ON ADT7461A Temperature monitor with series resistance cancellation Datasheet

ADT7461A
+15C Temperature Monitor
with Series Resistance
Cancellation
The ADT7461A is a dual-channel digital thermometer and
undertemperature/overtemperature alarm, intended for use in PCs and
thermal management systems. It is pin- and register-compatible with the
ADM1032 and ADT7461. A feature of the ADT7461A is series
resistance cancellation, where up to 1.5 kW (typical) of resistance in
series with the temperature monitoring diode can be automatically
cancelled from the temperature result, allowing noise filtering. The
ADT7461A has a configurable ALERT output and an extended,
switchable temperature measurement range.
The ADT7461A can measure the temperature of a remote thermal
diode accurate to ±1°C and the ambient temperature accurate to ±3°C.
The temperature measurement range defaults to 0°C to +127°C,
compatible with the ADM1032, but it can be switched to a wider
measurement range of −64°C to +191°C.
The ADT7461A communicates over a 2-wire serial interface,
compatible with system management bus (SMBus) standards. The
default SMBus address of the ADT7461A is 0x4C. An ADT7461A−2 is
available with an SMBus address of 0x4D. This is useful if more than one
ADT7461A is used on the same SMBus.
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.
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MSOP−8
CASE 846AB
PIN ASSIGNMENT
VDD
D+
D−
THERM
1
8
2
7
3
6
4
5
(Top View)
MARKING DIAGRAM
8
T1x
AYWG
G
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
On-Chip and Remote Temperature Sensor
0.25°C Resolution/1°C Accuracy on Remote Channel
1°C Resolution/1°C Accuracy on Local Channel
Automatically Cancels Up to 1.5 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 –64°C to +191°C
Pin- and Register-Compatible with ADM1032 and ADT7461
2-wire SMBus Serial Interface with SMBus Alert Support
Programmable Over/Undertemperature Limits
Offset Registers for System Calibration
Up to Two Overtemperature Fail-Safe THERM Outputs
Small 8-lead MSOP
240 mA Operating Current, 5 mA Standby Current
These are Pb-Free Devices*
SCLK
SDATA
ALERT/THERM2
GND
1
T1x
A
Y
W
G
= Refer to Ordering Information 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
•
•
•
•
• Embedded Systems
• Burn-In Applications
• Instrumentation
Desktop and Notebook Computers
Industrial Controllers
Smart Batteries
Automotive
* For additional information on our Pb-Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting
Techniques Reference Manual, SOLDERRM/D.
© Semiconductor Components Industries, LLC, 2013
April, 2013 − Rev. 8
1
Publication Order Number:
ADT7461A/D
ADT7461A
2
D−
3
ADDRESS POINTER
REGISTER
LOCAL TEMPERATURE
VALUE REGISTER
LOCAL TEMPERATURE
LOW-LIMIT REGISTER
A-TO-D
CONVERTER
ANALOG
MUX
BUSY
RUN/STANDBY
LOCAL TEMPERATURE
HIGH-LIMIT REGISTER
LIMIT
COMPARATOR
DIGITAL MUX
D+
CONVERSION RATE
REGISTER
DIGITAL MUX
ON-CHIP
TEMPERATURE
SENSOR
REMOTE TEMPERATURE
VALUE REGISTER
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
ADT7461A
6
ALERT/
THERM2
4
THERM
SMBUS INTERFACE
1
5
7
8
VDD
GND
SDATA
SCLK
Figure 1. Functional Block Diagram
Table 1. ABSOLUTE MAXIMUM RATINGS
Parameter
Positive Supply Voltage (VDD) to GND
Rating
Unit
−0.3, +3.6
V
−0.3 to VDD + 0.3
V
D− to GND
−0.3 to +0.6
V
SCLK, SDATA, ALERT, THERM
−0.3 to +3.6
V
Input Current, SDATA, THERM
−1, +50
mA
±1
mA
D+
Input Current, D−
ESD Rating, All Pins (Human Body Model)
1500
V
Maximum Junction Temperature (TJ Max)
150
°C
−65 to +150
°C
Storage Temperature Range
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.
Table 2. THERMAL CHARACTERISTICS
Package Type
8-Lead MSOP
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2
qJA
qJC
Unit
142
43.74
°C/W
ADT7461A
Table 3. PIN ASSIGNMENT
Pin No.
Mnemonic
Description
1
VDD
Positive Supply, 3.0 V to 3.6 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. Can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pullup resistor.
Supply Ground Connection.
Open-Drain Logic Output Used as Interrupt or SMBus ALERT. This can 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
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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STOP
ADT7461A
Table 5. ELECTRICAL CHARACTERISTICS (TA = −40°C to +125°C, VDD = 3.0 V to 3.6 V, unless otherwise noted.)
