Dual Channel Temperature Sensor and Overtemperature Alarm

ADT7482
Dual Channel Temperature
Sensor and Overtemperature
Alarm
The ADT7482 is a three-channel digital thermometer and
under/overtemperature alarm for PCs and thermal management
systems. It can measure the temperature in two remote locations, such
as in the remote thermal diode in a CPU or GPU, or using a discrete
diode connected transistor. This device also measures its own ambient
temperature.
One feature of the ADT7482 is series resistance cancellation where
up to 1.5 kW (typical) of resistance in series with each of the
temperature monitoring diodes can be automatically cancelled from
the temperature result, allowing noise filtering. The temperature of the
remote thermal diodes and ambient temperature can be measured
accurate to 1C. The temperature measurement range, which defaults
to 0C to 127C, can be switched to a wider measurement range of
from −55C to +150C.
The ADT7482 communicates over a 2-wire serial interface
compatible with System Management Bus (SMBus) standards. The
default address of the ADT7482 is 0x4C. An ALERT output signals
when the on-chip or remote temperature is outside the programmed
limits. 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−10
CASE 846AC
PIN ASSIGNMENT
VDD
1
D1+
2
D1−
3
THERM
GND








9
SDATA
8
ALERT/THERM2
4
7
D2+
5
6
D2−
ADT7482
MARKING DIAGRAM
Features




10 SCLK
10
1 Local and 2 Remote Temperature Sensors
0.25C Resolution/1C Accuracy on Remote Channels
1C Resolution/1C Accuracy on Local Channel
Automatically Cancels Up to 1.5 kW (Typ) of Resistance in Series
with the Remote Sensors
Extended, Switchable Temperature Measurement Range
0C to +127C (Default) or −55C to +150C
2-wire SMBus Serial Interface with SMBus Alert Support
Programmable Over/Undertemperature Limits
Offset Registers for System Calibration
Up to 2 Overtemperature Fail-Safe THERM Outputs
Small, 10-lead MSOP Package
240ĂmA Operating Current, 5ĂmA Standby Current
This Device is Pb-Free, Halogen Free and is RoHS Compliant
T0A
AYWG
G
1
T0A
A
Y
W
G
= Device Code
= 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 19 of this data sheet.
Applications







Desktop and Notebook Computers
Industrial Controllers
Smart Batteries
Automotive
Embedded Systems
Burn-in Applications
Instrumentation
 Semiconductor Components Industries, LLC, 2012
July, 2012 − Rev. 4
1
Publication Order Number:
ADT7482/D
ADT7482
ADDRESS POINTER
REGISTER
ONE-SHOT
REGISTER
LOCAL TEMPERATURE
VALUE REGISTER
LOCAL TEMPERATURE
THERM LIMIT REGISTER
2
D1−
3
D2+
7
D2−
6
SRC
11-BIT ADC
ANALOG
MUX
RUN/STANDBY
BUSY
LOCAL TEMPERATURE
LOW LIMIT REGISTER
LIMIT COMPARATOR
D1+
CONVERSION RATE
REGISTER
DIGITAL MUX
ON-CHIP
TEMPERATURE
SENSOR
REMOTE 1 AND 2 TEMP
VALUE REGISTER
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
REMOTE 1 & 2 TEMP.
THERM LIMIT REG.
REMOTE 1 & 2 TEMP.
LOW LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
OFFSET REGISTER
REMOTE 1 & 2 TEMP.
HIGH LIMIT REGISTERS
CONFIGURATION
REGISTER
EXTERNAL DIODE OPEN-CIRCUIT
INTERRUPT
MASKING
STATUS REGISTERS
ADT7482
SMBus INTERFACE
1
5
9
10
4
VDD
GND
SDATA
SCLK
THERM
8
ALERT/THERM2
Figure 1. Functional Block Diagram
Table 1. ABSOLUTE MAXIMUM RATINGS
Parameter
Positive Supply Voltage (VDD) to GND
D+
D− to GND
Rating
Unit
−0.3 to +3.6
V
−0.3 to VDD + 0.3
V
−0.3 to +0.6
V
SCLK, SDATA, ALERT, THERM
−0.3 to +3.6
V
Input Current, SDATA, THERM
−1 to +50
mA
1
mA
Input Current, D−
ESD Rating, All Pins (Human Body Model)
2,000
V
Maximum Junction Temperature (TJ MAX)
150
C
Storage Temperature Range
−65 to +150
C
IR Reflow Peak Temperature
220
C
IR Reflow Peak Temperature for Pb-Free
260
C
Lead Temperature, Soldering (10 sec)
300
C
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 (Note 1)
Package Type
10-lead MSOP
qJA
qJC
Unit
142
43.7
C/W
1. qJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages.
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ADT7482
Table 3. PIN ASSIGNMENT
Pin No.
Mnemonic
Description
1
VDD
Positive Supply, 3.0 V to 3.6 V.
2
D1+
Positive Connection to the First Remote (Remote 1) Temperature Sensor.
3
D1−
Negative Connection to the First Remote (Remote 1) Temperature Sensor.
4
THERM
5
GND
Supply Ground Connection.
6
D2−
Negative Connection to the Second Remote (Remote 2) Temperature Sensor.
7
D2+
Positive Connection to the Second Remote (Remote 2) Temperature Sensor.
8
ALERT/THERM2
9
SDATA
Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pullup resistor.
10
SCLK
Logic Input, SMBus Serial Clock. Requires pullup resistor.
