±1°C Temperature Monitor with
Series Resistance Cancellation
ADT7461
FEATURES
GENERAL DESCRIPTION
On-chip and remote temperature sensor
0.25°C resolution/1°C accuracy on remote channel
1°C resolution/3°C accuracy on local channel
Automatically cancels up to 3 kΩ (typ) of resistance in series
with remote diode to allow noise filtering
Extended, switchable temperature measurement range
0°C to +127°C (default) or –55°C to +150°C
Pin- and register-compatible with the ADM1032
2-wire SMBus serial interface with SMBus alert support
Two SMBus address versions available:
ADT7461 SMBus address is 0x4C
ADT7461-2 SMBus address is 0x4D
Programmable over/under temperature limits
Offset registers for system calibration
Up to two over temperature fail-safe THERM outputs
Small 8-lead SOIC or8-lead MSOP packages
170 μA operating current, 5.5 μA standby current
The ADT7461 1 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 accurately measure the temperature of a
remote thermal diode to ±1°C and the ambient temperature to
±3°C. The temperature measurement range defaults to 0°C to
+127°C, compatible with the ADM1032, but can be switched to
a wider measurement range of −55°C to +150°C. The ADT7461
communicates over a 2-wire serial interface compatible with
system management bus (SMBus) standards. An ALERT output
signals when the on-chip or remote temperature is out of range.
The THERM output is a comparator output that allows on/off
control of a cooling fan. The ALERT output can be reconfigured
as a second THERM output, if required.
APPLICATIONS
Desktop and notebook computers
Industrial controllers
Smart batteries
Embedded systems
Instrumentation
The SMBus address of the ADT7461 is 0x4C. An ADT7461-2
is also available, which uses SMBus Address 0x4D.
1
3
LOCAL TEMPERATURE
LOW LIMIT REGISTER
RUN/STANDBY
REMOTE TEMPERATURE
VALUE REGISTER
SRC
BLOCK
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
DIGITAL MUX
2
D–
LOCAL TEMPERATURE
VALUE REGISTER
ADC
BUSY
D+
ADDRESS POINTER
REGISTER
LIMIT
COMPARATOR
ANALOG
MUX
CONVERSION RATE
REGISTER
DIGITAL MUX
ON-CHIP
TEMPERATURE
SENSOR
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.
REMOTE TEMPERATURE
LOW LIMIT REGISTER
REMOTE TEMPERATURE
HIGH LIMIT REGISTER
LOCAL THERM LIMIT
REGISTER
REMOTE OFFSET
REGISTER
EXTERNAL THERM LIMIT
REGISTER
CONFIGURATION
REGISTER
EXTERNAL DIODE OPEN CIRCUIT
INTERRUPT
MASKING
STATUS REGISTER
ADT7461
1
5
7
8
4
6
VDD
GND
SDATA
SCLK
THERM
ALERT/
THERM2
04110-0-012
SMBus INTERFACE
Figure 1. Functional Block Diagram
Rev. B
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
© 2005 Analog Devices, Inc. All rights reserved.
ADT7461
TABLE OF CONTENTS
Specifications..................................................................................... 3
ADT7461 Registers .................................................................... 11
SMBus Timing Specifications ......................................................... 4
Serial Bus Interface..................................................................... 14
Absolute Maximum Ratings............................................................ 5
Addressing the Device ............................................................... 14
Thermal Characteristics .............................................................. 5
ALERT Output............................................................................ 16
ESD Caution.................................................................................. 5
Low Power Standby Mode......................................................... 16
Pin Configuration and Function Descriptions............................. 6
Sensor Fault Detection .............................................................. 16
Typical Performance Characteristics ............................................. 7
The ADT7461 Interrupt System............................................... 16
Functional Description .................................................................... 9
Application Information ........................................................... 18
Series Resistance Cancellation.................................................... 9
Thermal Inertia and Self-Heating............................................ 18
Temperature Measurement Method .......................................... 9
Layout Considerations............................................................... 19
Temperature Measurement Results.......................................... 10
Application Circuit..................................................................... 20
Temperature Measurement Range ........................................... 10
Outline Dimensions ....................................................................... 21
Temperature Data Format ......................................................... 10
Ordering Guide .......................................................................... 22
REVISION HISTORY
7/05—Rev. A to Rev. B
Changes to Features Section............................................................. 1
Changes to Applications Section ..................................................... 1
Changes to General Description Section ....................................... 1
Changes to Addressing the Device Section................................... 14
Updated Outline Dimensions ......................................................... 21
Changes to the Ordering Guide...................................................... 22
10/04—Rev. 0 to Rev. A
Change to SMBus Specifications .................................................... 4
Changes to Figure 6 and Figure 10................................................. 7
Added Figure 9 and Figure 13......................................................... 7
Changes to Temperature Measurement Section......................... 10
Changes to Figure 19 and Figure 25............................................. 16
Changes to Serial Bus Interface Section ...................................... 23
10/03—Revision 0: Initial Version
Rev. B | Page 2 of 24
ADT7461
SPECIFICATIONS
TA = −40°C to +120°C, VDD = 3 V to 5.5 V, unless otherwise noted.
Table 1.
Parameter
POWER SUPPLY
Supply Voltage, VDD
Average Operating Supply Current, IDD
Undervoltage Lockout Threshold
Power-On-Reset Threshold
TEMPERATURE-TO-DIGITAL CONVERTER
Local Sensor Accuracy
Resolution
Remote Diode Sensor Accuracy
Min
Typ
Max
Unit
Test Conditions
3.0
3.30
170
5.5
5.5
2.55
5.5
215
10
20
2.8
2.5
V
μA
μA
μA
V
V
0.0625 conversions/sec rate 1
Standby mode, –40°C ≤ TA ≤ +85°C
Standby mode, +85°C ≤ TA ≤ +120°C
VDD input, disables ADC, rising edge
±1
1
±3
2.2
1
32.13
114.6
°C
°C
°C
°C
°C
μA
μA
μA
ms
3.2
12.56
ms
±1
±3
Resolution
Remote Sensor Source Current
Conversion Time
Maximum Series Resistance Cancelled
OPEN-DRAIN DIGITAL OUTPUTS
(THERM, ALERT/THERM2)
Output Low Voltage, VOL
High Level Output Leakage Current, IOH
ALERT Output Low Sink Current
SMBus INTERFACE3, 4
Logic Input High Voltage, VIH
SCLK, SDATA
Logic Input Low Voltage, VIL
SCLK, SDATA
Hysteresis
SMBus Output Low Sink Current
Logic Input Current, IIH, IIL
SMBus Input Capacitance, SCLK, SDATA
SMBus Clock Frequency
SMBus Timeout 5
SCLK Falling Edge to SDATA Valid Time
0.25
96
36
6
−40°C ≤ TA ≤ +100°C, 3 V ≤ VDD ≤ 3.6 V
+60°C ≤ TA ≤ +100°C, −55°C ≤ TD 2 ≤ +150°C, 3 V ≤ VDD ≤ 3.6 V
−40°C ≤ TA ≤ +120°C, −55°C ≤ TD 2 ≤ +150°C, 3 V ≤ VDD ≤ 5.5 V
kΩ
High level 3
Middle level3
Low level3
From stop bit to conversion complete (both channels),
one-shot mode with averaging switched on
One-shot mode with averaging off (that is,
conversion rate = 16, 32, or 64 conversions per second)
Resistance split evenly on both the D+ and D– inputs
1
V
μA
mA
IOUT = −6.0 mA3
VOUT = VDD3
ALERT forced to 0.4 V
2.1
V
3 V ≤ VDD ≤ 3.6 V
V
3 V ≤ VDD ≤ 3.6 V
3
0.1
0.4
1
0.8
500
6
−1
+1
5
25
400
64
1
mV
mA
μA
pF
kHz
ms
μs
SDATA forced to 0.6 V
User programmable
Master clocking in data
1
See Table 8 for information on other conversion rates.