Parameter
Conditions
Min
Typ
Max
Unit
3.0
3.30
3.6
V
Power Supply
Supply Voltage, VDD
Average Operating Supply Current, IDD
0.0625 Conversions/Sec Rate (Note 1)
Standby Mode
−
−
240
5.0
350
30
mA
Undervoltage Lockout Threshold
VDD Input, Disables ADC, Rising Edge
−
2.55
−
V
1.0
−
2.5
V
−
−
−
−
−
−
±1.0
±1.5
±2.5
°C
−
1.0
−
°C
−
−
−
−
−
−
±1.0
±1.5
±2.5
°C
Power-On-Reset Threshold
Temperature-To-Digital Converter
Local Sensor Accuracy
0°C ≤ TA ≤ +70°C
0°C ≤ TA ≤ +85°C
−40°C ≤ TA ≤ +100°C
Resolution
Remote Diode Sensor Accuracy
0°C ≤ TA ≤ +70°C, −55°C ≤ TD (Note 2) ≤ +150°C
0°C ≤ TA ≤ +85°C, −55°C ≤ TD (Note 2) ≤ +150°C
−40°C ≤ TA ≤ +100°C, −55°C ≤ TD (Note 2) ≤ +150°C
Resolution
−
0.25
−
°C
Remote Sensor Source Current
High Level (Note 3)
Middle Level (Note 3)
Low Level (Note 3)
−
−
−
220
82
13.5
−
−
−
mA
Conversion Time
From Stop Bit to Conversion Complete, One-shot
Mode with Averaging Switched On
−
40
52
ms
One-shot Mode with Averaging Off (That Is,
Conversion Rate = 16-, 32-, or 64-conversions per
Second)
−
6.0
8.0
ms
Resistance Split Evenly on Both the D+ and D–
Inputs
−
1.5
−
kW
Maximum Series Resistance Cancelled
Open-Drain Digital Outputs (THERM, ALERT/THERM2)
Output Low Voltage, VOL
IOUT = −6.0 mA
−
−
0.4
V
High Level Output Leakage Current, IOH
VOUT = VDD
−
0.1
1.0
mA
SMBus Interface (Note 3 and 4)
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
−
500
−
mV
Hysteresis
SDA Output Low Voltage, VOL
−
−
0.4
mA
−1.0
−
+1.0
mA
SMBus Input Capacitance, SCLK, SDATA
−
5.0
−
pF
SMBus Clock Frequency
−
−
400
kHz
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 10 for information on other conversion rates.
Guaranteed by characterization, but not production tested.
Guaranteed by design, but not production tested.
See SMBus Timing Specifications section for more information.
Disabled by default. Detailed procedures to enable it are in the Serial Bus Interface section of the datasheet.
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ADT7461A
TYPICAL PERFORMANCE CHARACTERISTICS
3.5
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
3.0
2.5
2.0
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
MEAN
HIGH 4S
LOW 4S
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
3.5
1.5
1.0
0.5
0
−0.5
−1.0
−50
0
50
100
2.5
2.0
1.5
1.0
0.5
0
−0.5
−1.0
−50
150
0
TEMPERATURE (°C)
100
150
Figure 4. Remote Temperature Error vs. Actual
Temperature
10
0
5
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
50
TEMPERATURE (°C)
Figure 3. Local Temperature Error vs. Temperature
D+ To GND
0
−5
D+ To VCC
−10
−15
−20
−25
1
10
−2
−4
−6
−8
−10
DEV 2
−14
DEV 4
−16
−18
100
DEV 3
−12
0
5
Figure 5. Temperature Error vs. D+/D− Leakage
Resistance
1000
15
20
25
Figure 6. Temperature Error vs. D+/D− Capacitance
422
DEV 2BC
900
10
CAPACITANCE (nF)
LEAKAGE RESISTANCE (MW)
420
800
DEV 2BC
418
700
600
500
IDD (mA)
IDD (mA)
DEV 15
DEV 16
HIGH 4S
LOW 4S
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
3.0
DEV 4BC
400
300
414
DEV 3BC
DEV 4BC
412
DEV 3BC
200
416
410
100
0
0.01
0.1
1
10
408
3.0
100
3.1
3.2
3.3
3.4
3.5
3.6
VDD (V)
CONVERSION RATE (Hz)
Figure 7. Operating Supply Current vs.
Conversion Rate
Figure 8. Operating Supply Current vs. Voltage
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ADT7461A
TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d)
4.4
35
DEV 2
4.2
30
4.0
25
DEV 3
3.8
ISTBY (mA)
IDD (mA)
DEV 2BC
DEV 3BC
DEV 4BC
DEV 4
3.6
20
15
3.4
10
3.2
5
3.0
3.0
3.1
3.2
3.3
3.4
3.5
0
3.6
1
10
VDD (V)
Figure 10. Standby Supply Current vs. Clock
Frequency
80
25
100 mV
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
1000
FSCL (kHz)
Figure 9. Standby Supply Current vs. Voltage
20
15
50 mV
10
20 mV
5
0
100
0
100
200
300
400
500
70
50
40
30
50 mV
20
10
20 mV
0
−10
600
100 mV
60
0
100
200
Figure 11. Temperature Error vs. Common-Mode
Noise Frequency
TEMPERATURE ERROR (°C)
50
40
30
20
10
0
400
500
600
Figure 12. Temperature Error vs. Differential-Mode
Noise Frequency
60
0
300
NOISE FREQUENCY (MHz)
NOISE FREQUENCY (MHz)
500
1000
1500
2000
SERIES RESISTANCE (W)
Figure 13. Temperature Error vs. Series Resistance
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ADT7461A
Differences between the ADT7461A and the ADT7461
Series Resistance Cancellation
Although the ADT7461A is pin- and register-compatible
with the ADT7461, there are some specification differences
between the two devices. A summary of these differences is
shown below in Table 6.
