Page 1
General Description
The MAX6649 is a precise, two-channel digital temperature sensor. It accurately measures the temperature of
its own die and a remote PN junction, and reports the
temperature in digital form using a 2-wire serial interface. The remote PN junction is typically the emitterbase 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 MAX6649 includes 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 MAX6649 provides two system alarms: ALERT and
OVERT. ALERT asserts when any of four temperature
conditions are violated: local overtemperature, remote
overtemperature, local undertemperature, or remote
undertemperature. OVERT asserts when the temperature
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 singleshot mode. The adjustable conversion rate allows the
user to optimize supply current 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 MAX6649
operates from -55°C to +125°C, and measures temperatures between 0°C and +145°C. The MAX6649 is available in an 8-pin µMAX package.
Applications
Graphics Processors
Desktop Computers
Notebook Computers
Servers
Thin Clients
Workstations
Test and Measurement
Multichip Modules
Features
♦ Dual Channel: Measures Remote and Local
Temperature
♦ 0.125°C Resolution
♦ High Accuracy ±1°C (max) from +60°C to +145°C
(Remote), and ±2°C (max) from +60°C to +100°C
(Local)
♦ Two Alarm Outputs: ALERT and OVERT
♦ Programmable Under/Overtemperature Alarm
Temperature Thresholds
♦ Programmable Conversion Rate
♦ SMBus-Compatible Interface
♦ SMBus Timeout
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
V
CC
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
MAX6649
OVERT
Typical Operating Circuit
19-2540; Rev 0; 8/02
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
PART
TEMP RANGE
PIN-
MEASURED
TEMP RANGE
MAX6649MUA
8 µMAX
0°C to +145°C
SMBus is a trademark of Intel Corp.
Pin Configuration and Functional Diagram appear at end of
data sheet.
PACKAGE
-55°C to +125°C
Page 2
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor 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
V
CC
...........................................................................-0.3V to +6V
DXP.............................................................-0.3V to (V
CC
+ 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 (T
A
= +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)
PARAMETER
CONDITIONS
UNITS
Supply Voltage V
CC
3.0 5.5 V
°C
Temperature Resolution
10 Bits
VCC = 3.3V,
T
A
= +100°C
0.5
VCC = 3.3V, TA =
Remote Temperature Error
V
CC
= 3.3V, TA =
+0°C to +100°C
°C
Local Temperature Error V
CC
= 3.3V
T
A
= 0°C to +125°C
°C
Supply Sensitivity of Temperature
Error
°C/V
Undervoltage Lockout (UVLO)
Threshold
UVLO Falling edge of V
CC
disables ADC 2.4 2.7
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 12 µA
Operating Current During conversion
mA
0.25 conversions per second 40 80
Average Operating Current
2 conversions per second
400
µA
Conversion Time t
CONV
From stop bit to conversion completion 95
156 ms
Conversion Time Error -25
%
DXP and DXN Leakage Current Standby mode 100 nA
High level 80
120
Remote-Diode Source Current I
RJ
Low level 8 10 12
µA
SYMBOL
MIN TYP MAX
0.125
TRJ = +60°C to +145°C -1.0
+60°C to +100°C
T
= +25°C to +145°C -1.6 +1.6
RJ
TRJ = +0°C to +145°C -3.2 +3.2
TA = +60°C to +100°C -2.0 +2.0
-3.0 +3.0
±0.2
250
125
100
+1.0
2.95
0.08
+25
Page 3
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor 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= +100°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
ALERT, OVERT
I
SINK
= 1mA 0.4
Output Low Voltage
I
SINK
= 4mA 0.6
V
Output High Leakage Current VOH = 5.5V 1 µA
SMBus-COMPATIBLE INTERFACE (SCLK AND SDA)
Logic Input Low Voltage V
IL
0.8 V
VCC = 3.0V 2.2
Logic Input High Voltage V
IH
VCC = 5.5V 2.6
V
Input Leakage Current I
LEAK
VIN = GND or V
CC
-1 +1 µA
Output Low-Sink Current I
SINK
VOL = 0.6V 6 mA
Input Capacitance C
IN
5pF
SMBus-COMPATIBLE TIMING (Note 2)
Serial Clock Frequency f
SCLK
(Note 3) 100 kHz
Bus Free Time Between STOP
and START Condition
t
BUF
4.7 µs
START Condition Setup Time 4.7 µs
Repeat START Condition Setup
Time
t
SU:STA
90% to 90% 50 ns
START Condition Hold Time
10% of SDA to 90% of SCLK 4 µs
STOP Condition Setup Time
90% of SCLK to 90% of SDA 4 µs
Clock Low Period t
LOW
10% to 10% 4.7 µs
Clock High Period t
HIGH
90% to 90% 4 µs
Data Setup Time
(Note 4) 250 µs
Receive SCLK/SDA Rise Time t
R
1µs
Receive SCLK/SDA Fall Time t
F
300 ns
Pulse Width of Spike Suppressed
t
SP
050ns
SMBus Timeout
SDA low period for interface reset 25 37 45 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 t
TIMEOUT
.
