_______________________________________________________________ Maxim Integrated Products 1
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.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
19-5260; Rev 0; 8/10
Ordering Information/Selector Guide
General Description
The MAX6581 precision multichannel temperature
sensor monitors its own temperature and the tempera-
tures of up to seven external diode-connected transis-
tors. All temperature channels have programmable alert
and overtemperature thresholds. When the measured
temperature of a channel crosses the respective thresh-
old, a status bit is set in one of the status registers. Two
open-drain alarm outputs (ALERT and OVERT) assert
corresponding to these bits in the status register(s).
Resistance cancellation is available for all channels and
compensates for high series resistance in circuit-board
traces and thermal diodes.
The 2-wire serial interface accepts SMBus™ protocols
(write byte, read byte, send byte, and receive byte) for
reading the temperature data and programming the
alarm thresholds.
The MAX6581 is specified for an operating temperature
range of -40NC to +125NC and is available in a 24-pin,
4mm x 4mm thin QFN package with an exposed pad.
Features
S Eight Channels to Measure Seven Remote and
One Local Temperature
S 11-Bit, 0.125NC Resolution
S High Accuracy of ±1NC (max) from +60NC to
+100NC (Remote Channels)
S -64NC to +150NC Remote Temperature Range
S Programmable Undertemperature/
Overtemperature Alerts
S SMBus/I2C-Compatible Interface
S Two Open-Drain Alarm Outputs (ALERT and
OVERT)
S Resistance Cancellation on All Remote Channels
Applications
Desktop Computers
Notebook Computers
Workstations
Servers
Data Communications
SMBus is a trademark of Intel Corp.
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
**Future product—contact factory for availability.
Note: These devices operate over the -40NC to +125NC operating temperature range.
Typical Application Circuit appears at end of data sheet.
EVALUATION KIT
AVAILABLE
PART SLAVE ADDRESS PIN-PACKAGE OPERATING
TEMPERATURE RANGE
MEASURED
TEMPERATURE RANGE
MAX6581TG9A+ 0X9A 24 TQFN-EP* -40NC to +125NC -64NC to +150NC
MAX6581TG9C+** 0X9C 24 TQFN-EP* -40NC to +125NC -64NC to +150NC
MAX6581TG9E+** 0X9E 24 TQFN-EP* -40NC to +125NC -64NC to +150NC
MAX6581TG98+** 0X98 24 TQFN-EP* -40NC to +125NC -64NC to +150NC
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
2 ______________________________________________________________________________________
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, SMBCLK, SMBDATA, ALERT,
OVERT, STBY to GND ..........................................-0.3V to +4V
DXP_ to GND ............................................ -0.3V to (VCC + 0.3V)
DXN_ to GND ........................................... -0.3V to (VCC + 0.3V)
SMBDATA, ALERT, OVERT Current .................. -1mA to +50mA
DXN_ Current .................................................................... Q1mA
Continuous Power Dissipation (TA = +70NC)
24-Pin Thin QFN (derate 27.8mW/NC above +70NC) .. 2222mW
Package Junction-to-Ambient Thermal Resistance
(BJA) (Note 1) ............................................................36.0NC/W
Package Junction-to-Case Thermal Resistance
(BJC) (Note 1) ..............................................................3.0NC/W
ESD Protection (all pins, Human Body Model) ...................Q2kV
Operating Temperature Range ........................ -40NC to +125NC
Junction Temperature .....................................................+150NC
Storage Temperature Range .......................... -65NC to +150NC
Lead Temperature (soldering, 10s) ...............................+300NC
Soldering Temperature (reflow) ......................................+260NC
ELECTRICAL CHARACTERISTICS
(VCC = +3.0V to +3.6V, TA = -40NC to +125NC, unless otherwise noted. Typical values are at VCC = +3.3V and TA = +25NC.) (Note 2)
ABSOLUTE MAXIMUM RATINGS
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-
layer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage VCC 3.0 3.6 V
Standby Supply Current ISS SMBus static 4 15 FA
Operating Current ICC1 During conversion, RC off 500 600 FA
ICC2 During conversion, RC on 550 650
Temperature Resolution 11 Bits
0.125 NC
3-Sigma Temperature Accuracy
(Remote Channels 1–7) VCC = 3.3V