Open-Drain Output. This pin can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pullup resistor.
Open-Drain Logic Output. This pin is used as interrupt or SMBus alert. May also be configured as a
second THERM output. Requires pullup resistor.
Table 4. TIMING SPECIFICATIONS (Note 1)
1.
2.
3.
4.
Parameter
Limit at TMIN and TMAX
Unit
fSCLK
400
kHz max
Description
tLOW
1.3
ms min
Clock Low Period, between 10% Points
tHIGH
0.6
ms min
Clock High Period, between 90% Points
tR
300
ms max
Clock/Data Rise Time
tF
300
ns max
Clock/Data Fall Time
tSU; STA
600
ms min
Start Condition Setup Time
tHD; STA
(Note 2)
600
ms min
Start Condition Hold Time
tSU; DAT
(Note 3)
100
ns min
Data Setup Time
tSU; STO
(Note 4)
600
ms min
Stop Condition Setup Time
tBUF
1.3
ms min
Bus Free Time between Stop and Start Conditions
Guaranteed by design, 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
ADT7482
Table 5. ELECTRICAL CHARACTERISTICS (TA = −40C to +120C, VDD = 3.0 V to 3.6 V, unless otherwise noted.)
Parameter
Test Conditions
Min
Typ
Max
Unit
3.0
3.30
3.6
V
0.0625 Conversions/Sec Rate (Note 1)
−
240
350
mA
Standby Mode
−
5.0
30
mA
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 Supply
Supply Voltage, VDD
Average Operating Supply Current, IDD
Undervoltage Lockout Threshold
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 (Note 2)
0C  TA  +70C, −55C  TD  +150C
0C  TA  +85C, −55C  TD  +150C
−40C  TA  +100C, −55C  TD  +150C
Resolution
−
0.25
−
C
Remote Sensor Source Current
High Level (Note 3)
Mid Level (Note 3)
Low Level (Note 2)
−
−
−
220
82
13.5
−
−
−
mA
Maximum Series Resistance Cancelled
Resistance Split Evenly on D+ and D− Lines
−
1.5
−
kW
Conversion Time
From Stop Bit to Conversion Complete
(All Channels) One-shot Mode with
Averaging Switched On
−
71
93
ms
One-shot Mode with Averaging Off
(Conversion Rate = 16, 32, or 64
Conversions per Second)
−
11.5
15
ms
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
Logic Input High Voltage, VIH SCLK, SDATA
2.1
−
−
V
Logic Input Low Voltage, VIL SCLK, SDATA
−
−
0.8
V
Hysteresis
−
500
−
mV
SMBus Interface (Note 3 and 4)
SDA Output Low Voltage, VOL
IOUT = −6.0 mA
−
−
0.4
V
−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 11 for conversion rates.
Guaranteed by characterization, but not production tested.
Guaranteed by design, but not production tested.
See the Timing Specifications section for more information.
Disabled by default. For details on enabling the SMBus, see the Serial Bus Interface section.
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ADT7482
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
1.5
1.0
0.5
0
−0.5
−1.0
−50
0
50
100
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
3.0
TEMPERATURE ERROR (C)
TEMPERATURE ERROR (C)
3.5
2.5
2.0
1.0
0.5
0
−0.5
−1.0
−50
150
3.0
2.5
2.0
1.0
0.5
0
−0.5
0
5
50
100
0
−5
−10
D+ To VCC
−15
−20
−25
150
10
1
100
LEAKAGE RESISTANCE (MW)
Figure 5. Remote 2 Temperature Error
vs. Temperature
Figure 6. Temperature Error vs. D+/D− Leakage
Resistance
0
1000
−2
900
−4
800
DEV 2BC
700
−6
IDD (mA)
TEMPERATURE ERROR (C)
150
D+ To GND
TEMPERATURE (C)
−8
−10
DEV 3
−12
500
DEV 4BC
400
DEV 3BC
200
DEV 4
−16
600
300
DEV 2
−14
−18
100
10
DEV 15
DEV 16
MEAN
HIGH 4S
LOW 4S
1.5
−1.0
−50
50
Figure 4. Remote 1 Temperature Error
vs. Temperature
TEMPERATURE ERROR (C)
TEMPERATURE ERROR (C)
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
0
TEMPERATURE (C)
Figure 3. Local Temperature Error vs. Temperature
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 15
DEV 16
HIGH 4S
LOW 4S
1.5
TEMPERATURE (C)
3.5
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
100
0
5
10
15
20
0
0.01
25
CAPACITANCE (nF)
0.1
1
10
CONVERTION RATE (Hz)
Figure 7. Temperature Error vs. D+/D− Capacitance
Figure 8. Operating Supply Current
vs. Conversion Rate
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5
100
ADT7482
422
4.4
420
4.2
DEV 2BC
418
4.0
416
3.8
414
IDD (mA)
IDD (mA)
TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d)
DEV 3BC
DEV 4BC
412
DEV 2
DEV 3
DEV 4
3.6
3.4
3.2
410
408
3.0
3.1
3.2
3.3
3.4
3.5
3.0
3.0
3.6
3.1
3.2
Figure 9. Operating Supply Current vs. Voltage
TEMPERATURE ERROR (C)
ISTBY (mA)
20
15
10
5
10
1
100
20
100 mV
15
10
50 mV
5
0
1000
20 mV
0
80
80
70
70
100 mV
50
40
50 mV
20
10
20 mV
0
−10
0
100
200
300
200
300
400
500
600
Figure 12. Temperature Error vs. Common-Mode
Noise Frequency
TEMPERATURE ERROR (C)
TEMPERATURE ERROR (C)
Figure 11. Standby Supply Current vs. SCLK
Frequency
60
100
NOISE FREQUENCY (MHz)
FSCL (kHz)
30
3.6
25
25
0
3.5
Figure 10. Standby Supply Current vs. Voltage
DEV 2BC
DEV 3BC
DEV 4BC
30
3.4
VDD (V)
VDD (V)
35
3.3
400
500
60
50
40
30
20
10
0
600
0
500
1000
1500
2000
2500
TOTAL SERIES RESISTANCE ON D+/D− LINES (W)
NOISE FREQUENCY (MHz)
Figure 13. Temperature Error vs. Differential Mode
Noise Frequency
Figure 14. Temperature Error vs. Series Resistance
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ADT7482
Theory of Operation
Temperature Measurement Method
The ADT7482 is a local and 2 remote temperature sensor
and overtemperature/undertemperature alarm. When the
ADT7482 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 either of the remote
temperature sensors. The ADC digitizes these signals and the
results are stored in the local, Remote 1, and Remote 2
temperature value registers.