Guaranteed by characterization, but not production tested.
3
Guaranteed by design, but not production tested.
4
See the SMBUS Timing Specifications section for more information.
5
Disabled by default; see the Serial Bus Interface section for details on enabling it.
2
Rev. B | Page 3 of 24
ADT7461
SMBus TIMING SPECIFICATIONS
Table 2. SMBus Timing Specifications 1
Parameter
fSCLK
tLOW
tHIGH
tR
tF
tSU; STA
tHD; STA 2
tSU; DAT 3
tHD; DAT
tSU; STO 4
tBUF
Limit at TMIN and TMAX
400
1.3
0.6
300
300
600
600
100
300
600
1.3
Unit
kHz max
μs min
μs min
ns max
ns max
ns min
ns min
ns min
ns min
ns min
μs min
Description
Clock low period, between 10% points
Clock high period, between 90% points
Clock/data rise time
Clock/data fall time
Start condition setup time
Start condition hold time
Data setup time
Data hold time
Stop condition setup time
Bus free time between stop and start conditions
tF
tHD;STA
1
Guaranteed by design, but not production tested.
Time from 10% of SDATA to 90% of SCLK.
3
Time for 10% or 90% of SDATA to 10% of SCLK.
4
Time for 90% of SCLK to 10% of SDATA.
2
tR
tLOW
SCLK
tHD;STA
tHD;DAT
tHIGH
tSU;STA
tSU;STO
tSU;DAT
tBUF
STOP START
START
Figure 2. Serial Bus Timing
Rev. B | Page 4 of 24
STOP
04110-0-001
SDATA
ADT7461
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter
Positive Supply Voltage (VDD) to GND
D+
D− to GND
SCLK, SDATA, ALERT
THERM
Input Current, SDATA, THERM
Input Current, D−
ESD Rating, All Pins (Human Body Model)
Maximum Junction Temperature (TJ max)
Storage Temperature Range
IR Reflow Peak Temperature
Pb-Free Parts Only
Lead Temperature (Soldering 10 sec)
Rating
−0.3 V, +5.5 V
−0.3 V to VDD + 0.3 V
−0.3 V to +0.6 V
−0.3 V to +5.5 V
−0.3 V to VDD + 0.3 V
−1 mA, +50 mA
±1 mA
2000 V
150°C
−65°C to +150°C
220°C
260°C (±0.5°C)
300°C
THERMAL CHARACTERISTICS
8-lead SOIC package
θJA = 121°C/W
8-lead MSOP package
θJA = 142°C/W
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
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. B | Page 5 of 24
ADT7461
VDD
1
8
SCLK
D+
2
ADT7461
7
SDATA
D–
3
TOP VIEW
(Not to Scale)
6
ALERT/THERM2
THERM
4
5
GND
04110-0-013
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
Figure 3. Pin Configuration
Table 4. Pin Function Descriptions
Pin No.
1
2
3
Mnemonic
VDD
D+
D−
4
THERM
5
GND
6
ALERT/THERM2
7
8
SDATA
SCLK
Description
Positive Supply, 3 V to 5.5 V.
Positive Connection to Remote Temperature Sensor.
Negative Connection to Remote Temperature Sensor.
Open-drain output that can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pull-up to VDD.
Supply Ground Connection.
Open-Drain Logic Output Used as Interrupt or SMBus Alert. This may also be configured as a second THERM
output. Requires pull-up resistor.
Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pull-up resistor.
Logic Input, SMBus Serial Clock. Requires pull-up resistor.
