DATA SHEET
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+15C Temperature Monitor
with Series Resistance
Cancellation
ADT7461
The ADT7461 is a dual-channel digital thermometer and under/over
temperature alarm intended for use in PCs and thermal management
systems. It is pin- and register-compatible with the ADM1032. The
ADT7461 has three additional features: series resistance cancellation
(where up to 3 kW (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.
The SMBus address of the ADT7461 is 0x4C.
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
On-Chip and Remote Temperature Sensor
0.25°C Resolution/1°C Accuracy on Remote Channel
1°C Resolution/3°C Accuracy on Local Channel
Automatically Cancels Up to 3 kW (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
ADT7461 SMBus Address is 0x4C
Programmable Over/Undertemperature Limits
Offset Registers for System Calibration
Up to Two Overtemperature Fail−Safe THERM Outputs
Small 8-lead SOIC NB or 8-lead MSOP Packages
170 mA Operating Current, 5.5 mA Standby Current
These are Pb-Free Devices*
SOIC−8 NB
CASE 751
MSOP8
CASE 846AB
PIN ASSIGNMENT
VDD
D+
D−
THERM
1
8
2
7
3
6
4
5
SCLK
SDATA
ALERT/THERM2
GND
(Top View)
MARKING DIAGRAMS
8
ADT74
61A
#YWW
1
SOIC−8
ADT7461A
#
Y
W
= Specific Device Code
= Pb-Free Package
= Year
= Work Week
8
T1x
AYWG
G
1
MSOP8
T1x
A
Y
W
G
= Refer to Order Info Table
= Assembly Location
= Year
= Work Week
= Pb-Free Package
(Note: Microdot may be in either location)
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 18 of this data sheet.
Applications
•
•
•
•
•
Desktop and Notebook Computers
Industrial Controllers
Smart Batteries
Embedded Systems
Instrumentation
*For additional information on our Pb−Free strategy and soldering details, please download the onsemi Soldering and Mounting Techniques
Reference Manual, SOLDERRM/D.
© Semiconductor Components Industries, LLC, 2014
December, 2021 − Rev. 9
1
Publication Order Number:
ADT7461/D
ADT7461
2
D−
3
LOCAL TEMPERATURE
VALUE REGISTER
LOCAL TEMPERATURE
LOW LIMIT REGISTER
ADC
BUSY
D+
ADDRESS POINTER
REGISTER
RUN/STANDBY
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
LIMIT
COMPARATOR
DIGITAL MUX
ANALOG
MUX
CONVERSION RATE
REGISTER
DIGITAL MUX
ON-CHIP
TEMPERATURE
SENSOR
REMOTE TEMPERATURE
VALUE REGISTER
SRC
BLOCK
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
SMBUS INTERFACE
1
5
7
8
4
6
VDD
GND
SDATA
SCLK
THERM
ALERT/
THERM2
Figure 1. Functional Block Diagram
Table 1. ABSOLUTE MAXIMUM RATINGS
Parameter
Positive Supply Voltage (VDD) to GND
Rating
Unit
−0.3, +5.5
V
−0.3 to VDD + 0.3
V
D− to GND
−0.3 to +0.6
V
SCLK, SDATA, ALERT
−0.3 to +5.5
V
D+
THERM
Input Current, SDATA, THERM
Input Current, D−
−0.3 to VDD + 0.3
V
−1, +50
mA
±1
mA
ESD Rating, All Pins (Human Body Model)
2000
V
Maximum Junction Temperature (TJ Max)
150
°C
Storage Temperature Range
−65 to +150
°C
IR Reflow Peak Temperature
220
°C
260 (±0.5)
°C
300
°C
IR Reflow Peak Temperature for Pb-Free
Lead Temperature (Soldering 10 sec)
Stresses exceeding those listed in the Maximum Ratings table may damage the device. If any of these limits are exceeded, device functionality
should not be assumed, damage may occur and reliability may be affected.
NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.
Table 2. THERMAL CHARACTERISTICS
Package Type
qJA
Unit
8-lead SOIC NB Package
121
°C/W
8-lead MSOP Package
142
°C/W
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ADT7461
Table 3. PIN ASSIGNMENT
Pin No.
Mnemonic
Description
1
VDD
Positive Supply, 3.0 V to 5.5 V.
2
D+
Positive Connection to Remote Temperature Sensor.
3
D−
Negative Connection to Remote Temperature Sensor.