The local and remote measurement results are compared
with the corresponding high, low, and THERM temperature
limits, stored in 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 a remote diode open circuit 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 8 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, 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 ADT7482 is to measure the
change in VBE when the device is operated at three different
currents. Previous devices have used only two operating
currents. The use of a third current 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 is not grounded and should
be linked to the base. To prevent ground noise from
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.
Capacitor C1 can 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.
To measure DVBE, the operating current through the
sensor is switched among three related currents. Shown in
Figure 15, N1  I and N2  I are different multiples of the
current, I. The currents through the temperature diode are
switched between I and N1  I, giving DVBE1, and then
between I and N2  I, giving DVBE2. The temperature can
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/second, no digital averaging takes place.
Signal conditioning and measurement of the internal
temperature sensor are performed in the same manner.
Series Resistance Cancellation
Parasitic resistance to the D+ and D− inputs to the
ADT7482, 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 temperature measurement. This error
typically causes a 0.5C offset per ohm of parasitic resistance
in series with the remote diode.
The ADT7482 automatically cancels out the effect of this
series resistance on the temperature reading, providing a more
accurate result, without the need for user characterization of
this resistance. The ADT7482 is designed to automatically
cancel typically up to 1.5 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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ADT7482
*CAPACITOR C1 IS OPTIONAL.
IT SHOULD ONLY BE USED IN
NOISY ENVIRONMENTS.
I
N1  I
N2  I
IBIAS
VDD
D+
REMOTE
SENSING
TRANSISTOR
VOUT+
To ADC
C1*
D−
VOUT−
BIAS
DIODE
LOW-PASS FILTER
fC = 65 kHz
Figure 15. Input Signal Conditioning
Temperature Measurement Results
When reading the full remote temperature value, both the
high and low byte, the two registers should be read LSB first
and then MSB. Reading the LSB causes the MSB to be
locked until it is read. This guarantees that the two values are
read as a result of the same temperature measurement.
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 measurement is an 8-bit measurement
with 1C resolution. The remote temperature measurements
are 10-bit measurements, with the 8 MSBs stored in one
register and the 2 LSBs stored in another register. Table 6 is a
list of the temperature measurement registers.
Temperature Measurement Range
The temperature measurement range for both local and
remote measurements is, by default, 0C to +127C.
However, the ADT7482 can be operated using an extended
temperature range. It can measure the full temperature range
of a remote thermal diode, from −55C to +150C. Switch
between these two temperature ranges by setting or clearing
Bit 2 in the Configuration 1 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 measured by the ADT7482 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.
Note that while both local and remote temperature
measurements can be made while the part is in extended
temperature mode, the ADT7482 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.
Table 6. REGISTER ADDRESS FOR THE
TEMPERATURE VALUES
Temperature
Channel
Register Address,
MSBs
Register Address,
LSBs
Local
0x00
N/A
Remote 1
0x01
0x10 (2 MSBs)
Remote 2
0x30
0x33 (2 MSBs)
Set Bit 3 of the Configuration 1 register to 1, to read the
Remote 2 temperature values from the following register
addresses:
Remote 2, MSBs = 0x01
Remote 2, LSBs = 0x10
The above is true only when Bit 3 of the Configuration 1
register is set. To read the Remote 1 temperatures, switch
this bit back to 0.
Only the two MSBs in the remote temperature low byte
are used. This gives the remote temperature measurement a
resolution of 0.25C. Table 7 shows the data format for the
remote temperature low byte.
Temperature Data Format
When the measurement range is in extended mode, an
offset binary data format is used for both local and remote
results. Temperature values in the offset binary data format
are offset by +64. Examples of temperatures in both data
formats are shown in Table 8.
Switching between measurement ranges can be done 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 do not change. Ensure that
when the data format changes, the limit registers are
reprogrammed as necessary. For more information, refer to
the Limit Registers section.
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
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ADT7482
is to this register address that the second byte of a write
operation is written to 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, since its register address is 0x00.
The ADT7482 has two temperature data formats. When
the temperature measurement range is from 0C to 127C
(default), the temperature data format for both local and
remote temperature results is binary.
Table 8. 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
+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
+150C
0 111 1111
(Note 3)
1 101 0110
Configuration Registers
There are two configuration registers used to control the
operation of the ADT7482. Configuration 1 register is at
Address 0x03 for reads and Address 0x09 for writes. See
Table 9 for details regarding the operation of this register.