Parasitic resistance to the D+ and D− inputs to the
ADT7461A, seen in series with the remote diode, is caused
by a variety of factors, including PCB track resistance and
track length. This series resistance appears as a temperature
offset in the remote sensor’s temperature measurement. This
error typically causes a 0.5°C offset per ohm of parasitic
resistance in series with the remote diode.
The ADT7461A automatically cancels the effect of this
series resistance on the temperature reading, giving a more
accurate result, without the need for user characterization of
this resistance. The ADT7461A is designed to automatically
cancel typically up to 1.5 kW of resistance. By using an
advanced temperature measurement method, this process is
transparent to the user. This feature permits resistances to be
added to the sensor path to produce a filter, allowing the part
to be used in noisy environments. See the section on Noise
Filtering for more details.
Table 6. DIFFERENCES BETWEEN THE ADT7461A
AND THE ADT7461
Specification
ADT7461A
ADT7461
Unit
3.0 to 3.6
3.0 to 5.5
V
Maximum Local Sensor
Accuracy
1.0
3.0
°C
Maximum Series
Resistance Cancellation
1.5
3.0
kW
Supply Voltage
Average Operating
Supply Current
16 Conversions/Sec
Standby Mode
240
5.0
170
5.5
Max Conversion Time
One Shot, Averaging On
One Shot, Averaging Off
52
8.0
114.6
12.56
Remote Sensor Current
Levels
High
Mid
Low
mA
ms
Temperature Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, measuring
the base emitter voltage (VBE) of a transistor operated at
constant current. However, this technique requires
calibration to null the effect of the absolute value of VBE,
which varies from device to device.
The technique used in the ADT7461A measures the
change in VBE when the device operates at three different
currents. Previous devices used only two operating currents,
but it is the use of a third current that allows automatic
cancellation of resistances in series with the external
temperature sensor.
Figure 14 shows the input signal conditioning used to
measure the output of an external temperature sensor. This
figure shows the external sensor as a substrate transistor, but
it can equally be a discrete transistor. If a discrete transistor
is used, the collector is not grounded but is linked to the base.
To prevent ground noise interfering with the measurement,
the more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at
the D− input. C1 may be added as a noise filter
(a recommended maximum value of 1000 pF). However, a
better option in noisy environments is to add a filter, as
described in the Noise Filtering section. See the Layout
Considerations section for more information on C1.
To measure DVBE, the operating current through the
sensor is switched among three related currents. As shown
in Figure 14, N1 × I and N2 × I are different multiples of the
current, I. The currents through the temperature diode are
switched between I and N1 × I, giving DVBE1; and then
between I and N2 × I, giving DVBE2. The temperature is
then calculated using the two DVBE measurements. This
method also cancels the effect of any series resistance on the
temperature measurement.
mA
220
82
13.5
96
36
6.0
Theory of Operation
The ADT7461A is a local and remote temperature sensor
and over/under temperature alarm, with the added ability to
automatically cancel the effect of 1.5 kW (typical) of
resistance in series with the temperature monitoring diode.
When the ADT7461A is operating normally, the on-board
ADC operates in a free running mode. The analog input
multiplexer alternately selects either the on-chip
temperature sensor to measure its local temperature or the
remote temperature sensor. The ADC digitizes these signals
and the results are stored in the local and remote temperature
value registers.
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 or
the low temperature limit causes the ALERT output to
assert. The ALERT output also asserts if an external diode
fault is detected. Exceeding the THERM temperature limits
causes the THERM output to assert low. The ALERT output
can be reprogrammed as a second THERM output.
The limit registers are programmed and the device
controlled and configured via the serial SMBus. The
contents of any register are also read back via the SMBus.
Control and configuration functions consist of switching
the device between normal operation and standby mode,
selecting the temperature measurement range, masking or
enabling the ALERT output, switching Pin 6 between
ALERT and THERM2, and selecting the conversion rate.
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ADT7461A
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 producing a temperature
measurement. To reduce the effects of noise, digital filtering
I
N1 × I
N2 × I
is performed by averaging the results of 16 measurement
cycles for low conversion rates. At rates of 16-, 32-, and
64-conversions/second, no digital averaging occurs.
Signal conditioning and measurement of the internal
temperature sensor are performed in the same manner.
IBIAS
VDD
D+
REMOTE
SENSING
TRANSISTOR
VOUT+
To ADC
C1*
D−
VOUT−
BIAS
DIODE
LOW-PASS FILTER
fC = 65 kHz
*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000 pF MAX.
Figure 14. Input Signal Conditioning
Temperature Measurement Results
When reading the full external temperature value, read the
LSB first. This causes the MSB to be locked (that is, the
ADC does not write to it) until it is read. This feature ensures
that the results read back from the two registers come from
the same measurement.
used to measure the full temperature range of an external
diode, from −55°C to +150°C.
