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MAX6657MSA-MAX6658MSA-MAX6659MEE
1C / SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature Alarms
General DescriptionThe MAX6657/MAX6658/MAX6659 are precise, two-
channel digital temperature sensors. Each accurately
measures the temperature of its own die and one
remote PN junction, and reports the temperature in digi-
tal form on a 2-wire serial interface. The remote junction
can be a diode-connected transistor like the low-cost
NPN type 2N3904 or 2N3906 PNP type. The remote
junction can also be a common-collector PNP, such as
a substrate PNP of a microprocessor.
The 2-wire serial interface accepts standard System
Management Bus (SMBus™) commands such as Write
Byte, Read Byte, Send Byte, and Receive Byte to read
the temperature data and program the alarm thresholds
and conversion rate. The MAX6657/MAX6658/
MAX6659 can function autonomously with a program-
mable conversion rate, which allows the control of sup-
ply current and temperature update rate to match
system needs. For conversion rates of 4Hz or less, the
temperature is represented in extended mode as 10
bits + sign with a resolution of 0.125°C. When the con-
version rate is faster than 4Hz, output data is 7 bits +
sign with a resolution of 1°C. The MAX6657/
MAX6658/MAX6659 also include an SMBus timeout
feature to enhance system reliability.
Remote accuracy is ±1°C between +60°C and +100°C
with no calibration needed. The MAX6657 measures
temperatures from 0°C to +125°C and the MAX6658/
MAX6659 from -55°C to +125°C. The MAX6659 has the
added benefit of being able to select one of three
addresses through an address pin, and a second over-
temperature alarm pin for greater system reliability.
ApplicationsDesktop ComputersWorkstations
Notebook Computers
Servers
FeaturesDual Channel: Measures Remote and Local
Temperature11-Bit, 0.125°C ResolutionHigh Accuracy ±1°C (max) from +60°C to +100°C
(Remote)No Calibration RequiredProgrammable Under/Overtemperature AlarmsProgrammable Conversion Rate
(0.0625Hz to 16Hz)SMBus/I2C™-Compatible InterfaceTwo Alarm Outputs: ALERTand OVERT1
(MAX6657 and MAX6658)Three Alarm Outputs: ALERT, OVERT1,
and OVERT2(MAX6659)
MAX6657/MAX6658/MAX6659
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms19-2034; Rev 2; 3/02
Ordering Information
Pin Configurations
Typical Operating Circuit appears at the end of the
data sheet.
MAX6657/MAX6658/MAX6659
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
SMBCLK, SMBDATA, ALERT, OVERT1,OVERT2..............................................................-0.3V to +6V
SMBDATA, ALERT, OVERT1, OVERT2
Current ..........................................................-1mA to +50mA
DXN Current ......................................................................±1mA
Continuous Power Dissipation (TA= +70°C)
8-Pin SO (derate 5.9mW/°C above +70°C) .................471mW
16-Pin QSOP (derate 8.3mW/°C above +70°C) ..........664mW
Junction Temperature .....................................................+150°C
Storage Temperature Range ............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................+300°C
ELECTRICAL CHARACTERISTICS(Circuit of Typical Operating Circuit, VCC= +3.0V to +5.5V, TA= 0°C to +125°C, unless otherwise specified. Typical values are at
VCC= +3.3V and TA= +25°C.)
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
MAX6657/MAX6658/MAX6659
Note 2:If both the local and the remote junction are below TA= -20°C, then VCC> 3.15V.
Note 3:For conversion rates of 4Hz or slower, the conversion time doubles.
Note 4:Timing specifications guaranteed by design.
Note 5:The serial interface resets when SMBCLK is low for more than tTIMEOUT.
Note 6:A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SMBCLK's falling edge.
ELECTRICAL CHARACTERISTICS (continued)(Circuit of Typical Operating Circuit, VCC= +3.0V to +5.5V, TA= 0°C to +125°C, unless otherwise specified. Typical values are at
VCC= +3.3V and TA= +25°C.)
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
MAX6657/MAX6658/MAX6659
Typical Operating Characteristics(VCC= +3.3V, TA= +25°C, unless otherwise noted.)