Configuration 2 Register is at Address 0x24 for both reads
and writes. Setting Bit 7 of this register locks all lockable
registers. The affected registers can only be modified if the
ADT7482 is powered down and powered up again. See
Table 16 for a list of the registers affected by the lock bit.
Temperature Value Registers
The ADT7482 has five 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
over the SMBus.
 The local temperature value register is at Address 0x00.
 The Remote 1 temperature value high byte register is at
Address 0x01; the Remote 1 low byte register is at
Address 0x10.
 The Remote 2 temperature value high byte register is at
Address 0x30; the Remote 2 low byte register is at
Address 0x33.
 The Remote 2 temperature values can be read from
Addresses 0x01 for the high byte and Address 0x10 for
the low byte if Bit 3 of Configuration Register 1 is set
to 1.
 To read the Remote 1 temperature values, set Bit 3 of
Configuration Register 1 to 0.
 The power-on default for all five registers is 0x00.
1. Offset binary scale temperature values are offset by +64.
2. Binary scale temperature measurement returns 0 for all
temperatures < 0C.
3. Binary scale temperature measurement returns 127 for all
temperatures > 127C.
Registers
The registers in the ADT7482 are 8-bits wide. 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.
Address Pointer Register
The address pointer register itself 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
ADT7482, which is stored in the address pointer register. It
Table 9. CONFIGURATION 1 REGISTER (READ ADDRESS 0x03, WRITE ADDRESS 0x09)
Bit
Mnemonic
Function
7
Mask
Setting this bit to 1 masks all ALERTs on the ALERT pin. Default = 0 = ALERT enabled. This applies only if Pin 8 is
configured as ALERT, otherwise it has no effect.
6
Mon/STBY
Setting this bit to 1 places the ADT7482 in standby mode, that is, it suspends all temperature measurements
(ADC). The SMBus remains active and values can be written to, and read from, the registers. THERM and ALERT
are also active in standby mode. Changes made to the limit registers in standby mode that effect the THERM or
ALERT outputs cause these signals to be updated. Default = 0 = temperature monitoring enabled.
5
AL/TH
4
Reserved
This bit selects the function of Pin 8. Default = 0 = ALERT. Setting this bit to 1 configures Pin 8 as the THERM2 pin.
Reserved for future use.
3
Remote 1
/Remote2
Setting this bit to 1 enables the user to read the Remote 2 values from the Remote 1 registers. When default = 0,
Remote 1 temperature values and limits are read from these registers.
2
Temp
Range
1
Mask R1
Setting this bit to 1 masks ALERTs due to the Remote 1 temperature exceeding a programmed limit. Default = 0.
0
Mask R2
Setting this bit to 1 masks ALERTs due to the Remote 2 temperature exceeding a programmed limit. Default = 0.
Setting this bit to 1 enables the extended temperature measurement range of −50C to +150C.
Default = 0 = 0C to +127C.
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ADT7482
Table 10. CONFIGURATION 2 REGISTER (ADDRESS 0x24)
Bit
Mnemonic
7
Lock Bit
<6:0>
Res
Function
Setting this bit to 1 locks all lockable registers to their current values. This prevents tampering with settings
until the device is powered down. Default = 0.
Reserved for future use.
Conversion Rate Register
When Pin 8 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 8 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.
It is important to remember that the temperature limits
data format is the same as the temperature measurement data
format. If the temperature measurement uses the default
binary scale, then the temperature limits also use the binary
scale. If the temperature measurement scale is switched,
however, the temperature limits do not switch automatically.
The limit registers must be reprogrammed 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 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 at Address 0x04 for reads
and Address 0x0A for writes. The four LSBs of this register
are used to program the conversion times from 15.5 ms
(Code 0x0A) to 16 seconds (Code 0x00). To program the
ADT7482 to perform continuous measurements, set the
conversion rate register to 0x0B. For example, a conversion
rate of 8 conversions/second means that, beginning at
125 ms intervals, the device performs a conversion on the
local and the remote temperature channels. The four MSBs
of this register are reserved and should not be written to.
This register can be written to and read back over the
SMBus. The default value of this register is 0x07, giving a
rate of 8 conversions per second. Use of slower conversion
times greatly reduces the device power consumption.
Limit Registers
The ADT7482 has three limits for each temperature
channel: high, low, and THERM temperature limits for
local, Remote 1, and Remote 2 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 16 for limit register addresses and power-on
default values.
Table 11. CONVERSION RATE/CHANNEL SELECTOR REGISTER (READ ADDRESS 0x04, WRITE ADDRESS 0x0A)
Bit
Mnemonic
Function
7
Reserved
Reserved for Future Use. Do Not Write to this Bit.
6
Reserved
Reserved for Future Use. Do Not Write to this Bit.
5
Reserved
Reserved for Future Use. Do Not Write to this Bit.
4
Reserved
Reserved for Future Use. Do Not Write to this Bit.
<3:0>
Conversion Rates
These Bits Set how often the ADT7482 Measures each Temperature Channel.
Conversions/sec
Time (seconds)
0000 = 0.0625
0001 = 0.125
0010 = 0.25
0011 = 0.5
0100 = 1
0101 = 2
0110 = 4
0111 = 8 = Default
1000 = 16
1001 = 32
1010 = 64
16
8
4
2
1
500 m
250 m
125 m
62.5 m
31.25 m
15.5 m
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ADT7482
Status Registers
The ALERT interrupt latch is not reset by reading the
status register. It is reset 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 have been reset.