The extended temperature range is selected by setting
Bit 2 of the configuration register to 1. The temperature
range is 0°C to 127°C when Bit 2 equals 0. 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 ADT7461A is
limited by the remote diode selection. The temperature
registers 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. Therefore,
the temperature measurement on the external channel may
not be accurate for temperatures that are outside the
operating range of the remote sensor.
It should be noted that although both local and remote
temperature measurements can be made while the part is in
extended temperature mode, the ADT7461A itself should
not be exposed to temperatures greater than those specified
in the absolute maximum ratings section. Further, the device
is only guaranteed to operate as specified at ambient
temperatures from −40°C to +120°C.
Temperature Measurement Range
Temperature Data Format
The results of the local and remote temperature
measurements are stored in the local and remote temperature
value registers and compared with limits programmed into
the local and remote high and low limit registers.
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 giving the external
temperature measurement a resolution of 0.25°C. Table 7
lists the data format for the external temperature low byte.
Table 7. EXTENDED TEMPERATURE RESOLUTION
(REMOTE TEMPERATURE LOW BYTE)
Extended Resolution
Remote Temperature
Low Byte
0.00°C
0 000 0000
0.25°C
0 100 0000
0.50°C
1 000 0000
0.75°C
1 100 0000
The ADT7461A 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
The temperature measurement range for both internal and
external measurements is, by default, 0°C to +127°C.
However, the ADT7461A can be operated using an
extended temperature range. The extended measurement
range is −64°C to +191°C. Therefore, the ADT7461A can be
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ADT7461A
register on the ADT7461A that is stored in the address
pointer register. It is to this register address that the second
byte of a write operation is written, or to which a subsequent
read operation is performed.
The power-on default value of the address pointer register
is 0x00. Therefore, if a read operation is performed
immediately after power-on, without first writing to the
address pointer, the value of the local temperature is returned
because its register address is 0x00.
for both internal and external results. Temperature values are
offset by 64°C in the offset binary data format. Examples of
temperatures in both data formats are shown in Table 8.
Table 8. TEMPERATURE DATA FORMAT
(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
+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
+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
Configuration Register
+150°C
0 111 1111
(Note 3)
1 101 0110
The configuration register is Address 0x03 at read and
Address 0x09 at write. Its power-on default is 0x00. Only
four bits of the configuration register are used. Bit 0, Bit 1,
Bit 3, and Bit 4 are reserved; the user does not write to them.
Bit 7 of the configuration register masks 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 applies only if Pin 6 is configured as ALERT.
If Pin 6 is configured as THERM2, then the value of Bit 7
has no effect.
If Bit 6 is set to 0, which is power-on default, the device
is in operating mode with ADC converting. If Bit 6 is set to
1, the device is in standby mode and the ADC does not
convert. The SMBus does, however, remain active in
standby mode; therefore, values can be read from or written
to the ADT7461A via the SMBus. 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
ADT7461A. 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 value), the temperature measurement range is set
between 0°C to +127°C. Setting Bit 2 to 1 sets the
measurement range to the extended temperature range
(−64°C to +191°C).
Temperature Value Registers
The ADT7461A has three registers to store the results of
local and remote temperature measurements. These
registers can only be written to by the ADC and can be read
by the user over the SMBus. The local temperature value
register is at Address 0x00.
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.
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 likewise switches the data format.
The next temperature result following the switching is
reported back to the register in the new format. However, the
contents of the limit registers do not change. It is up to the
user to ensure that when the data format changes, the limit
registers are reprogrammed as necessary. More information
on this is found in the Limit Registers section.
ADT7461A Registers
The ADT7461A contains 22, 8-bit registers in total. These
registers store the results of remote and local temperature
measurements, high and low temperature limits, and
configure and control the device. See the Address Pointer
Register section through the Consecutive ALERT Register
section of this data sheet for more information on the
ADT7461A registers. Additional details are shown in
Table 9 through Table 13. The entire register map is
available in Table 14.
Address Pointer Register
The address pointer register itself does not have, nor does
it require, an address because the first byte of every write
operation is automatically written to this register. The data
in this first byte always contains the address of another
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ADT7461A
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 14 for details of the
limit register addresses 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 can 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. Therefore, if the temperature measurement uses
default binary, then the temperature limits also use the
binary scale. If the temperature measurement scale is
switched, however, the temperature limits do not
automatically switch. The user must reprogram the limit
registers to the desired value in the correct data format. For
example, if the remote low limit is set at 10°C with the
default binary scale, the limit register value is 0000 1010b.
If the scale is switched to offset binary, the value in the low
temperature limit register needs to be reprogrammed to
0100 1010b.
Table 9. CONFIGURATION REGISTER BIT
ASSIGNMENTS
Bit
Name
Power-On
Default
Function
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
Temperature
Range Select
1, 0
0
0 = 0°C to 127°C
1 = Extended Range
0
Reserved
0
Conversion Rate Register
The conversion rate register is Address 0x04 at read and
Address 0x0A at write. The lowest four bits of this register
are used to program the conversion rate by dividing the
internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256,
512, or 1024 to give conversion times from 15.5 ms
(Code 0x0A) to 16 seconds (Code 0x00). For example, a
conversion rate of eight conversions per second means that
beginning at 125 ms intervals, the device performs a
conversion on the internal and the external temperature
channels.
