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ADM1021AARQ-REEL-ADM1021AARQ-REEL7
Remote Temperature Sensor with Serial Interface
REV.D
FUNCTIONAL BLOCK DIAGRAM
Low-Cost Microprocessor
System Temperature Monitor*
FEATURES
Alternative to the ADM1021
On-Chip and Remote Temperature Sensing
No Calibration Necessary
1�C Accuracy for On-Chip Sensor
3�C Accuracy for Remote Sensor
Programmable Overtemperature/Undertemperature
Limits
Programmable Conversion Rate
2-Wire SMBus Serial Interface
Supports System Management Bus (SMBus) Alert
200 mA Max Operating Current
1 mA Standby Current
3 V to 5.5 V Supply
Small 16-Lead QSOP Package
APPLICATIONS
Desktop Computers
Notebook Computers
Smart Batteries
Industrial Controllers
Telecom Equipment
Instrumentation
PRODUCT DESCRIPTIONThe ADM1021A is a two-channel digital thermometer and
undertemperature/overtemperature alarm, intended for use in
personal computers and other systems requiring thermal monitor-
ing and management. The device can measure the temperature
of a microprocessor using a diode-connected PNP transistor, which
may be provided on-chip in the case of the Pentium® III or similar
processors, or can be a low-cost discrete NPN/PNP device such
as the 2N3904/2N3906. A novel measurement technique cancels
out the absolute value of the transistor’s base emitter voltage, so
that no calibration is required. The second measurement channel
measures the output of an on-chip temperature sensor, to monitor
the temperature of the device and its environment.
The ADM1021A communicates over a two-wire serial interface
compatible with SMBus standards. Undertemperature and
overtemperature limits can be programmed into the devices over
the serial bus, and an ALERT output signals when the on-chip
or remote temperature is out of range. This output can be used
as an interrupt, or as an SMBus alert.
*Patents Pending
ADM1021A–SPECIFICATIONS(TA = TMIN to TMAX1, VDD = 3.0 V to 3.6 V, unless otherwise noted.)NOTES
1TMAX = 100°C; TMIN = 0°C.
2Operation at VDD = 5 V guaranteed by design, not production tested.
3Guaranteed by design, not production tested.
Specifications subject to change without notice.
PIN FUNCTION DESCRIPTIONS
ABSOLUTE MAXIMUM RATINGS*Positive Supply Voltage (VDD) to GND . . . . . . –0.3 V to +6 V
D+, ADD0, ADD1 . . . . . . . . . . . . . . . –0.3 V to VDD + 0.3 V
D– to GND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +0.6 V
SCLK, SDATA, ALERT, STBY . . . . . . . . . . . –0.3 V to +6 V
Input Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .±50 mA
Input Current, D– . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±1 mA
ESD Rating, All Pins (Human Body Model) . . . . . . . . 2000 V
Continuous Power Dissipation
Up to 70°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 mW
Derating above 70°C . . . . . . . . . . . . . . . . . . . . . 6.7 mW/°C
Operating Temperature Range . . . . . . . . . . –55°C to +125°C
Maximum Junction Temperature (TJ max) . . . . . . . . . . 150°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature, (Soldering 10 sec) . . . . . . . . . . . . .300°C
IR Reflow Peak Temperature . . . . . . . . . . . . . . . . . . . . . 220°C
*Stresses above those listed under Absolute Maximum Ratings may cause perma-nent 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.
THERMAL CHARACTERISTICS16-Lead QSOP Package: θJA = 150°C/W.
PIN CONFIGURATIONFigure 1.Diagram for Serial Bus Timing
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
ORDERING GUIDE* Z = Pb-Lead free
LEAKAGE RESISTANCE – M�
TEMPERATURE ERROR – 1
–30TPC 1.Temperature Error vs. PC Board Track Resistance
FREQUENCY – Hz
TEMPERATURE ERROR –
100M1k10k100k1M10MTPC 2.Temperature Error vs. Power Supply Noise
Frequency
FREQUENCY – Hz
TEMPERATURE ERROR – 1k10k10M100M
100100k1MTPC 3.Temperature Error vs. Common-Mode Noise
Frequency
TPC 4.Temperature Error of ADM1021A vs. Pentium
III Temperature
CAPACITANCE – nF
TEMPERATURE ERROR – 81012141618202224TPC 5.Temperature Error vs. Capacitance Between D+
and D–
SCLK FREQUENCY – kHz
SUPPLY CURRENT –
5102550751001000250500750TPC 6.Standby Supply Current vs. Clock Frequency
ADM1021A–Typical Performance Characteristics
FREQUENCY – Hz
TEMPERATURE ERROR –
100k1M10M100M1GTPC 7.Temperature Error vs. Differential-Mode Noise
Frequency
TPC 8.Operating Supply Current vs. Conversion Rate
SUPPLY VOLTAGE – V
SUPPLY CURRENT –
–20TPC 9.Standby Supply Current vs. Supply Voltage
TPC 10.Response to Thermal Shock
FUNCTIONAL DESCRIPTIONThe ADM1021A contains a two-channel A-to-D converter with
special input-signal conditioning to enable operation with remote and
on-chip diode temperature sensors. When the ADM1021A is operat-
ing normally, the A-to-D converter 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. These signals are digitized by the ADC and
the results stored in the Local and Remote Temperature Value
Registers as 8-bit, twos complement words.