4
THERM
5
GND
6
ALERT/THERM2
7
SDATA
Logic Input/Output, SMBus Serial Data. Open-Drain output. Requires pullup resistor.
8
SCLK
Logic Input, SMBus Serial Clock. Requires pullup resistor.
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 pullup 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 pullup resistor.
Table 4. SMBus TIMING SPECIFICATIONS (Note 1)
1.
2.
3.
4.
Parameter
Limit at TMIN and TMAX
Unit
fSCLK
400
kHz max
tLOW
1.3
ms min
Clock low period, between 10% points.
tHIGH
0.6
ms min
Clock high period, between 90% points.
tR
300
ns max
Clock/data rise time.
tF
300
ns max
Clock/data fall time.
tSU; STA
600
ns min
Start condition setup time.
tHD; STA (Note 2)
600
ns min
Start condition hold time.
tSU; DAT (Note 3)
100
ns min
Data setup time.
tHD; DAT
300
ns min
Data hold time.
tSU; STO (Note 4)
600
ns min
Stop condition setup time.
tBUF
1.3
ms min
Bus free time between stop and start conditions.
Description
−
Guaranteed by design, but not production tested.
Time from 10% of SDATA to 90% of SCLK.
Time for 10% or 90% of SDATA to 10% of SCLK.
Time for 90% of SCLK to 10% of SDATA.
t LOW
tF
t HD; STA
tR
SCLK
t HD; STA
t HD; DAT
t HIGH
t SU; STA
t SU; DAT
t SU; STO
SDATA
t BUF
STOP START
START
Figure 2. Serial Bus Timing
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3
STOP
ADT7461
Table 5. ELECTRICAL CHARACTERISTICS (TA = −40°C to +120°C, VDD = 3.0 V to 5.5 V, unless otherwise noted).
Parameter
Conditions
Min
Typ
Max
Unit
3.0
3.30
5.5
V
−
−
−
170
5.5
5.5
215
10
20
mA
2.2
2.55
2.8
V
1.0
−
2.5
V
−
±1.0
±3.0
°C
−
1.0
−
°C
−
−
±1.0
°C
−
−
±3.0
−
0.25
−
°C
−
−
−
96
36
6.0
−
−
−
mA
ms
Power Supply
Supply Voltage, VDD
Average Operating Supply Current, IDD
0.0625 Conversions/Sec Rate (Note 1)
Standby Mode, –40°C ≤ TA ≤ +85°C
Standby Mode, +85°C ≤ TA ≤ +120°C
Undervoltage Lockout Threshold
VDD input, disables ADC, rising edge
Power-On-Reset Threshold
Temperature-To-Digital Converter
Local Sensor Accuracy
−40°C ≤ TA ≤ +100°C, 3.0 V ≤ VDD ≤ 3.6 V
Resolution
Remote Diode Sensor Accuracy
+60°C ≤ TA ≤ +100°C,
−55°C ≤ TD (Note 2) ≤ +150°C, 3.0 V ≤ VDD ≤ 3.6 V
−40°C ≤ TA ≤ +120°C,
−55°C ≤ TD (Note 2) ≤ +150°C, 3.0 V ≤ VDD ≤ 5.5 V
Resolution
Remote Sensor Source Current
High Level (Note 3)
Middle Level (Note 3)
Low Level (Note 3)
Conversion Time
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)
32.13
−
114.6
3.2
−
12.56
Resistance split evenly on both the D+ and D– inputs
−
3.0
−
kW
Maximum Series Resistance Cancelled
Open-Drain Digital Outputs (THERM, ALERT/THERM2)
Output Low Voltage, VOL
IOUT = −6.0 mA (Note 3)
−
−
0.4
V
High Level Output Leakage Current, IOH
VOUT = VDD (Note 3)
−
0.1
1.0
mA
ALERT Output Low Sink Current
ALERT Forced to 0.4 V
1.0
−
−
mA
Logic Input High Voltage, VIH SCLK, SDATA
3.0 V ≤ VDD ≤ 3.6 V
2.1
−
−
V
Logic Input Low Voltage, VIL SCLK, SDATA
3.0 V ≤ VDD ≤ 3.6 V
−
−
0.8
V
SMBus Interface (Note 3 and 4)
Hysteresis
−
500
−
mV
6.0
−
−
mA
−1.0
−
+1.0
mA
SMBus Input Capacitance, SCLK, SDATA
−
5.0
−
pF
SMBus Clock Frequency
−
−
400
kHz
SMBus Output Low Sink Current
SDATA Forced to 0.6 V
Logic Input Current, IIH, IIL
1.
