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MAX6646MUA-MAX6647MUA-MAX6649MUA
+145C Precision SMBus-Compatible Remote/ Local Sensors with Overtemperature Alarms
General DescriptionThe MAX6646/MAX6647/MAX6649 are precise, two-
channel digital temperature sensors. The devices accu-
rately measure the temperature of their own die and a
remote PN junction, and report the temperature in digital
form using a 2-wire serial interface. The remote PN junc-
tion is typically the emitter-base junction of a common-
collector PNP on a CPU, FPGA, or ASIC.
The 2-wire serial interface accepts standard system man-
agement bus (SMBus) write byte, read byte, send byte,
and receive byte commands to read the temperature
data and to program the alarm thresholds. To enhance
system reliability, the MAX6646/MAX6647/MAX6649
include an SMBus timeout. A fault queue prevents the
ALERTand OVERToutputs from setting until a fault has
been detected one, two, or three consecutive times
(programmable).
The MAX6646/MAX6647/MAX6649provide two system
alarms: ALERTand OVERT. ALERTasserts when any of
four temperature conditions are violated: local overtem-
perature, remote overtemperature, local undertempera-
ture, or remote undertemperature. OVERTasserts when
the temperature rises above the value in either of the two
OVERTlimit registers. The OVERToutput can be used to
activate a cooling fan, or to trigger a system shutdown.
Measurements can be done autonomously, at the pro-
grammed conversion rate, or in a single-shot mode. The
adjustable conversion rate allows optimizing supply cur-
rent and temperature update rate to match system needs.
Remote accuracy is ±1°C maximum error between +60°C
and +145°C with no calibration needed. The
MAX6646/MAX6647/MAX6649 operate from -55°C to
+125°C, and measure temperatures between 0°C and
+145°C. The MAX6646/MAX6647/MAX6649 are available
in an 8-pin µMAX package.
ApplicationsGraphics Processors
Desktop Computers
Notebook Computers
Servers
Thin Clients
Workstations
Test and Measurement
Multichip Modules
FeaturesDual Channel: Measures Remote and Local
Temperature0.125°C ResolutionHigh Accuracy ±1°C (max) from +60°C to +145°C
(Remote), and ±2°C (max) from +60°C to +100°C
(Local)Measures High-Ideality Thermal Diodes Up to
+170°C (Apparent)
+145°C (Real)Two Alarm Outputs: ALERTand OVERTProgrammable Under/Overtemperature Alarm
Temperature ThresholdsProgrammable Conversion RateSMBus-Compatible InterfaceSMBus Timeout
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
Ordering Informationypical Operating Circuit19-2540; Rev 1; 7/03
Selector Guide, Pin Configurations, and Functional Diagram
appear at end of data sheet.
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
ABSOLUTE MAXIMUM RATINGSStresses 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 Referenced to GND
VCC...........................................................................-0.3V to +6V
DXP.............................................................-0.3V to (VCC+ 0.3V)
DXN.......................................................................-0.3V to +0.8V
SCLK, SDA, ALERT, OVERT.....................................-0.3V to +6V
SDA, ALERT, OVERTCurrent.............................-1mA to +50mA
DXN Current.......................................................................±1mA
Continuous Power Dissipation (TA= +70°C)
8-Pin µMAX (derate 5.9mW/°C above +70°C).............471mW
ESD Protection (all pins, Human Body Model)................±2000V
Junction Temperature......................................................+150°C
Operating Temperature Range.........................-55°C to +125°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
ELECTRICAL CHARACTERISTICS(VCC= 3.0V to 5.5V, TA= -55°C to +125°C, unless otherwise specified. Typical values are at VCC= 3.3V and TA= +100°C.) (Note 1)
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
ELECTRICAL CHARACTERISTICS (continued)(VCC= 3.0V to 5.5V, TA= -55°C to +125°C, unless otherwise specified. Typical values are at VCC= 3.3V and TA= +100°C.) (Note 1)
Note 1:All parameters tested at a single temperature. Specifications over temperature are guaranteed by design.
Note 2:Timing specifications guaranteed by design.
Note 3:The serial interface resets when SCLK is low for more than tTIMEOUT.
Note 4:A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SCLK’s falling edge.
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
Typical Operating Characteristics(VCC= 3.3V, TA= +25°C, unless otherwise noted.)