Rev. B | Page 6 of 24
ADT7461
TYPICAL PERFORMANCE CHARACTERISTICS
60
20
40
15
TEMPERATURE ERROR (°C)
D+ TO GND
20
0
–20
D+ TO VCC
–40
10
100mV INTERNAL
5
0
–5
100mV EXTERNAL
–80
0
20
40
60
80
04110-0-015
04110-0-017
250mV INTERNAL
–60
–10
–15
100
0
20
LEAKAGE RESISTANCE (MΩ)
Figure 7. Temperature Error vs. Power Supply Noise Frequency
0
–0.1
–10
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–10
10
30
50
70
90
110
130
–20
–30
–40
–50
04110-0-018
TEMPERATURE ERROR (°C)
0
–60
04110-0-022
TEMPERATURE ERROR (°C)
Figure 4. Temperature Error vs. Leakage Resistance
–0.8
–3
–70
0
150
5
10
15
20
25
CAPACITANCE (nF)
TEMPERATURE (°C)
Figure 8. Temperature Error vs. Capacitance Between D+ and D−
Figure 5. Temperature Error vs. Actual Temperature Using 2N3906
180
4
40mV NO FILTER
60mV NO FILTER
40mV WITH FILTER
60mV WITH FILTER
160
TEMPERATURE ERROR (°C)
3
TEMPERATURE ERROR (°C)
40
FREQUENCY (MHz)
2
1
0
140
100mV NO FILTER
120
100
80
60
40
20
04110-0-027
–1
–2
0
100
200
300
400
500
0
0
600
100
200
300
400
500
600
FREQUENCY (MHz)
FREQUENCY (MHz)
Figure 6. Temperature Error vs. Differential Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
100mV WITH FILTER
–20
04110-0-024
TEMPERATURE ERROR (°C)
250mV EXTERNAL
Figure 9. Temperature Error vs. 100 mV Differential Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
Rev. B | Page 7 of 24
ADT7461
55
5
40mV NO FILTER
60mV NO FILTER
40mV WITH FILTER
60mV WITH FILTER
45
TEMPERATURE ERROR (°C)
3
2
1
0
100
200
300
400
500
15
100mV WITH FILTER
–5
600
0
100
200
FREQUENCY (MHz)
300
400
500
600
FREQUENCY (MHz)
Figure 10. Temperature Error vs. Common-Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
Figure 13. Temperature Error vs. 100 mV Common-Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
40
800
35
700
600
30
5.5V
5.5V
500
IDD (μA)
25
IDD (μA)
04110-0-026
–1
25
5
04110-0-025
0
100mV NO FILTER
35
20
400
300
15
200
10
3V
3V
04110-0-020
5
0
50
0
150
100
200
250
300
350
04110-0-019
TEMPERATURE ERROR (°C)
4
100
0
0.01
400
0.1
1
10
100
CONVERSION RATE (Hz)
SCL CLOCK FREQUENCY (kHz)
Figure 11. Standby Supply Current vs. Clock Frequency
Figure 14. Operating Supply Current vs. Conversion Rate
50
7
45
6
TEMPERATURE ERROR (°C)
40
4
3
2
0
3.0 3.2
3.4
3.6
3.8
4.0
4.2 4.4
4.6
4.8
5.0
3.3V T = –30
30
3.3V T = +25
25
3.3V T = +120
20
5.5V T = –30
15
5.5V T = +25
10
5.5V T = +120
04110-0-023
1
35
5
04110-0-021
IDD (μA)
5
0
–5
5.2 5.4
0
VDD (V)
2
10
200
1k
2k
3k
SERIES RESISTANCE (Ω)
Figure 12. Standby Current vs. Supply Voltage
Figure 15. Temperature Error vs. Series Resistance
Rev. B | Page 8 of 24
4k
ADT7461
TEMPERATURE MEASUREMENT METHOD
FUNCTIONAL DESCRIPTION
The ADT7461 is a local and remote temperature sensor and
over/under temperature alarm, with the added ability to automatically cancel the effect of 3 kΩ (typical) of resistance in
series with the temperature monitoring diode. When the
ADT7461 is operating normally, the on-board ADC operates
in a free-running mode. The analog input multiplexer
alternately selects either the on-chip temperature sensor to
measure its local temperature or the remote temperature sensor.
The ADC digitizes these signals and the results are stored in the
local and remote temperature value registers.
The local and remote measurement results are compared with
the corresponding high, low, and THERM temperature limits,
stored in eight on-chip registers. Out-of-limit comparisons
generate flags that are stored in the status register. A result that
exceeds the high temperature limit, the low temperature limit,
or an external diode fault causes the ALERT output to assert
low. Exceeding THERM temperature limits causes the THERM
output to assert low. The ALERT output can be reprogrammed
as a second THERM output.
The limit registers can be programmed and the device controlled and configured via the serial SMBus. The contents
of any register can also be read back via the SMBus.
Control and configuration functions consist of switching the
device between normal operation and standby mode, selecting
the temperature measurement scale, masking or enabling the
ALERT output, switching Pin 6 between ALERT and THERM2,
and selecting the conversion rate.
SERIES RESISTANCE CANCELLATION
Parasitic resistance to the D+ and D− inputs to the ADT7461,
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 0.5°C offset per ohm of parasitic resistance in series with the
remote diode.
The ADT7461 automatically cancels out the effect of this series
resistance on the temperature reading, giving a more accurate
result, without the need for user characterization of this resistance. The ADT7461 is designed to automatically cancel typically
up to 3 kΩ of resistance. By using an advanced temperature
measurement method, this is transparent to the user. This
feature allows resistances to be added to the sensor path to
produce a filter, allowing the part to be used in noisy environments. See the Noise Filtering section for more details.
A simple method of 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. However, 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 16 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, but it could equally
be a discrete transistor. If a discrete transistor is used, the collector will not be grounded and should be linked to the base. To
prevent ground noise interfering with the measurement, the
more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at the
D− input. C1 may be added as a noise filter (a recommended
maximum value of 1,000 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.
To measure ΔVBE, the operating current through the sensor is
switched among three related currents. Figure 16 shows
N1 × I and N2 × I as different multiples of the current, I. The
currents through the temperature diode are switched between
I and N1 × I, giving ΔVBE1, and then between I and N2 × I,
giving ΔVBE2. The temperature may then be calculated using the
two ΔVBE measurements. This method can also be shown to
cancel the effect of any series resistance on the temperature
measurement.
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 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 per second, no digital
averaging takes place.
Signal conditioning and measurement of the internal temperature sensor is performed in the same manner.
Rev. B | Page 9 of 24
ADT7461
VDD
I
N1×I
N2×I
IBIAS
D+
VOUT+
D–
TO ADC
BIAS
DIODE
VOUT–
LOW-PASS FILTER
fC = 65kHz
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
04110-0-002
REMOTE
SENSING
TRANSISTOR
C1*
Figure 16. Input Signal Conditioning
TEMPERATURE MEASUREMENT RESULTS
The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers
and are compared with limits programmed into the local and
remote high and low limit registers.
The local temperature value is in Register 0x00 and has a resolution of 1°C. 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. This gives the external temperature
measurement a resolution of 0.25°C. Table 5 shows the data
format for the external temperature low byte.
Table 5. Extended Temperature Resolution (Remote
Temperature Low Byte)
Extended Resolution
0.00°C
0.25°C
0.50°C
0.75°C
Remote Temperature Low Byte
0 000 0000
0 100 0000
1 000 0000
1 100 0000
Above 150°C, 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. The user should be
aware that the temperature measurement on the external channel
may not be accurate for temperatures that are outside the
operating range of the remote sensor.
While both local and remote temperature measurements can be
made while the part is in extended temperature mode, the
ADT7461 itself should not be exposed to temperatures greater than
those specified in the Absolute Maximum Ratings section. Also,
the device is guaranteed to operate only as specified at ambient
temperatures from −40°C to +120°C.
TEMPERATURE DATA FORMAT
When reading the full external temperature value, both the high
and low byte, the two registers should be read in succession.
Reading one register does not lock the other, so both should be
read before the next conversion finishes. In practice, there is
more than enough time to read both registers, as transactions
over the SMBus are significantly faster than a conversion time.