2.
3.
4.
5.
SMBus Timeout (Note 5)
User Programmable
−
25
64
ms
SCLK Falling Edge to SDATA Valid Time
Master Clocking in Data
−
−
1.0
ms
See Table 9 for information on other conversion rates.
Guaranteed by characterization, but not production tested.
Guaranteed by design, but not production tested.
See the SMBUS Timing Specifications section for more information.
Disabled by default; see the Serial Bus Interface section for details on enabling it.
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ADT7461
TYPICAL PERFORMANCE CHARACTERISTICS
0
40
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
60
D+ To GND
20
0
−20
D+ To VCC
−40
−60
−80
0
20
40
60
80
−0.1
−0.2
−0.3
−0.4
−0.5
−0.6
−0.7
−0.8
−3
100
−10
10
LEAKAGE RESISTANCE (MW)
Figure 3. Temperature Error vs. Leakage Resistance
1
−0
−1
0
100
200
300
400
500
10
100 mV INTERNAL
5
0
−5
100 mV EXTERNAL
250 mV INTERNAL
−10
−15
600
20
0
40
FREQUENCY (MHz)
Figure 6. Temperature Error vs. Power Supply
Noise Frequency
Figure 5. Temperature Error vs. Differential Mode
Noise Frequency (With and Without R-C-R Filter
of 100 W - 2.2 nF - 100 W)
0
180
−10
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
110 130 150
15
FREQUENCY (MHz)
−20
−30
−40
−50
−60
−70
90
250 mV EXTERNAL
2
−2
70
20
40mV NO FILTER
60mV NO FILTER
40mV WITH FILTER
60mV WITH FILTER
3
50
Figure 4. Temperature Error vs. Actual
Temperature Using 2N3906
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
4
30
TEMPERATURE (°C)
0
5
10
15
20
160
120
100
80
60
40
20
0
−20
25
100 mV
NO FILTER
140
100 mV WITH FILTER
0
100
200
300
400
500
600
FREQUENCY (MHz)
CAPACITANCE (nF)
Figure 7. Temperature Error vs. Capacitance
Between D+ and D−
Figure 8. Temperature Error vs. 100 mV
Differential Mode Noise Frequency (With and
Without R-C-R Filter of 100 W - 2.2 nF - 100 W)
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ADT7461
TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d)
4
3
TEMPERATURE ERROR (°C)
40
40mV NO FILTER
60mV NO FILTER
40mV WITH FILTER
60mV WITH FILTER
35
30
2
IDD (mA)
1
5.5 V
25
20
15
−0
10
−1
−2
3V
5
0
100
200
300
400
500
0
600
50
0
FREQUENCY (MHz)
TEMPERATURE ERROR (°C)
6
IDD (mA)
5
4
3
2
1
250
300
350
400
3.4
3.8
4.2
4.6
5.0
100 mV
NO FILTER
45
35
25
15
5
−5
5.4
100 mV WITH FILTER
100
0
VDD (V)
200
300
400
500
600
FREQUENCY (MHz)
Figure 11. Standby Current vs. Supply Voltage
Figure 12. Temperature Error vs. 100 mV
Common-Mode Noise Frequency (With and
Without R-C-R Filter of 100 W - 2.2 nF - 100 W)
800
50
45
600
TEMPERATURE ERROR (°C)
700
IDD (mA)
200
55
7
5.5 V
500
400
300
200
3V
100
0
0.01
150
Figure 10. Standby Supply Current vs. Clock
Frequency
Figure 9. Temperature Error vs. Common-Mode
Noise Frequency (With and Without R-C-R Filter
of 100 W Ć 2.2 nF - 100 W)
0
3.0
100
SCL CLOCK FREQUENCY (kHz)
40
35
3.3 V T = −30
30
3.3 V T = +25
25
3.3 V T = +120
20
5.5 V T = −30
15
5.5 V T = +25
10
5.5 V T = +120
5
0
0.1
1
10
−5
100
0
2
10
200
1k
2k
3k
SERIES RESISTANCE (W)
CONVERSION RATE (Hz)
Figure 13. Operating Supply Current vs.
Conversion Rate
Figure 14. Temperature Error vs. Series
Resistance
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4k
ADT7461
Functional Description
Temperature Measurement Method
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 kW (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.