The measurement results are compared with local and remote,
high and low temperature limits, stored in four on-chip registers.
Out-of-limit comparisons generate flags that are stored in the
status register, and one or more out-of-limit results will cause
the ALERT output to pull low.
The limit registers can be programmed, and the device con-
trolled and configured, via the serial System Management Bus.
The contents of any register can also be read back via the SMBus.
Control and configuration functions consist of:
On initial power-up, the remote and local temperature values
default to –128°C. Since the device normally powers up converting,
a measurement of local and remote temperature is made and these
values are then stored before a comparison with the stored limits
is made. However, if the part is powered up in standby mode
(STBY pin pulled low), no new values are written to the register
before a comparison is made. As a result, both RLOW and LLOW
are tripped in the Status Register, thus generating an ALERT out-
put. This may be cleared in one of two ways:Change both the local and remote lower limits to –128°C
and read the status register (which in turn clears the ALERT
output).Take the part out of standby and read the status register
(which in turn clears the ALERT output). This will work only
if the measured values are within the limit values.
MEASUREMENT METHODA simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, or the base-emitter
voltage of a transistor, operated at constant current. Unfortu-
ADM1021AFigure 2.Input Signal Conditioning
The technique used in the ADM1021A is to measure the change
in VBE when the device is operated at two different currents.
This is given by:
∆VBE = KT/q × ln(N)
where:
K is Boltzmann’s constant,
q is charge on the electron (1.6 × 10–19 coulombs),
T is absolute temperature in kelvins,
N is ratio of the two currents.
Figure 2 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, provided for tempera-
ture monitoring on some microprocessors, but it could equally
well be a discrete transistor. If a discrete transistor is used, the
collector will not be grounded and should be linked to the base.
To prevent ground noise interfering with the measurement, the
more negative terminal of the sensor is not referenced to ground,
but is biased above ground by an internal diode at the D– input.
If the sensor is operating in a noisy environment, C1 may optionally
be added as a noise filter. Its value is typically 2200pF, but should
be no more than 3000pF. See the section on layout considerations
for more information on C1.
To measure ∆VBE, the sensor is switched between operating currents
of I and N × I. The resulting waveform is passed through a 65kHz
low-pass filter to remove noise, then to a chopper-stabilized ampli-
fier that performs the functions of amplification and rectification of
the waveform to produce a dc voltage proportional to ∆VBE. This
voltage is measured by the ADC to give a temperature output in
8-bit twos complement format. To reduce the effects of noise
further, digital filtering is performed by averaging the results of
16 measurement cycles.
Signal conditioning and measurement of the internal tempera-
ture sensor is performed in a similar manner.
DIFFERENCES BETWEEN THE ADM1021 AND THE
ADM1021AAlthough the ADM1021A is pin-for-pin compatible with the
ADM1021, there are some differences between the two devices.
for the ADM1021. The main reason for this is to improve
the noise immunity of the part.As a result of the greater Remote Sensor Source Current the
operating current of the ADM1021A is higher than that of
the ADM1021, typically 205 mA versus 160 mA.The temperature measurement range of the ADM1021A is
0°C to 127°C, compared with –128°C to +127°C for the
ADM1021. As a result, the ADM1021 should be used if
negative temperature measurement is required.The power-on reset values of the remote and local tempera-
ture values are –128°C in the ADM1021A as compared with
0°C in the ADM1021. As the part is powered up converting
(except when the part is in standby mode, i.e., Pin 15 is
pulled low) the part will measure the actual values of remote
and local temperature and write these to the registers.The four MSBs of the Revision Register may be used to
identify the part. The ADM1021 Revision Register reads
0xh and the ADM1021A reads 3xh.The power-on default value of the Address Pointer Register
is undefined in the ADM1021A and is equal to 00h in the
ADM1021. As a result, a value must be written to the Address
Pointer Register before a read is done in the ADM1021A.
The ADM1021 is capable of reading back local temperature
without writing to the Address Pointer Register as it defaulted
to the local temperature measurement register at power-up.Setting the mask bit (Bit 7 Config Reg) on the ADM1021A
will mask current and future ALERTs. On the ADM1021
the mask bit will only mask future ALERTs. Any current
ALERT will have to be cleared using an ARA.
TEMPERATURE DATA FORMATOne LSB of the ADC corresponds to 1°C, so the ADC can theo-
retically measure from –128°C to +127°C, although the device
does not measure temperatures below 0°C so the actual range is
0°C to 127°C. The temperature data format is shown in Table I.
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.
Table III.List of ADM1021A Registers
Table I.Temperature Data Format
REGISTERSThe ADM1021A contains nine registers that are used to store
the results of remote and local temperature measurements,
high and low temperature limits, and to configure and control
the device. A description of these registers follows, and further
details are given in Tables II to IV. It should be noted that the
ADM1021A’s registers are dual port, and have different addresses
for read and write operations. Attempting to write to a read address,
or to read from a write address, will produce an invalid result.