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
MAX6657/MAX6658/MAX6659
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
MAX6657/MAX6658/MAX6659
Detailed DescriptionThe MAX6657/MAX6658/MAX6659 are temperature
sensors designed to work in conjunction with a micro-
processor or other intelligence in thermostatic,
process-control, or monitoring applications. Com-
munication with the MAX6657/MAX6658/MAX6659
occurs through the SMBus serial interface and dedicat-
ed alert pins. Two independent overtemperature alarms
(OVERT1and OVERT2) are asserted if their software
programmed temperature thresholds are exceeded.
OVERT1and OVERT2can be connected to fans, a sys-
tem shutdown, or other thermal management circuitry.
The MAX6657/MAX6658/MAX6659 convert tempera-
tures to digital data either at a programmed rate or a
single conversion. Conversions have a 0.125°C resolu-
tion (extended resolution) or 1°C resolution (legacy res-
olution). Extended resolution represents temperature as
10 bits + sign bit and is available for autonomous con-
versions that are 4Hz and slower and single-shot con-
versions. Legacy resolution represents temperature as
7 bits + sign bit and allows for faster autonomous con-
version rates of 8Hz and 16Hz.
ADC and MultiplexerThe averaging ADC integrates over a 60ms period
(each channel, typically, in the 7-bit + sign legacy
mode). Using an averaging ADC attains 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. If the remote channel is not used, connect DXP to
DXN. Do not leave DXP and DXN unconnected. When a
conversion is initiated, both channels are converted
Functional Diagram
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
whether they are used or not. The DXN input is biased
at one VBEabove ground by an internal diode to set up
the ADC inputs for a differential measurement.
Resistance in series with the remote diode causes
about +1/2°C error per ohm.
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 register shows that
the device is actually performing a new conversion. The
results of the previous conversion sequence are still
available when the ADC is busy.
Remote-Diode SelectionThe MAX6657/MAX6658/MAX6659 can directly mea-
sure the die temperature of CPUs and other ICs that
have on-board temperature-sensing diodes (see
Typical Operating Circuit) or they can measure the tem-
perature of a discrete diode-connected transistor. The
type of remote diode used is set by bit 5 of the
Configuration Byte. If bit 5 is set to zero, the remote
sensor is a diode-connected transistor, and if bit 5 is set
to 1, the remote sensor is a substrate or common collec-
tor PNP transistor. For best accuracy, the discrete tran-
sistor should be a small-signal device with its collector
and base connected together. Accuracy has been
experimentally verified for all the devices listed in Table1.
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 at
the highest expected temperature must be greater than
0.25V at 10µA, and at the lowest expected tempera-
ture, forward voltage must be less than 0.95V at 100µA.
Large power transistors must not be used. Also, ensure
that the base resistance is less than 100Ω. Tight speci-
fications for forward current gain (50 < β< 150, for
example) indicate that the manufacturer has good
process controls and that the devices have consistent
VBEcharacteristics.
Thermal Mass and Self-HeatingWhen sensing local temperature, these devices are
intended to measure the temperature of the PC board
to which they are soldered. The leads provide a good
thermal path between the PC board traces and the die.
Thermal conductivity between the die and the ambient
air is poor by comparison, making air temperature mea-
surements impractical. Because the thermal mass of
the PC board is far greater than that of the MAX6657/
MAX6658/MAX6659, the devices follow temperature
changes on the PC board with little or no perceivable
delay.
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, thermal mass has virtu-
ally no effect; the measured temperature of the junction
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sen-
sors, smaller packages (i.e., a SOT23) yield the best
thermal response times. Take care to account for ther-
mal gradients between the heat source and the sensor,
and ensure that stray air currents across the sensor
package do not interfere with measurement accuracy.
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, with VCC=
+5.0V, a 16Hz conversion rate and ALERTsinking
1mA, the typical power dissipation is:
VCCx 450µA + 0.4V x 1mA = 2.65mW
θJ-Afor the 8-pin SO package is about +170°C/W, so
assuming no copper PC board heat sinking, the result-
ing temperature rise is:
∆T = 2.65mW x +170°C/W = +0.45°C
Even under these engineered circumstances, it is diffi-
cult to introduce significant self-heating errors.
ADC Noise FilteringThe integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-sup-
ply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote mea-
MAX6657/MAX6658/MAX6659
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
MAX6657/MAX6658/MAX6659surements. The noise can be reduced with careful PC
board layout and proper external noise filtering.
High-frequency EMI is best filtered at DXP and DXN
with an external 2200pF capacitor. Larger capacitor
values can be used for added filtering, but do not
exceed 3300pF because it can introduce errors due to
the rise time of the switched current source.