Note 4: A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SCLK’s falling edge.
t
HD:STA
t
SU:STO
t
HD:DAT
t
TIMEOUT
Page 4
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC= 3.3V, TA= +25°C, unless otherwise noted.)
MAX6649 toc01
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT (µA)
5.04.54.03.5
3.0
3.5
4.0
4.5
5.0
2.5
3.0 5.5
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6649 toc02
CONVERSION RATE (Hz)
OPERATING SUPPLY CURRENT (µA)
4.002.001.000.500.250.13
100
200
300
400
0
0.63
OPERATING SUPPLY CURRENT
vs. CONVERSION RATE
MAX6649 toc03
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
125100755025
-1.5
-0.5
0
0.5
1.0
1.5
2.0
-2.0
0
REMOTE TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
-1.0
TA = +85°C
FAIRCHILD 2N3906
MAX6649 toc04
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
100755025
-1.5
-1.0
-0.5
0
0.5
1.0
-2.0
0 125
LOCAL TEMPERATURE ERROR
vs. DIE TEMPERATURE
MAX6649 toc05
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
10k1k100101
-1
0
1
2
3
4
5
-2
0.1 100k
VCC = SQUARE WAVE APPLIED TO
V
CC
WITH NO BYPASS CAPACITOR
LOCAL ERROR
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
REMOTE ERROR
MAX6649 toc06
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
10k1k10010
-1
0
1
2
3
4
5
6
7
8
9
-2
1 100k
LOCAL TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCY
LOCAL ERROR
REMOTE ERROR
VIN = AC-COUPLED TO DXN
V
IN
= 100mV
P-P
MAX6649 toc07
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
10k1k10010
-0.5
0
0.5
1.0
1.5
2.0
-1.0
1 100k
TEMPERATURE ERROR
vs. DIFFERENTIAL-MODE NOISE FREQUENCY
MAX6649 toc08
DXP-DXN CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
10.0001.000
-4
-3
-2
-1
0
1
-5
0.100 100.000
TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCE
Page 5
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
_______________________________________________________________________________________ 5
Detailed Description
The MAX6649 is a temperature sensor designed to
work in conjunction with a microprocessor or other intelligence in thermostatic, process-control, or monitoring
applications. Communication with the MAX6649 occurs
through the SMBus-compatible 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 softwareprogrammed OVERT limit is exceeded. OVERT can be
connected to fans, a system shutdown, a clock throttle
control, or other thermal-management circuitry.
The MAX6649 converts 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 registers 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 “extend-
ed” 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 voltage and compute the temperature based on this voltage. Both channels are automatically 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 performs 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 leaving the pins open.
Pin Description
PIN NAME FUNCTION
1V
CC
Supply Voltage Input, 3V to 5.5V. Bypass V
CC
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 FLOATING; tie DXP to DXN if no remote diode is used. Place a 2200pF
capacitor between DXP and DXN for noise filtering.
3 DXN
Combined Remote-Diode Current Sink and A/D Negative Input. DXN is internally connected to
ground.
4 OVERT
Overtemperature Alert/Interrupt Output, Open Drain. OVERT is logic low when the temperature is
above the software-programmed threshold.
5 GND Ground
6 ALERT
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
Table 1. Main Temperature Data Register
Format (00h, 01h)
TEMP (°C) DIGITAL OUTPUT
145 1001 0001
130 1000 0010
128 1000 0000
25 0001 1001
0 0000 0000
<0 0000 0000
Diode fault
(short or open)
1111 1111
Page 6
MAX6649
The worst-case 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.
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 automatically 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 configuration byte register (Table 6). All data is retained in memory, 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 MAX6649 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 temperature register. The previous data is not changed and
remains available.
Supply-current drain during the 125ms conversion period 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 MAX6649 appears 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. This device responds to the same
SMBus slave address for access to all functions.
The MAX6649 employs 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 protocols 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 registers 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 (Table 2). An additional 3 bits can be read from
the read internal extended temperature register (10h),
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.
To ensure valid extended data, read extended resolution temperature data using one of the following
approaches:
1) Put the 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 contents of the
temperature data registers.
2) If the MAX6649 is 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
+145°C Precision SMBus-Compatible Remote/
Local Sensor 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)
Page 7
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
_______________________________________________________________________________________ 7
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.