TA = +30NC to +85NC,
TRJ = +60NC to +100NC-0.85 +0.85
NCTA, TRJ = -40NC to +125NC-1.2 +1.2
TA = +30NC to +85NC,
TRJ = +100NC to +150NC-2.5 +2.5
3-Sigma Temperature Accuracy
(Local) VCC = 3.3V
TA = +30NC to +85NC-1 +1
NCTA = -40NC to +125NC-2 +2
TA = 0NC to +150NC-3 +3
6-Sigma Temperature Accuracy
(Remote Channels 1–7) VCC = 3.3V
TA = +30NC to +85NC,
TRJ = +60NC to +100NC-1 +1
NCTA, TRJ = -40NC to +125NC-2 +2
TA = +30NC to +85NC,
TRJ = +100NC to +125NC-2.75 +2.75
6-Sigma Temperature Accuracy
(Local) VCC = 3.3V
TA = +30NC to +85NC-1.5 +1.5
NCTA = -40NC to +125NC-2.5 +2.5
TA = 0NC to +150NC-3.5 +3.5
Supply Sensitivity of
Temperature Accuracy Q0.2 NC/V
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +3.0V to +3.6V, TA = -40NC to +125NC, unless otherwise noted. Typical values are at VCC = +3.3V and TA = +25NC.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Conversion Time per Channel tCONV
Resistance cancellation mode off 95 125 156
ms
Resistance cancellation mode on or beta
compensation on 190 250 312
Remote-Diode Source Current IRJ
High level Resistance cancellation
mode off
80 100 120
FA
Low level 8 10 12
High level Resistance cancellation
mode on or beta
compensation on
160 200 240
Low level 16 20 24
DXP_ and DXN_ Leakage
Current Standby mode 100 nA
Undervoltage Lockout Threshold UVLO Falling edge of VCC disables ADC 2.25 2.80 2.95 V
Undervoltage Lockout Hysteresis 90 mV
Power-On-Reset (POR)
Threshold VCC falling edge 1.3 2.0 2.2 V
POR Threshold Hysteresis 90 mV
ALERT and OVERT
Output Low Voltage VOL ISINK = 1mA 0.01 V
ISINK = 6mA 0.3
Input Leakage Current ILEAK -1 +1 FA
SMBus INTERFACE, STBY
Logic Input Low Voltage VIL VCC = 3.6V 0.8 V
Logic Input High Voltage VIH VCC = 3.0V 2.2 V
Input Leakage Current -1 +1 FA
Output Low Voltage VOL ISINK = 6mA 0.1 V
Input Capacitance CIN 5 pF
SMBus-COMPATIBLE TIMING (Figures 3 and 4) (Note 3)
Serial-Clock Frequency fSMBCLK (Note 4) 400 kHz
Bus Free Time Between STOP
and START Condition tBUF fSMBCLK = 400kHz 1.6 Fs
START Condition Setup Time fSMBCLK = 400kHz 0.6 Fs
Repeated START Condition
Setup Time tSU:STA 90% of SMBCLK to 90% of SMBDATA,
fSMBCLK = 400kHz 50 ns
START Condition Hold Time tHD:STA 10% of SMBDATA to 90% of SMBCLK,
fSMBCLK = 400kHz 0.6 Fs
STOP Condition Setup Time tSU:STO 90% of SMBCLK to 90% of SMBDATA,
fSMBCLK = 400kHz 0.6 Fs
Clock Low Period tLOW 10% to 10%, fSMBCLK = 400kHz 1 Fs
Clock High Period tHIGH 90% to 90% 0.6 Fs
Data-In Hold Time tHD:DAT 0 0.9 us
Data-In Setup Time tSU:DAT (Note 5) 100 ns
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
4 ______________________________________________________________________________________
Typical Operating Characteristics
(VCC = +3.3V, VSTBY = VCC, TA = +25NC, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +3.0V to +3.6V, TA = -40NC to +125NC, unless otherwise noted. Typical values are at VCC = +3.3V and TA = +25NC.) (Note 2)
Note 2: All parameters are tested at TA = +85NC. Specifications over temperature are guaranteed by design.
Note 3: Timing specifications are guaranteed by design.
Note 4: The serial interface resets when SMBCLK is low for more than tTIMEOUT.
Note 5: A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SMBCLK’s falling
edge.
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6581 toc01
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT (µA)
3.53.43.33.23.1
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0
3.0 3.6
HARDWARE OR SOFTWARE
STANDBY SUPPLY CURRENT
AVERAGE OPERATING SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6581 toc02
SUPPLY VOLTAGE (V)
AVERAGE OPERATING SUPPLY CURRENT (µA)
3.53.43.1 3.2 3.3
365
370
375
380
385
390
395
400
360
3.0 3.6
RESISTANCE
CANCELLATION OFF
REMOTE-DIODE TEMPERATURE ERROR
vs. REMODE-DIODE TEMPERATURE
MAX6581 toc03
REMOTE-DIODE TEMPERATURE (°C)
REMOTE-DIODE TEMPERATURE ERROR (°C)
1109050 703010
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
-4
-5
-6
-7
-8
-9
-10 -10 130
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Receive SMBCLK/SMBDATA
Rise Time tR300 ns
Receive SMBCLK/SMBDATA Fall
Time tF300 ns
Data-Out Hold Time tDH 50 ns
Pulse Width of Spike Suppressed tSP 0 50 ns
SMBus Timeout tTIMEOUT SMBDATA low period for interface reset 25 37 45 ms
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(VCC = +3.3V, VSTBY = VCC, TA = +25NC, unless otherwise noted.)
