Figure 6. Temperature Error vs. D+/D− Leakage Resistance
Figure 9. Operating Supply Current vs. Voltage
Rev. A | Page 8 of 24
C
AD 746±A
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
80
70
60
50
40
30
20
10
0
DEV 2
100mV
DEV 3
DEV 4
50mV
20mV
–10
3.0
3.1
3.2
3.3
(V)
3.4
3.5
3.6
0
100
200
300
400
500
600
NOISE FREQUENCY (MHz)
V
DD
Figure 13. Temperature Error vs. Differential-Mode Noise Frequency
Figure 10. Standby Supply Current vs. Voltage
35
30
25
20
15
10
5
60
50
40
30
20
10
0
DEV 2BC
DEV 3BC
DEV 4BC
0
1
10
100
1000
0
500
1000
1500
2000
FSCL (kHz)
SERIES RESISTANCE (Ω)
Figure 11. Standby Supply Current vs. Clock Frequency
Figure 14. Temperature Error vs. Series Resistance
25
20
15
10
5
100mV
50mV
20mV
0
0
100
200
300
400
500
600
NOISE FREQUENCY (MHz)
Figure 12. Temperature Error vs. Common-Mode Noise Frequency
Rev. A | Page 9 of 24
AD 746±AC
HEORYCOFCOPERA IONC
The ADT7461A is a local and remote temperature sensor and
over/under temperature alarm, with the added ability to auto-
matically cancel the effect of 1.5 kΩ (typical) of resistance in
series with the temperature monitoring diode. When the
ADT7461A is operating normally, the on-board ADC operates
in a free running mode. The analog input multiplexer alternately
selects either the on-chip temperature sensor to measure its
local temperature or the remote temperature sensor. The ADC
digitizes these signals and the results are stored in the local and
remote temperature value registers.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the base
emitter voltage (VBE) of a transistor operated at constant current.
However, this technique requires calibration to null the effect of
the absolute value of VBE, which varies from device to device.
The technique used in the ADT7461A measures the change in VBE
when the device operates at three different currents. Previous
devices used only two operating currents, but it is the use of a third
current that allows automatic cancellation of resistances in series
with the external temperature sensor.
The local and remote measurement results are compared with
THERM
the corresponding high, low, and
temperature limits,
Figure 15 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, but it can equally be
a discrete transistor. If a discrete transistor is used, the collector
is not grounded but is linked to the base. To prevent ground
noise interfering with the measurement, the more negative
terminal of the sensor is not referenced to ground, but is biased
above ground by an internal diode at the D− input. C1 may be
added as a noise filter (a recommended maximum value of
1000 pF). However, a better option in noisy environments is to
add a filter, as described in the Noise Filtering section. See the
Layout Considerations section for more information on C1.
stored in eight on-chip registers. Out-of-limit comparisons
generate flags that are stored in the status register. A result that
exceeds the high temperature limit or the low temperature limit
ALERT
ALERT
causes the
output to assert. The
output also
THERM
asserts if an external diode fault is detected. Exceeding the
THERM
temperature limits causes the
output to assert low. The
ALERT
THERM
output.
output can be reprogrammed as a second
The limit registers are programmed and the device controlled
and configured via the serial SMBus. The contents of any
register are also read back via the SMBus.
Control and configuration functions consist of switching the
device between normal operation and standby mode, selecting
the temperature measurement range, masking or enabling the
To measure ΔVBE, the operating current through the sensor is
switched among three related currents. As shown in Figure 15,
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 ΔVBE1; and then between I and N2 × I, giving
ΔVBE2. The temperature is then calculated using the two ΔVBE
measurements. This method also cancels the effect of any series
resistance on the temperature measurement.
ALERT
ALERT
THERM2
and ,
output, switching Pin 6 between
and selecting the conversion rate.
SERIES RESISTANCE CANCELLATION
Parasitic resistance to the D+ and D− inputs to the ADT7461A,
seen in series with the remote diode, is caused by a variety of
factors, including PCB track resistance and track length. This
series resistance appears as a temperature offset in the remote
sensor’s temperature measurement. This error typically causes a
0.5ꢀC offset per ohm of parasitic resistance in series with the
remote diode.
The resulting ΔVBE 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 ΔVBE. The ADC digitizes this voltage
producing a temperature measurement. To reduce the effects of
noise, digital filtering is performed by averaging the results of
16 measurement cycles for low conversion rates. At rates of 16-,
32-, and 64-conversions/second, no digital averaging occurs.
The ADT7461A automatically cancels the effect of this series
resistance on the temperature reading, giving a more accurate
result, without the need for user characterization of this resistance.
The ADT7461A is designed to automatically cancel typically up
to 1.5 kΩ of resistance. By using an advanced temperature
measurement method, this process is transparent to the user.
This feature permits resistances to be added to the sensor path to
produce a filter, allowing the part to be used in noisy environments.
See the section on Noise Filtering for more details.
Signal conditioning and measurement of the internal
temperature sensor are performed in the same manner.
Rev. A | Page 10 of 24
C
AD 746±A
V
DD
I
BIAS
I
N2 × I
N1 × I
V
OUT+
D+
1
TO ADC
C1
REMOTE
SENSING
TRANSISTOR
LPF
fC = 65kHz
D–
V
OUT–
BIAS
DIODE
1
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.