Alarm Threshold Registers
Four registers store ALERT threshold values—one hightemperature (T
HIGH
) and one low-temperature (T
LOW
)
register each for the local and remote channels. If
either measured temperature equals or exceeds the
corresponding ALERT threshold value, the ALERT interrupt asserts.
SMBCLK
AB CDEFG HIJ
K
SMBDATA
t
SU:STA
t
HD:STA
t
LOWtHIGH
t
SU:DAT
t
HD:DAT
t
SU:STO
t
BUF
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
L
M
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: equivalent 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: equivalent to chip-select line
Command Byte: selects
which register you are
reading from
Slave Address: repeated
due to change in dataflow direction
Data Byte: reads from
the register set by the
command byte
Command Byte: sends command 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
S ADDRESS WR ACK COMMAND
7 bits 8 bits
ACK DATA
8 bits
ACK P
1
S ADDRESS WR ACK COMMAND ACK S ADDRESS RD ACK DATA /// P
8 bits7 bits8 bits7 bits
S ADDRESS WR ACK COMMAND ACK P
7 bits 8 bits
S ADDRESS RD ACK DATA /// P
8 bits7 bits
Page 8
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
8 _______________________________________________________________________________________
The POR state of both ALERT T
HIGH
registers is full
scale (0101 0101, or +85°C). The POR state of both
T
LOW
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 0101 0101 or +85°C.
Diode Fault Alarm
A continuity fault detector at DXP detects an open circuit 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 status byte 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 external temperature reading exceeds a high- or low-temperature 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 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 common interrupt line.
The MAX6649 responds 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 generated 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
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 protocol clears the
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 CDEFG
HIJ
SMBDATA
t
SU:STAtHD:STA
t
LOW
t
HIGH
t
SU:DAT
t
SU:STOtBUF
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
I
2
C is a trademark of Philips Corp.
Page 9
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
_______________________________________________________________________________________ 9
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 component 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
MAX6649. 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 MAX6649 incorporates collision avoidance so that
completely asynchronous operation is allowed between
SMBus operations and temperature conversions.
One-Shot
The one-shot command immediately forces a new conversion cycle to begin. If the one-shot command is
received while the MAX6649 is 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 command 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 (disable) interrupts. Bit 6 puts the MAX6649 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 conversion is complete
and if the fault condition 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 T
HIGH
and T
LOW
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 (especially when converting at the fastest rate). In these circumstances, 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 portableequipment applications. The conversion rate byte’s
POR state is 07h or 4Hz. The MAX6649 looks 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.
BIT
NAME
POR
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 1
Fault queue-length control
bit (see Table 4).
1 FQ0 1
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 1
Table 4. Fault Queue Length Bit Definition
STATE
Page 10
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
10 ______________________________________________________________________________________
Valid A/D conversion results for both channels are available one total conversion time (125ms nominal, 156ms
maximum) after initiating a conversion, whether conversion is initiated through the RUN bit, one-shot command, or initial power-up. Changing the conversion rate
can also affect the delay until new results are available.
Slave Addresses
The MAX6649 has a fixed address of 1001 100. The
MAX6649 also responds 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 VCCand
clears the memory if VCCfalls below 2.0V (typ). When
power is first applied and VCCrises above 2.0V (typ),
the logic blocks begin operating, although reads and
REGISTER
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
RWOE 19h 0101 0101 +85°C Read/write remote (external) OVERT limit (MAX6649)
RWOI 20h 0101 0101 +85°C Read/write local (internal) OVERT limit
HYS 21h 0000 1010 10°C Overtemperature hysteresis
QUEUE 22h 1000 0110 — Fault queue
— FEh 0100 1101 — Read manufacture ID
— FFh 0101 1001 — Read revision ID
Table 5. Command-Byte Bit Assignments
BIT NAME
FUNCTION
MASK 0 Masks ALERT interrupts when set to 1.
6 RUN 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)
ADDRESS
7 (MSB)
POR STATE
Page 11
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
______________________________________________________________________________________ 11
writes at VCClevels below 3V are not recommended. A
second VCCcomparator, the ADC UVLO comparator,
prevents the ADC from converting until there is sufficient 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) T
HIGH
limit is set to +85°C.
• Local (internal) T
LOW
limit is set to 0°C.
• Remote (external) T
HIGH
limit is set to +85°C.
• Remote (external) T
LOW
limit is set to 0°C.
• OVERT limit is set to +85°C.
Applications Information
Remote-Diode Selection
The MAX6649 can directly measure the die temperature of CPUs and other ICs that have on-board temperature-sensing diodes (see Typical Operating Circuit),
or it can measure the temperature of a discrete diodeconnected transistor.
