For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
General Description
The MAX6648/MAX6692 are precise, two-channel digi-
tal temperature sensors. They accurately 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 junction 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
Management 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 MAX6648/MAX6692
include an SMBus timeout. A fault queue prevents the
ALERT and OVERT outputs from setting until a fault has
been detected one, two, or three consecutive times
(programmable).
The MAX6648/MAX6692 provide two system alarms:
ALERT and OVERT. ALERT asserts when any of four tem-
perature conditions are violated: local overtemperature,
remote overtemperature, local undertemperature, or
remote undertemperature. OVERT asserts when the tem-
perature rises above the value in either of the two OVERT
limit registers. The OVERT output can be used to activate
a cooling fan, or to trigger a system shutdown.
Measurements can be done autonomously, with the
conversion rate programmed by the user, or in a single-
shot mode. The adjustable conversion rate allows the
user to optimize supply current and temperature
update rate to match system needs.
Remote accuracy is ±0.8°C maximum error between
+25°C and +125°C with no calibration needed. The
MAX6648/MAX6692 operate from -55°C to +125°C, and
measure temperatures between 0°C and +125°C. The
MAX6648 is available in an 8-pin µMAX®package, and the
MAX6692 is available in 8-pin µMAX and SO packages.
Applications
Desktop Computers
Notebook Computers
Servers
Thin Clients
Workstations
Test and Measurement
Multichip Modules
Features
oDual Channel Measures Remote and Local
Temperature
o+0.125°C Resolution
oHigh Accuracy ±0.8°C (max) from +25°C to +125°C
(Remote), and ±2°C (max) from +60°C to +100°C
(Local)
oTwo Alarm Outputs: ALERT and OVERT
oTwo Default OVERT Thresholds Available
MAX6648: +110°C
MAX6692: +85°C
oProgrammable Conversion Rate
oSMBus-Compatible Interface
oSMBus Timeout
oProgrammable Under/Overtemperature Alarm
Thresholds
oCompatible with 90nm, 65nm, and 45nm Process
Technology
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
________________________________________________________________________________________________________________________________
Maxim Integrated Products
1
Ordering Information
VCC
DXP
DXN
10kΩ EACH
CLOCK
TO FAN DRIVER OR
SYSTEM SHUTDOWN
3.3V
DATA
INTERRUPTED TO μP
200Ω
0.1μF
SDA
SCLK
ALERT
GND
2200pF
μP
OVERT
MAX6648
MAX6692
Typical Operating Circuit
19-2545; Rev 4; 6/08
PART PIN-
PACKAGE
MEASURED
TEMP RANGE
MAX6648MUA 8 µMAX 0°C to +125°C
MAX6648YMUA 8 µMAX 0°C to +125°C
MAX6692MUA 8 µMAX 0°C to +125°C
MAX6692MSA 8 SO 0°C to +125°C
MAX6692YMUA 8 µMAX 0°C to +125°C
MAX6692YMSA 8 SO 0°C to +125°C
SMBus is a trademark of Intel Corp.
µMAX is a registered trademark of Maxim Integrated Products, Inc.
Pin Configuration and Functional Diagram appear at end of
data sheet.
Note: All devices operate over the -55°C to +125°C temperature
range.
