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MAX6654MEE
1C Accurate Remote/Local Temperature Sensor with SMBus Serial Interface
General DescriptionThe MAX6654†is a precise digital thermometer that
reports the temperature of both a remote P-N junction
and its own die. The remote junction can be a diode-con-
nected transistor—typically a low-cost, easily mounted
2N3904 NPN type or 2N3906 PNP type—that replaces
conventional thermistors or thermocouples. Remote
accuracy is ±1°C for multiple transistor manufacturers,
with no calibration needed. The remote junction can also
be a common-collector PNP, such as a substrate PNP of
a microprocessor (µP).
The 2-wire serial interface accepts standard System
Management Bus (SMBus)TM, Write Byte, Read Byte,
Send Byte, and Receive Byte commands to program the
alarm thresholds and to read temperature data.
Measurements can be done automatically and
autonomously, with the conversion rate programmed by
the user, or programmed to operate in a single-shot
mode. The adjustable conversion rate allows the user to
optimize supply current and temperature update rate to
match system needs. When the conversion rate is faster
than 1Hz, the conversion results are available as a 7-bit-
plus-sign byte with a 1°C LSB. When the conversion rate
is 1Hz or slower, the MAX6654 enters the extended
mode. In this mode, 3 additional bits of temperature data
are available in the extended resolution register, provid-
ing 10-bit-plus-sign resolution with a 0.125°C LSB.
Single-shot conversions also have 0.125°C per LSB reso-
lution when the conversion rate is 1Hz or slower.
A parasitic resistance cancellation (PRC) mode can also
be invoked for conversion rates of 1Hz or slower by set-
ting bit 4 of the configuration register to 1. In PRC mode,
the effect of series resistance on the leads of the external
diode is canceled. The 11-bit conversion in PRC mode is
performed in <500ms and is disabled for conversion
rates faster than 1Hz. The one-shot conversion is also 11
bits in <500ms.
The MAX6654 default low-temperature measurement
limit is 0°C. This can be extended to -64°C by setting bit
5 of the configuration register to 1.
The MAX6654 is available in a small, 16-pin QSOP sur-
face-mount package.
________________________Applications
FeaturesHigh Accuracy ±1°C (max) from +70°C to +100°C
(Remote)11-Bit, 0.125°C ResolutionDual Channel: Measures Remote and Local
TemperatureNo Calibration RequiredProgrammable Under/Overtemperature AlarmsI2C™-Compatible/SMBus Interface+3V to +5.5V Supply Range
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Pin Configurationypical Operating Circuit19-1836; Rev 2; 1/02
Ordering InformationDesktop Computers
Notebook Computers
Servers
Thin Clients
Workstations
Test and Measurement
Multichip Modules
SMBus is a trademark of Intel Corp.
I2C is a trademark of Philips Corp.Patents pending
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICSStresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
(All voltages are referenced to GND unless otherwise noted.)
VCC..........................................................................-0.3V to +6V
DXP, ADD_................................................-0.3V to (VCC + 0.3V)
DXN ......................................................................-0.3V to +0.8V
SMBCLK, SMBDATA, ALERT, STBY........................-0.3V to +6V
SMBDATA, ALERTCurrent.................................-1mA to +50mA
DXN Current ......................................................................±1mA
ESD Protection (all pins, Human Body Model)..................2000V
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.30mW/°C above +70°C).........667mW
Operating Temperature Range ........................-55°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range ............................-65°C to +165°C
Lead Temperature (soldering, 10s).................................+300°C
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
ELECTRICAL CHARACTERISTICS (continued)
Note 1:+25°C ≤TA≤+85°C.
Note 2:If both the MAX6654 and the remote junction are below TA= -20°C, then VCC> 3.15V.
Note 3:The conversion time doubles for the extended resolution mode. This causes the average operating current to approximately
double.
Note 4:The serial interface resets when SMBCLK is low for more than tTIMEOUT.
Note 5:Note that a transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SMBCLK’s
falling edge.
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Typical Operating Characteristics(VCC= +3.3V to +5.5V, TA= +25°C, unless otherwise noted.)
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Detailed DescriptionThe MAX6654 is a temperature sensor that communi-
cates through an SMBus/I2C-compatible interface with a
µP in thermal-management applications. Essentially an
11-bit serial analog-to-digital converter (ADC) with a
sophisticated front end, the MAX6654 measures the
change in diode voltage at different current levels to cal-
culate temperature. It contains a current source, a multi-
plexer, an ADC, an SMBus interface, and associated
control logic (Figure 1). Temperature data from the ADC
is loaded into data registers, where it is automatically
compared with data previously stored in four
over/undertemperature alarm registers.
ADC and MultiplexerThe ADC is an averaging type that integrates over a
60ms period (each channel, typically, in the 8-bit “lega-
cy” mode), with excellent noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure their forward voltages and
compute their temperatures. Both channels are auto-
matically converted once the conversion process has
started, either in free-running or single-shot mode. If
one of the two channels is not used, the device still per-
forms both measurements, and the user can ignore the
results of the unused channel. If the remote-diode
channel is unused, connect DXP to DXN rather than
leave the pins open.
The DXN input is biased at 1VBEabove ground by an
internal diode to set up the ADC inputs for a differential
measurement. The worst-case DXP-DXN differential
input voltage range is 0.28V to 0.9V.
Excess resistance in series with the remote diode caus-
es about +1/2°C error per ohm when the parasitic resis-
tance cancellation mode is not being used. When the
parasitic resistance cancellation mode is being used,
excess resistance of up to 100Ωwill not cause any dis-
cernable error. A 200µV offset voltage forced on DXP-
DXN causes about 1°C error.