Effect of Ideality Factor
The accuracy of the remote temperature measurements
depends on the ideality factor (n) of the remote “diode”
(actually a transistor). The MAX6649 is 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
different. Fortunately, the difference is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor n
NOMINAL
is used to measure the tem-
perature of a diode with a different ideality factor n
1
.
The measured temperature TMcan be corrected using:
Where temperature is measured in Kelvin.
TT
n
n
M ACTUAL
NOMINAL
=
1
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)
BIT
NAME
POR
STATE
FUNCTION
7 (MSB)
0 A/D is busy converting when 1.
6
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
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
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
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
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)
Intel and Pentium are registered trademarks of Intel Corp.
BUSY
LHIGH
LLOW
RHIGH
RLOW
FAULT
Page 12
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
12 ______________________________________________________________________________________
As mentioned above, the nominal ideality factor of the
MAX6649 is 1.008. As an example, assume you want to
use the MAX6649 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 measured 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 additional 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 connected together. Table 9 lists examples of discrete transistors that are appropriate for use with the MAX6649.
The transistor must be a small-signal type with a relatively 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 temperature,
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 consistent
VBEcharacteristics.
ADC Noise Filtering
The integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-supply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote measurements. 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 larger values can introduce errors due
to the rise time of the switched current source.
PC Board Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6649 as close as is practical 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 introduce 30°C error, even with good filtering.
3 0 453 1 36ΩΩ×
°
=°..
C
C
90
198 6
0 453
µ
µ
°
=
°
V
V
C
C
Ω
Ω
.
.
∆VR A A AR
MS S
=µµ=µ×−()100 10 90
TT
n
n
TT
ACTUAL M
NOMINAL
MM
=
=
=
.
.
(. )
1
1 008
1 002
1 00599
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).
Page 13
3) 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 20MΩ leakage 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 possible 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 negligible 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 200Ω resistor in series with V
CC
for best noise
filtering (see Typical Operating Circuit).
8) Copper cannot be used as an EMI shield; only ferrous 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 microphones. For example, Belden 8451 works well for distances 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 capacitance 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 1Ω of series resistance, the error is approximately 0.5°C.
Thermal Mass and Self-Heating
When sensing local temperature, this device is intended
to measure the temperature of the PC board to which it
is 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 measurements
impractical. Because the thermal mass of the PC board
is far greater than that of the MAX6649, the device
follows 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 virtually no effect; the measured temperature of the junction
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sensors, smaller packages, such as SOT23s, yield 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 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 V
CC
=
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
ø
J-A
for the 8-pin µMAX package is about +221°C/W,
so assuming no copper PC board heat sinking, the
resulting temperature rise is:
∆T = 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.
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
______________________________________________________________________________________ 13______________________________________________________________________________________ 13
MINIMUM
10MILS
10MILS
10MILS
10MILS
GND
DXN
DXP
GND
Figure 4. Recommended DXP-DXN PC Traces
Page 14
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
14 ______________________________________________________________________________________
1
2
3
4
8
7
6
5
SCLK
SDA
GND
DXN
DXP
V
CC
MAX6649
µMAX
TOP VIEW
OVERT
ALERT
Pin Configuration
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
V
CC
S
R
Q
OVERT
ALERT
MAX6649
OVERT THRESHOLD
Functional Diagram
Chip Information
TRANSISTOR COUNT: 14,764
PROCESS: BiCMOS
Page 15
MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor 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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
8LUMAXD.EPS
PACKAGE OUTLINE, 8L uMAX/uSOP
1
1
21-0036
J
REV.DOCUMENT CONTROL NO.APPROVAL
PROPRIETARY INFORMATION
TITLE:
MAX
0.043
0.006
0.014
0.120
0.120
0.198
0.026
0.007
0.037
0.0207 BSC
0.0256 BSC
A2
A1
c
e
b
A
L
FRONT VIEW
SIDE VIEW
E H
0.6±0.1
0.6±0.1
ÿ 0.50±0.1
1
TOP VIEW
D
8
A2
0.030
BOTTOM VIEW
1
6∞
S
b
L
H
E
D
e
c
0∞
0.010
0.116
0.116
0.188
0.016
0.005
8
4X S
INCHES
-
A1
A
MIN
0.002
0.950.75
0.5250 BSC
0.25 0.36
2.95 3.05
2.95 3.05
4.78
0.41
0.65 BSC
5.03
0.66
6∞0∞
0.13 0.18
MAX
MIN
MILLIMETERS
- 1.10
0.05 0.15
α
α
DIM
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