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
2______________________________________________________________________________________________________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Stresses 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, OVERT Current .............................-1mA to +50mA
DXN Current .......................................................................±1mA
Continuous Power Dissipation (TA= +70°C)
8-Pin µMAX (derate 5.9mW/°C above +70°C) .............471mW
8-Pin SO (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= +85°C.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage VCC 3.0 5.5 V
0.125 °C
Temperature Resolution 10 Bits
VCC = 3.3V,
TA = +85°C TRJ = +25°C to +125°C -0.8 +0.8
TRJ = +60°C to +100°C -1.0 +1.0
VCC = 3.3V,
+60°C ≤ TA ≤
+100°C TRJ = 0°C to +125°C -1.6 +1.6
Remote Temperature Error
n = 1.008
VCC = 3.3V, +0°C
≤ TA ≤ +100°C TRJ = 0°C to +125°C -3.0 +3.0
°C
TA = +60°C to +100°C -2.0 +2.0
Local Temperature Error VCC = 3.3V TA = 0°C to +125°C -3.0 +3.0 °C
TA = + 60°C to + 100°C - 4.0
Local Temperature Error
(MAX6648Y/MAX6692Y) V
C C = 3.3V TA = 0°C to +125°C -4.4 °C
Supply Sensitivity of Temperature
Error ±0.2 °C/V
Undervoltage Lockout (UVLO)
Threshold UVLO Falling edge of VCC disables ADC 2.4 2.7 2.95 V
UVLO Hysteresis 90 mV
Power-On-Reset (POR) Threshold VCC falling edge 2.0 V
POR Threshold Hysteresis 90 mV
Standby Supply Current SMBus static 3.5 12 µA
Operating Current During conversion 0.45 0.8 mA
0.25 conversions per second 40 80
Average Operating Current 2 conversions per second 250 400 µA
Conversion Time tCONV From stop bit to conversion completion 95 125 156 ms
Conversion Time Error -25 +25 %
DXP and DXN Leakage Current Standby mode 100 nA
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
______________________________________________________________________________________________________________________________________________________________________________3
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= +85°C.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
High level 80 100 120
Remote-Diode Source Current IRJ Low level 8 10 12 µA
ALERT, OVERT
ISINK = 1mA 0.4
Output Low Voltage ISINK = 4mA 0.6 V
Output High Leakage Current VOH = 5.5V 1 µA
SMBus-COMPATIBLE INTERFACE (SCLK AND SDA)
Logic Input Low Voltage VIL 0.8 V
VCC = 3.0V 2.2
Logic Input High Voltage VIH VCC = 5.5V 2.6 V
Input Leakage Current ILEAK VIN = GND or VCC -1 +1 µA
Output Low-Sink Current ISINK VOL = 0.6V 6 mA
Input Capacitance CIN 5pF
SMBus-COMPATIBLE TIMING (Note 2)
Serial Clock Frequency fSCLK (Note 3) 100 kHz
Bus Free Time Between STOP and
START Condition tBUF 4.7 µs
START Condition Setup Time 4.7 µs
Repeat START Condition Setup
Time tSU:STA 90% to 90% 50 ns
START Condition Hold Time tHD:STA 10% of SDA to 90% of SCLK 4 µs
STOP Condition Setup Time tSU:STO 90% of SCLK to 90% of SDA 4 µs
Clock Low Period tLOW 10% to 10% 4.7 µs
Clock High Period tHIGH 90% to 90% 4 µs
Data Setup Time tHD:DAT (Note 4) 250 µs
Receive SCLK/SDA Rise Time tR1µs
Receive SCLK/SDA Fall Time tF300 ns
Pulse Width of Spike Suppressed tSP 050ns
SMBus Timeout tTIMEOUT SDA low period for interface reset 25 37 55 ms
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.
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
4______________________________________________________________________________________________________________________________________________________________________________
Typical Operating Characteristics
(VCC = 3.3V, TA= +25°C, unless otherwise noted.)
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6648/92 toc01
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT (μA)
5.04.54.03.5
2.8
3.2
3.6
4.0
2.4
3.0 5.5
OPERATING SUPPLY CURRENT
vs. CONVERSION RATE
MAX6648/92 toc02
CONVERSION RATE (Hz)
OPERATING SUPPLY CURRENT (μA)
4.002.001.000.500.250.13
100
200
300
400
500
600
0
0.63
REMOTE TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
MAX6648/92 toc03
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
100755025
-1.5
-0.5
0.5
1.5
2.5
-2.5
0125
TA = +85°C
FAIRCHILD 2N3906
LOCAL TEMPERATURE ERROR
vs. DIE TEMPERATURE
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1.0
-1.0
MAX6648/92 toc05
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
1007550250125
REMOTE TEMPERATURE ERROR
vs. 45nm REMOTE DIODE TEMPERATURE
MAX6648/92 toc04
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
90807060
-4
-2
0
2
4
6
-6
50 100
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6648/92 toc06
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
10k1k110 100
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0
0.1 100k
LOCAL ERROR
REMOTE ERROR