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 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 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 DVBE, the operating current through the
sensor is switched among three related currents. Figure 15
shows N1 × I and N2 × I as different multiples of the
current, I. The currents through the temperature diode are
switched between I and N1 × I, giving DVBE1, and then
between I and N2 × I, giving DVBE2. The temperature may
then be calculated using the two DVBE measurements. This
method can also be shown to cancel the effect of any series
resistance on the temperature measurement.
The resulting DVBE 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 DVBE. The
ADC digitizes this voltage and a temperature measurement is
produced. To reduce the effects of noise, digital filtering is
performed by averaging the results of 16 measurement cycles
for low conversion rates. 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.
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 kW 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.
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7
ADT7461
I
N1 × I
N2 × I
IBIAS
VDD
VOUT+
D+
REMOTE
SENSING
TRANSISTOR
To ADC
C1*
D−
VOUT−
BIAS
DIODE
LOW-PASS FILTER
fC = 65 kHz
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
Figure 15. Input Signal Conditioning
Temperature Measurement Results
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.
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.
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 6
shows the data format for the external temperature low byte.
Table 6. EXTENDED TEMPERATURE RESOLUTION
(REMOTE TEMPERATURE LOW BYTE)
Extended Resolution
Remote Temperature
Low Byte
0.00°C
0 000 0000
0.25°C
0 100 0000
0.50°C
1 000 0000
0.75°C
1 100 0000
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 Data Format
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 7.
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
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ADT7461
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.
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.
Table 7. TEMPERATURE DATA FORMAT
(LOCAL AND REMOTE TEMPERATURE HIGH BYTE)
Offset Binary
(Note 1)
Temperature
Binary
–55°C
0 000 0000
(Note 2)
0 000 1001
0°C
0 000 0000
0 100 0000
Configuration Register
+1°C
0 000 0001
0 100 0001
+10°C
0 000 1010
0 100 1010
+25°C
0 001 1001
0 101 1001
+50°C
0 011 0010
0 111 0010
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.
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.
+75°C
0 100 1011
1 000 1011
+100°C
0 110 0100
1 010 0100
+125°C
0 111 1101
1 011 1101
+127°C
0 111 1111
1 011 1111
+150°C
0 111 1111
(Note 3)
1 101 0110
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.
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 8 to Table 12.
Table 8. CONFIGURATION REGISTER BIT
ASSIGNMENTS
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.
Bit
Function
Power-On
Default
7
MASK1
0 = ALERT Enabled
1 = ALERT Masked
0
6
RUN/STOP
0 = Run
1 = Standby
0
5
ALERT/
THERM2
0 = ALERT
1 = THERM2
0
4, 3
Reserved
2
1, 0
Temperature Value Registers
The ADT7461 has three registers to store the results of
local and remote temperature measurements. These
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9
Name
Temperature
Range Select
Reserved
0
0 = 0°C to 127°C
1 = Extended Range
0
0
ADT7461
Conversion Rate Register
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
powerup (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.
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 9.
Table 9. CONVERSION RATE REGISTER CODES
Code
Conversion/
Sec
Average Supply Current
mA Typ at VDD = 5.5 V
0x00
0.0625
121.33
0x01
0.125
128.54
0x02
0.25
131.59
0x03
0.5
146.15
0x04
1
169.14
0x05
2
233.12
0x06
4
347.42
0x07
8
638.07
0x08
16
252.44
0x09
32
417.58
0x0A
64
816.87
0x0B to 0xFF
Reserved
Status Register
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-of-limit 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.
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.
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 13 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,
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10
ADT7461
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 11. SAMPLE OFFSET REGISTER CODES
Offset Value
0x11
0x12
−128°C
1000 0000
00 00 0000
−4°C
1111 1100
00 00 0000
−1°C
1111 1111
00 000000
−0.25°C
1111 1111
11 00 0000
0°C
0000 0000
00 00 0000
+0.25°C
0000 0000
01 00 0000
+1°C
0000 0001
00 00 0000
+4°C
0000 0100
00 00 0000
0111 1111
11 00 0000
Table 10. STATUS REGISTER BIT ASSIGNMENTS
Bit
Name
Function
7
BUSY
(Note 1)
1 when ADC is converting
6
LHIGH
(Note 2)
1 when local high temperature limit is
tripped
+127.75°C
5
LLOW
(Note 2)
1 when local low temperature limit is
tripped
One-Shot Register
4
RHIGH
(Note 2)
1 when remote high temperature limit is
tripped
3
RLOW
(Note 2)
1 when remote low temperature limit is
tripped
2
OPEN
(Note 2)
1 when remote sensor is an open circuit
1
RTHRM
1 when remote THERM limit is tripped
0
LTHRM
1 when local THERM limit is tripped
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.