Figure 15. Input Signal Conditioning
The extended temperature range is selected by setting Bit 2 of
the configuration register to 1. The temperature range is 0ꢀC
to 127ꢀC when Bit 2 equals 0. A valid result is available in the
next measurement cycle after changing the temperature range.
TEMPERATURE MEASUREMENT RESULTS
The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers
and compared with limits programmed into the local and
remote high and low limit registers.
In extended temperature mode, the upper and lower temperature
that can be measured by the ADT7461A is limited by the
remote diode selection. The temperature registers can have
values from −64ꢀC to +191ꢀC. However, most temperature
sensing diodes have a maximum temperature range of −55ꢀC to
+150ꢀC. Above +150ꢀC, they may lose their semiconductor
characteristics and approximate conductors instead. This results
in a diode short. In this case, a read of the temperature result
register gives the last good temperature measurement. Therefore,
the temperature measurement on the external channel may not
be accurate for temperatures that are outside the operating range
of the remote sensor.
The local temperature value is in Register 0x00 and has a
resolution of 1ꢀC. The external temperature value is stored in
two registers, with the upper byte in Register 0x01 and the
lower byte in Register 0x10. Only the two MSBs in the external
temperature low byte are used giving the external temperature
measurement a resolution of 0.25ꢀC. Table 7 lists the data
format for the external temperature low byte.
Table 7. Extended Temperature Resolution
(Remote Temperature Low Byte)
Extended Resolution
Remote Temperature Low Byte
0.00°C
0.25°C
0.50°C
0.75°C
0 000 0000
0 100 0000
1 000 0000
1 100 0000
It should be noted that although both local and remote temperature
measurements can be made while the part is in extended
temperature mode, the ADT7461A itself should not be exposed to
temperatures greater than those specified in the absolute maximum
ratings section. Further, the device is only guaranteed to operate as
specified at ambient temperatures from −40ꢀC to +120ꢀC.
When reading the full external temperature value, read the LSB
first. This causes the MSB to be locked (that is, the ADC does
not write to it) until it is read. This feature ensures that the
results read back from the two registers come from the same
measurement.
TEMPERATURE DATA FORMAT
The ADT7461A has two temperature data formats. When the
temperature measurement range is from 0ꢀC to 127ꢀC (default),
the temperature data format for both internal and external
temperature results is binary. When the measurement range is
in extended mode, an offset binary data format is used for both
internal and external results. Temperature values are offset by
64ꢀC in the offset binary data format. Examples of temperatures
in both data formats are shown in Table 8.
TEMPERATURE MEASUREMENT RANGE
The temperature measurement range for both internal and
external measurements is, by default, 0ꢀC to +127ꢀC. However,
the ADT7461A can be operated using an extended temperature
range. The extended measurement range is −64ꢀC to +191ꢀC.
Therefore, the ADT7461A can be used to measure the full
temperature range of an external diode, from −55ꢀC to +150ꢀC.
Rev. A | Page 11 of 24
AD 746±AC
Table 8. Temperature Data Format (Temperature High Byte)
The external temperature value high byte register is at
Address 0x01, with the low byte register at Address 0x10.
The power-on default for all three registers is 0x00.
Temperature
–55°C
0°C
+1°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
+127°C
+150°C
Binary
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
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
Configuration Register
The configuration register is Address 0x03 at read and
Address 0x09 at write. Its power-on default is 0x00. Only
four bits of the configuration register are used. Bit 0, Bit 1,
Bit 3, and Bit 4 are reserved; the user does not write to them.
ALERT
Bit 7 of the configuration register masks the
output.
output is enabled. This is the power-on
ALERT
If Bit 7 is 0, the
1 Offset binary scale temperature values are offset by 64°C.
2 Binary scale temperature measurement returns 0°C for all
temperatures <0°C.
3 Binary scale temperature measurement returns 127°C for all
temperatures >127°C.
ALERT
default. If Bit 7 is set to 1, the
output is disabled. This
ALERT
applies only if Pin 6 is configured as
. If Pin 6 is config-
THERM2
, then the value of Bit 7 has no effect.
ured as
If Bit 6 is set to 0, which is power-on default, the device is in
operating mode with ADC converting. If Bit 6 is set to 1, the
device is in standby mode and the ADC does not convert. The
SMBus does, however, remain active in standby mode; therefore,
values can be read from or written to the ADT7461A via the
The user can switch between measurement ranges at any time.
Switching the range likewise switches the data format. The next
temperature result following the switching is reported back to
the register in the new format. However, the contents of the limit
registers do not change. It is up to the user to ensure that when
the data format changes, the limit registers are reprogrammed
as necessary. More information on this is found in the Limit
Registers section.
ALERT
THERM
SMBus. The
and
outputs are also active in
standby mode. Changes made to the registers in standby mode
THERM ALERT
or
that affect the
be updated.
outputs cause these signals to
ADT7461A REGISTERS
Bit 5 determines the configuration of Pin 6 on the ADT7461A.