VIN = SQUARE WAVE APPLIED TO VCC
WITH NO 0.1μF VCC CAPACITOR
-1
0
1
2
3
4
5
6
7
8
9
-2
TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCY
MAX6648/92 toc07
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
100k10k10 100 1k1
REMOTE ERROR
LOCAL ERROR
VIN = AC-COUPLED TO DXN
VIN = 100mVP-P
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
-2.0
TEMPERATURE ERROR
vs. DIFFERENTIAL-MODE NOISE FREQUENCY
MAX6648/92 toc08
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
100k10k10 100 1k1
VIN = 20mVP-P SQUARE WAVE
APPLIED TO DXP-DXN
TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCE
MAX6648/92 toc09
DXP-DXN CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
10.0001.000
-5
-4
-3
-2
-1
0
1
-6
0.100 100.000
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
______________________________________________________________________________________________________________________________________________________________________________5
Detailed Description
The MAX6648/MAX6692 are temperature sensors
designed to work in conjunction with a microprocessor
or other intelligence in thermostatic, process-control, or
monitoring applications. Communication with the
MAX6648/MAX6692 occurs through the SMBus-com-
patible serial interface and dedicated alert pins. ALERT
asserts if the measured local or remote temperature is
greater than the software-programmed ALERT high
limit or less than the ALERT low limit. ALERT also
asserts if the remote-sensing diode pins are shorted or
unconnected. The overtemperature alarm, OVERT,
asserts if the software-programmed OVERT limit is
exceeded. OVERT can be connected to fans, a system
shutdown, a clock throttle control, or other thermal-
management circuitry.
The MAX6648/MAX6692 convert temperatures to digital
data either at a programmed rate or in single conver-
sions. Temperature data is represented as 10 bits plus
sign, with the LSB equal to 0.125°C. The “main” tempera-
ture data registers (at addresses 00h and 01h) are 8-bit
registers that represent the data as 7 bits with the final
MSB 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 Multiplexer
The 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 user can ignore the results of the unused chan-
Pin Description
PIN NAME FUNCTION
1V
CC Supply Voltage Input, 3V to 5.5V. Bypass VCC to GND with a 0.1µF capacitor. A 200Ω series
resistor is recommended but not required for additional noise filtering.
2 DXP
Combined Remote-Diode Current Source and A/D Positive Input for Remote-Diode Channel. DO
NOT LEAVE DXP DISCONNECTED; connect DXP to DXN if no remote diode is used. Place a
2200pF capacitor between DXP and DXN for noise filtering.
3DXN
Combined Remote-Diode Current Sink and A/D Negative Input. DXN is internally biased to one
diode drop above ground.
4OVERT Overtemperature Alert/Interrupt Output, Open Drain. OVERT is logic low when the temperature is
above the software-programmed threshold.
5 GND Ground
6ALERT
SMBus Alert (Interrupt) Output, Open Drain. ALERT asserts when temperature exceeds user-set
limits (high or low temperature). ALERT stays asserted until acknowledged by either reading the
status register or by successfully responding to an alert response address, provided that the fault
condition no longer exists. See the
ALERT
Interrupts section.
7 SDA SMBus Serial-Data Input/Output, Open Drain
8 SCLK SMBus Serial-Clock Input
TEMP (°C) DIGITAL OUTPUT
130 0 111 1111
127 0 111 1111
126 0 111 1111
25 0 001 1001
0 0 000 0000
<0 0 000 0000
-1 0 000 0000
-25 0 000 0000
Diode fault
(short or open) 1 000 0000
Table 1. Main Temperature Data Register
Format (00h, 01h)
MAX6648/MAX6692
nel. If the remote-diode channel is unused, connect
DXP to DXN rather than leaving the pins open.
The DXN input is biased to one VBE above ground by
an internal diode to prepare the ADC inputs for a differ-
ential measurement. The worst-case DXP-DXN differen-
tial 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.
A/D Conversion Sequence
A 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 Mode
Standby mode reduces the supply current to less than
10µA by disabling the ADC and timing circuitry. Enter
standby mode by setting the RUN bit 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 MAX6648/MAX6692 can be forced to
perform A/D conversions through the one-shot command,
regardless of the RUN bit 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 peri-
od is 500µA (typ). Slowing down the conversion rate
reduces the average supply current (see
Typical
Operating Characteristics
). Between conversions, the
conversion rate timer consumes about 25µA of supply
current. In standby mode, supply current drops to
about 3µA.