PC Board LayoutFollow these guidelines to reduce the measurement
error of the temperature sensors:Place the MAX6657/MAX6658/MAX6659 as close
as is practical to the remote diode. In noisy environ-
ments, such as a computer motherboard, this dis-
tance can be 4in to 8in (typ). This length can be
increased if the worst noise sources are avoided.
Noise sources include CRTs, clock generators,
memory buses, and ISA/PCI buses.Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce +30°C error, even with good filtering.Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any higher
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. If high-voltage
traces are unavoidable, connect guard traces to GND
on either side of the DXP-DXN traces (Figure 1).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. 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. Adding a few thermocouples causes a negli-
gible error.Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10mil widths
and spacings that are recommended in Figure 1 are
not absolutely necessary, as they offer only a minor
improvement in leakage and noise over narrow
traces. Use wider traces when practical.Add a 200Ωresistor in series with VCCfor best
noise filtering (see Typical Operating Circuit).
Twisted-Pair and Shielded CablesUse a twisted-pair cable to connect the remote sensor
for remote-sensor distances longer than 8in or in very
noisy environments. Twisted-pair cable lengths can be
between 6ft and 12ft before noise introduces excessive
errors. For longer distances, the best solution is a
shielded twisted pair like that used for audio micro-
phones. For example, Belden #8451 works well for dis-
tances up to 100ft in a noisy environment. At the
device, connect the twisted pair to DXP and DXN and
the shield to GND. Leave the shield unconnected at the
remote sensor.
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the 2200pF
capacitor can often be removed or reduced in value.
Cable resistance also affects remote-sensor accuracy.
For every 1Ωof series resistance, the error is approxi-
mately +1/2°C.
Low-Power Standby ModeStandby mode reduces the supply current to less than
10µA by disabling the ADC. Enter hardware standby
(MAX6659 only) by forcing the STBYpin low, or enter
software standby by setting the RUN/STOP bit to 1 in
the Configuration Byte register. Hardware and software
standbys are very similar—all data is retained in memo-
ry,and the SMB interface is alive and listening for
SMBus commands. The only difference is that in soft-
ware standby mode, the one-shot command initiates a
conversion. With hardware standby, the one-shot com-
mand is ignored. Activity on the SMBus causes the
device to draw extra supply current.
Driving the STBYpin low overrides any software con-
version command. If a hardware or software standby
command is received while a conversion is in progress,
the conversion cycle is interrupted, and the tempera-
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
ture registers are not updated. The previous data is not
changed and remains available.
SMBus Digital Interface From a software perspective, each of the MAX6657/
MAX6658/MAX6659 appears as a series of 8-bit regis-
ters 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
device responds to the same SMBus slave address for
access to all functions.
The MAX6657/MAX6658/MAX6659 employ four stan-
dard SMBus protocols: Write Byte, Read Byte, Send
Byte, and Receive Byte (Figures 2, 3, and 4). The short-
er Receive Byte protocol allows quicker transfers, pro-
vided that the correct data register was previously
selected by a Read Byte instruction. Use caution with
the shorter protocols in multimaster systems, since a
second master could overwrite the command byte with-
out informing the first master.
When the conversion rate is greater than 4Hz, temperature
data can be read from the Read InternalTemperature
(00h) and Read External Temperature (01h) registers.
The temperature data format is 7 bits + sign in two's-
complement form for each channel, with the LSB repre-
senting 1°C (Table 2). The MSB is transmitted first.
When the conversion rate is 4Hz or less, the first 8 bits
of temperature data can be read from the Read Internal
Temperature (00h) and Read External Temperature
(01h) registers, the same as for faster conversion rates.
An additional 3 bits can be read from the Read External
Extended Temperature (10h) and Read Internal
Extended Temperature (11h) registers, which extends
the data to 10 bits + sign and the resolution to
+0.125°C per LSB (Table 3).
When a conversion is complete, the Main register and
the Extended register are updated almost simultane-
ously. Ensure that no conversions are completed
between reading the Main and Extended registers so
that when data that is read, both registers contain the
result of the same conversion.
To ensure valid extended data, read extended resolu-
tion temperature data using one of the following
approaches:Put the MAX6657/MAX6658/MAX6659 into standby
mode by setting bit 6 of the Configuration register to
MAX6657/MAX6658/MAX6659
±1°C, SMBus-Compatible Remote/Local Temperature
Sensorswith Overtemperature Alarms