When Flag 1 and/or Flag 0 of Status Register 1 or Flag 1
of Status Register 2 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
reset automatically, and the THERM output goes high. To
add hysteresis, program Register 0x21. The THERM output
is reset only when the temperature falls below the THERM
limit minus hysteresis.
When Pin 8 is configured as THERM2, only the high
temperature limits are relevant. If Flag 6 or Flag 4 of Status
Register 1 or Flag 4 of Status Register 2 are set, the
THERM2 output goes low to indicate that the temperature
measurements are outside the programmed limits. Flag 5
and Flag 3 of Status Register 1 and Flag 3 of Status
Register 2 have no effect on THERM2. The behavior of
THERM2 is otherwise the same as THERM.
Bit 0 of the Status Register 2 is set whenever the
ADT7482 ALERT output is asserted low. Read Status
Register 2 to determine if the ADT7482 is responsible for
the ALERT. This bit is reset when the ALERT output is reset.
If the ALERT output is masked, then this bit is not set.
The status registers are read-only registers at
Addresses 0x02 (Status Register 1) and Address 0x23
(Status Register 2). They contain status information for the
ADT7482.
Table 12. STATUS REGISTER 1 BIT ASSIGNMENTS
Bit
Mnemonic
7
BUSY
6
Function
ALERT
1 when ADC Converting
No
LHIGH
(Note 1)
1 when Local High
Temperature Limit Tripped
Yes
5
LLOW
(Note 1)
1 when Local Low
Temperature Limit Tripped
Yes
4
R1HIGH
(Note 1)
1 when Remote 1 High
Temperature Limit Tripped
Yes
3
R1LOW
(Note 1)
1 when Remote 1 Low
Temperature Limit Tripped
Yes
2
D1 OPEN
(Note 1)
1 when Remote 1 Sensor
Open Circuit
Yes
1
R1THRM1
1 when Remote 1 THERM
Limit Tripped
No
0
LTHRM1
1 when Local THERM Limit
Tripped
No
1. These flags stay high until the status register is read, or they are
reset by POR.
Table 13. STATUS REGISTER 2 BIT ASSIGNMENTS
Bit
Mnemonic
Function
ALERT
7
Res
Reserved for Future Use
No
Offset Register
6
Res
Reserved for Future Use
No
5
Res
Reserved for Future Use
No
4
R2HIGH
(Note 1)
1 when Remote 2 High
Temperature Limit Tripped
Yes
3
R2LOW
(Note 1)
1 when Remote 2 Low
Temperature Limit Tripped
Yes
2
D2 OPEN
(Note 1)
1 when Remote 2 Sensor
Open Circuit
Yes
1
R2THRM1
1 when Remote 2 THERM
Limit Tripped
No
0
ALERT
1 when ALERT Condition
Exists
No
Offset errors can 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 values are stored as 10-bit, twos complement
values.
 The Remote 1 Offset MSBs are stored in Register 0x11
and the LSBs are stored in Register 0x12 (low byte, left
justified). The Remote 2 Offset MSBs are stored in
Register 0x34 and the LSBs are stored in Register 0x35
(low byte, left justified).
 The Remote 2 Offset can be written to or read from the
Remote 1 Offset Registers if Bit 3 of the
Configuration 1 register is set to 1. This bit should be
set to 0 (default) to read the Remote 1 offset values.
1. These flags stay high until the status register is read, or they are
reset by POR.
The eight flags that can generate an ALERT are NOR’d
together. When any flags are high, the ALERT interrupt
latch is set and the ALERT output goes low (provided they
are not masked out).
Reading the Status 1 register clears the 5 flags, (Bit 6
through Bit 2) in Status Register 1, provided the error
conditions that caused the flags to be set have gone away.
Reading the Status 2 Register clears the three flags, (Bit 4
through Bit 2) in Status Register 2, provided the error
conditions that caused the flags to be set have gone away. A
flag bit can only be reset if the corresponding value register
contains an in-limit measurement or if the sensor is good.
Only the upper 2 bits of the LSB registers are used. The
MSB of MSB offset registers 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 or
subtracted 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 a different value is written to it.
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ADT7482
Consecutive ALERT Register
Table 14. SAMPLE OFFSET REGISTER CODES
Offset Value
0x11/0x34
0x12/0x35
−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 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. This register allows some filtering
of the output. This is particularly useful at the fastest three
conversion rates, where no averaging takes place. This
register address is 0x22. For more information, refer to
Table 15.
Table 15. CONSECUTIVE ALERT REGISTER BIT
One-shot Register
The one-shot register initiates a conversion and
comparison cycle when the ADT7482 is in standby mode,
after which the device returns to standby. Writing to the
one-shot register address (0x0F) causes the ADT7482 to
perform a conversion and comparison on both the local and
the remote 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.
Register Value
Amount of Out-of-Limit
Measurements Required
yza 000x
1
yza 001x
2
yza 011x
3
yza 111x
4
NOTES: y = SMBus SCL timeout bit. Default = 0. See the Serial
Bus Interface section for more information.
z = SMBus SDA timeout bit. Default = 0. See the Serial
Bus Interface section for more information.
a = Mask Internal ALERTs.
x = Don’t care bit.