SMBus Digital Interface
From a software perspective, the MAX6648/MAX6692
appear as a set of byte-wide registers that contain tem-
perature 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. These devices respond to the
same SMBus slave address for access to all functions.
The MAX6648/MAX6692 employ four standard SMBus
protocols: write byte, read byte, send byte, and receive
byte (Figures 1, 2, and 3). The shorter receive byte proto-
col allows quicker transfers, provided that the correct
data register was previously selected by a read byte
instruction. Use caution when using the shorter protocols
in multimaster systems, as a second master could over-
write 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 7 bits plus 1 bit, indicating the diode fault status
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 10 bits plus sign and the resolution to 0.125°C
per LSB (Table 2). An additional 3 bits can be read
from the read internal extended temperature register
(11h), which extends the data to 10 bits (plus 1 bit indi-
cating the diode fault status) 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.
To ensure valid extended data, read extended resolu-
tion temperature data using one of the following
approaches:
1) Put the MAX6648/MAX6692 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 contents
of the temperature data registers.
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
6______________________________________________________________________________________________________________________________________________________________________________
FRACTIONAL TEMP (°C) DIGITAL OUTPUT
0.000 000X XXXX
0.125 001X XXXX
0.250 010X XXXX
0.375 011X XXXX
0.500 100X XXXX
0.625 101X XXXX
0.750 110X XXXX
0.875 111X XXXX
Table 2. Extended Resolution Temperature
Register Data Format (10h, 11h)
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
______________________________________________________________________________________________________________________________________________________________________________7
2) If the MAX6648/MAX6692 are 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 completion, immediately
read the contents of the temperature 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.
SMBCLK
AB CD
EFG H
IJK
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tHD:DAT tSU:STO tBUF
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW
LM
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 2. SMBus Write Timing Diagram
Write Byte Format
Read Byte Format
Send Byte Format Receive Byte Format
Slave Address: equiva-
lent to chip-select line of
a 3-wire interface
Command Byte: selects which
register you are writing to
Data Byte: data goes into the register
set by the command byte (to set
thresholds, configuration masks, and
sampling rate)
Slave Address: equiva-
lent to chip-select line
Command Byte: selects
which register you are
reading from
Slave Address: repeated
due to change in data-
flow direction
Data Byte: reads from
the register set by the
command byte
Command Byte: sends com-
mand with no data, usually
used for one-shot command
Data Byte: reads data from
the register commanded
by the last Read Byte or
Write Byte transmission;
also used for SMBus Alert
Response return address
S = Start condition Shaded = Slave transmission
P = Stop condition /// = Not acknowledged
Figure 1. SMBus Protocols
SADDRESS RD ACK DATA /// P
7 bits 8 bits
WRSACK COMMAND ACK P
8 bits
ADDRESS
7 bits
P
1
ACKDATA
8 bits
ACKCOMMAND
8 bits
ACKWRADDRESS
7 bits
S
SADDRESS WR ACK COMMAND ACK SADDRESS
7 bits8 bits7 bits
RD ACK DATA
8 bits
/// P
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
8______________________________________________________________________________________________________________________________________________________________________________
SMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
AB CD
EFG HIJ
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tSU:STO tBUF
LMK
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 3. SMBus Read Timing Diagram
Alarm Threshold Registers
Four registers store ALERT threshold 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
corresponding ALERT threshold value, the ALERT inter-
rupt asserts.
The power-on-reset (POR) state of both ALERT THIGH
registers is full scale (0101 0101, or +85°C). The POR
state of both TLOW registers is 0000 0000, or 0°C.
Two additional registers store remote and local alarm
threshold data corresponding to the OVERT output. 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 OVERT output asserts. The POR state of the
OVERT threshold is 0110 1110 or +110°C for the
MAX6648, and 0101 0101 or +85°C for the MAX6692.
Diode Fault Alarm
A 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 1000 0000.
If the fault is an open-circuit fault bit 2 (OPEN) of the
status byte, it is set to 1 and the ALERT condition is
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.
ALERT
Interrupts
The ALERT interrupt occurs when the internal or exter-
nal temperature reading exceeds a high- or low-tem-
perature limit (user programmed) or when the remote
diode is disconnected (for continuity fault detection).