Register addresses above 0Fh are reserved for future use or used
for factory test purposes and should not be written to.
Address Pointer RegisterThe Address Pointer Register itself does not have, nor does it
require, an address, as it is the register to which the first data
byte of every write operation is written automatically. This data
byte is an address pointer that sets up one of the other registers
for the second byte of the write operation, or for a subsequent
read operation.
Value RegistersThe ADM1021A has two registers to store the results of local
and remote temperature measurements. These registers are
written to by the ADC and can only be read over the SMBus.
Status RegisterBit 7 of the Status Register indicates when it is high that the
ADC is busy converting. Bits 5 to 3 are flags that indicate the
results of the limit comparisons.
If the local and/or remote temperature measurement is above the
corresponding high temperature limit or below the corresponding
low temperature limit, then one or more of these flags will be set.
Bit 2 is a flag that is set if the remote temperature sensor is open-
circuit. These five flags are NOR’d together, so that if any of them
is high, the ALERT interrupt latch will be set and the ALERT
output will go low. Reading the Status Register will clear the five
flag bits, provided the error conditions that caused the flags to be
set have gone away. While a limit comparator is tripped due to a
value register containing an out-of-limit measurement, or the sen-
sor is open-circuit, the corresponding flag bit cannot be reset. A
flag bit can only be reset if the corresponding value register con-
tains an in-limit measurement, or the sensor is good.
Table II.Status Register Bit Assignments*These flags stay high until the status register is read or they are reset by POR.
ADM1021AThe ALERT interrupt latch is not reset by reading the Status
Register, but will be 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.
Configuration RegisterTwo bits of the configuration register are used. If Bit 6 is 0, which
is the power-on default, the device is in operating mode with the
ADC converting. If Bit 6 is set to 1, the device is in standby
mode and the ADC does not convert. Standby mode can also
be selected by taking the STBY pin low. In standby mode the val-
ues stored in the Remote and Local Temperature Registers remain
at the value they were when the part was placed in standby.
Bit 7 of the configuration register is used to mask the ALERT out-
put. If Bit 7 is 0, which is the power-on default, the ALERT output
is enabled. If Bit 7 is set to 1, the ALERT output is disabled.
Table IV.Configuration Register Bit Assignments
Conversion Rate RegisterThe lowest three bits of this register are used to program the con-
version rate by dividing the ADC clock by 1, 2, 4, 8, 16, 32, 64,
or 128, to give conversion times from 125ms (Code 07h) to 16
seconds (Code 00h). This register can be written to and read back
over the SMBus. The higher five bits of this register are unused
and must be set to zero. Use of slower conversion times greatly
reduces the device power consumption, as shown in Table V.
Table V.Conversion Rate Register Codes00h
01h
02h
03h
04h
05h
06h
07h
Limit RegistersThe ADM1021A has four limit registers to store local and remote,
high and low temperature limits. These registers can be written
to and read back over the SMBus. The high limit registers
perform a >comparison while the low limit registers perform acomparison. For example, if the high limit register is pro-
grammed as a limit of 80°C, measuring 81°C will result in an
alarm condition. Even though the temperature measurement
One-Shot RegisterThe one-shot register is used to initiate a single conversion and
comparison cycle when the ADM1021A is in standby mode,
after which the device returns to standby. This is not a data
register as such and it is the write operation that causes the one-
shot conversion. The data written to this address is irrelevant and
is not stored.
SERIAL BUS INTERFACEControl of the ADM1021A is carried out via the serial bus. The
ADM1021A is connected to this bus as a slave device, under the
control of a master device. Note that the SMBus and SCL pins
are three-stated when the ADM1021A is powered down and will
not pull down the SMBus.
ADDRESS PINSIn 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 will respond. The ADM1021A has two
address pins, ADD0 and ADD1, to allow selection of the device
address, so that several ADM1021As can be used on the same
bus, and/or to avoid conflict with other devices. Although only
two address pins are provided, these are three-state, and can be
grounded, left unconnected, or tied to VDD, so that a total of
nine different addresses are possible, as shown in Table VI.
It should be noted that the state of the address pins is only sampled
at power-up, so changing them after power-up will have no effect.
Table VI.Device AddressesADD0, ADD1 sampled at power-up only.
The serial bus protocol operates as follows:The master initiates data transfer by establishing a START
condition, defined as a high-to-low transition on the serial
data line SDATA, while the serial clock line SCLK remains
high. This indicates that an address/data stream will follow.
All slave peripherals connected to the serial bus respond to
the START condition and shift in the next eight bits, consisting
of a 7-bit address (MSB first) plus an R/W bit, which deter-
mines the direction of the data transfer, i.e., whether data
will be written to or read from the slave device.
The peripheral whose address corresponds to the transmitted
address responds by pulling the data line low during the low
period before the ninth clock pulse, known as the Acknowl-
edge Bit. All other devices on the bus now remain idle while