TEMPERATURE MEASUREMENT RANGE
The temperature measurement range for both internal and
external measurements is, by default, 0°C to +127°C. However,
the ADT7461 can be operated using an extended temperature
range. It can measure the full temperature range of an external
diode, from −55°C to +150°C. The user can switch between
these two temperature ranges by setting or clearing Bit 2 in the
configuration register. A valid result is available in the next
measurement cycle after changing the temperature range.
In extended temperature mode, the upper and lower temperature that can be measured by the ADT7461 is limited by the
remote diode selection. The temperature registers themselves
can have values from −64°C to +191°C. However, most
temperature sensing diodes have a maximum temperature
range of −55°C to +150°C.
The ADT7461 has two temperature data formats. When the
temperature measurement range is from 0°C to +127°C
(default), the temperature data format for both internal and
external temperature results is binary. When the measurement
range is in extended mode, an offset binary data format is used
for both internal and external results. Temperature values in the
offset binary data format are offset by 64°C. Examples of temperatures in both data formats are shown in Table 6.
Table 6. Temperature Data Format (Local and Remote
Temperature High Byte)
Temperature
–55°C
0°C
+1°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
+127°C
+150°C
1
Binary
0 000 0000 2
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 1111 3
Offset Binary 1
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
Offset binary scale temperature values are offset by 64°C.
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.
2
Rev. B | Page 10 of 24
ADT7461
The user can switch between measurement ranges at any time.
Switching the range also 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 are not changed. The user must ensure that the
limit registers are reprogrammed, as necessary, when the data
format changes. See the Limit Registers section for more
information.
ADT7461 REGISTERS
The ADT7461 contains a total of 22 8-bit registers. These registers are used to store the results of remote and local temperature
measurements and high and low temperature limits and to
configure and control the device. A description of these registers
follows. Additional details are provided in Table 7 to Table 11.
Address Pointer Register
The address pointer register does not have or require an
address, as the first byte of every write operation is automatically written to this register. The data in this first byte always
contains the address of another register on the ADT7461, which
is stored in the address pointer register. This register address is
written to by the second byte of a write operation or is used for
a subsequent read operation.
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, since its register
address is 0x00.
Temperature Value Registers
The ADT7461 has three registers to store the results of local and
remote temperature measurements. These registers can only be
written to by the ADC and can be read by the user over the
SMBus. The local temperature value register is at Address 0x00.
If Bit 6 is set to 0 (the power-on default), the device is in
operating mode with the ADC converting. If Bit 6 is set to 1, the
device is in standby mode and the ADC does not convert. The
SMBus does, however, remain active in standby mode, so values
can be read from or written to the ADT7461 via the SMBus in
this mode. The ALERT and THERM outputs are also active in
standby mode. Changes made to the registers in standby mode
that affect the THERM or ALERT outputs cause these signals to
be updated.
Bit 5 determines the configuration of Pin 6 on the ADT7461. If
Bit 5 is 0 (default), then Pin 6 is configured as an ALERT
output. If Bit 5 is 1, then Pin 6 is configured as a THERM2
output. Bit 7, the ALERT mask bit, is only active when Pin 6 is
configured as an ALERT output. If Pin 6 is set up as a THERM2
output, then Bit 7 has no effect.
Bit 2 sets the temperature measurement range. If Bit 2 is 0
(default), the temperature measurement range is set between
0°C to +127°C. Setting Bit 2 to 1 means that the measurement
range is set to the extended temperature range.
Table 7. Configuration Register Bit Assignments
Bit
Name
7
MASK1
6
RUN/STOP
5
ALERT/THERM2
4, 3
Reserved
Temperature
Range Select
Reserved
2
1, 0
Function
0 = ALERT enabled
1 = ALERT masked
0 = Run
1 = Standby
0 = ALERT
1 = THERM2
Power-On
Default
0
0
0
0
0 = 0°C to 127°C
1 = Extended range
0
0
Conversion Rate Register
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.
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. Bits 0, 1, 3, and 4 are
reserved and should not be written to by the user.
Bit 7 of the configuration register is used to mask the ALERT
output. If Bit 7 is 0, the ALERT output is enabled. This is the
power-on default. If Bit 7 is set to 1, the ALERT output is
disabled. This only applies if Pin 6 is configured as ALERT. If
Pin 6 is configured as THERM2, the value of Bit 7 has no effect.
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 8 conversions
per second means that beginning at 125 ms intervals, the device
performs a conversion on the internal and external temperature
channels.
This register can be written to and read back over the SMBus.
The higher four bits of this register are unused and must be set
to 0. The default value of this register is 0x08, giving a rate of
16 conversions per second. Use of slower conversion times
greatly reduces the device power consumption, as shown
in Table 8.
Rev. B | Page 11 of 24
ADT7461
Status Register
Table 8. Conversion Rate Register Codes
Code
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x07
0x08
0x09
0x0A
0x0B to 0xFF
Conversion/Second
0.0625
0.125
0.25
0.5
1
2
4
8
16
32
64
Reserved
Average Supply
Current μA Typ
at VDD = 5.5 V
121.33
128.54
131.59
146.15
169.14
233.12
347.42
638.07
252.44
417.58
816.87
The status register is a read-only register at Address 0x02. It
contains status information for the ADT7461.
Bit 7 of the status register indicates the ADC is busy converting
when it is high. The other bits in this register flag the out-oflimit temperature measurements (Bits 6 to 3 and Bits 1 to 0) and
the remote sensor open circuit (Bit 2).
If Pin 6 is configured as an ALERT output, the following applies. If
the local temperature measurement exceeds its limits, Bit 6 (high
limit) or Bit 5 (low limit) of the status register asserts to flag this
condition. If the remote temperature measurement exceeds its
limits, then Bit 4 (high limit) or Bit 3 (low limit) asserts. Bit 2
asserts to flag an open-circuit condition on the remote sensor.
These five flags are NOR’d together so if any of them is high, the
ALERT interrupt latch is set and the ALERT output goes low.
Limit Registers
The ADT7461 has eight limit registers: high, low, and THERM
temperature limits for both local and remote temperature
measurements. The remote temperature high and low limits
span two registers each to contain an upper and lower byte for
each limit. There is also a THERM hysteresis register. All limit
registers can be written to and read back over the SMBus. See
Table 12 for address details of the limit registers and their
power-on default values.
When Pin 6 is configured as an ALERT output, the high limit
registers perform a > comparison while the low limit registers
perform a ≤ comparison. For example, if the high limit register
is programmed with 80°C, then measuring 81°C results in an
out-of-limit condition, setting a flag in the status register. If the
low limit register is programmed with 0°C, measuring 0°C or
lower results in an out-of-limit condition.
