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ADT7461-ADT7461AR-ADT7461ARM-ADT7461ARM-REEL-ADT7461ARM-REEL7-ADT7461ARMZ-ADT7461ARMZ-REEL-ADT7461ARMZ-REEL7-ADT7461ARZ-ADT7461ARZ-REEL-ADT7461ARZ-REEL7
±1°C Temperature Monitor with Series Resistance Cancellation
±1°C Temperature Monitor with
Series Resistance Cancellation
Rev. A
FEATURES
On-chip and remote temperature sensor
0.25°C resolution/1°C accuracy on remote channel
1°C resolution/3°C accuracy on local channel
Automatically cancels up to 3 kΩ (typ) of resistance in series
with remote diode to allow noise filtering
Extended, switchable temperature measurement range 0°C
to +127°C (default) or –55°C to +150°C
Pin- and register-compatible with ADM1032
2-wire SMBus serial interface with SMBus alert support
Programmable over/under temperature limits
Offset registers for system calibration
Up to two overtemperature fail-safe THERM outputs
Small 8-lead SOIC or MSOP package
170 µA operating current, 5.5 µA standby current
APPLICATIONS
Desktop and notebook computers
Industrial controllers
Smart batteries
Automotive
Enbedded systems
Burn-in applications
Instrumentation
GENERAL DESCRIPTION The ADT74611 is a dual-channel digital thermometer and
under/over temperature alarm, intended for use in PCs and
thermal management systems. It is pin- and register-compatible
with the ADM1032. The ADT7461 has three additional features:
series resistance cancellation (where up to 3 kΩ (typical) of
resistance in series with the temperature monitoring diode may
be automatically cancelled from the temperature result, allowing
noise filtering); configurable ALERT output; and an extended,
switchable temperature measurement range.
The ADT7461 can accurately measure the temperature of a
remote thermal diode to ±1°C and the ambient temperature to
±3°C. The temperature measurement range defaults to 0°C to
+127°C, compatible with the ADM1032, but can be switched to
a wider measurement range of−55°C to +150°C. The ADT7461
communicates over a 2-wire serial interface compatible with
system management bus (SMBus) standards. An ALERT output
signals when the on-chip or remote temperature is out of range.
The THERM output is a comparator output that allows on/off
control of a cooling fan. The ALERT output can be reconfigured
as a second THERM output, if required.
1 . Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239;
6,133,753; 6,169,442; other patents pending.
SCLKSDATAGNDVDD
THERMALERT/
THERM2Figure 1. Functional Block Diagram
TABLE OF CONTENTS Specifications....................................................................................3
SMBus Timing Specifications........................................................4
Absolute Maximum Ratings...........................................................5
ESD Caution.................................................................................5
Pin Configuration and Function Descriptions............................6
Typical Performance Characteristics............................................7
Functional Description...................................................................9
Series Resistance Cancellation...................................................9
Temperature Measurement Method.........................................9
Temperature Measurement Results.........................................10
Temperature Measurement Range..........................................10
Temperature Data Format........................................................10
ADT7461 Registers...................................................................11
Serial Bus Interface....................................................................14
Addressing the Device..............................................................14
ALERT Output...........................................................................16
Low Power Standby Mode........................................................16
Sensor Fault Detection.............................................................16
The ADT7461 Interrupt System..............................................16
Application Information..........................................................18
Factors Affecting Diode Accuracy..........................................18
Thermal Inertia and Self-Heating...........................................18
Layout Considerations..............................................................19
Application Circuit....................................................................20
Outline Dimensions......................................................................21
Ordering Guide.........................................................................21
REVISION HISTORY
10/04—Changed from Rev. 0 to Rev. A Change to SMBus specifications................................................4
Changes to Figure 6 and Figure 10............................................7
Added Figure 9 and Figure 13....................................................7
Changes to Temperature Measurement section....................10
Changes to Figure 19 and Figure 25........................................16
Changes to Serial Bus Interface section..................................14
10/03—Revision 0: Initial Version SPECIFICATIONS TA = −40°C to +120°C, VDD = 3 V to 5.5 V, unless otherwise noted.
Table 1. See for information on other conversion rates. Table 8 Guaranteed by characterization, but not production tested. Guaranteed by design, but not production tested.
4 See section for more information. SMBUS Timing Specifications Disabled by default. Details on how to enable it are in the SMBus section of this data sheet.
SMBus TIMING SPECIFICATIONS
Table 2. SMBus Timing Specifications1 Guaranteed by design, but not production tested.