The conversion rate register can be written to and read
back over the SMBus. The higher four bits of this register are
unused and must be set to 0. The default value of this register
is 0x08, giving a rate of 16 conversions per second. Use of
slower conversion times greatly reduces the device power
consumption.
Status Register
Table 10. CONVERSION RATE REGISTER CODES
Code
Conversion/
Second
Time (Seconds)
0x00
0.0625
16
0x01
0.125
8
0x02
0.25
4
0x03
0.5
2
0x04
1
1
0x05
2
500 m
0x06
4
250 m
0x07
8
125 m
0x08
16
62.5 m
0x09
32
31.25 m
0x0A
64
15.5 m
0x0B to 0xFF
Reserved
The status register is a read-only register at Address 0x02.
It contains status information for the ADT7461A.
When Bit 7 of the status register is high, it indicates that the
ADC is busy converting. The other bits in this register flag the
out-of-limit temperature measurements (Bit 6 to Bit 3, and
Bit 1 to Bit 0) and the remote sensor open circuit (Bit 2).
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’ed 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, Bit 6 to
Bit 2, provided the error conditions causing the flags to be
set have gone away. A flag bit can be reset only if the
corresponding value register contains an in-limit
measurement or if the sensor is good.
Limit Registers
The ADT7461A 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
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ADT7461A
maximum is +127.75°C. The value in the offset register is
added to, or subtracted from, the measured value of the
remote temperature.
The offset register powers up with a default value of 0°C
and has no effect unless the user writes a different value to it.
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 that 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 automatically reset 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 the 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 that 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 12. SAMPLE OFFSET REGISTER CODES
Name
BUSY
6
LHIGH
(Note 1)
1 when local high temperature limit is
tripped
5
LLOW
(Note 1)
1 when local low temperature limit is
tripped
4
RHIGH
(Note 1)
1 when remote high temperature limit is
tripped
3
RLOW
(Note 1)
1 when remote low temperature limit is
tripped
2
OPEN
(Note 1)
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
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
10 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
+127.75°C
0111 1111
11 00 0000
The one-shot register is used to initiate a conversion and
comparison cycle when the ADT7461A is in standby mode,
after which the device returns to standby. Writing to the
one-shot register address (0x0F) causes the ADT7461A to
perform a conversion and comparison on both the internal
and the external temperature channels. This is not a data
register as such, and it is the write operation to Address 0x0F
that causes the one-shot conversion. The data written to this
address is irrelevant and is not stored.
Function
7
0x11
One-Shot Register
Table 11. STATUS REGISTER BIT ASSIGNMENTS
Bit
Offset Value
1 when ADC is converting
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.
1. These flags stay high until the status register is read or they are
reset by POR unless Pin 6 is configured as THERM2. Then, only
Bit 2 remains high until the status register is read or is reset by
POR.
Offset Register
Table 13. CONSECUTIVE ALERT REGISTER CODES
Offset errors can be introduced into the remote
temperature measurement by clock noise or when the
thermal diode is located away from the hot spot. To achieve
the specified accuracy on this channel, these offsets must be
removed.
The offset value is stored as a 10-bit, twos complement
value in Register 0x11 (high byte) and Register 0x12 (low
byte, left justified). Only the upper two bits of Register 0x12
are used. The MSB of Register 0x11 is the sign bit. The
minimum, programmable offset is −128°C, and the
NOTE:
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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.
ADT7461A
Table 14. LIST OF REGISTERS
Read Address (Hex)
Write Address (Hex)
Name
Power-On Default
Not Applicable
Not Applicable
Address Pointer
Undefined
00
Not Applicable
Local Temperature Value
0000 0000 (0x00)
01
Not Applicable
External Temperature Value High Byte
0000 0000 (0x00)
02
Not Applicable
Status
Undefined
03
09
Configuration
0000 0000 (0x00)
04
0A
Conversion Rate
0000 1000 (0x08)
05
0B
Local Temperature High Limit
0101 0101 (0x55) (85°C)
06
0C
Local Temperature Low Limit
0000 0000 (0x00) (0°C)
07
0D
External Temperature High Limit High Byte
0101 0101 (0x55) (85°C)
08
0E
External Temperature Low Limit High Byte
0000 0000 (0x00) (0°C)
Not Applicable
0F (Note 1)
10
Not Applicable
External Temperature Value Low Byte
0000 0000
11
11
External Temperature Offset High Byte
0000 0000
12
12
External Temperature Offset Low Byte
0000 0000
13
13
External Temperature High Limit Low Byte
0000 0000
14
14
External Temperature Low Limit Low Byte
0000 0000
19
19
External THERM Limit
0101 0101 (0x55) (85°C)
20
20
Local THERM Limit
0101 0101 (0x55) (85°C)
21
21
THERM Hysteresis
0000 1010 (0x0A) (10°C)
22
22
Consecutive ALERT
0000 0001 (0x01)
FE
Not Applicable
Manufacturer ID
0100 0001 (0x41)
FF
Not Applicable
Die Revision Code
0101 0111 (0x57)
One-Shot
1. Writing to Address 0x0F causes the ADT7461A to perform a single measurement. It is not a data register, and it does not matter what data
is written to it.