LOCAL TEMPERATURE ERROR
vs. DIE TEMPERATURE
MAX6581 toc04
DIE TEMPERATURE (°C)
LOCAL TEMPERATURE ERROR (°C)
908060 7010 20 30 40 500
-4
-3
-2
-1
0
1
2
3
4
5
-5
-10 100
REMOTE-DIODE TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6581 toc05
POWER-SUPPLY NOISE FREQUENCY (MHz)
REMOTE-DIODE TEMPERATURE ERROR (°C)
10.10.01
-4
-3
-2
-1
0
1
2
3
4
5
-5
0.001 10
100mVP-P
TRJ = +85°C
LOCAL TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6581 toc06
POWER-SUPPLY NOISE FREQUENCY (MHz)
LOCAL TEMPERATURE ERROR (°C)
10.10.01
-4
-3
-2
-1
0
1
2
3
4
5
-5
0.001 10
100mVP-P
REMOTE-DIODE TEMPERATURE ERROR
vs. CAPACITANCE
MAX6581 toc07
CAPACITANCE (nF)
REMOTE-DIODE TEMPERATURE ERROR (°C)
10
-4
-3
-2
-1
0
1
2
3
4
5
-5
1 100
100mVP-P
TRJ = +85°C
REMOTE-DIODE TEMPERATURE ERROR
vs. RESISTANCE
MAX6581 toc08
RESISTANCE (I)
REMOTE-DIODE TEMPERATURE ERROR (°C)
908060 7020 30 40 5010
0
5
10
15
20
25
30
35
40
45
50
-5
0 100
TRJ = +85°C
RESISTANCE
CANCELLATION OFF
RESISTANCE
CANCELLATION ON
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
6 ______________________________________________________________________________________
Pin Configuration
Pin Description
PIN NAME FUNCTION
1 DXP2
Combined Current Source and ADC Positive Input for Channel 2 Remote Diode. Connect DXP2 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP2 unconnected
or connect to DXN2 if a remote diode is not used. Connect a 100pF capacitor between DXP2 and
DXN2 for noise filtering.
2 DXN2
Cathode Input for Channel 2 Remote Diode. Connect the cathode of the channel 2 remote-diode-
connected transistor to DXN2. If the channel 2 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN2. Leave DXN2 unconnected or connect to DXP2 if a
remote diode is not used. Connect a 100pF capacitor between DXP2 and DXN2 for noise filtering.
3 DXP3
Combined Current Source and ADC Positive Input for Channel 3 Remote Diode. Connect DXP3 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP3 unconnected
or connect to DXN3 if a remote diode is not used. Connect a 100pF capacitor between DXP3 and
DXN3 for noise filtering.
4 DXN3
Cathode Input for Channel 3 Remote Diode. Connect the cathode of the channel 3 remote-diode-
connected transistor to DXN3. If the channel 3 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN3. Leave DXN3 unconnected or connect to DXP3 if a
remote diode is not used. Connect a 100pF capacitor between DXP3 and DXN3 for noise filtering.
5 DXP4
Combined Current Source and ADC Positive Input for Channel 4 Remote Diode. Connect DXP4 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP4 unconnected
or connect to DXN4 if a remote diode is not used. Connect a 100pF capacitor between DXP4 and
DXN4 for noise filtering.
6, 22 N.C. No Connection. Connect to other N.C. or leave unconnected.
23
24
22
21
8
7
9
DXN2
DXN3
DXP4
N.C.
10
DXP2
VCC
I.C.
STBY
ALERT
DXP7
1 2
N.C.
4 5 6
1718 16 14 13
DXP1
DXN1
DXN6
DXN5
DXP5
DXN4
MAX6581
DXP3 OVERT
3
15
GND
20 11 DXP6
SMBCLK
19 12 DXN7
SMBDATA
TOP VIEW
*EP = EXPOSED PAD, CONNECT TO GND
*EP
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
_______________________________________________________________________________________ 7
Pin Description (continued)
PIN NAME FUNCTION
7 DXN4
Cathode Input for Channel 4 Remote Diode. Connect the cathode of the channel 4 remote-diode-
connected transistor to DXN4. If the channel 4 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN4. Leave DXN4 unconnected or connect to DXP4 if a
remote diode is not used. Connect a 100pF capacitor between DXP4 and DXN4 for noise filtering.