2 Time from 10% of SDATA to 90% of SCLK. Time for 10% or 90% of SDATA to 10% of SCLK.
4 Time for 90% of SCLK to 10% of SDATA.
SCLK
SDATAtF
tSU;DAT
STOPSTARTSTOPSTART
Figure 2. Serial Bus Timing
ABSOLUTE MAXIMUM RATINGS
Table 3.
Thermal Characteristics 8-Lead SOIC Package
θJA = 121°C/W
8-Lead MSOP Package
θJA = 142°C/W
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS 04110-0-013
VDDD+Figure 3. Pin Configuration
Table 4. Pin Function Descriptions TYPICAL PERFORMANCE CHARACTERISTICS LEAKAGE RESISTANCE (MΩ)
RATURE
RROR (
°C)
–20
Figure 4. Temperature Error vs. Leakage Resistance
TEMPERATURE (°C)
RATURE
RROR (
°C)
–0.1Figure 5. Temperature Error vs. Actual Temperature Using 2N3906
FREQUENCY (MHz)
RATURE
RROR (
Figure 6. Temperature Error vs. Differential Mode Noise Frequency
FREQUENCY (MHz)
RATURE
RROR (
°C)
–10
Figure 7. Temperature Error vs. Power Supply Noise Frequency
CAPACITANCE (nF)
RATURE
RROR (
–10
Figure 8. Temperature Error vs. Capacitance between D+ and D−
FREQUENCY (MHz)
RATURE
RROR (
°C)
–20
Figure 9. Temperature Error vs. 100 mV Differential Mode Noise Frequency
FREQUENCY (MHz)
RATURE
RROR (
Figure 10. Temperature Error vs. Common-Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
SCL CLOCK FREQUENCY (kHz)
IDD
Figure 11. Standby Supply Current vs. Clock Frequency
VDD (V)
IDD
Figure 12. Standby Current vs. Supply Voltage
FREQUENCY (MHz)
RATURE
RROR (
Figure 13. Temperature Error vs. 100 mV Common-Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
CONVERSION RATE (Hz)
IDD
700Figure 14. Operating Supply Current vs. Conversion Rate
04110-0-023
2102001k2k3k4k
SERIES RESISTANCE (Ω)
RATURE
RROR (Figure 15. Temperature Error vs. Series Resistance
FUNCTIONAL DESCRIPTION The ADT7461 is a local and remote temperature sensor and
over/under temperature alarm, with the added ability to auto-
matically cancel the effect of 3 kΩ (typical) of resistance in
series with the temperature monitoring diode. When the
ADT7461 is operating normally, the on-board ADC operates in
a free-running mode. The analog input multiplexer alternately
selects either the on-chip temperature sensor to measure its
local temperature or the remote temperature sensor. The ADC
digitizes these signals and the results are stored in the local and
remote temperature value registers.
The local and remote measurement results are compared with
the corresponding high, low, and THERM temperature limits,
stored in eight on-chip registers. Out-of-limit comparisons
generate flags that are stored in the status register. A result that
exceeds the high temperature limit, the low temperature limit,
or an external diode fault will cause 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 con-
trolled and configured via the serial SMBus. The contents of any
register can also be read back via the SMBus.
Control and configuration functions consist of switching the
device between normal operation and standby mode, selecting
the temperature measurement scale, masking or enabling the
ALERT output, switching Pin 6 between ALERT and THERM2,
and selecting the conversion rate.
SERIES RESISTANCE CANCELLATION Parasitic resistance to the D+ and D− inputs to the ADT7461,
seen in series with the remote diode, is caused by a variety of
factors, including PCB track resistance and track length. This
series resistance appears as a temperature offset in the remote
sensor’s temperature measurement. This error typically causes a
0.5°C offset per ohm of parasitic resistance in series with the
remote diode.
The ADT7461 automatically cancels out the effect of this series
resistance on the temperature reading, giving a more accurate
result, without the need for user characterization of this resis-
tance. The ADT7461 is designed to automatically cancel typically
up to 3 kΩ of resistance. By using an advanced temperature
measurement method, this is transparent to the user. This
feature allows resistances to be added to the sensor path to
produce a filter, allowing the part to be used in noisy environ-
ments. See the section on Noise Filtering for more details.
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 out
the effect of the absolute value of VBE, which varies from device
to device.
The technique used in the ADT7461 is to measure the change
in VBE when the device is operated at three different currents.
Previous devices have used only two operating currents, but it is
the use of a third current that allows automatic cancellation of
resistances in series with the external temperature sensor.