A/D Conversion SequenceA conversion sequence consists of a local temperature
measurement and a remote temperature measurement.
Each time a conversion begins, whether initiated auto-
matically in the free-running autoconvert mode
(RUN/STOP = 0) or by writing a “One-Shot” command,
both channels are converted, and the results of both
measurements are available after the end of conver-
sion. A BUSY status bit in the status byte shows that the
device is actually performing a new conversion; howev-
er, even if the ADC is busy, the results of the previous
conversion are always available.
Remote-Diode SelectionThe MAX6654 can directly measure the die tempera-
ture of CPUs and other ICs having on-board tempera-
ture-sensing diodes as shown in the Typical Operating
Circuit, or it can measure the temperature of a discrete
diode-connected transistor. For best accuracy, the dis-
crete transistor should be a small-signal device with its
collector and base connected together. Accuracy has
been experimentally verified for all of the devices listed
in Table 1.
The transistor must be a small-signal type with a rela-
tively high forward voltage; otherwise, the A/D input
voltage range can be violated. The forward voltage
must be >0.28V at 10µA; check to ensure this is true at
the highest expected temperature. The forward voltage
must be <0.9V at 100µA; check to ensure this is true at
the lowest expected temperature. Large power transis-
tors don’t work at all. Also, ensure that the base resis-
tance is <100Ω. Tight specifications for forward-current
gain (+50 to +150, for example) indicate that the manu-
facturer has good process controls and that the
devices have consistent VBE characteristics.
For heat-sink mounting, the 500-32BT02-000 thermal
sensor from Fenwal Electronics is a good choice. This
device consists of a diode-connected transistor, an alu-
minum plate with screw hole, and twisted-pair cable
(Fenwal Inc., Milford, MA, 508-478-6000).
Thermal Mass and Self-HeatingThermal mass can significantly affect the time required
for a temperature sensor to respond to a sudden
change in temperature. The thermal time constant of
the 16-pin QSOP package is about 140s in still air. For
the junction temperature of a MAX6654 in still air to set-
tle to within +1°C after a sudden +100°C change in air
temperature, about five time constants or 12 minutes
are required. However, the MAX6654 is not intended to
Table 1. Remote-Sensor Transistor
Manufacturers
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interfacemeasure ambient temperature; when measuring local
temperature, it senses the temperature of the PC board
to which it is soldered. The leads provide a good ther-
mal path between the PC board traces and the
MAX6654’s die. Thermal conductivity between the
MAX6654’s die and the ambient air is poor by compari-
son. Because the thermal mass of the PC board is far
greater than that of the MAX6654, the device follows
temperature changes on the PC board with little or no
perceivable delay.
When measuring temperature with discrete remote sen-
sors, the use of smaller packages, such as SOT23s,
yields the best thermal response times. Take care to
account for thermal gradients between the heat source
and the sensor, and ensure that stray air currents
across the sensor package do not interfere with mea-
surement accuracy. When measuring the temperature
of a CPU or other IC with an on-chip sense junction,
thermal mass has virtually no effect; the measured tem-
perature of the junction tracks the actual temperature
within a conversion cycle.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
worst-case error occurs when autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERToutput. For example, at an 8Hz rate
and with ALERTsinking 1mA, the typical power dissi-
pation is VCCx 450µA + 0.4V x 1mA. Package theta J-
A is about 150°C/Ω, so with VCC= 5V and no copper
PC board heat sinking, the resulting temperature rise is:
∆T = 2.7mW x 150°C/W = 0.4°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
ADC Noise FilteringThe ADC is an integrating type with inherently good
noise rejection, especially of low-frequency signals such
as 60Hz/120Hz power-supply hum. Micropower opera-
tion places constraints on high-frequency noise rejection;
therefore, careful PC board layout and proper external
noise filtering are required for high-accuracy remote
measurements in electrically noisy environments.
High-frequency EMI is best filtered at DXP and DXN with
an external 2200pF capacitor. This value can be
increased to about 3300pF (max), including cable
capacitance. Capacitance >3300pF introduces errors
due to the rise time of the switched current source.
Nearly all noise sources tested cause the ADC measure-
ments to be higher than the actual temperature, typically
by +1°C to +10°C, depending on the frequency and
amplitude (see Typical Operating Characteristics).
PC Board LayoutPlace the MAX6654 as close as practical to the
remote diode. In a noisy environment, such as a
computer motherboard, this distance can be 4
inches to 8 inches (typ) or more, as long as the
worst noise sources (such as CRTs, clock genera-
tors, memory buses, and ISA/PCI buses) are avoid-
ed.Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across a fast memory bus, which can easily intro-
duce +30°C error, even with good filtering.
Otherwise, most noise sources are fairly benign.Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any high-
voltage traces, such as +12VDC. Leakage currents
from PC board contamination must be dealt with
carefully since a 20MΩleakage path from DXP to
ground causes about +1°C error.Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 2). With guard traces in
place, routing near high-voltage traces is no longer
an issue.Route through as few vias and crossunders as pos-
sible to minimize copper/solder thermocouple
effects.When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PC-board-induced ther-
mocouples are not a serious problem. A copper-
solder thermocouple exhibits 3µV/°C, and it takes
about 200µV of voltage error at DXP-DXN to cause
a +1°C measurement error. So, most parasitic ther-
mocouple errors are swamped out.Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10mil
widths and spacings recommended in Figure 2
aren’t absolutely necessary (as they offer only a
Figure 2. Recommended DXP/DXN PC Traces