1. Polling of the BUSY bit is not recommended.
2. These flags stay high until the status register is read or they are
reset by POR.
Consecutive ALERT Register
The value written to this register determines how many
out-of-limit 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.
Offset 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 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 12. CONSECUTIVE ALERT REGISTER CODES
NOTE:
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11
Register Value
Number of Out-of-Limit
Measurements Required
yxxx 000x
1
yxxx 001x
2
yxxx 011x
3
yxxx 111x
4
x = don’t care bits, and y = SMBus timeout bit.
Default = 0. See SMBus section for more information.
ADT7461
Table 13. LIST OF REGISTERS
Read Address (Hex)
Write Address (Hex)
Not Applicable
Not Applicable
Address Pointer
Name
Undefined
Power-On Default
0x00
Not Applicable
Local Temperature Value
0000 0000 (0x00)
0x01
Not Applicable
External Temperature Value High Byte
0000 0000 (0x00)
0x02
Not Applicable
Status
Undefined
0x03
0x09
Configuration
0000 0000 (0x00)
0x04
0x0A
Conversion Rate
0000 1000 (0x08)
0x05
0x0B
Local Temperature High Limit
0101 0101 (0x55) (85°C)
0x06
0x0C
Local Temperature Low Limit
0000 0000 (0x00) (0°C)
0x07
0x0D
External Temperature High Limit High Byte
0101 0101 (0x55) (85°C)
0x08
0x0E
External Temperature Low Limit High Byte
0000 0000 (0x00) (0°C)
Not Applicable
0x0F (Note 1)
One-Shot
0x10
Not Applicable
External Temperature Value Low Byte
0000 0000
0x11
0x11
External Temperature Offset High Byte
0000 0000
0x12
0x12
External Temperature Offset Low Byte
0000 0000
0x13
0x13
External Temperature High Limit Low Byte
0000 0000
0x14
0x14
External Temperature Low Limit Low Byte
0000 0000
0x19
0x19
External THERM Limit
0110 1100 (0x55) (85°C)
0x20
0x20
Local THERM Limit
0101 0101 (0x55) (85°C)
0x21
0x21
THERM Hysteresis
0000 1010 (0x0A) (10°C)
0x22
0x22
Consecutive ALERT
0000 0001 (0x01)
0xFE
Not Applicable
Manufacturer ID
0100 0001 (0x41)
0xFF
Not Applicable
Die Revision Code
0101 0001 (0x51)
1. Writing to Address ox0F causes the ADT7461 to perform a single measurement. It is not a data register, therefore, data written to it is
irrelevant.
Serial Bus Interface
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.
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
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.
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).
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12
ADT7461
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.
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 16. 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 16 to
Figure 18 use the ADT7461 SMBus Address 0x4C.
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.
Any number of bytes of data may be transferred over the
serial bus in one operation, but it is not possible to mix read
1
9
1
9
SCLK
SDATA
A6
A5
START BY
MASTER
A4
A3
A2
A0
A1
D6
D7
R/W
ACK. BY
ADT7461
FRAME 1
SERIAL BUS ADDRESS BYTE
D4
D5
D2
D3
D1
D0
ACK. BY
ADT7461
FRAME 2
ADDRESS POINTER REGISTER BYTE
1
9
SCLK (CONTINUED)
D7
SDATA (CONTINUED)
D4
D5
D6
D2
D3
D1
D0
ACK. BY
ADT7461
FRAME 3
DATA BYTE
STOP BY
MASTER
Figure 16. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
1
9
9
1
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
START BY
MASTER
D7
R/W
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
Figure 17. Writing to the Address Pointer Register Only
9
1
9
1
SCLK
SDATA
START BY
MASTER
A6
A5
A4
A3
A2
A1
A0
FRAME 1
SERIAL BUS ADDRESS BYTE
R/W
D7
D6
ACK. BY
ADT7461
D5
D4
D3
D2
FRAME 2
DATA BYTE FROM ADT7461
Figure 18. Reading Data from a Previously Selected Register
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13
D1
D0
NACK. BY
MASTER
STOP BY
MASTER
ADT7461
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 17.