Figure 16 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, but it could equally
be a discrete transistor. If a discrete transistor is used, the collec-
tor will not be grounded and should be linked to the base. To
prevent ground noise interfering with the measurement, the
more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at the
D− input. C1 may be added as a noise filter (a recommended
maximum value of 1,000 pF). However, a better option in noisy
environments is to add a filter, as described in the Noise
Filtering section. See the Layout Considerations section for
more information on C1.
To measure ∆VBE, the operating current through the sensor is
switched among three related currents. Shown in Figure 16,
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 ∆VBE1, and then between I and N2 × I, giving
∆VBE2. The temperature may then be calculated using the two
∆VBE measurements. This method can also be shown to cancel
the effect of any series resistance on the temperature measurement.
The resulting ∆VBE 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 ∆VBE. The ADC digitizes this vol-
tage 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 tempera-
ture sensor is performed in the same manner.
VOUT+
TO ADC
VOUT–
REMOTESENSINGTRANSISTOR
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
Figure 16. Input Signal Conditioning
TEMPERATURE MEASUREMENT RESULTS The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers
and are compared with limits programmed into the local and
remote high and low limit registers.
The local temperature value is in Register 0x00 and has a reso-
lution 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 temp-
erature low byte are used. This gives the external temperature
measurement a resolution of 0.25°C. Table 5 shows the data
format for the external temperature low byte.
Table 5. Extended Temperature Resolution (Remote
Temperature Low Byte) When reading the full external temperature value, both the high
and low byte, the two registers should be read in succession.
Reading one register does not lock the other, so both should be
read before the next conversion finishes. In practice, there is
more than enough time to read both registers, as transactions
over the SMBus are significantly faster than a conversion time.
TEMPERATURE MEASUREMENT RANGE The temperature measurement range for both internal and
external measurements is, by default, 0°C to +127°C. However,
the ADT7461 can be operated using an extended temperature
range. It can measure the full temperature range of an external
diode, from −55°C to +150°C. The user can switch between
these two temperature ranges by setting or clearing Bit 2 in the
configuration register. A valid result is available in the next
measurement cycle after changing the temperature range.
In extended temperature mode, the upper and lower temp-
erature that can be measured by the ADT7461 is limited by the
erature sensing diodes have a maximum temperature range of
−55°C to +150°C. Above 150°C, they may lose their semicon-
ductor characteristics and approximate conductors instead. This
results in a diode short. In this case, a read of the temperature
result register will give the last good temperature measurement.
The user should be aware that the temperature measurement on
the external channel may not be accurate for temperatures that
are outside the operating range of the remote sensor.
It should be noted that while both local and remote temperature
measurements can be made while the part is in extended temp-
erature mode, the ADT7461 itself should not be exposed to temp-
eratures 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 DATA FORMAT The ADT7461 has two temperature data formats. When the
temperature measurement range is from 0°C to +127°C
(default), the temperature data format for both internal and
external temperature results is binary. When the measurement
range is in extended mode, an offset binary data format is used
for both internal and external results. Temperature values in the
offset binary data format are offset by 64°C. Examples of temp-
eratures in both data formats are shown in Table 6.
Table 6. Temperature Data Format (Local and Remote
Temperature High Byte) Offset binary scale temperature values are offset by 64°C.
2 Binary scale temp. measurement returns 0°C for all temperatures < 0°C.
The user may switch between measurement ranges at any time.
Switching the range will also switch the data format. The next
temperature result following the switching will be reported back
to the register in the new format. However, the contents of the
limit registers will not change. It is up to the user to ensure that
when the data format changes, the limit registers are repro-
grammed as necessary. More information on this can be found
in the Limit Registers section.
ADT7461 REGISTERS The ADT7461 contains 22 8-bit registers in total. These regis-
ters are used to store the results of remote and local temperature
measurements and high and low temperature limits and to confi-
gure and control the device. A description of these registers fol-
lows. Additional details are given in Table 7 through Table 11.
Address Pointer Register The address pointer register itself does not have or require an
address, as the first byte of every write operation is automa-
tically written to this register. The data in this first byte always
contains the address of another register on the ADT7461, which
is stored in the address pointer register. It 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, so if a read operation is performed immediately after
power-on, without first writing to the address pointer, the value
of the local temperature will be returned, since its register
address is 0x00.
Temperature Value Registers The ADT7461 has three registers to store the results of local and
remote temperature measurements. These registers can only be
written to by the ADC and can be read by the user over the
SMBus. The local temperature value register is at Address 0x00.