Exceeding either the local or remote THERM limit asserts
THERM low. When Pin 6 is configured as THERM2, exceeding
either the local or remote high limit asserts THERM2 low. A
default hysteresis value of 10°C is provided that applies to both
THERM channels. This hysteresis value may be reprogrammed
to any value after power-up (Register Address 0x21).
It is important to remember that the temperature limits data
format is the same as the temperature measurement data
format. So, if the temperature measurement uses default binary,
the temperature limits also use the binary scale. If the
temperature measurement scale is switched, however, the
temperature limits do not switch automatically. The user must
reprogram the limit registers to the desired value in the correct
data format. For example, if the remote low limit is set at 10°C
and the default binary scale is being used, the limit register
value should be 0000 1010b. If the scale is switched to offset
binary, the value in the low temperature limit register should be
reprogrammed to be 0100 1010b.
Reading the status register clears the five flags, Bits 6 to 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.
The ALERT interrupt latch is not reset by reading the status
register. It resets when the ALERT output has been serviced by
the master reading the device address, provided the error condition has gone away and the status register flag bits are reset.
When Flag 1 and/or Flag 0 are set, the THERM output goes low
to indicate the temperature measurements are outside the
programmed limits. The THERM output does not need to be
reset, unlike the ALERT output. Once the measurements are
within the limits, the corresponding status register bits are reset
automatically and the THERM output goes high. The user may
add hysteresis by programming Register 0x21. The THERM
output is reset only when the temperature falls to limit value
minus hysteresis value.
When Pin 6 is configured as THERM2, only the high temperature limits are relevant. If Flag 6 and/or Flag 4 are set, the
THERM2 output goes low to indicate the temperature
measurements are outside the programmed limits. Flag 5 and
Flag 3 have no effect on THERM2. The behavior of THERM2 is
otherwise the same as THERM.
Table 9. Status Register Bit Assignments
Bit
Name
Function
7
6
5
BUSY
LHIGH 1
LLOW1
1 when ADC is converting
1 when local high temperature limit is tripped
1 when local low temperature limit is tripped
4
RHIGH1
1 when remote high temperature limit is tripped
3
RLOW1
1 when remote low temperature limit is tripped
2
OPEN1
1
RTHRM
1 when remote sensor is an open circuit
1 when remote THERM limit is tripped
0
LTHRM
1 when local THERM limit is tripped
1
These flags stay high until the status register is read or they are reset by POR.
Rev. B | Page 12 of 24
ADT7461
Offset Register
One-Shot Register
Offset errors may be introduced into the remote temperature
measurement by clock noise or by the thermal diode being
located away from the hot spot. To achieve the specified
accuracy on this channel, these offsets must be removed.
The one-shot register is used to initiate a conversion and
comparison cycle when the ADT7461 is in standby mode, after
which the device returns to standby. Writing to the one-shot
register address (0x0F) causes the ADT7461 to perform a
conversion and comparison on both the internal and the
external temperature channels. This is not a data register as
such; the write operation to Address 0x0F causes the one-shot
conversion. The data written to this address is irrelevant and is
not stored.
The offset value is stored as a 10-bit, twos complement value in
Registers 0x11 (high byte) and 0x12 (low byte, left justified).
Only the upper 2 bits of Register 0x12 are used. The MSB of
Register 0x11 is the sign bit. The minimum offset that can be
programmed is −128°C, and the maximum is +127.75°C. The
value in the offset register is added to 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.
Table 10. Sample Offset Register Codes
Offset Value
−128°C
−4°C
−1°C
−0.25°C
0°C
+0.25°C
+1°C
+4°C
+127.75°C
0x11
1000 0000
1111 1100
1111 1111
1111 1111
0000 0000
0000 0000
0000 0001
0000 0100
0111 1111
0x12
00 00 0000
00 00 0000
00 000000
10 00 0000
00 00 0000
01 00 0000
00 00 0000
00 00 0000
11 00 0000
Consecutive ALERT Register
The value written to this register determines how many out-oflimit measurements must occur before an ALERT is generated.
The default value is that one out-of-limit measurement
generates an ALERT. The maximum value that can be chosen is
4. The purpose of this register is to allow the user to perform
some filtering of the output. This is particularly useful at the
fastest three conversion rates, where no averaging takes place.
This register is at Address 0x22.
Table 11. Consecutive ALERT Register Bit
Register Value 1, 2
yxxx 000x
yxxx 001x
yxxx 011x
yxxx 111x
1
2
Number of Out-of-Limit
Measurements Required
1
2
3
4
x = don’t care bit.
y = SMBus timeout bit; default = 0. See the Serial Bus Interface section.
Table 12. List of Registers
Read Address (Hex)
Write Address (Hex)
Name
Power-On Default
Not applicable
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x07
0x08
Not applicable
0x10
0x11
0x12
0x13
0x14
0x19
Not applicable
Not applicable
Not applicable
Not applicable
0x09
0x0A
0x0B
0x0C
0x0D
0x0E
0x0F 1
Not applicable
0x11
0x12
0x13
0x14
0x19
Address Pointer
Local Temperature Value
External Temperature Value High Byte
Status
Configuration
Conversion Rate
Local Temperature High Limit
Local Temperature Low Limit
External Temperature High Limit High Byte
External Temperature Low Limit High Byte
One-Shot
External Temperature Value Low Byte
External Temperature Offset High Byte
External Temperature Offset Low Byte
External Temperature High Limit Low Byte
External Temperature Low Limit Low Byte
External THERM Limit
Undefined
0000 0000 (0x00)
0000 0000 (0x00)
Undefined
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)
0000 0000
0000 0000
0000 0000
0000 0000
0000 0000
0110 1100 (0x55) (85°C)
0x20
0x20
0x21
0x21
Local THERM Limit
THERM Hysteresis
0000 1010 (0x0A) (10°C)
0x22
0xFE
0xFF
0x22
Not applicable
Not applicable
Consecutive ALERT
Manufacturer ID
Die Revision Code
0000 0001 (0x01)
0100 0001 (0x41)
0101 0001 (0x51)
1
0101 0101 (0x55) (85°C)
Writing to Address 0x0F causes the ADT7461 to perform a single measurement. It is not a data register; therefore, data written to it is irrelevant.
Rev. B | Page 13 of 24
ADT7461
SERIAL BUS INTERFACE
Control of the ADT7461 is carried out via the serial bus. The
ADT7461 is connected to this bus as a slave device, under the
control of a master device.