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 18.
2. 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.
MASTER
RECEIVES
SMBALERT
START
ALERT RESPONSE
ADDRESS
MASTER SENDS
ARA AND READ
COMMAND
RD ACK
DEVICE
ADDRESS
NO
STOP
ACK
DEVICE SENDS
ITS ADDRESS
Figure 19. 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.
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.
Low Power Standby Mode
The ADT7461 can be put into low power standby mode
by set-ting 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 SMBus is still enabled. Power consumption in the
standby mode is reduced to less than 10 mA if there is no
SMBus activity or 100 mA if there are clock and data signals
on the bus.
When the device is in standby mode, it is still 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, 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.
ALERT Output
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 pullup 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 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 19 occurs.
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14
ADT7461
Sensor Fault Detection
Figure 20 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.
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.
TEMPERATURE
1005C
905C
THERM LIMIT
805C
THERM LIMIT − HYSTERESIS
705C
HIGH TEMP LIMIT
605C
The ADT7461 Interrupt System
505C
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.
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
405C
RESET BY MASTER
ALERT
0°C
0 000 0000
1°C
0 000 0001
10°C
0 000 1010
3
Figure 20. 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.
3. 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 20.
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 21 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.
Table 14. THERM HYSTERESIS
Binary Representation
4
2
hovers around the THERM limit, and the fan is constantly
being switched.
THERM Hysteresis
1
THERM
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15
ADT7461
100 W
TEMPERATURE
905C
REMOTE
TEMPERATURE
SENSOR
THERM LIMIT
805C
705C
605C
1 nF
D−
Figure 22. Filter Between Remote Sensor and
ADT7461 Factors Affecting Diode Accuracy
THERM2 LIMIT
505C
100 W
D+
405C
Remote Sensing Diode
305C
THERM2
1
THERM
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+.
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.
4
2
3
Figure 21. Operation of the THERM and THERM2
Interrupts
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 21.
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.
Both the external and internal temperature
measurements cause THERM and THERM2 to
operate as described.
DT + (nF * 1.008)ń1.008
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 22 shows a low-pass R-C-R filter with the following
values:
(273.15 Kelvin ) T)
(eq. 1)
To factor this in, the user can write the DT value to the
offset register. It is then automatically added to or
subtracted from the temperature measurement by the
ADT7461.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7461, IHIGH, is 96 mA, and the
low level current, ILOW, is 6 mA. 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 mA, at the
highest operating temperature.
• Base-emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature.
• Base resistance less than 100 W.
• Small variation in hFE (50 to 150) that indicates tight
control of VBE characteristics.
R = 100 W and C = 1 nF
This filtering reduces both common-mode noise and
differential noise.
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16
ADT7461
Transistors, such as the 2N3904, 2N3906, or equivalents
in SOT-23 packages are suitable devices to use.
GND
5 MIL
5 MIL
Thermal Inertia and Self-Heating
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.
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. 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, qJA, of the SOIC-8 package is about 121°C/W.
5 MIL
5 MIL
D−
5 MIL
5 MIL
GND
5 MIL
Figure 23. Typical Arrangement of Signal Tracks
3. Try to minimize the number of copper/solder
joints that can cause thermocouple effects. Where
copper/solder joints are used, make sure that they
are in both the D+ and D− path and at the same
temperature.
Thermocouple effects should not be a major
problem as 1°C corresponds to about 200 mV, and
thermocouple voltages are about 3 mV/°C of
temperature difference. Unless there are two
thermocouples with a big temperature differential
between them, thermocouple voltages should be
much less than 200 mV.
4. Place a 0.1 mF 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 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.
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.
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.
Application Circuit
Figure 24 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.
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17
ADT7461
0.1 mF
VDD
3 V to 3.6 V
TYP 10 kW
ADT7461
D+
D−
SCLK
SMBUS
CONTROLLER
SDATA
ALERT/
THERM2
2N3906
SHIELD
or
CPU THERMAL
DIODE
VDD
5 V or 12 V
THERM
TYP 10 kW
GND
FAN
CONTROL
CIRCUIT
FAN
ENABLE
Figure 24. Typical Application Circuit
Table 15. ORDERING INFORMATION
Package
Description
Package
Option
Branding
SMBus
Address
Shipping†
ADT7461ARZ−REEL
8-lead SOIC NB
R−8
−
4C
2,500 Tape & Reel
ADT7461ARMZ−R7
8-lead MSOP
RM−8
T1B
4C
1,000 Tape & Reel
Device Order Number*
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
*The “Z’’ suffix indicates Pb-Free package available.