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
Detailed DescriptionThe MAX6646/MAX6647/MAX6649 are temperature sen-
sors designed to work in conjunction with a microproces-
sor or other intelligence in thermostatic, process-control,
or monitoring applications. Communication with the
MAX6646/MAX6647/MAX6649 occurs through the
SMBus-compatible serial interface and dedicated alert
and overtemperature outputs. ALERTasserts if the mea-
sured local or remote temperature is greater than the
software-programmed ALERThigh limit or less than theALERTlow limit in the MAX6649. ALERTalso asserts, in
the MAX6649, if the remote-sensing diode pins are short-
ed or unconnected. The overtemperature alarm, OVERT,
asserts if the software-programmed OVERTlimit is
exceeded. OVERTcan be connected to fans, a system
shutdown, a clock throttle control, or other thermal-man-
agement circuitry.
The MAX6646/MAX6647/MAX6649 convert temperatures
to digital data either at a programmed rate or in single
conversions. Temperature data is represented as 11 bits,
with the LSB equal to 0.125°C. The “main” temperature
data registers (at addresses 00h and 01h) are 8-bit regis-
ters that represent the data as 8 bits with the full-scale
reading indicating the diode fault status (Table 1). The
remaining 3 bits of temperature data are available in the
“extended” registers at addresses 11h and 10h (Table 2).
ADC and MultiplexerThe averaging ADC integrates over a 60ms period
(each channel, typically), with excellent noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure each diode’s forward volt-
age and compute the temperature based on this volt-
age. Both channels are automatically converted once
the conversion process has started, either in free-run-
ning or single-shot mode. If one of the two channels is
not used, the device still performs both measurements,
and the results of the unused channel can be ignored.
If the remote-diode channel is unused, connect DXP to
DXN rather than leaving the inputs open.
Pin Description
MAX6646/MAX6647/MAX6649The DXP-DXN differential input voltage range is 0.25V to
0.95V. Excess resistance in series with the remote diode
causes +0.5°C (typ) error per ohm.
Remote Temperature Measurement
RangeThe MAX6646/MAX6647/MAX6649 measure remote
temperatures significantly above the +120°C limit of
many temperature sensors. External diode-connected
transistors work well as temperature sensors up to
approximately +145°C, where accuracy begins to
degrade. Thermal diodes on some CPUs have charac-
teristics that produce “apparent temperatures” far
above actual operating temperatures. The MAX6646/
MAX6647/MAX6649 measure apparent temperatures as
high as +170°C, as long as the actual temperature is
less than +145°C.
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 autonomous mode (RUN=
0) or by writing a one-shot command, both channels are
converted, and the results of both measurements are
available after the end of a conversion. A BUSY status bit
in the status byte indicates that the device is performing a
new conversion. The results of the previous conversion
are always available, even if the ADC is busy.
Low-Power Standby ModeStandby mode reduces the supply current to less than
12µA by disabling the ADC and timing circuitry. Enter
standby mode by setting the RUNbit to 1 in the configu-
ration byte register (Table 6). All data is retained in mem-
ory, and the SMBus interface is active and listening for
SMBus commands. Standby mode is not a shutdown
mode. With activity on the SMBus, the device draws more
supply current (see Typical Operating Characteristics). In
standby mode, the MAX6646/MAX6647/MAX6649 can be
forced to perform A/D conversions through the one-shot
command, regardless of the RUNbit status.
If a standby command is received while a conversion is
in progress, the conversion cycle is truncated, and the
data from that conversion is not latched into a tempera-
ture register. The previous data is not changed and
remains available.
Supply-current drain during the 125ms conversion period
is 250µA (typ). Slowing down the conversion rate reduces
the average supply current (see Typical Operating
Characteristics). Between conversions, the conversion
rate timer consumes 25µA (typ) of supply current. In
standby mode, supply current drops to 3µA (typ).
SMBus Digital Interface From a software perspective, the MAX6646/MAX6647/
MAX6649appear as a set of byte-wide registers that
contain temperature data, alarm threshold values, and
control bits. A standard SMBus-compatible 2-wire serial
interface is used to read temperature data and write
control bits and alarm threshold data.
The MAX6646/MAX6647/MAX6649 employ four standard
SMBus protocols: write byte, read byte, send byte, and
receive byte (Figures 1, 2, and 3). The shorter receive
byte protocol allows quicker transfers, provided that the
correct data register was previously selected by a read
byte instruction. Use caution when using the shorter pro-
tocols in multimaster systems, as a second master could
overwrite the command byte without informing the first
master.
Temperature data can be read from the read internal
temperature (00h) and read external temperature (01h)
registers. The temperature data format for these regis-
ters is 8 bits for each channel, with the LSB representing
1°C (Table 1). The MSB is transmitted first.