ALERT
The ADT7461A contains 22, 8-bit registers in total. These registers
store the results of remote and local temperature measurements,
high and low temperature limits, and configure and control the
device. See the Address Pointer Register section through the
If Bit 5 is 0 (default), then Pin 6 is configured as an
THERM2
output. If Bit 5 is 1, then Pin 6 is configured as a
ALERT
ALERT
output. Bit 7, the
configured as an
mask bit, is only active when Pin 6 is
ALERT
Consecutive
Register section of this data sheet for more
THERM2
output. If Pin 6 is set up as a
information on the ADT7461A registers. Additional details are
shown in Table 9 through Table 13. The entire register map is
available in Table 14.
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 0ꢀC to +127ꢀC. Setting Bit 2 to 1 sets the measurement
range to the extended temperature range (−64ꢀC to +191ꢀC).
Address Pointer Register
The address pointer register itself does not have, nor does it
require, an address because the first byte of every write operation is
automatically written to this register. The data in this first byte
always contains the address of another register on the ADT7461A
that is stored in the address pointer register. It is to this register
address that the second byte of a write operation is written, or
to which a subsequent read operation is performed.
Table 9. Configuration Register Bit Assignments
Power-On
Default
Bit
Name
Function
7
MASK1
0 = ALERT Enabled
1 = ALERT Masked
0 = Run
1 = Standby
0 = ALERT
0
6
5
RUN/STOP
0
0
The power-on default value of the address pointer register is
0x00. Therefore, if a read operation is performed immediately
after power-on, without first writing to the address pointer, the
value of the local temperature is returned because its register
address is 0x00.
ALERT/THERM2
1 = THERM2
4, 3 Reserved
Temperature Range 0 = 0°C to 127°C
Select 1 = Extended Range
1, 0 Reserved
0
0
2
Temperature Value Registers
0
The ADT7461A has three registers to store the results of local
and remote temperature measurements. These registers can
only be written to by the ADC and can be read by the user
over the SMBus. The local temperature value register is at
Address 0x00.
Rev. A | Page 12 of 24
C
AD 746±A
Conversion Rate Register
THERM
limit asserts
Exceeding either the local or remote
THERM
THERM2
low. When Pin 6 is configured as
, exceeding
THERM2
The conversion rate register is Address 0x04 at read and
Address 0x0A at write. The lowest four bits of this register are
used to program the conversion rate by dividing the internal
oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, or 1024 to
give conversion times from 15.5 ms (Code 0x0A) to 16 seconds
(Code 0x00). For example, a conversion rate of eight conversions
per second means that beginning at 125 ms intervals, the device
performs a conversion on the internal and the external temperature
channels.
either the local or remote high limit asserts
low. A
default hysteresis value of 10ꢀC is provided that applies to both
channels. This hysteresis value can be reprogrammed
to any value after power-up (Register Address 0x21).
THERM
It is important to remember that the temperature limits data
format is the same as the temperature measurement data
format. Therefore, if the temperature measurement uses default
binary, then the temperature limits also use the binary scale. If
the temperature measurement scale is switched, however, the
temperature limits do not automatically switch. The user must
reprogram the limit registers to the desired value in the correct
data format. For example, if the remote low limit is set at 10ꢀC
with the default binary scale, the limit register value is 0000 1010b.
If the scale is switched to offset binary, the value in the low tem-
perature limit register needs to be reprogrammed to 0100 1010b.
The conversion rate register can be written to and read back
over the SMBus. The higher four bits of this register are unused
and must be set to 0. The default value of this register is 0x08,
giving a rate of 16 conversions per second. Use of slower con-
version times greatly reduces the device power consumption.
Table 10. Conversion Rate Register Codes
Code
Conversion/Second Time (Seconds)
Status Register
0x00
0.0625
16
0x01
0x02
0x03
0x04
0x05
0x06
0x07
0x08
0.125
0.25
0.5
1
2
4
8
16
8
4
2
1
The status register is a read-only register at Address 0x02. It
contains status information for the ADT7461A.
When Bit 7 of the status register is high, it indicates that the
ADC is busy converting. The other bits in this register flag the
out-of-limit temperature measurements (Bit 6 to Bit 3, and Bit 1
to Bit 0) and the remote sensor open circuit (Bit 2).
500 m
250 m
125 m
62.5 m
31.25 m
15.5 m
ALERT
If Pin 6 is configured as an
output, the following applies:
0x09
0x0A
0x0B to 0xFF
32
64
Reserved
If the local temperature measurement exceeds its limits, Bit 6 (high
limit) or Bit 5 (low limit) of the status register asserts to flag this
condition. If the remote temperature measurement exceeds its
limits, then Bit 4 (high limit) or Bit 3 (low limit) asserts. Bit 2
asserts to flag an open circuit condition on the remote sensor.
These five flags are NOR’ed together, so if any of them is high, the
Limit Registers
THERM
The ADT7461A has eight limit registers: high, low, and
ALERT
ALERT
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
interrupt latch is set and the
output goes low.
Reading the status register clears the five flags, Bit 6 to Bit 2,
provided the error conditions causing the flags to be set have
gone away. A flag bit can be reset only if the corresponding
value register contains an in-limit measurement or if the sensor
is good.
THERM
each limit. There is also a
hysteresis register. All limit
registers can be written to, and read back over, the SMBus. See
Table 14 for details of the limit register addresses and their power-
on default values.
ALERT
register. It resets when the
the master reading the device address, provided the error condi-
tion has gone away and the status register flag bits are reset.