Table 16. LIST OF REGISTERS
Read
Address
(Hex)
Write
Address
(Hex)
N/A
N/A
Address Pointer
Undefined
00
N/A
Local Temperature Value
0000 0000 (0x00)
01
N/A
Remote 1 Temperature Value High Byte
0000 0000 (0x00)
Bit 3 Conf. Reg. = 0
No
01
N/A
Remote 2 Temperature Value High Byte
0000 0000 (0x00)
Bit 3 Conf. Reg. = 1
No
02
N/A
Status Register 1
Undefined
No
03
09
Configuration Register 1
0000 0000 (0x00)
Yes
04
0A
Conversion Rate
0000 0111 (0x07)
Yes
05
0B
Local Temperature High Limit
0101 0101 (0x55) (85C)
Yes
06
0C
Local Temperature Low Limit
0000 0000 (0x00) (0C)
Yes
07
0D
Remote 1 Temperature High Limit High Byte
0101 0101 (0x55) (85C)
Bit 3 Conf. Reg. = 0
Yes
07
0D
Remote 2 Temperature High Limit High Byte
0101 0101 (0x55) (85C)
Bit 3 Conf. Reg. = 1
Yes
08
0E
Remote 1 Temperature Low Limit High Byte
0000 0000 (0x00) (0C)
Bit 3 Conf. Reg. = 0
Yes
Remote 2 Temperature Low Limit High Byte
0000 0000 (0x00) (0C)
Bit 3 Conf. Reg. = 1
Mnemonic
Power-On Default
Comment
Lock
No
No
08
0E
N/A
0F
(Note 1)
10
N/A
Remote 1 Temperature Value Low Byte
0000 0000
Bit 3 Conf. Reg. = 0
No
10
N/A
Remote 2 Temperature Value Low Byte
0000 0000
Bit 3 Conf. Reg. = 1
No
11
11
Remote 1 Temperature Offset High Byte
0000 0000
Bit 3 Conf. Reg. = 0
Yes
11
11
Remote 2 Temperature Offset High Byte
0000 0000
Bit 3 Conf. Reg. = 1
Yes
12
12
Remote 1 Temperature Offset Low Byte
0000 0000
Bit 3 Conf. Reg. = 0
Yes
12
12
Remote 2 Temperature Offset Low Byte
0000 0000
Bit 3 Conf. Reg. = 1
Yes
13
13
Remote 1 Temperature High Limit Low Byte
0000 0000
Bit 3 Conf. Reg. = 0
Yes
13
13
Remote 2 Temperature High Limit Low Byte
0000 0000
Bit 3 Conf. Reg. = 1
Yes
14
14
Remote 1 Temperature Low Limit Low Byte
0000 0000
Bit 3 Conf. Reg. = 0
Yes
One Shot
Yes
N/A
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ADT7482
Table 16. LIST OF REGISTERS (continued)
Read
Address
(Hex)
Write
Address
(Hex)
Comment
Lock
14
14
Remote 2 Temperature Low Limit Low Byte
0000 0000
Bit 3 Conf. Reg. = 1
Yes
19
19
Remote 1 THERM Limit
0101 0101 (0x55) (85C)
Bit 3 Conf. Reg. = 0
Yes
19
19
Remote 2 THERM Limit
0101 0101 (0x55) (85C)
Bit 3 Conf. Reg. = 1
Yes
20
20
Local THERM Limit
0101 0101 (0x55) (85C)
Yes
21
21
THERM Hysteresis
0000 1010 (0x0A) (10C)
Yes
22
22
Consecutive ALERT
0000 0001 (0x01)
Yes
23
N/A
Status Register 2
0000 0000 (0x00)
No
24
24
Configuration 2 Register
0000 0000 (0x00)
Yes
30
N/A
Remote 2 Temperature Value High Byte
0000 0000 (0x00)
No
31
31
Remote 2 Temperature High Limit High Byte
0101 0101 (0x55) (85C)
Yes
32
32
Remote 2 Temperature Low Limit High Byte
0000 0000 (0x00) (0C)
Yes
33
N/A
Remote 2 Temperature Value Low Byte
0000 0000 (0x00)
No
34
34
Remote 2 Temperature Offset High Byte
0000 0000 (0x00)
Yes
35
35
Remote 2 Temperature Offset Low Byte
0000 0000 (0x00)
Yes
36
36
Remote 2 Temperature High Limit Low Byte
0000 0000 (0x00) (0C)
Yes
37
37
Remote 2 Temperature Low Limit Low Byte
0000 0000 (0x00) (0C)
Yes
Mnemonic
Power-On Default
39
39
Remote 2 THERM Limit
0101 0101 (0x55) (85C)
Yes
FE
N/A
Manufacturer ID
0100 0001 (0x41)
N/A
FF
N/A
Die Revision Code
0110 0101 (0x65)
N/A
1. Writing to address 0F causes the ADT7482 to perform a single measurement. It is not a data register as such and it does not matter what
data is written to it.
Serial Bus Interface
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 to 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.
Data is sent over the serial bus in a sequence of nine clock
pulses, eight bits of data followed by an acknowledge bit
from the slave device. Transitions on the data line must
occur during the low period of the clock signal and remain
stable during the high period, since a low-to-high transition
when the clock is high can be interpreted as a stop signal. The
number of data bytes that can be transmitted over the serial
bus in a single read or write operation is limited only by what
the master and slave devices can handle.
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
Control of the ADT7482 is achieved via the serial bus.
The ADT7482 is connected to this bus as a slave device,
under the control of a master device.
The ADT7482 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. Set
Bit 7 (SCL Timeout Bit) of the consecutive alert register
(Address = 0x22) to enable the SCL Timeout. Set Bit 6 (SDA
Timeout Bit) of the consecutive alert register (Address = 0x22)
to enable the SDA Timeout.
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 ADT7482 is available with one device address, 0x4C
(1001 100b). The address mentioned in this data sheet is a
7-bit address. The R/W bit needs to be added to arrive at an
8-bit address.