Serial Bus Interface
indicates that an address/data stream follows. All
slave peripherals connected to the serial bus
respond to the start condition and shift in the next
eight 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 is written to,
or read from, the slave device. The peripheral
whose address corresponds to the transmitted
address responds by pulling the data line low
during the low period before the ninth clock pulse,
known as the acknowledge bit. All other devices
on the bus remain idle while the selected device
waits for data to be read from 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
of the clock signal and remain stable during the
high period, since a low-to-high transition when
the clock is high can be interpreted as a stop
signal. The number of data bytes that can be
transmitted over the serial bus in a single read or
Control of the ADT7461A is carried out via the serial bus.
The ADT7461A is connected to this bus as a slave device,
under the control of a master device.
The ADT7461A has an SMBus timeout feature. When
this is enabled, the SMBus times out after typically 25 ms of
no activity. However, this feature is not enabled by default.
Bit 7 of the consecutive alert register (Address = 0x22)
should be set to enable it.
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 ADT7461Ais available with one device address, 0x4C
(1001 100b). An ADT7461A−2 is also available.
The ADT7461A−2 has an SMBus address of 0x4D
(1001 101b). This is to allow two ADT7461A devices on the
same bus, or if the default address conflicts with an existing
device on the SMBus. The serial bus protocol operates as
follows:
1. The master initiates a data transfer by establishing
a start condition, defined as a high-to-low
transition on SDATA, the serial data line, while
SCLK, the serial clock line, remains high. This
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ADT7461A
without starting a new operation. For the
ADT7461A, write operations contain either one or
two bytes, while read operations contain one byte.
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 no acknowledge. The master
takes the data line low during the low period
before the tenth clock pulse, then high during the
tenth clock pulse to assert a stop condition.
Any number of bytes of data are transferable over
the serial bus in one operation, but it is not
possible to mix read and write in one operation
because the type of operation is determined at the
beginning and cannot subsequently be changed
To write data to one of the device data registers, or to read
data from it, the address pointer register must be set so that
the correct data register is addressed. The first byte of a write
operation always contains a valid address that is stored in the
address pointer register. If data is to be written to the device,
the write operation contains a second data byte that is written
to the register selected by the address pointer register.
This procedure is illustrated in Figure 15. 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.
1
9
1
9
SCLK
SDATA
A6
A5
START BY
MASTER
A4
A3
A2
A0
A1
D6
D7
R/W
ACK. BY
ADT7461A
FRAME 1
SERIAL BUS ADDRESS BYTE
D4
D5
D2
D3
D1
D0
ACK. BY
ADT7461A
FRAME 2
ADDRESS POINTER REGISTER BYTE
1
9
SCLK (CONTINUED)
D7
SDATA (CONTINUED)
D4
D5
D6
D2
D3
D1
D0
ACK. BY STOP BY
ADT7461A MASTER
FRAME 3
DATA BYTE
Figure 15. 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
ADT7461A
ACK. BY
ADT7461A
STOP BY
MASTER
FRAME 2
ADDRESS POINTER REGISTER BYTE
FRAME 1
SERIAL BUS ADDRESS BYTE
Figure 16. 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
ADT7461A
D5
D4
D3
D2
FRAME 2
ADDRESS POINTER REGISTER BYTE
Figure 17. Reading Data from a Previously Selected Register
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D1
D0
NACK BY
MASTER
STOP BY
MASTER
ADT7461A
When reading data from a register there are two
possibilities.
1. If the address pointer register value of the
ADT7461A is unknown or not the desired value, it
is first necessary to set it to the correct value
before data can be read from the desired data
register. This is done by writing to the ADT7461A
as before, but only the data byte containing the
register read address is sent, because data is not to
be written to the register see Figure 16.
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 see
Figure 17.
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 16 can be omitted.
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 it can be interrogated in
the usual way.
4. If more than one device’s ALERT output is low,
the one with the lowest device address takes
priority, in accordance with normal SMBus
arbitration.
Once the ADT7461A has responded to the alert response
address, it resets its ALERT output, provided that the error
condition that caused the ALERT no longer exists. If the
SMBALERT line remains low, the master sends the ARA
again, and so on until all devices whose ALERT outputs
were low have responded.
NOTES:
1. It is possible to read a data byte from a data register without
first writing to the address pointer register. However, if the
address pointer register is already at the correct value, it is
not possible to write data to a register without writing to the
address pointer register because the first data byte of a write
is always written to the address pointer register.
2. 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 ADT7461A can be put into low power standby mode
by setting Bit 6 of the configuration register. When Bit 6 is
low, the ADT7461A operates normally. When Bit 6 is high,
the ADC is inhibited, and any conversion in progress is
terminated without writing the result to the corresponding
value register. However, the SMBus is still enabled. Power
consumption in the standby mode is reduced to 5 mA if there
is no SMBus activity, or 30 mA if there are clock and data
signals on the bus.