The external temperature value high byte register is at
Address 0x01, with the low byte register at Address 0x10.
The power-on default for all three registers is 0x00.
Configuration Register The configuration register is Address 0x03 at read and Address
0x09 at write. Its power-on default is 0x00. Only four bits of the
configuration register are used. Bits 0, 1, 3, and 4 are reserved
and should not be written to by the user.
Bit 7 of the configuration register is used to mask the ALERT
output. If Bit 7 is 0, the ALERT output is enabled. This is the
power-on default. If Bit 7 is set to 1, the ALERT output is
disabled. This only applies if Pin 6 is configured as ALERT. If
Pin 6 is configured as THERM2, 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 the ADC converting. If Bit 6 is set to 1, the
device is in standby mode and the ADC does not convert. The
SMBus does, however, remain active in standby mode, so values
can be read from or written to the ADT7461 via the SMBus in
this mode. The ALERT and THERM outputs are also active in
standby mode. Changes made to the registers in standby mode
that affect the THERM or ALERT outputs will cause these
signals to be updated.
Bit 5 determines the configuration of Pin 6 on the ADT7461. If
Bit 5 is 0, (default) then Pin 6 is configured as an ALERT output.
If Bit 5 is 1, then Pin 6 is configured as a THERM2 output. Bit 7,
the ALERT mask bit, is only active when Pin 6 is configured as
an ALERT output. If Pin 6 is setup 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 means that the
measurement range is set to the extended temperature range.
Table 7. Configuration Register Bit Assignments
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 8 conversions
per second means that beginning at 125 ms intervals the device
performs a conversion on the internal and the external
temperature channels.
This register can be written to and read back over the SMBus.
The higher four bits of this register are unused and must be set
to 0. The default value of this register is 0x08, giving a rate of
16 conversions per second. Use of slower conversion times
greatly reduces the device power consumption, as shown
in Table 8.
Table 8. Conversion Rate Register Codes
Limit Registers The ADT7461 has eight limit registers: high, low, and THERM
temperature limits for both local and remote temperature
measurements. The remote temperature high and low limits
span two registers each, to contain an upper and lower byte for
each limit. There is also a THERM hysteresis register. All limit
registers can be written to and read back over the SMBus. See
Table 12 for details of the limit registers’ 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 will result 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 will result in an out-of-limit condition.
Exceeding either the local or remote THERM limit asserts
THERM low. When Pin 6 is configured as THERM2, exceeding
either the local or remote high limit asserts THERM2 low. A
default hysteresis value of 10°C is provided that applies to both
THERM channels. This hysteresis value may be reprogrammed
to any value after power-up (Register Address 0x21).
It is important to remember that the temperature limits data
format is the same as the temperature measurement data
format. So if the temperature measurement uses default binary,
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 user must
reprogram the limit registers to the desired value in the correct
data format. For example, if the remote low limit is set at 10°C
and the default binary scale is being used, the limit register
value should be 0000 1010b. If the scale is switched to offset
binary, the value in the low temperature limit register should be
Status Register The status register is a read-only register, at Address 0x02. It
contains status information for the ADT7461.
Bit 7 of the status register indicates that the ADC is busy con-
verting when it is high. The other bits in this register flag the
out-of-limit temperature measurements (Bits 6 to 3 and Bits 1
to 0) and the remote sensor open circuit (Bit 2).
If Pin 6 is configured as an ALERT output, the following applies. If
the local temperature measurement exceeds its limits, Bit 6 (high
limit) or Bit 5 (low limit) of the status register asserts to flag this
condition. If the remote temperature measurement exceeds its
limits, then Bit 4 (high limit) or Bit 3 (low limit) asserts. Bit 2 asserts
to flag an open-circuit condition on the remote sensor. These five
flags are NOR’d together, so if any of them is high, the ALERT
interrupt latch will be set and the ALERT output will go low.
Reading the status register clears the five flags, Bits 6 to 2, pro-
vided the error conditions causing the flags to be set have gone
away. A flag bit can be reset only if the corresponding value reg-
ister contains an in-limit measurement or if the sensor is good.
The ALERT interrupt latch is not reset by reading the status
register. It resets when the ALERT output has been serviced by
the master reading the device address, provided the error condi-
tion 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 reset
automatically and the THERM output goes high. The user may
add hysteresis by programming Register 0x21. The THERM
output will be reset only when the temperature falls to limit
value–hysteresis value.
When Pin 6 is configured as THERM2, only the high temp-
erature 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 9. Status Register Bit Assignments