The ALERT interrupt 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 only if the
fault condition no longer exists. Asserting ALERT does
not halt automatic conversion. The ALERT output pin is
open drain, allowing multiple devices to share a com-
mon interrupt line.
The MAX6648/MAX6692 respond 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 2 and 3 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 Address
The 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 ALERT interrupt 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.
The alert response can activate several different slave
devices simultaneously, similar to the I2C general call. If
more than one slave attempts to respond, bus arbitration
rules apply, and the device with the lower address
code wins. The losing device does not generate an
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
______________________________________________________________________________________________________________________________________________________________________________9
acknowledge and continues to hold the ALERT line low
until cleared. (The conditions for clearing an ALERT
vary, depending on the type of slave device).
Successful completion of the read alert response proto-
col clears the interrupt latch, provided the condition
that caused the alert no longer exists.
OVERT
Overtemperature Alarm/Warning
Outputs
OVERT asserts when the temperature rises to a value
stored in one of the OVERT limit registers (19h, 20h). It
deasserts when the temperature drops below the
stored limit, minus hysteresis. OVERT can be used to
activate a cooling fan, send a warning, invoke clock
throttling, or trigger a system shutdown to prevent com-
ponent damage.
Command Byte Functions
The 8-bit command byte register (Table 5) is the master
index that points to the various other registers within the
MAX6648/MAX6692. The register’s POR state is 0000
0000, so a receive byte transmission (a protocol that
lacks the command byte) that occurs immediately after
POR, returns the current local temperature data.
The MAX6648/MAX6692 incorporate collision avoid-
ance so that completely asynchronous operation is
allowed between SMBus operations and temperature
conversions.
One-Shot
The one-shot command immediately forces a new con-
version cycle to begin. If the one-shot command is
received while the MAX6648/MAX6692 are in standby
mode (RUN bit = 1), a new conversion begins, after
which the device returns to standby mode. If a one-shot
conversion is in progress when a one-shot command is
received, the command is ignored. If a one-shot com-
mand is received in autonomous mode (RUN bit = 0)
between conversions, a new conversion begins, the
conversion rate timer is reset, and the next automatic
conversion takes place after a full delay elapses.
Configuration Byte Functions
The configuration byte register (Table 6) is a read-write
register with several functions. Bit 7 is used to mask (dis-
able) interrupts. Bit 6 puts the MAX6648/MAX6692 into
standby mode (STOP) or autonomous (RUN) mode.
Status Byte Functions
The status byte register (Table 7) indicates which (if
any) temperature thresholds have been exceeded. This
byte also indicates whether the ADC is converting and
whether there is an open-circuit fault detected in the
external sense junction. After POR, the normal state of
all flag bits is zero, assuming no alarm conditions are
present. The status byte is cleared by any successful
read of the status byte, after a conversion is complete
and the fault no longer exists. Note that the ALERT
interrupt latch is not automatically cleared when the
status flag bit indicating the ALERT is cleared. The fault
condition must be eliminated before the ALERT output
can be cleared.
When autoconverting, if the THIGH and TLOW limits are
close together, it is possible for both high-temp and
low-temp status bits to be set, depending on the
amount of time between status read operations (espe-
cially when converting at the fastest rate). In these cir-
cumstances, it is best not to rely on the status bits to
indicate reversals in long-term temperature changes.
Instead use a current temperature reading to establish
the trend direction.
Conversion Rate Byte
The conversion rate register (Table 8) programs the
time interval between conversions in free-running
autonomous mode (RUN = 0). This variable rate control
can be used to reduce the supply current in portable-
equipment applications. The conversion rate byte’s
POR state is 07h or 4Hz. The MAX6648/MAX6692 look
BIT NAME POR
STATE FUNCTION
7 RFU 1 Reserved. Always write 1 to
this bit.
6 to 3 RFU 0 Reserved. Always write
zero to this bit.
2 FQ1 0 Fault queue-length control
bit (see Table 4).
1 FQ0 0 Fault queue-length control
bit (see Table 4).
0 RFU 0 Reserved. Always write
zero to this bit.