The
interrupt latch is not reset by reading the status
ALERT
ALERT
When Pin 6 is configured as an
output, the high limit
output has been serviced by
registers perform a > comparison, while the low limit registers
perform a ≤ comparison. For example, if the high limit register
is programmed with 80ꢀC, then measuring 81ꢀC results in an
out-of-limit condition, setting a flag in the status register. If the
low limit register is programmed with 0ꢀC, measuring 0ꢀC or
lower results in an out-of-limit condition.
Rev. A | Page 13 of 24
ADT7461A
Table 12. Sample Offset Register Codes
THERM
When Flag 1 and/or Flag 0 are set, the
output goes low
Offset Value
−128°C
−4°C
−1°C
−0.25°C
0°C
+0.25°C
+1°C
+4°C
0x11
0x12
to indicate that the temperature measurements are outside the
THERM
1000 0000
1111 1100
1111 1111
1111 1111
0000 0000
0000 0000
0000 0001
0000 0100
0111 1111
00 00 0000
00 00 0000
00 00 0000
11 00 0000
00 00 0000
01 00 0000
00 00 0000
00 00 0000
11 00 0000
programmed limits. The
output does not need to be
output. Once the measurements are
within the limits, the corresponding status register bits are
THERM
ALERT
reset, unlike the
automatically reset and the
output goes high. The user
THERM
may add hysteresis by programming Register 0x21. The
output is reset only when the temperature falls to limit value
minus the hysteresis value.
+127.75°C
THERM2
When Pin 6 is configured as
ture limits are relevant. If Flag 6 and/or Flag 4 are set, the
THERM2
, only the high tempera-
One-Shot Register
output goes low to indicate that the temperature
measurements are outside the programmed limits. Flag 5 and
THERM2 THERM2
is
The one-shot register is used to initiate a conversion and
comparison cycle when the ADT7461A is in standby mode,
after which the device returns to standby. Writing to the one-
shot register address (0x0F) causes the ADT7461A to perform a
conversion and comparison on both the internal and the
external temperature channels. This is not a data register as
such, and it is the write operation to Address 0x0F that causes
the one-shot conversion. The data written to this address is
irrelevant and is not stored.
Flag 3 have no effect on
THERM
. The behavior of
otherwise the same as
.
Table 11. Status Register Bit Assignments
Bit Name
Function
7
6
5
4
3
2
1
0
BUSY
1 when ADC converting
LHIGH1
LLOW1
RHIGH1
RLOW1
OPEN1
RTHRM
LTHRM
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
ALERT
Consecutive
The value written to this register determines how many out-of-
ALERT
Register
1 when remote THERM limit tripped
limit measurements must occur before an
The default value is that one out-of-limit measurement generates
ALERT
is generated.
1 when local THERM limit tripped
1 These flags stay high until the status register is read or they are reset by POR
THERM2
an
. The maximum value that can be chosen is 4. The
unless Pin 6 is configured as
status register is read or is reset by POR.
. Then, only Bit 2 remains high until the
purpose of this register is to allow the user to perform some
filtering of the output. This is particularly useful at the fastest
three conversion rates, where no averaging takes place. This
register is at Address 0x22.
Offset Register
Offset errors can be introduced into the remote temperature
measurement by clock noise or when the thermal diode is
located away from the hot spot. To achieve the specified
accuracy on this channel, these offsets must be removed.
ALERT
Table 13. Consecutive
Register Bit
Number of Out-of-Limit
Measurements Required
Register Value1
yxxx 000x
yxxx 001x
yxxx 011x
The offset value is stored as a 10-bit, twos complement value in
Register 0x11 (high byte) and Register 0x12 (low byte, left
justified). Only the upper two bits of Register 0x12 are used.
The MSB of Register 0x11 is the sign bit. The minimum,
programmable offset is −128°C, and the maximum is
1
2
3
4
yxxx 111x
1 x = don’t care bit.
y = SMBus timeout bit.
Default = 0. See the Serial Bus Interface section.
+127.75°C. The value in the offset register is added to, or
subtracted from, the measured value of the remote temperature.
The offset register powers up with a default value of 0°C and has
no effect unless the user writes a different value to it.
Rev. A | Page 14 of 24
C
AD 746±A
Table 14. List of Registers
Read Address (Hex)
Write Address (Hex)
Name
Power-On Default
Not Applicable
Not Applicable
Address Pointer
Undefined
00
01
02
Not Applicable
Not Applicable
Not Applicable
Local Temperature Value
External Temperature Value High Byte
Status
0000 0000 (0x00)
0000 0000 (0x00)
Undefined
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F1
Configuration
Conversion Rate
0000 0000 (0x00)
0000 1000 (0x08)
0101 0101 (0x55) (85°C)
0000 0000 (0x00) (0°C)
0101 0101 (0x55) (85°C)
0000 0000 (0x00) (0°C)
Local Temperature High Limit
Local Temperature Low Limit
External Temperature High Limit High Byte
External Temperature Low Limit High Byte
One-Shot
Not Applicable
10
11
12
13
14
19
20
21
22
FE
FF
Not Applicable
11
12
13
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
0000 0000
0000 0000
0000 0000
0000 0000
14
0000 0000
19
0101 0101 (0x55) (85°C)
0101 0101 (0x55) (85°C)
0000 1010 (0x0A) (10°C)
0000 0001 (0x01)
0100 0001 (0x41)
0101 0110 (0x56)
Local THERM Limit
20
THERM Hysteresis
21
Consecutive ALERT
22
Not Applicable
Not Applicable
Manufacturer ID
Die Revision Code
1 Writing to Address 0x0F causes the ADT7461A to perform a single measurement. It is not a data register, and it does not matter what data is written to it.