When the device is in standby mode, it is possible to
initiate a one-shot conversion of both channels by writing to
the one-shot register (Address 0x0F), after which the device
returns to standby. It does not matter what is written to the
one-shot register, all data written to it is ignored. It is also
possible to write new values to the limit register while in
standby mode. If the values stored in the temperature value
registers are outside the new limits, an ALERT is generated,
even though the ADT7461A 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 resistor. Several ALERT outputs can
be wire-OR’ed together, so that 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 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 following procedure occurs (see Figure 18):
Sensor Fault Detection
At its D+ input, the ADT7461A contains internal sensor
fault detection circuitry. This circuit can detect situations
where an external remote diode is either not connected or
incorrectly connected to the ADT7461A. A simple voltage
comparator trips if the voltage at D+ exceeds VDD − 1 V
(typical), signifying an open circuit between D+ and D−.
The output of this comparator is checked when a conversion
is initiated. Bit 2 of the status register (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
ADT7461A, tie the D+ and D− inputs together to prevent
continuous setting of the open flag.
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 18. Use of SMBALERT
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ADT7461A
The ADT7461A Interrupt System
TEMPERATURE
The ADT7461A 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 and local limits are set by
default to 85°C. A hysteresis value can be programmed; in
which case, 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 can be
reprogrammed to any value after powerup.
The hysteresis loop on the THERM outputs is useful when
THERM is used, for example, as an on/off controller for a
fan. The user’s system can be set up so that when THERM
asserts, a fan is switched on to cool the system. When
THERM goes high again, the fan can be switched off.
Programming a hysteresis value protects from fan jitter,
where the temperature hovers around the THERM limit, and
the fan is constantly switched.
1005C
905C
Binary Representation
0°C
0 000 0000
1°C
0 000 0001
10°C
0 000 1010
THERM LIMIT − HYSTERESIS
705C
HIGH TEMP LIMIT
605C
505C
405C
RESET BY MASTER
ALERT
1
4
THERM
2
3
Figure 19. Operation of the ALERT and THERM
Interrupts
• If the measured temperature exceeds the high
•
•
•
•
•
Table 15. THERM HYSTERESIS
THERM Hysteresis
THERM LIMIT
805C
•
temperature limit, the ALERT output asserts low.
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.
The THERM output deasserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In ,
the default hysteresis value of 10°C is shown.
The ALERT output deasserts only when the
temperature has fallen below the high temperature
limit, and the master has read the device address and
cleared the status register.
Pin 6 on the ADT7461A 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 is
not maskable.
The programmed hysteresis value also applies to
THERM2.
Figure 20 shows how THERM and THERM2 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 is used to turn on a
fan. If the temperature continues to rise and exceeds the
THERM limits, the THERM output provides additional
cooling by throttling the CPU.
Figure 19 shows how the THERM and ALERT outputs
operate. The ALERT output can be used as a SMBALERT
to signal to the host via the SMBus that the temperature has
risen. The user can use the THERM output to turn on a fan
to cool the system, if the temperature continues to increase.
This method ensures that there is a fail-safe mechanism to
cool the system, without the need for host intervention.
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ADT7461A
100 W
TEMPERATURE
905C
REMOTE
TEMPERATURE
SENSOR
THERM LIMIT
805C
705C
605C
1 nF
D−
Figure 21. Filter Between Remote Sensor and
ADT7461A Factors Affecting Diode Accuracy
THERM2 LIMIT
505C
100 W
D+
405C
Remote Sensing Diode
305C
THERM2
1
THERM
The ADT7461A is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types are either
PNP or NPN transistors connected as diodes (base-shorted
to collector). If an NPN transistor is used, the collector and
base are connected to D+ and the emitter to D−. If a PNP
transistor is used, the collector and base are connected to D−
and the emitter to D+.
To reduce the error due to variations in both substrate and
discrete transistors, consider several factors:
• The ideality factor, nF, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7461A is trimmed for an nF value of 1.008.
The following equation may be used to calculate the
error introduced at a temperature, T (°C), when using a
transistor whose nF does not equal 1.008. Consult the
processor data sheet for the nF values.
4
2
3
Figure 20. Operation of the THERM and THERM2
Interrupts
• When the THERM2 limit is exceeded, the THERM2
signal asserts low.
• If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
• The THERM output deasserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 20, there is no hysteresis value shown.
• As the system cools further, 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 1000 pF. Although this
capacitor reduces the noise, it does not eliminate it, making
it difficult to use the sensor in a very noisy environment.
The ADT7461A has a major advantage over other devices
when it comes to eliminating the effects of noise on the
external sensor. The series resistance cancellation feature
allows a filter to be constructed between the external
temperature sensor and the part. The effect of any filter
resistance seen in series with the remote sensor is
automatically cancelled from the temperature result.
The construction of a filter allows the ADT7461A and the
remote temperature sensor to operate in noisy environments.
Figure 21 shows a low-pass R−C−R filter, where R = 100 W
and C = 1 nF. This filtering reduces both common-mode and
differential noise.
•
(273.15 Kelvin ) T)
(eq. 1)
To factor this in, the user writes the DT value to the offset
register. It is then automatically added to, or subtracted
from, the temperature measurement.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7461A, IHIGH, is 220 mA and
the low level current, ILOW, is 13.5 mA. If the
ADT7461A current levels do not match the current
levels specified by the CPU manufacturer, it may
become necessary to remove an offset. The CPU data
sheet should advise whether this offset needs to be
removed and how to calculate it. This offset is
programmed to the offset register. It is important to
note that if more than one offset must be considered,
the algebraic sum of these offsets must be programmed
to the offset register.