2.
Data is sent over the serial bus in a sequence of nine clock
pulses, eight bits of data followed by an acknowledge bit
from the slave device. Transitions on the data line must
occur during the low period of the clock signal and remain
stable during the high period, since a low-to-high transition
when the clock is high may be interpreted as a stop signal.
The number of data bytes that can be transmitted over the
serial bus in a single read or write operation is limited only
by what the master and slave devices can handle.
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
a no acknowledge. The master then 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.
After a conversion sequence completes, there should be no
SMBus transactions to the ADT7461 for at least one conversion
time, to allow the next conversion to complete. The conversion
time depends on the value programmed in the conversion rate
register.
The ADT7461 has an SMBus timeout feature. When this is
enabled, the SMBus times out typically after 25 ms of inactivity.
However, this feature is not enabled by default. Bit 7 of the
consecutive alert register (Address = 0x22) should be set to
enable it.
Consult the SMBus 1.1 specification for more information
(www.smbus.org).
ADDRESSING THE DEVICE
In general, every SMBus device has a 7-bit device address,
except for some devices that have extended 10-bit addresses.
When the master device sends a device address over the bus,
the slave device with that address responds. The ADT7461 is
available with one device address, 0x4C (1001 100b). The
ADT7461-2 is also available with one device address, 0x4D
(1001 101b)
Any number of bytes of data may be transferred over the serial
bus in one operation, but it is not possible to mix read and write
in one operation because the type of operation is determined at
the beginning and cannot subsequently be changed without
starting a new operation. With the ADT7461, write operations
contain either one or two bytes, while read operations contain
one byte.
The serial bus protocol operates as follows:
1.
The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial
data line SDATA, while the serial clock line SCLK remains
high. This indicates that an address/data stream will follow.
All slave peripherals connected to the serial bus respond to
the start condition and shift in the next eight bits, consisting of a 7-bit address (MSB first) plus an R/W bit, which
determines the direction of the data transfer, that is,
whether data will be written to or read from the slave
device. The peripheral whose address corresponds to the
transmitted address responds by pulling the data line low
during the low period before the ninth clock pulse, known
as the acknowledge bit. All other devices on the bus now
remain idle while the selected device waits for data to be
read from or written to it. If the R/W bit is a 0, the master
writes to the slave device. If the R/W bit is a 1, the master
reads from the slave device.
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 is illustrated in Figure 17. The device address is sent over
the bus followed by R/W set to 0. This is followed by two data
bytes. The first data byte is the address of the internal data
register to be written to, which is stored in the address pointer
register. The second data byte is the data to be written to the
internal data register. The examples shown in Figure 17 to
Figure 19 use the ADT7461 SMBus Address 0x4C.
Rev. B | Page 14 of 24
ADT7461
1
9
9
1
SCLK
A6
SDATA
A5
A4
A3
A2
A1
A0
R/W
START BY
MASTER
D6
D7
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7461
ACK. BY
ADT7461
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
1
9
SCLK (CONTINUED)
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7461
STOP BY
MASTER
FRAME 3
DATA BYTE
04110-0-003
D7
SDATA (CONTINUED)
Figure 17. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
1
9
1
9
SCLK
A6
A5
A4
A3
A2
A1
A0
R/W
START BY
MASTER
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7461
ACK. BY
ADT7461
FRAME 1
SERIAL BUS ADDRESS BYTE
STOP BY
MASTER
FRAME 2
ADDRESS POINTER REGISTER BYTE
04110-0-004
SDATA
Figure 18. Writing to the Address Pointer Register Only
1
9
1
9
SCLK
A6
A5
A4
A3
A2
A1
A0
R/W
START BY
MASTER
D7
D6
D5
D4
D3
D2
D1
ACK. BY
ADT7461
D0
NACK. BY STOP BY
MASTER MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
DATA BYTE FROM ADT7461
04110-0-005
SDATA
Figure 19. Reading from a Previously Selected Register
When reading data from a register there are two possibilities.
1.
If the ADT7461’s address pointer register value is unknown
or not the desired value, it is 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 ADT7461 as before,
but only the data byte containing the register read address
is sent, since data is not to be written to the register. This is
shown in Figure 18.
A read operation is then performed consisting of the serial
bus address, R/W bit set to 1, followed by the data byte
read from the data register. This is shown in Figure 19.
2.
Although it is possible to read a data byte from a data register
without first writing to the address pointer register, if the
address pointer register is already at the correct value, it is not
possible to write data to a register without writing to the
address pointer register because the first data byte of a write is
always written to the address pointer register.
Also, 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.
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 18 can be omitted.
Rev. B | Page 15 of 24
ADT7461
ALERT OUTPUT
LOW POWER STANDBY MODE
This is applicable when Pin 6 is configured as an ALERT
output. The ALERT output goes low whenever an out-of-limit
measurement is detected, or if the remote temperature sensor is
open circuit. It is an open-drain output and requires a pull-up
to VDD. Several ALERT outputs can be wire-ORed together, so
the common line goes low if one or more of the ALERT outputs
goes low.
The ADT7461 can be put into low power standby mode by setting Bit 6 of the configuration register. When Bit 6 is low, the
ADT7461 operates normally. When Bit 6 is high, the ADC is
inhibited, and any conversion in progress is terminated without
writing the result to the corresponding value register.
The ALERT output can be used as an interrupt signal to a
processor, or it may be used as an SMBALERT. Slave devices on
the SMBus cannot normally signal to the bus master that they
want to talk, but the SMBALERT function allows them to do so.
One or more ALERT outputs can be connected to a common
SMBALERT line that is connected to the master. When the
SMBALERT line is pulled low by one of the devices, the
procedure shown in Figure 20 occurs.
MASTER
RECEIVES
SMBALERT
ALERT RESPONSE
ADDRESS
RD ACK
MASTER SENDS
ARA AND READ
COMMAND
DEVICE
ADDRESS
When the device is in standby mode, it is still possible to initiate
a one-shot conversion of both channels by writing to the oneshot register (Address 0x0F), after which the device returns to
standby. It does not matter what is written to the one-shot
register, as all data written to it is ignored. It is also possible to
write new values to the limit register while in standby mode. If
the values stored in the temperature value registers are now
outside the new limits, an ALERT is generated even though the
ADT7461 is still in standby.
SENSOR FAULT DETECTION
NO
ACK STOP
04110-0-006
START
The SMBus is still enabled. Power consumption in the standby
mode is reduced to less than 10 μA if there is no SMBus activity
or 100 μA if there are clock and data signals on the bus.