1. The master initiates a data transfer by establishing a start
condition, defined as a high-to-low transition on SDATA,
the serial data line, while SCLK, the serial clock line,
remains high. This indicates that an address/data stream
follows. All slave peripherals connected to the serial bus
respond to the start condition and shift in the next eight
SERIAL BUS INTERFACE
Control of the ADT7461A is carried out via the serial bus. The
ADT7461A is connected to this bus as a slave device, under the
control of a master device.
The ADT7461A has an SMBus timeout feature. When this is
enabled, the SMBus times out after typically 25 ms of no activity.
However, this feature is not enabled by default. Bit 7 of the
consecutive alert register (Address = 0x22) should be set to
enable it.
W
bits, consisting of a 7-bit address (MSB first) plus an R/
bit, which determines the direction of the data transfer,
that is, whether data is written to, or read from, the slave
device. The peripheral whose address corresponds to the
transmitted address responds by pulling the data line low
during the low period before the ninth clock pulse, known
as the acknowledge bit. All other devices on the bus remain
idle while the selected device waits for data to be read from
ADDRESSING THE DEVICE
In general, every SMBus device has a 7-bit device address,
except for some devices that have extended 10-bit addresses.
When the master device sends a device address over the bus,
the slave device with that address responds. The ADT7461A
is available with one device address, 0x4C (1001 100b).
An ADT7461A-2 is also available.
W
or written to it. If the R/ bit is a 0, the master writes to
W
the slave device. If the R/ bit is a 1, the master reads
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 can be interpreted as a stop signal.
The number of data bytes that can be transmitted over the
serial bus in a single read or write operation is limited only
by what the master and slave devices can handle.
The ADT7461A-2 has an SMBus address of 0x4D (1001 101b).
This is to allow two ADT7461A devices on the same bus, or if
the default address conflicts with an existing device on the
SMBus. The serial bus protocol operates as follows:
Rev. A | Page 15 of 24
AD 746±AC
3. When all data bytes have been read or written, stop
conditions are established. In write mode, the master pulls
the data line high during the tenth clock pulse to assert a
stop condition. In read mode, the master device overrides
the acknowledge bit by pulling the data line high during
the low period before the ninth clock pulse. This is known
as no acknowledge. The master takes the data line low
during the low period before the tenth clock pulse, then
high during the tenth clock pulse to assert a stop condition.
To write data to one of the device data registers, or to read data
from it, the address pointer register must be set so that the
correct data register is addressed. The first byte of a write
operation always contains a valid address that is stored in the
address pointer register. If data is to be written to the device, the
write operation contains a second data byte that is written to the
register selected by the address pointer register.
This procedure is illustrated in Figure 16. The device address is
W
sent over the bus followed by R/ set to 0. This is followed by
Any number of bytes of data are transferable over the serial bus
in one operation, but it is not possible to mix read and write in
one operation because the type of operation is determined at
the beginning and cannot subsequently be changed without
starting a new operation. For the ADT7461A, write operations
contain either one or two bytes, while read operations contain
one byte.
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.
1
9
1
9
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
R/W
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7461A
ACK. BY
ADT7461A
START BY
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
1
9
SCLK (CONTINUED)
D7
D6
D5
D4
D3
D2
D1
D0
SDATA (CONTINUED)
ACK. BY STOP BY
ADT7461A MASTER
FRAME 3
DATA BYTE
Figure 16. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
1
9
1
9
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
R/W
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7461A
ACK. BY STOP BY
ADT7461A MASTER
START BY
MASTER
FRAME 2
FRAME 1
SERIAL BUS ADDRESS BYTE
ADDRESS POINTER REGISTER BYTE
Figure 17. Writing to the Address Pointer Register Only
1
9
1
9
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
R/W
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7461A
ACK. BY STOP BY
ADT7461A MASTER
START BY
MASTER
FRAME 2
FRAME 1
SERIAL BUS ADDRESS BYTE
ADDRESS POINTER REGISTER BYTE
Figure 18. Reading from a Previously Selected Register
Rev. A | Page 16 of 24
C
AD 746±A
When reading data from a register there are two possibilities.
ALERT
One or more
SMBALERT
SMBALERT
outputs can be connected to a common
line that is connected to the master. When the
line is pulled low by one of the devices, the
following procedure occurs (see Figure 19):
•
If the address pointer register value of the ADT7461A is
unknown or not the desired value, it is first necessary to set
it to the correct value before data can be read from the desired
data register. This is done by writing to the ADT7461A as
before, but only the data byte containing the register read
address is sent, because data is not to be written to the
register (see Figure 17).
MASTER
RECEIVES
SMBALERT
ALERT RESPONSE
ADDRESS
DEVICE
ADDRESS
NO
START
RD ACK
STOP
ACK
MASTER SENDS
ARA AND READ
COMMAND
DEVICE SENDS
ITS ADDRESS
A read operation is then performed consisting of the serial
W
bus address, R/ bit set to 1, followed by the data byte
SMBALERT
Figure 19. Use of
read from the data register (see Figure 18).