Serial Bus Protocol Operation
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 follows.
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ADT7482
the correct data register is addressed. The first byte of a write
operation always contains a valid address that is stored in the
address pointer register. If data is to be written to the device,
the write operation contains a second data byte that is written
to the register selected by the address pointer register.
This procedure is illustrated in Figure 16. The device
address is 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.
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 can be transferred over the
serial bus in one operation, but it is not possible to mix read
and write in one operation because the type of operation is
determined at the beginning and cannot subsequently be
changed without starting a new operation. In the case of the
ADT7482, 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
1
9
9
1
SCLK
1
SDATA
0
START BY
MASTER
0
1
1
0
0
R/W
D7
FRAME 1
SERIAL BUS ADDRESS BYTE
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7482
FRAME 2
ADDRESS POINTER REGISTER BYTE
ACK. BY
ADT7482
1
9
SCLK (CONTINUED)
SDATA (CONTINUED)
D7
D6
D5
D4
D3
D2
D1
FRAME 3
DATA BYTE
D0
ACK. BY STOP BY
ADT7482 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
1
START BY
MASTER
0
0
1
1
0
0
FRAME 1
SERIAL BUS ADDRESS BYTE
D7
R/W
ACK. BY
ADT7482
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7482
FRAME 2
ADDRESS POINTER REGISTER BYTE
STOP BY
MASTER
Figure 17. Writing to the Address Pointer Register Only
9
1
1
9
SCLK
SDATA
START BY
MASTER
1
0
0
1
1
0
0
FRAME 1
SERIAL BUS ADDRESS BYTE
R/W
D7
D6
ACK. BY
ADT7482
D5
D4
D3
D2
D0
NO ACK.
BY MASTER
FRAME 2
DATA BYTE FROM ADT7482
Figure 18. Reading Data from a Previously Selected Register
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14
D1
STOP BY
MASTER
ADT7482
Reading Data from a Register
MASTER
RECEIVES
SMBALERT
When reading data from a register there are two
possibilities:
1. If the ADT7482 address pointer register value 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 performing a write to the ADT7482 as
before, but only the data byte containing the
register read address is sent, as 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
already 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.
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 should 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 has a low ALERT, the one
with the lowest device address has priority, in
accordance with normal SMBus arbitration.
5. Once the ADT7482 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 with low ALERT outputs
respond.
When reading data from a register, it is important to note
the following points:
 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. This is
because the first data byte of a write is always written
to the address pointer register.
 Remember that some of the ADT7482 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 ADT7482 can be put into low power standby mode
by setting Bit 6 (Mon/STBY bit) of the Configuration 1
register (Read Address = 0x03, Write Address = 0x09) to 1.
When Bit 6 is 0, the ADT7482 operates normally. When
Bit 6 is 1, 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 in low power standby mode.
Power consumption in this standby mode is reduced to a
typical of 5 mA if there is no SMBus activity or up to 30 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 all 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 ADT7482 is still in standby
mode.
ALERT Output
Pin 8 can be 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 an open
circuit. It is an open-drain output and requires a pullup to
VDD. 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 it can be used as a 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 connected to the master. When
the SMBALERT line is pulled low by one of the devices, the
following procedure occurs as illustrated in Figure 19.
Sensor Fault Detection
The ADT7482 has sensor fault detection circuitry
internally at its D+ inputs. This circuit can detect situations
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15
ADT7482
where a remote diode is not connected, or is incorrectly
connected, to the ADT7482. 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 (D1 OPEN flag) of the Status Register 1
(Address 0x02) is set if a fault is detected on the Remote 1
channel. Bit 2 (D2 OPEN flag) of the Status Register 2
(Address 0x23) is set if a fault is detected on the Remote 2
channel. If the ALERT pin is enabled, setting this flag causes
ALERT to assert low.
If a remote sensor is not used with the ADT7482, then the
D+ and D− inputs of the ADT7482 need to be tied together
to prevent the OPEN flag from being set continuously.
Most temperature sensing diodes have an operating
temperature range of −55C to +150C. Above 150C, they
lose their semiconductor characteristics and approximate
conductors instead. This results in a diode short, setting the
open flag. The remote diode in this case no longer gives an
accurate temperature measurement. A read of the
temperature result register gives the last good temperature
measurement. Be aware that while the diode fault is
triggered, the temperature measurement on the remote
channels may not be accurate.
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 being
switched on and off.
Table 17. THERM HYSTERESIS
THERM Hysteresis
Binary Representation
0C
0 000 0000
1C
0 000 0001
10C
0 000 1010
If the ADT7482 is in the default temperature range (0C
to 127C), then THERM hysteresis must be less than the
THERM limit.
Figure 20 shows how the THERM and ALERT outputs
operate. If desired, use the ALERT output as a SMBALERT
to signal to the host via the SMBus that the temperature has
risen. 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.
TEMPERATURE
1005C
905C
Interrupt System
THERM LIMIT
805C
The ADT7482 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 remote diode. THERM is intended as a fail-safe interrupt
output that cannot be masked.
If the Remote 1, Remote 2, 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 remote 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 Remote 1,
Remote 2, or local temperature exceeds the programmed
THERM limits. The 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 − hysteresis) limit. The local
and remote THERM 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 on/off control of a fan. The 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
THERM LIMIT − HYSTERESIS
705C
HIGH TEMP LIMIT
605C
505C
405C
RESET BY MASTER
ALERT
1
4
THERM
2
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 de-asserts (goes high) when
the temperature falls to THERM limit minus
hysteresis. In Figure 20, the default hysteresis
value of 10C is shown.