DEVICE SENDS
ITS ADDRESS
Figure 20. Use of SMBALERT
1.
SMBALERT is pulled low.
2.
Master initiates a read operation and sends the alert
response address (ARA = 0001 100). This is a general call
address that must not be used as a specific device address.
3.
The device whose ALERT output is low responds to the
alert response address and the master reads its device
address. As the device address is seven bits, an LSB of 1 is
added. The address of the device is now known and can be
interrogated in the usual way.
4.
If the ALERT output is low on more than one device, the
one with the lowest device address has priority, in
accordance with normal SMBus arbitration.
5.
Once the ADT7461 has responded to the alert response
address, it resets its ALERT output, provided the error
condition that caused the ALERT no longer exists. If the
SMBALERT line remains low, the master sends the ARA
again; this sequence continues until all devices whose
ALERT out-puts were low have responded.
At its D+ input, the ADT7461 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 ADT7461. A simple voltage comparator trips
if the voltage at D+ exceeds VDD −1 V (typical), signifying an
open circuit between D+ and D−. The output of this
comparator is checked when a conversion is initiated. Bit 2 of
the status register (open flag) is set if a fault is detected. If the
ALERT pin is enabled, setting this flag causes ALERT to assert
low.
If the user does not wish to use an external sensor with the
ADT7461, then to prevent continuous setting of the OPEN flag,
the user should tie the D+ and D− inputs together.
THE ADT7461 INTERRUPT SYSTEM
The ADT7461 has two interrupt outputs, ALERT and THERM.
Both have different functions and behavior. ALERT is maskable
and responds to violations of software-programmed temperature limits or an open-circuit fault on the external diode.
THERM is intended as a fail-safe interrupt output that cannot
be masked.
If the external or local temperature exceeds the programmed
high temperature limits or equals or exceeds the low temperature limits, the ALERT output is asserted low. An open-circuit
fault on the external diode also causes ALERT to assert. ALERT
is reset when serviced by a master reading its device address,
provided the error condition has gone away and the status
register has been reset.
Rev. B | Page 16 of 24
ADT7461
The THERM output asserts low if the external or local temperature exceeds the programmed THERM limits. THERM
temperature limits should normally be equal to or greater than
the high temperature limits. THERM is reset automatically
when the temperature falls back within the THERM limit. The
external limit is set by default to 85°C, as is the local THERM
limit. A hysteresis value can be programmed so that THERM
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
10°C, but this may be reprogrammed to any value after powerup.
The hysteresis loop on the THERM outputs is useful when
THERM is used for on/off control of a fan. The user’s system
can be set up so that when THERM asserts, a fan can be
switched on to cool the system. When THERM goes high again,
the fan can be switched off. Programming an hysteresis value
protects from fan jitter where the temperature hovers around
the THERM limit, and the fan is constantly being switched.
Table 13. THERM Hysteresis
0°C
1°C
10°C
The THERM output deasserts (goes high) when the
temperature falls to THERM limit minus hysteresis. The
default hysteresis value of 10°C is shown in Figure 21.
4.
The ALERT output deasserts only when the temperature
falls below the high temperature limit, and the master has
read the device address and cleared the status register.
Pin 6 on the ADT7461 can be configured as either an ALERT
output or as an additional THERM output. THERM2 asserts
low when the temperature exceeds the programmed local
and/or remote high temperature limits. It is reset in the same
manner as THERM, and it is not maskable. The programmed
hysteresis value applies to THERM2 also.
Figure 22 shows how THERM and THERM2 might operate
together to implement two methods of cooling the system. In
this example, the THERM2 limits are set lower than the
THERM limits. The THERM2 output could be used to turn on
a fan. If the temperature continues to rise and exceeds the
THERM limits, the THERM output could provide additional
cooling by throttling the CPU.
Binary Representation
0 000 0000
0 000 0001
0 000 1010
TEMPERATURE
90°C
80°C
THERM LIMIT
70°C
Figure 21 shows how the THERM and ALERT outputs operate.
A user may choose to use the ALERT output as an SMBALERT
to signal to the host via the SMBus that the temperature has
risen. The user could use the THERM output to turn on a fan to
cool the system, if the temperature continues to increase. This
method would ensure there is a fail-safe mechanism to cool the
system without the need for host intervention.
60°C
40°C
30°C
1
4
THERM2
THERM
TEMPERATURE
100°C
THERM2 LIMIT
50°C
2
3
04110-0-008
THERM Hysteresis
3.
Figure 22. Operation of the THERM and THERM2 Interrupts
90°C
80°C
THERM LIMIT
70°C
THERM LIMIT-HYSTERESIS
60°C
HIGH TEMP LIMIT
50°C
1.
When the THERM2 limit is exceeded, the THERM2 signal
asserts low.
2.
If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
3.
The THERM output deasserts (goes high) when the
temperature falls to THERM limit minus hysteresis. No
hysteresis value is shown in Figure 22.
4.
As the system continues to cool and the temperature falls
below the THERM2 limit, the THERM2 signal resets.
Again, no hysteresis value is shown for THERM2.
40°C
ALERT
THERM
1
4
2
3
04110-0-007
RESET BY MASTER
Figure 21. Operation of the ALERT and THERM Interrupts
1.
If the measured temperature exceeds the high temperature
limit, the ALERT output asserts low.
2.
If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low. This can be
used to throttle the CPU clock or switch on a fan.
Both the external and internal temperature measurements cause
THERM and THERM2 to operate as described.
Rev. B | Page 17 of 24
ADT7461
•
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. While this capacitor reduces the noise, it does
not eliminate it, making it difficult to use the sensor in a very
noisy environment.
The ADT7461 has a major advantage over other devices for
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.
The construction of a filter allows the ADT7461 and the remote
temperature sensor to operate in noisy environments. Figure 23
shows a low-pass R-C-R filter with the following values:
R = 100 Ω and C = 1 nF. This filtering reduces both commonmode noise and differential noise.
100Ω
1nF
100Ω
D–
04110-0-009
D+
REMOTE
TEMPERATURE
SENSOR
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7461, IHIGH, is 96 μA, and the low level current,
ILOW, is 6 μA. If the ADT7461 current levels do not match
the current levels specified by the CPU manufacturer, it
may become necessary to remove an offset. The CPUs data
sheet advises whether this offset needs to be removed and
how to calculate it. This offset may be programmed to the
offset register. It is important to note that if more than one
offset must be considered, the algebraic sum of these
offsets must be programmed to the offset register.
If a discrete transistor is being used with the ADT7461, the best
accuracy is obtained by choosing devices according to the
following criteria:
•
Base-emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature.