SMBALERT
1.
is pulled low.
•
If the address pointer register is known to be at the desired
address, data can be read from the corresponding data
register without first writing to the address pointer register
and the bus transaction shown in Figure 17 can be omitted.
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.
ALERT
3. The device whose
output is low responds to the
Notes
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.
•
It is possible to read a data byte from a data register
without first writing to the address pointer register.
However, if the address pointer register is already at the
correct value, it is not possible to write data to a register
without writing to the address pointer register because the
first data byte of a write is always written to the address
pointer register.
ALERT
4. If more than one device’s
output is low, the one
with the lowest device address takes priority, in accordance
with normal SMBus arbitration.
Once the ADT7461A has responded to the alert response address,
•
Some of the registers have different addresses for read and
write operations. The write address of a register must be
written to the address pointer if data is to be written to that
register, but it may not be possible to read data from that
address. The read address of a register must be written to
the address pointer before data can be read from that
register.
ALERT
it resets its
that caused the
remains low, the master sends the ARA again, and so on until
ALERT
output, provided that the error condition
ALERT SMBALERT
no longer exists. If the
line
all devices whose
outputs were low have responded.
LOW POWER STANDBY MODE
The ADT7461A can be put into low power standby mode by
setting Bit 6 of the configuration register. When Bit 6 is low, the
ADT7461A operates normally. When Bit 6 is high, the ADC is
inhibited, and any conversion in progress is terminated without
writing the result to the corresponding value register. However,
the SMBus is still enabled. Power consumption in the standby
mode is reduced to 5 μA if there is no SMBus activity, or 30 μA
if there are clock and data signals on the bus.
ALERT OUTPUT
ALERT
This is applicable when Pin 6 is configured as an
output. The
measurement is detected, or if the remote temperature sensor is
open circuit. It is an open-drain output and requires a pull-up
resistor. Several
ALERT
output goes low whenever an out-of-limit
ALERT
outputs can be wire-OR’ed together, so
ALERT
that the common line goes low if one or more of the
outputs goes low.
When the device is in standby mode, it is possible to initiate a
one-shot conversion of both channels by writing to the one-shot
register (Address 0x0F), after which the device returns to
standby. It does not matter what is written to the one-shot
register, all data written to it is ignored. It is also possible to
write new values to the limit register while in standby mode. If
the values stored in the temperature value registers are outside
ALERT
The
processor, or as an
cannot normally signal to the bus master that they want to talk,
SMBALERT
output can be used as an interrupt signal to a
SMBALERT
. Slave devices on the SMBus
but the
function allows them to do so.
ALERT
the new limits, an
is generated, even though the
ADT7461A is still in standby.
Rev. A | Page 17 of 24
AD 746±AC
THERM
THERM Hysteresis
Table 15.
Hysteresis
SENSOR FAULT DETECTION
Binary Representation
At its D+ input, the ADT7461A contains internal sensor fault
detection circuitry. This circuit can detect situations where an
external remote diode is either not connected or incorrectly
connected to the ADT7461A. A simple voltage comparator trips
if the voltage at D+ exceeds VDD − 1 V (typical), signifying an
open circuit between D+ and D−. The output of this comparator is
checked when a conversion is initiated. Bit 2 of the status register
0°C
1°C
10°C
0 000 0000
0 000 0001
0 000 1010
THERM
ALERT
Figure 20 shows how the
and
outputs operate.
SMBALERT
to signal to the
ALERT
The
output can be used as a
host via the SMBus that the temperature has risen. The user can
THERM
ALERT
(open flag) is set if a fault is detected. If the
ALERT
pin is enabled,
use the
output to turn on a fan to cool the system, if
setting this flag causes
to assert low.
the temperature continues to increase. This method ensures
that there is a fail-safe mechanism to cool the system, without
the need for host intervention.
If the user does not wish to use an external sensor with the
ADT7461A, tie the D+ and D− inputs together to prevent
continuous setting of the open flag.
TEMPERATURE
100°C
90°C
THE ADT7461A INTERRUPT SYSTEM
80°C
70°C
60°C
50°C
40°C
THERM LIMIT
ALERT
The ADT7461A has two interrupt outputs,
THERM
and
ALERT
THERM LIMIT-HYSTERESIS
HIGH TEMP LIMIT
. Both have different functions and behavior.
is
maskable and responds to violations of software programmed
temperature limits or an open-circuit fault on the external
RESET BY MASTER
THERM
diode.
is intended as a fail-safe interrupt output that
cannot be masked.
1
4
ALERT
THERM
2
3
If the external or local temperature exceeds the programmed
high temperature limits, or equals or exceeds the low tempera-
ALERT
ALERT
THERM
Interrupts
Figure 20. Operation of the
and
ture limits, the
output is asserted low. An open-circuit
ALERT ALERT
•
•
If the measured temperature exceeds the high temperature
ALERT
fault on the external diode also causes
to assert.
limit, the
If the temperature continues to increase and exceeds the
THERM THERM
output asserts low.
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.
limit, the
used to throttle the CPU clock or switch on a fan.