4. The ALERT output de-asserts 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 8 on the ADT7482 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
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16
ADT7482
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 ADT7482 and the
remote temperature sensor to operate in noisy environments.
Figure 22 shows a low-pass R-C-R filter, with the following
values:
R = 100 W and C = 1 nF
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.
This filtering reduces both common-mode noise and
differential noise.
TEMPERATURE
905C
THERM LIMIT
805C
100 W
705C
REMOTE
TEMPERATURE
SENSOR
605C
THERM2 LIMIT
505C
405C
D−
Figure 22. Filter Between Remote Sensor
and ADT7482
305C
THERM2
100 W
D+
1 nF
1
THERM
4
2
Factors Affecting Diode Accuracy
3
Remote Sensing Diode
The ADT7482 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 transistors 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, a number of 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 ADT7482 is trimmed for an nf value of 1.008. The
following equation can 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.
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 de-asserts (goes high) when
the temperature falls to THERM limit minus
hysteresis. In Figure 21, there is no hysteresis
value shown.
4. 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.
The temperature measurement could be either the local or
the remote temperature measurement.
Applications Information
DT + ǒn f * 1.008Ǔń1.008
Noise Filtering
For temperature sensors operating in noisy environments,
the previous practice was to place a capacitor across the D+
pin and the D− pins to help combat the effects of noise.
However, large capacitance’s 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 ADT7482 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

(eq. 1)
To factor this in, write the DT value to the offset register.
It is then automatically added to or subtracted from the
temperature measurement by the ADT7482.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7482, IHIGH, is 220 mA and the
low level current, ILOW, is 13.5 mA. If the ADT7482
current levels do not match the current levels specified
by the CPU manufacturer, it may be necessary to
remove an offset. The CPU data sheet advises whether
this offset needs to be removed and how to calculate it.
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17
ǒ273.15 Kelvin ) TǓ
ADT7482
noise sources, that is, 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 pickup,
a 5 mil track width and spacing is recommended.
Provide a ground plane under the tracks, if
possible.
This offset can 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 ADT7482,
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 (such as 50 to 150) that indicates
tight control of VBE characteristics.
GND
5 MIL
5 MIL
D+
5 MIL
5 MIL
D−
Transistors, such as 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 23. Typical Arrangement of Signal Tracks
Accuracy depends on the temperature of the remote
sensing diode and/or the local temperature sensor being at
the same temperature as that being measured. A number of
factors can affect this. Ideally, the sensor should be in good
thermal contact with the part of the system being measured.
If it is not, the thermal inertia caused by the sensor’s mass
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 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. In practice, the ADT7482
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 ADT7482 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 ADT7482, 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 MSOP−10
package is about 142C/W.
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 can be placed across D+ and D−,
close to the ADT7482. This capacitance can effect
the temperature measurement, so care must be
taken to ensure that any capacitance seen at D+
and D− is a maximum of 1000 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. A total of 6 feet to 12 feet is
needed.
For long distances (up to 100 feet), use shielded
twisted pair, such as Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D− and
the shield to GND close to the ADT7482. Leave
the remote end of the shield unconnected to avoid
ground loops.
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADT7482 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:
1. Place the ADT7482 as close as possible to the
remote sensing diode. Provided that the worst
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.
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18
ADT7482
Application Circuit
The SCLK pin and the SDATA pin of the ADT7482 can
be interfaced directly to the SMBus of an I/O controller, such
as the Intel 820 chipset.
Figure 24 shows a typical application circuit for the
ADT7482, using discrete sensor transistors. The pullups on
SCLK, SDATA, and ALERT are required only if they are not
already provided elsewhere in the system.
VDD
ADT7482
D1+
2N3904/06
or
CPU THERMAL
DIODE
D1−
D2+
3.0 V to 3.6 V
0.1 mF
TYP 10 kW
SCLK
SMBus
CONTROLLER
SDATA
ALERT
D2−
VDD
5.0 V or 12 V
THERM
TYP 10 kW
GND
FAN
CONTROL
CIRCUIT
FAN
ENABLE
Figure 24. Typical Application Circuit
Table 18. ORDERING INFORMATION
Device Order Number*
Temperature Range
Package Type
Shipping†
SMBus Address
ADT7482ARMZ−REEL
−40C to +125C
10-lead MSOP
3,000 Tape & Reel
4C
†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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19
ADT7482
PACKAGE DIMENSIONS
MSOP10
CASE 846AC−01
ISSUE O
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. 846B−01 OBSOLETE. NEW STANDARD
846B−02
−A−
−B−
K
D 8 PL
0.08 (0.003)
PIN 1 ID
G
0.038 (0.0015)
−T− SEATING
PLANE
M
T B
S
A
S
C
H
L
J
MILLIMETERS
MIN
MAX
2.90
3.10
2.90
3.10
0.95
1.10
0.20
0.30
0.50 BSC
0.05
0.15
0.10
0.21
4.75
5.05
0.40
0.70
DIM
A
B
C
D
G
H
J
K
L
INCHES
MIN
MAX
0.114
0.122
0.114
0.122
0.037
0.043
0.008
0.012
0.020 BSC
0.002
0.006
0.004
0.008
0.187
0.199
0.016
0.028
SOLDERING FOOTPRINT*
10X
1.04
0.041
0.32
0.0126
3.20
0.126
8X
10X
4.24
0.167
0.50
0.0196
SCALE 8:1
5.28
0.208
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.
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks,
copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. 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
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