Table 3. Fault Queue Register Bit Definition
(22h)
FQ1 FQ0 FAULT QUEUE LENGTH (SAMPLES)
00 1
01 2
11 3
10 —
Table 4. Fault Queue Length Bit Definition
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
10 ____________________________________________________________________________________________________________________________________________________________________________
only at the 3 LSBs of this register, so the upper 5 bits
are don’t care bits, which should be set to zero. The
conversion rate tolerance is ±25% at any rate setting.
Valid A/D conversion results for both channels are avail-
able one total conversion time (125ms nominal, 156ms
maximum) after initiating a conversion, whether conver-
sion is initiated through the RUN bit, one-shot com-
mand, or initial power-up. Changing the conversion rate
can also affect the delay until new results are available.
Slave Addresses
The MAX6648/MAX6692 have a fixed address of 1001
100. The MAX6648/MAX6692 also respond to the
SMBus alert response slave address (see the
Alert
Response Address
section).
POR and UVLO
To prevent ambiguous power-supply conditions from
corrupting the data in memory and causing erratic
behavior, a POR voltage detector monitors VCC and
REGISTER ADDRESS POR STATE FUNCTION
RLTS 00h 0000 0000 0°C Read local (internal) temperature
RRTE 01h 0000 0000 0°C Read remote (external) temperature
RSL 02h N/A — Read status byte
RCL 03h 0000 0000 — Read configuration byte
RCRA 04h 0000 0111 — Read conversion rate byte
RLHN 05h 0101 0101 +85°C Read local (internal) ALERT high limit
RLLI 06h 0000 0000 0°C Read local (internal) ALERT low limit
RRHI 07h 0101 0101 +85°C Read remote (external) ALERT high limit
RRLS 08h 0000 0000 0°C Read remote (external) ALERT low limit
WCA 09h N/A — Write configuration byte
WCRW 0Ah N/A — Write conversion rate byte
WLHO 0Bh N/A — Write local (internal) ALERT high limit
WLLM 0Ch N/A — Write local (internal) ALERT low limit
WRHA 0Dh N/A — Write remote (external) ALERT high limit
WRLN 0Eh N/A — Write remote (external) ALERT low limit
OSHT 0Fh N/A — One-shot
REET 10h 0000 0000 0°C Read remote (external) extended temperature
RIET 11h 0000 0000 0°C Read local (internal) extended temperature
0110 1110 +110°C Read/write remote (external) OVERT limit (MAX6648)
RWOE 19h 0101 0101 +85°C Read/write remote (external) OVERT limit (MAX6692)
RWOI 20h 0101 0101 +85°C Read/write local (internal) OVERT limit
HYS 21h 0000 1010 10°C Overtemperature hysteresis
QUEUE 22h 1000 0000 — Fault queue
— FEh 0100 1101 — Read manufacture ID
— FFh 0101 1001 — Read revision ID
Table 5. Command-Byte Bit Assignments
BIT NAME POR STATE FUNCTION
7 (MSB) MASK 0 Masks ALERT interrupts when set to 1.
6RUN 0 Standby mode control bit; if set to 1, standby mode is initiated.
5 to 0 RFU 0 Reserved.
Table 6. Configuration-Byte Bit Assignments (03h)
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
____________________________________________________________________________________________________________________________________________________________________________11
clears the memory if VCC falls below 2.0V (typ). When
power is first applied and VCC rises above 2.0V (typ),
the logic blocks begin operating, although reads and
writes at VCC levels below 3V are not recommended. A
second VCC comparator, the ADC UVLO comparator
prevents the ADC from converting until there is suffi-
cient headroom (VCC = 2.8V typ).
Power-Up Defaults
Power-up defaults include:
• Interrupt latch is cleared.
• ADC begins autoconverting at a 4Hz rate.
• Command byte is set to 00h to facilitate quick local
temperature receive byte queries.
• Local (internal) THIGH limit set to +85°C.
• Local (internal) TLOW limit set to 0°C.
• Remote (external) THIGH limit set to +85°C.
• Remote (external) TLOW limit set to 0°C.
•OVERT internal limit is set to +85°C; every external
limit is set to +110°C (MAX6648).
•OVERT limits are set to +85°C (MAX6692).