THERM
output asserts low. This can be
THERM
The
temperature exceeds the programmed
temperature limits should normally be equal to or greater than
THERM
output asserts low if the external or local
•
•
The
temperature falls to
Figure 20, the default hysteresis value of 10ꢀC is shown.
ALERT
output deasserts (goes high) when the
THERM
THERM
limits.
THERM
limit minus hysteresis. In
the high temperature limits.
when the temperature falls back within the
external and local limits are set by default to 85ꢀC. A hysteresis
THERM
is reset automatically
THERM
limit. The
The
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.
value can be programmed; in which case,
resets when
the temperature falls to the limit value minus the hysteresis
value. This applies to both local and remote measurement
channels. The power-on hysteresis default value is 10ꢀC, but this
can be reprogrammed to any value after power-up.
•
•
Pin 6 on the ADT7461A can be configured as either an
ALERT
THERM
output or as an additional
output.
THERM2
asserts low when the temperature exceeds the
programmed local and/or remote high temperature limits.
THERM
THERM
The hysteresis loop on the
THERM
outputs is useful when
It is reset in the same manner as
and is not maskable.
is used, for example, as an on/off controller for a fan.
THERM
THERM
The user’s system can be set up so that when
fan is switched on to cool the system. When
asserts, a
goes high
THERM2
•
The programmed hysteresis value also applies to
.
again, the fan can be switched off. Programming a hysteresis
value protects from fan jitter, where the temperature hovers
THERM
THERM2
operate together
Figure 21 shows how
and
to implement two methods of cooling the system. In this example,
THERM2 THERM
THERM
around the
limit, and the fan is constantly switched.
the
limits are set lower than the
limits. The
THERM2
output is used to turn on a fan. If the temperature
THERM
THERM
limits, the
continues to rise and exceeds the
output provides additional cooling by throttling the CPU.
Rev. A | Page 18 of 24
C
AD 746±A
TEMPERATURE
90°C
Remote Sensing Diode
The ADT7461A is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the substrate. Discrete types are either PNP or NPN transistors
connected as diodes (base-shorted to collector). If an NPN
transistor is used, the collector and base are connected to D+
and the emitter to D−. If a PNP transistor is used, the collector
and base are connected to D− and the emitter to D+.
80°C
70°C
60°C
50°C
40°C
30°C
THERM LIMIT
THERM2 LIMIT
1
4
THERM2
THERM
To reduce the error due to variations in both substrate and
discrete transistors, consider several factors:
3
2
THERM
THERM2
Interrupts
Figure 21. Operation of the
and
•
The ideality factor, nF, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7461A is trimmed for an nF value of 1.008. The
following equation may be used to calculate the error
introduced at a temperature, T (ꢀC), when using a
transistor whose nF does not equal 1.008. Consult the
processor data sheet for the nF values.
THERM2
When the
asserts low.
THERM2
signal
•
•
•
limit is exceeded, the
If the temperature continues to increase and exceeds the
THERM THERM
limit, the
output asserts low.
output deasserts (goes high) when the
THERM
The
ΔT = (nF − 1.008)/1.008 × (273.15 Kelvin + T)
THERM
temperature falls to
limit minus hysteresis. In
To factor this in, the user writes the ΔT value to the offset
register. It is then automatically added to, or subtracted
from, the temperature measurement.
Figure 21, there is no hysteresis value shown.
•
As the system cools further, and the temperature falls
THERM2
THERM2
below the
limit, the
signal resets.
THERM2
•
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7461A, IHIGH, is 220 μA and the low level current,
Again, no hysteresis value is shown for
.
Both the external and internal temperature measurements cause
THERM THERM2
I
LOW, is 13.5 μA. If the ADT7461A current levels do not
and
to operate as described.
match the current levels specified by the CPU manufacturer,
it may become necessary to remove an offset. The CPU
data sheet should advise whether this offset needs to be
removed and how to calculate it. This offset is programmed
to the offset register. It is important to note that if more
than one offset must be considered, the algebraic sum of
these offsets must be programmed to the offset register.
APPLICATION INFORMATION
Noise Filtering
For temperature sensors operating in noisy environments, the
industry standard practice was to place a capacitor across the D+
and D− pins to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature measurement,
leading to a recommended maximum capacitor value of 1,000 pF.
Although this capacitor reduces the noise, it does not eliminate it,
making it difficult to use the sensor in a very noisy environment.
If a discrete transistor is used with the ADT7461A, the best
accuracy is obtained by choosing devices according to the
following criteria:
The ADT7461A has a major advantage over other devices when it
comes to eliminating the effects of noise on the external sensor.
The series resistance cancellation feature allows a filter to be
constructed between the external temperature sensor and the
part. The effect of any filter resistance seen in series with the remote
sensor is automatically cancelled from the temperature result.
•
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 (50 to 150) that indicates tight
control of VBE characteristics
The construction of a filter allows the ADT7461A and the remote
temperature sensor to operate in noisy environments. Figure 22
shows a low-pass R-C-R filter, where R = 100 Ω and C = 1 nF.
This filtering reduces both common-mode and differential noise.
Transistors, such as the 2N3904, 2N3906, or equivalents in
SOT-23 packages are suitable devices to use.