If a discrete transistor is used with the ADT7461A, 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
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16
ADT7461A
• Base-emitter voltage less than 0.95 V at 100 mA, at the
•
•
GND
lowest operating temperature
Base resistance less than 100 W
Small variation in hFE (50 to 150) that indicates tight
control of VBE characteristics
5 MIL
5 MIL
D+
5 MIL
5 MIL
D−
Transistors, such as the 2N3904, 2N3906, or equivalents
in SOT−23 packages are suitable devices to use.
5 MIL
5 MIL
GND
Thermal Inertia and Self-heating
5 MIL
Figure 22. Typical Arrangement of Signal Tracks
Accuracy depends on the temperature of the remote
sensing diode and/or the internal temperature sensor being
at the same temperature as that being measured. Many
factors can affect this. Ideally, place the sensor in good
thermal contact with the part of the system being measured.
If it is not, the thermal inertia caused by the sensor’s mass
causes a lag in the response of the sensor to a temperature
change. In the case of the remote sensor, this should not be
a problem since it is either a substrate transistor in the
processor or a small package device, such as the SOT−23,
placed in close proximity to it.
The on-chip sensor, however, is often remote from the
processor and only monitors the general ambient
temperature around the package. How accurately the
temperature of the board and/or the forced airflow reflects
the temperature to be measured dictates the accuracy of the
measurement. Self-heating due to the power dissipated in
the ADT7461A or the remote sensor causes the chip
temperature of the device or remote sensor to rise above
ambient. However, the current forced through the remote
sensor is so small that self-heating is negligible. In the case
of the ADT7461A, the worst-case condition occurs when the
device is converting at 64 conversions per second while
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 8-lead MSOP is approximately 142°C/W.
• Try to minimize the number of copper/solder joints that
•
•
•
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADT7461A is measuring very small voltages from the
remote sensor, so care must be taken to minimize noise
induced at the sensor inputs. Take the following precautions:
• Place the ADT7461A as close as possible to the remote
sensing diode. Provided that the worst noise sources,
that is, clock generators, data/address buses, and CRTs
are avoided, this distance can be 4 inches to 8 inches.
• Route the D+ and D– tracks close together, in parallel,
with grounded guard tracks on each side. To minimize
inductance and reduce noise pickup, a 5 mil track width
and spacing is recommended. Provide a ground plane
under the tracks, if possible.
can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+
and D− path and at the same temperature.
Thermocouple effects should not be a major problem as
1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
Unless there are two thermocouples with a big
temperature differential between them, thermocouple
voltages should be much less than 200 mV.
Place a 0.1 mF bypass capacitor close to the VDD pin. In
extremely noisy environments, place an input filter
capacitor across D+ and D− close to the ADT7461A.
This capacitance can effect the temperature
measurement, so ensure that any capacitance seen at D+
and D− is, at maximum, 1000 pF. This maximum value
includes the filter capacitance, plus any cable or stray
capacitance between the pins and the sensor diode.
If the distance to the remote sensor is more than
8 inches, the use of twisted pair cable is recommended.
A total of 6 feet to 12 feet is needed.
For really long distances (up to 100 feet), use a shielded
twisted pair, such as the Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D− and the
shield to GND close to the ADT7461A. Leave the
remote end of the shield unconnected to avoid ground
loops.
Because the measurement technique uses switched
current sources, excessive cable or filter capacitance can
affect the measurement. When using long cables, the filter
capacitance can be reduced or removed.
Application Circuit
Figure 23 shows a typical application circuit for the
ADT7461A, using a discrete sensor transistor connected via
a shielded, twisted pair cable. The pullups on SCLK,
SDATA, and ALERT are required only if they are not
provided elsewhere in the system.
The SCLK pin and the SDATA pin of the ADT7461A can
be interfaced directly to the SMBus of an I/O controller, such
as the Intel® 820 chipset.
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17
ADT7461A
0.1 mF
VDD
3 V to 3.6 V
TYP 10 kW
ADT7461A
D+
SCLK
D−
2N3906
or
CPU THERMAL
DIODE
SMBUS
CONTROLLER
SDATA
ALERT/
THERM2
SHIELD
VDD
5 V or 12 V
THERM
TYP 10 kW
GND
FAN
ENABLE
FAN
CONTROL
CIRCUIT
Figure 23. Typical Application Circuit
Table 16. ORDERING INFORMATION
Package Description
Branding
SMBus Address
Shipping†
ADT7461AARMZ−R
8-lead MSOP
T1K
4C
3,000 Tape & Reel
ADT7461AARMZ−2RL
8-lead MSOP
T1L
4D
3,000 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
ADT7461A
PACKAGE DIMENSIONS
MSOP8
CASE 846AB−01
ISSUE O
D
HE
PIN 1 ID
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
−T− PLANE
0.038 (0.0015)
A
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 ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239; 6,133,753; 6,169,442; 7,010,440; other patents pending.
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC 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.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death
associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
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Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
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ON Semiconductor Website: www.onsemi.com
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For additional information, please contact your local
Sales Representative
ADT7461A/D