An additional 3 bits can be read from the read external
extended temperature register (10h), which extends
the data to 11 bits and the resolution to 0.125°C per
LSB. An additional 3 bits can be read from the read
internal extended temperature register (11h), which
extends the data to 11 bits and the resolution to
0.125°C per LSB (Table 2).
When a conversion is complete, the main temperature
register and the extended temperature register are
updated simultaneously. Ensure that no conversions
are completed between reading the main register and
the extended register, so that both registers contain the
result of the same conversion.
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature AlarmsTo ensure valid extended data, read extended resolu-
tion temperature data using one of the following
approaches:Put the MAX6646/MAX6647/MAX6649 into standby
mode by setting bit 6 of the configuration register to 1.
Initiate a one-shot conversion using command byte
0Fh. When this conversion is complete, read the con-
tents of the temperature data registers.If the MAX6646/MAX6647/MAX6649is in run mode,
read the status byte. If the BUSY bit indicates that a
conversion is in progress, wait until the conversion is
complete (BUSY bit set to zero) before reading the
temperature data. Following a conversion comple-
tion, immediately read the contents of the tempera-
ture data registers. If no conversion is in progress,
the data can be read within a few microseconds,
which is a sufficiently short period of time to ensure
that a new conversion cannot be completed until
after the data has been read.
MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
Alarm Threshold RegistersFour registers store ALERTthreshold values—one high-
temperature (THIGH) and one low-temperature (TLOW)
register each for the local and remote channels. If either
measured temperature equals or exceeds the corre-
sponding ALERTthreshold value, the ALERTinterrupt
asserts.
The MAX6646/MAX6647 local (internal) ALERTTHIGH
register POR state is 0101 0101, or +85°C, while the
remote (external) ALERTTHIGHregister POR state is
0101 1111, or +95°C.The MAX6649 POR state of both
ALERTTHIGHregisters is 0101 0101, or +85°C. The POR
state of the local and remote TLOWregisters for all
devices is 0000 0000, or 0°C.
Two additional registers store remote and local alarm
threshold data corresponding to the OVERToutput. The
values stored in these registers are high-temperature
thresholds. If either of the measured temperatures equals
or exceeds the corresponding alarm threshold value, an
OVERToutput asserts. The MAX6646/MAX6647 local
(internal) OVERTregister POR state is 0101 0101, or
+85°C, while the remote (external) OVERTregister POR
state is 0111 1101, or +125°C. The MAX6649 POR state
of bothOVERTregisters is 0101 0101, or +85°C.
Diode Fault AlarmA continuity fault detector at DXP detects an open cir-
cuit between DXP and DXN, or a DXP short to VCC,
GND, or DXN. If an open or short circuit exists, the
external temperature register is loaded with 1111 1111.
If the fault is an open-circuit fault bit 2 (OPEN), the sta-
tus byte is set to 1. In the MAX6649, ALERTis activated
at the end of the conversion. Immediately after POR,
the status register indicates that no fault is present. If a
fault is present upon power-up, the fault is not indicated
until the end of the first conversion.
ALERTInterruptsThe ALERTinterrupt occurs when the internal or external
temperature reading exceeds a high- or low-temperature
limit (programmed) or in the MAX6649, when the remote
diode is disconnected (for continuity fault detection). The
ALERTinterrupt output signal is latched and can be
cleared only by either reading the status register or by
successfully responding to an alert response address. In
both cases, the alert is cleared if the fault condition no
longer exists. Asserting ALERTdoes not halt automatic
conversion. The ALERToutput is open drain, allowing
multiple devices to share a common interrupt line.
The MAX6646/MAX6647/MAX6649respond to the
SMBus alert response address, an interrupt pointer
return-address feature (see the Alert Response
Address section). Prior to taking corrective action,
always check to ensure that an interrupt is valid by
reading the current temperature.
Fault Queue Register In some systems, it may be desirable to ignore a single
temperature measurement that falls outside the ALERT
limits. Bits 1 and 2 of the fault queue register (address
22h) determine the number of consecutive temperature
faults necessary to set ALERT(see Tables 3 and 4).
Alert Response AddressThe SMBus alert response interrupt pointer provides
quick fault identification for simple slave devices that
lack the complex, expensive logic needed to be a bus
master. Upon receiving an ALERTinterrupt signal, the
host master can broadcast a receive byte transmission
to the alert response slave address (0001 100).
Following such a broadcast, any slave device that gen-
erated an interrupt attempts to identify itself by putting
its own address on the bus.