Applications Information
Remote-Diode Selection
The MAX6648/MAX6692 can directly measure the die
temperature of CPUs and other ICs that have on-board
temperature-sensing diodes (see
Typical Operating
Circuit
), or they can measure the temperature of a dis-
crete diode-connected transistor.
Effect of Ideality Factor
The accuracy of the remote temperature measurements
depends on the ideality factor (n) of the remote “diode”
BIT NAME POR
STATE FUNCTION
7 (MSB) BUSY 0 A/D is busy converting when 1.
6 LHIGH 0 Local (internal) high-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
5 LLOW 0 Local (internal) low-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
4 RHIGH 0 Remote (external) high-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
3 RLOW 0 Remote (external) low-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
2 FAULT 0 A 1 indicates DXN and DXP are either shorted or open; cleared by POR or readout of the status
byte if the fault condition no longer exists.
1 EOT 0 A 1 indicates the remote (external) junction temperature exceeds the external OVERT threshold.
0 IOT 0 A 1 indicates the local (internal) junction temperature exceeds the internal OVERT threshold.
Table 7. Status Register Bit Assignments (02h)
DATA CONVERSION
RATE (Hz)
00h 0.0625
01h 0.125
02h 0.25
03h 0.5
04h 1
05h 2
06h 4
07h 4
08h-FFh Reserved
Table 8. Conversion-Rate Control Byte
(04h)
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
12 ____________________________________________________________________________________________________________________________________________________________________________
(actually a transistor). The MAX6648/MAX6692 (not the
MAX6648Y/MAX6692Y) are optimized for n = 1.008,
which is the typical value for the Intel®Pentium®III and
the AMD Athlon MP model 6. If a sense transistor with a
different ideality factor is used, the output data is differ-
ent. Fortunately, the difference is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor nNOMINAL is used to measure the tem-
perature of a diode with a different ideality factor n1.
The measured temperature TMcan be corrected using:
where temperature is measured in Kelvin.
As mentioned above, the nominal ideality factor of the
MAX6648/MAX6692 is 1.008. As an example, assume
you want to use the MAX6648/MAX6692 with a CPU
that has an ideality factor of 1.002.
If the diode has no series resistance, the measured
data is related to the real temperature as follows:
For a real temperature of +85°C (358.15 K), the mea-
sured temperature is +82.91°C (356.02 K), which is an
error of -2.13°C.
Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, change in the measured voltage is:
Since 1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
Assume that the diode being measured has a series
resistance of 3Ω. The series resistance contributes an
offset of:
The effects of the ideality factor and series resistance
are additive. If the diode has an ideality factor of 1.002
and series resistance of 3Ω, the total offset can be cal-
culated by adding error due to series resistance with
error due to ideality factor:
1.36°C - 2.13°C = -0.77°C
for a diode temperature of +85°C.
In this example, the effect of the series resistance and
the ideality factor partially cancel each other.
For best accuracy, the discrete transistor should be a
small-signal device with its collector and base connect-
ed together. Table 9 lists examples of discrete transis-
tors that are appropriate for use with the MAX6648/
MAX6692.
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, the 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 specifications for forward current gain (50 < ß
<150, for example) indicate that the manufacturer has
good process controls and that the devices have con-
sistent VBE characteristics.
Operation with 45nm Substrate PNPs
Small transistor geometries and specialized processes
can affect temperature measurement accuracy.
Parasitic series resistance can be higher, which
increases the measured temperature value. Beta may
3 0 453 1 36ΩΩ
×°=°..
CC
90
198 6
0 453
μ
μ
°
=°
V
V
C
C
Ω
Ω
.
.
ΔVR A A AR
MS S
=μμ=μ×−()100 10 90
TT
n
nTT
ACTUAL M NOMINAL MM
=⎛
⎝
⎜
⎞
⎠
⎟=⎛
⎝
⎜⎞
⎠
⎟=
.
.(. )
1
1 008
1 002 1 00599
TT n
n
M ACTUAL
NOMINAL
=⎛
⎝
⎜
⎞
⎠
⎟
1
Intel and Pentium are registered trademarks of Intel Corp.
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3904
Rohm Semiconductor (USA) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Table 9. Remote-Sensor Transistor
Manufacturers
Note: Transistors must be diode connected (base shorted to
collector).
MAX6648/MAX6692
be low enough to alter the effective ideality factor.