•
Base-emitter voltage less than 0.95 V at 100 μA, at the
lowest operating temperature.
•
Base resistance less than 100 Ω.
•
Small variation in hFE (50 to 150) that indicates tight
control of VBE characteristics.
Transistors, such as the 2N3904, 2N3906, or equivalents in
SOT-23 packages are suitable devices to use.
Figure 23. Filter Between Remote Sensor and ADT7461
Factors Affecting Diode Accuracy
Remote Sensing Diode
THERMAL INERTIA AND SELF-HEATING
The ADT7461 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 can be either PNP or NPN
transistor connected as a diode (base-shorted to collector). If an
NPN transistor is used, the collector and base are connected to
D+ and the emitter to D−. If a PNP transistor is used, the
collector and base are connected to D− and the emitter to D+.
Accuracy depends on the temperature of the remote sensing
diode and/or the internal temperature sensor being at the same
temperature as the environment being measured; many 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 causes a lag in
the response of the sensor to a temperature change. With a
remote sensor, this should not be a problem since it will be
either a substrate transistor in the processor or a small package
device, such as the SOT-23, placed in close proximity to it.
To reduce the error due to variations in both substrate and
discrete transistors, several factors should be taken into
consideration:
•
The ideality factor, nF, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7461 is trimmed for an nF value of 1.008. The
following equation may be used to calculate the error
introduced at a temperature T (°C), when using a transistor
whose nf does not equal 1.008. Consult the processor data
sheet for the nF values.
ΔT = (nF − 1.008)/1.008 × (273.15 Kelvin + T)
To factor this in, the user can write the ΔT value to the
offset register. It is then automatically added to or
subtracted from the temperature measurement by
the ADT7461.
The on-chip sensor, however, is often remote from the
processor and only monitors 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 is 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 also affects 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.
Rev. B | Page 18 of 24
ADT7461
However, the current forced through the remote sensor is so
small that self-heating is negligible. With the ADT7461, the
worst-case condition occurs when the device is converting at 64
conversions per second while sinking the maximum current of
1 mA at the ALERT and THERM output. In this case, the total
power dissipation in the device is about 4.5 mW. The thermal
resistance, θJA, of the SOIC-8 package is about 121°C/W.
3.
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.
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:
1.
Place the ADT7461 as close as possible to the remote
sensing diode. Provided the worst noise sources, such as
clock generators, data/address buses, and CRTs, are
avoided, this distance can be 4 inches to 8 inches.
2.
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.
4.
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 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.
5.
If the distance to the remote sensor is more than 8 inches,
the use of twisted pair cable is recommended. This works
up to about 6 feet to 12 feet.
For extremely 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 ADT7461. Leave the
remote end of the shield unconnected to avoid ground
loops.
5MIL
GND
5MIL
D+
5MIL
5MIL
D–
5MIL
5MIL
04110-0-010
5MIL
GND
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.
Because the measurement technique uses switched current
sources, excessive cable or filter capacitance can affect the
measurement. When using long cables, the filter capacitance
may be reduced or removed.
Figure 24. Typical Arrangement of Signal Tracks
Rev. B | Page 19 of 24
ADT7461
APPLICATION CIRCUIT
Figure 25 shows a typical application circuit for the ADT7461
using a discrete sensor transistor connected via a shielded,
twisted pair cable. The pull-ups on SCLK, SDATA, and ALERT
are required only if they are not already provided elsewhere in
the system.
The SCLK and SDATA pins of the ADT7461 can be interfaced
directly to the SMBus of an I/O controller, such as the Intel® 820
chipset.
ADT7461
VDD
3V TO 3.6V
0.1μF
D+
TYP 10kΩ
SCLK
D–
SMBUS
CONTROLLER
SDATA
SHIELD
ALERT/
THERM2
VDD
THERM
5V OR 12V
TYP 10kΩ
GND
FAN
ENABLE
Figure 25. Typical Application Circuit
Rev. B | Page 20 of 24
FAN
CONTROL
CIRCUIT
04110-0-011
2N3906
OR
CPU THERMAL
DIODE
ADT7461
OUTLINE DIMENSIONS
5.00 (0.1968)
4.80 (0.1890)
8
5
4.00 (0.1574)
3.80 (0.1497) 1
6.20 (0.2440)
4 5.80 (0.2284)
1.27 (0.0500)
BSC
0.25 (0.0098)
0.10 (0.0040)
0.50 (0.0196)
× 45°
0.25 (0.0099)
1.75 (0.0688)
1.35 (0.0532)
0.51 (0.0201)
COPLANARITY
SEATING 0.31 (0.0122)
0.10
PLANE
8°
0.25 (0.0098) 0° 1.27 (0.0500)
0.40 (0.0157)
0.17 (0.0067)
COMPLIANT TO JEDEC STANDARDS MS-012-AA
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
Figure 26. 8-Lead Standard Small Outline Package [SOIC_N]
Narrow Body
(R-8)
Dimensions shown in millimeters and (inches)
3.00
BSC
8
3.00
BSC
1
5
4.90
BSC
4
PIN 1
0.65 BSC
1.10 MAX
0.15
0.00
0.38
0.22
COPLANARITY
0.10
0.23
0.08
8°
0°
SEATING
PLANE
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 27. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
Rev. B | Page 21 of 24
0.80
0.60
0.40
ADT7461
ORDERING GUIDE
Model
ADT7461AR
ADT7461AR-REEL
ADT7461AR-REEL7
ADT7461ARZ 1
ADT7461ARZ-REEL1
ADT7461ARZ-REEL71
ADT7461ARM
ADT7461ARM-REEL
ADT7461ARM-REEL7
ADT7461ARMZ1
ADT7461ARMZ-REEL1
ADT7461ARMZ-REEL71
ADT7461ARMZ-21
ADT7461ARMZ-2REEL1
ADT7461ARMZ-2REEL71
EVAL-ADT7461EB
1
Temperature Range
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−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
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
Package Description
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
Evaluation Board
Z = Pb-free part.
Rev. B | Page 22 of 24
Package Option
R-8
R-8
R-8
R-8
R-8
R-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
Branding
T1B
T1B
T1B
T1B
T1B
T1B
T1F
T1F
T1F
SMBus Address
4C
4C
4C
4C
4C
4C
4C
4C
4C
4C
4C
4C
4D
4D
4D
ADT7461
NOTES
Rev. B | Page 23 of 24
ADT7461
NOTES
© 2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C04110-0-7/05(B)
Rev. B | Page 24 of 24