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18
ADT7461
PACKAGE DIMENSIONS
SOIC−8 NB
CASE 751−07
ISSUE AK
−X−
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION A AND B DO NOT INCLUDE
MOLD PROTRUSION.
4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.
5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.127 (0.005) TOTAL
IN EXCESS OF THE D DIMENSION AT
MAXIMUM MATERIAL CONDITION.
6. 751−01 THRU 751−06 ARE OBSOLETE. NEW
STANDARD IS 751−07.
A
8
5
S
B
0.25 (0.010)
M
Y
M
1
4
−Y−
K
G
C
N
DIM
A
B
C
D
G
H
J
K
M
N
S
X 45 _
SEATING
PLANE
−Z−
0.10 (0.004)
H
D
0.25 (0.010)
M
Z Y
S
X
M
J
S
SOLDERING FOOTPRINT*
1.52
0.060
7.0
0.275
4.0
0.155
0.6
0.024
1.270
0.050
SCALE 6:1
mm Ǔ
ǒinches
*For additional information on our Pb-Free strategy and soldering
details, please download the onsemi Soldering and Mounting
Techniques Reference Manual, SOLDERRM/D.
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19
MILLIMETERS
MIN
MAX
4.80
5.00
3.80
4.00
1.35
1.75
0.33
0.51
1.27 BSC
0.10
0.25
0.19
0.25
0.40
1.27
0_
8_
0.25
0.50
5.80
6.20
INCHES
MIN
MAX
0.189
0.197
0.150
0.157
0.053
0.069
0.013
0.020
0.050 BSC
0.004
0.010
0.007
0.010
0.016
0.050
0 _
8 _
0.010
0.020
0.228
0.244
ADT7461
PACKAGE DIMENSIONS
MSOP8
CASE 846AB−01
ISSUE O
D
HE
PIN 1 ID
−T−
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION A DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE
BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED
0.15 (0.006) PER SIDE.
4. DIMENSION B DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSION.
INTERLEAD FLASH OR PROTRUSION SHALL NOT EXCEED 0.25 (0.010) PER SIDE.
5. 846A-01 OBSOLETE, NEW STANDARD 846A-02.
E
e
b 8 PL
0.08 (0.003)
M
T B
S
A
DIM
A
A1
b
c
D
E
e
L
HE
S
SEATING
PLANE
A
0.038 (0.0015)
A1
MILLIMETERS
NOM
MAX
−−
1.10
0.08
0.15
0.33
0.40
0.18
0.23
3.00
3.10
3.00
3.10
0.65 BSC
0.40
0.55
0.70
4.75
4.90
5.05
MIN
−−
0.05
0.25
0.13
2.90
2.90
INCHES
NOM
−−
0.003
0.013
0.007
0.118
0.118
0.026 BSC
0.016
0.021
0.187
0.193
MIN
−−
0.002
0.010
0.005
0.114
0.114
MAX
0.043
0.006
0.016
0.009
0.122
0.122
0.028
0.199
L
c
SOLDERING FOOTPRINT*
8X
1.04
0.041
0.38
0.015
3.20
0.126
6X
8X
4.24
0.167
0.65
0.0256
5.28
0.208
SCALE 8:1
mm Ǔ
ǒinches
*For additional information on our Pb-Free strategy and soldering
details, please download the onsemi Soldering and Mounting
Techniques Reference Manual, SOLDERRM/D.
onsemi,
, and other names, marks, and brands are registered and/or common law trademarks of Semiconductor Components Industries, LLC dba “onsemi” or its affiliates
and/or subsidiaries in the United States and/or other countries. onsemi owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property.
A listing of onsemi’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. onsemi reserves the right to make changes at any time to any
products or information herein, without notice. The information herein is provided “as−is” and onsemi makes no warranty, representation or guarantee regarding the accuracy of the
information, product features, availability, functionality, or suitability of its products for any particular purpose, nor does onsemi assume any liability arising out of the application or use
of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products
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provided by onsemi. “Typical” parameters which may be provided in onsemi data sheets and/or specifications can and do vary in different applications and actual performance may
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