100
Ω
D+
REMOTE
TEMPERATURE
SENSOR
1nF
100
Ω
D–
Figure 22. Filter Between Remote Sensor and ADT7461A
Factors Affecting Diode Accuracy
Rev. A | Page 19 of 24
AD 746±AC
THERMAL INERTIA AND SELF-HEATING
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
GND
D+
Accuracy depends on the temperature of the remote sensing
diode and/or the internal temperature sensor being at the same
temperature as that being measured. Many factors can affect
this. Ideally, place the sensor in good thermal contact with the
part of the system being measured. If it is not, the thermal
inertia caused by the sensor’s mass causes a lag in the response
of the sensor to a temperature change. In the case of the remote
sensor, this should not be a problem since it is either a substrate
transistor in the processor or a small package device, such as the
SOT-23, placed in close proximity to it.
D–
GND
Figure 23. Typical Arrangement of Signal Tracks
•
•
Try to minimize the number of copper/solder joints
that can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+ and
D− path and at the same temperature.
The on-chip sensor, however, is often remote from the processor
and only monitors the general ambient temperature around the
package. How accurately the temperature of the board and/or
the forced airflow reflects the temperature to be measured
dictates the accuracy of the measurement. Self-heating due to
the power dissipated in the ADT7461A or the remote sensor
causes the chip temperature of the device or remote sensor to
rise above ambient. However, the current forced through the
remote sensor is so small that self-heating is negligible. In the
case of the ADT7461A, the worst-case condition occurs when
the device is converting at 64 conversions per second while
Thermocouple effects should not be a major problem as
1ꢀC 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, place an input filter
capacitor across D+ and D− close to the ADT7461A. This
capacitance can effect the temperature measurement, so
ensure that any capacitance seen at D+ and D− is, at
maximum, 1,000 pF. This maximum value includes the
filter capacitance, plus any cable or stray capacitance
between the pins and the sensor diode.
ALERT
sinking the maximum current of 1 mA at the
and
THERM
output. In this case, the total power dissipation in the
device is about 4.5 mW. The thermal resistance, θJA, of the
8-lead MSOP is approximately 142ꢀC/W.
LAYOUT CONSIDERATIONS
If the distance to the remote sensor is more than 8 inches,
the use of twisted pair cable is recommended. A total of
6 feet to 12 feet is needed.
Digital boards can be electrically noisy environments, and the
ADT7461A is measuring very small voltages from the remote
sensor, so care must be taken to minimize noise induced at the
sensor inputs. Take the following precautions:
For really long distances (up to 100 feet), use a shielded
twisted pair, such as the Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D− and the
shield to GND close to the ADT7461A. Leave the remote
end of the shield unconnected to avoid ground loops.
•
Place the ADT7461A as close as possible to the remote
sensing diode. Provided that the worst noise sources, that
is, clock generators, data/address buses, and CRTs are
avoided, this distance can be 4 inches to 8 inches.
Because the measurement technique uses switched current
sources, excessive cable or filter capacitance can affect the
measurement. When using long cables, the filter capacitance
can be reduced or removed.
•
Route the D+ and D– tracks close together, in parallel, with
grounded guard tracks on each side. To minimize
inductance and reduce noise pickup, a 5 mil track width
and spacing is recommended. Provide a ground plane
under the tracks, if possible.
Rev. A | Page 20 of 24
C
AD 746±A
APPLICATION CIRCUIT
Figure 24 shows a typical application circuit for the ADT7461A,
using a discrete sensor transistor connected via a shielded, twisted
The SCLK pin and the SDATA pin of the ADT7461A can be
interfaced directly to the SMBus of an I/O controller, such as
the Intel® 820 chipset.
ALERT
pair cable. The pull-ups on SCLK, SDATA, and
are
required only if they are not provided elsewhere in the system.
V
3V TO 3.6V
DD
0.1µF
ADT7461A
TYP 10kΩ
D+
SCLK
5V OR 12V
SMBUS
CONTROLLER
D–
SDATA
SHIELD
2N3906
OR
ALERT/
THERM2
CPU THERMAL
DIODE
V
DD
THERM
GND
TYP 10kΩ
FAN CONTROL
CIRCUIT
FAN ENABLE
Figure 24. Typical Application Circuit
Rev. A | Page 21 of 24
AD 746±AC
OU LINECDIꢁENꢄIONꢄC
3.20
3.00
2.80
8
1
5
4
5.15
4.90
4.65
3.20
3.00
2.80
PIN 1
0.65 BSC
0.95
0.85
0.75
1.10 MAX
0.80
0.60
0.40
8°
0°
0.15
0.00
0.38
0.22
0.23
0.08
SEATING
PLANE
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 25. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
ORDERING GUIDE
Model
ADT7461AARMZ1
ADT7461AARMZ-REEL1
ADT7461AARMZ-REEL71
ADT7461AARMZ-21
ADT7461AARMZ-2REEL1
ADT7461AARMZ-2RL71
Temperature Range Package Description
Package Option
RM-8
RM-8
RM-8
RM-8
Branding SMBus Address
−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
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
T1K
T1K
T1K
T1L
T1L
T1L
4C
4C
4C
4D
4D
4D
RM-8
RM-8
1 Z = Pb-free part.
Rev. A | Page 22 of 24
C
AD 746±A
NO EꢄC
Rev. A | Page 23 of 24
AD 746±AC
NO EꢄC
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05571-0-5/06(A)
Rev. A | Page 24 of 24
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