Good results can be obtained if the process is consis-
tent and well behaved. For example, the curve shown
in the Remote Temperature Error vs. 45nm Remote
Diode Temperature graph in the
Typical Operating
Characteristics
section shows the temperature mea-
surement error of the MAX6648/MAX6692 when used
with a typical 45nm CPU thermal diode. Note that the
error is effectively a simple +4°C offset.
ADC Noise Filtering
The 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-
surements. The noise can be reduced with careful PCB
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 larger values can introduce errors due
to the rise time of the switched current source.
PCB Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6648/MAX6692 as close as is practi-
cal to the remote diode. In noisy environments, such
as a computer motherboard, this distance 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.
2) 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.
3) Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any higher
voltage traces, such as 12V DC. Leakage currents
from PCB 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 4).
4) Route through as few vias and crossunders as pos-
sible to minimize copper/solder thermocouple
effects.
5) 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 takes about 200µV of voltage
error at DXP-DXN to cause a 1°C measurement
error. Adding a few thermocouples causes a negligi-
ble error.
6) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10mil widths
and spacing recommended in Figure 4 are not
absolutely necessary, as they offer only a minor
improvement in leakage and noise over narrow
traces. Use wider traces when practical.
7) Add a 200Ωresistor in series with VCC for best noise
filtering (see
Typical Operating Circuit
).
8) Copper cannot be used as an EMI shield; only fer-
rous materials such as steel work well. Placing a
copper ground plane between the DXP-DXN traces
and traces carrying high-frequency noise signals
does not help reduce EMI.
Twisted-Pair and Shielded Cables
Use a twisted-pair cable to connect the remote sensor
for remote-sensor distance 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 0.5°C.
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
______________________________________________________________________________________ 13
MINIMUM
10MILS
10MILS
10MILS
10MILS
GND
DXN
DXP
GND
Figure 4. Recommended DXP-DXN PC Traces
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
14 ____________________________________________________________________________________________________________________________________________________________________________
Thermal Mass and Self-Heating
When sensing local temperature, these devices are
intended to measure the temperature of the PCB to
which they are soldered. The leads provide a good ther-
mal path between the PCB traces and the die. Thermal
conductivity between the die and the ambient air is poor
by comparison, making air temperature measurements
impractical. Because the thermal mass of the PCB is far
greater than that of the MAX6648/MAX6692, the devices
follow temperature changes on the PCB 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, such as SOT23s, 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 current
at the ALERT output. For example, with VCC = 5.0V, at a
4Hz conversion rate and with ALERT sinking 1mA, the
typical power dissipation is:
5.0V x 500µA + 0.4V x 1mA = 2.9mW
θJ-A for the 8-pin µMAX package is about +221°C/W,
so assuming no copper PCB heat sinking, the resulting
temperature rise is:
ΔT = 2.9mW x (+221°C/W) = +0.6409°C
Even under nearly worst-case conditions, it is difficult to
introduce a significant self-heating error.
MUX
REMOTE
LOCAL
ADC
2
CONTROL
LOGIC
SMBus
READ
WRITE
8
8
ADDRESS
DECODER
7
S
R
Q
DIODE
FAULT
DXP
DXN
SMBCLK
SMBDATA
REGISTER BANK
COMMAND BYTE
REMOTE TEMPERATURE
LOCAL TEMPERATURE
ALERT THRESHOLD
ALERT RESPONSE ADDRESS
VCC
S
R
Q
OVERT
ALERT
MAX6648
MAX6692
OVERT THRESHOLD
Functional Diagram
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
____________________________________________________________________________________________________________________________________________________________________________15
1
2
3
4
8
7
6
5
SCLK
SDA
ALERT
GNDOVERT
DXN
*SO PACKAGE AVAILABLE FOR MAX6692 ONLY.
DXP
VCC
MAX6648
MAX6692
μMAX/SO*
TOP VIEW
Pin Configuration Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
8 µMAX U8-1 21-0036
8 SO S8-4 21-0041
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2008 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0—— —
1—— —
2 11/05 — —
3 12/07 Changed max SMBus timeout from 45 to 55; and various style edits. 3, 8, 13, 14
4 6/08 Updated to include 4nm CPU compatibility. 1, 5, 12, 15