8 DXP5
Combined Current Source and ADC Positive Input for Channel 5 Remote Diode. Connect DXP5 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP5 unconnected
or connect to DXN5 if a remote diode is not used. Connect a 100pF capacitor between DXP5 and
DXN5 for noise filtering.
9 DXN5
Cathode Input for Channel 5 Remote Diode. Connect the cathode of the channel 5 remote-diode-
connected transistor to DXN5. If the channel 5 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN5. Leave DXN5 unconnected or connect to DXP5 if a
remote diode is not used. Connect a 100pF capacitor between DXP5 and DXN5 for noise filtering.
10 DXN6
Cathode Input for Channel 6 Remote Diode. Connect the cathode of the channel 6 remote-diode-
connected transistor to DXN6. If the channel 6 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN6. Leave DXN6 unconnected or connect to DXP6 if a
remote diode is not used. Connect a 100pF capacitor between DXP6 and DXN6 for noise filtering.
11 DXP6
Combined Current Source and ADC Positive Input for Channel 6 Remote Diode. Connect DXP6 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP6 unconnected
or connect to DXN6 if a remote diode is not used. Connect a 100pF capacitor between DXP6 and
DXN6 for noise filtering.
12 DXN7
Cathode Input for Channel 7 Remote Diode. Connect the cathode of the channel 7 remote-diode-
connected transistor to DXN7. If the channel 7 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN7. Leave DXN7 unconnected or connect to DXP7 if a
remote diode is not used. Connect a 100pF capacitor between DXP7 and DXN7 for noise filtering.
13 DXP7
Combined Current Source and ADC Positive Input for Channel 7 Remote Diode. Connect DXP7 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP7 unconnected
or connect to DXN7 if a remote diode is not used. Place a 100pF capacitor between DXP7 and
DXN7 for noise filtering.
14 STBY Active-Low Standby Input. Drive STBY logic-low to place the MAX6581 in standby mode, or logic-
high for normal mode. Temperature and threshold data are retained in standby mode.
15 I.C. Internally Connected. I.C. is internally connected to VCC. Connect I.C. to VCC or leave unconnected.
16 OVERT Overtemperature Active-Low, Open-Drain Output. OVERT asserts low when the temperature of any
remote channel exceeds the programmed threshold limit.
17 VCC Supply Voltage Input. Bypass to GND with a 0.1FF capacitor.
18 ALERT SMBus Alert (Interrupt), Active-Low, Open-Drain Output. ALERT asserts low when the temperature of
any channel crosses a programmed ALERT high or low threshold.
19 SMBDATA SMBus Serial-Data Input/Output. Connect SMBDATA to a pullup resistor.
20 SMBCLK SMBus Serial-Clock Input. Connect SMBCLK to a pullup resistor.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
8 ______________________________________________________________________________________
Detailed Description
The MAX6581 is a precision multichannel temperature
monitor that features one local and seven remote tem-
perature-sensing channels with a programmable alert
threshold for each temperature channel and a program-
mable overtemperature threshold for channels 1–7 (see
Figure 1). Communication with the MAX6581 is achieved
through the SMBus serial interface and a dedicated alert
pin (ALERT). The alarm outputs, (OVERT and ALERT)
assert if the software-programmed temperature thresh-
olds are exceeded. ALERT also asserts if the measured
temperature falls below the ALERT low limits. ALERT
typically serves as an interrupt, while OVERT can be
connected to a fan, system shutdown, or other thermal-
management circuitry.
ADC Conversion Sequence
The MAX6581 starts the conversion sequence by
measuring the temperature on channel 1, followed by 2,
local channel, 3–7. The conversion result for each active
channel is stored in the corresponding temperature data
register. No conversion is performed on any channel that
does not have a diode.
Low-Power Standby Mode
Enter software-standby mode by setting the STOP
bit to 1 in the Configuration register. Enter hardware-
standby by pulling STBY low. Software-standby mode
disables the ADC and reduces the supply current to
approximately 4FA. During either software or hardware
standby, data is retained in memory. During hardware
standby, the SMBus interface is inactive. During software
standby, the SMBus interface is active and listening for
commands. The timeout is enabled if a START condition
is recognized on SMBus. Activity on the SMBus causes
the supply current to increase. If a standby command is
received while a conversion is in progress, the conver-
sion cycle is interrupted, and the temperature registers
are not updated. The previous data is not changed and
remains available.
Operating-Current Calculation
The MAX6581 operates at different operating-current
levels depending on how many external channels are in
use and how many of those are in resistance cancella-
tion (RC) mode. The average operating current is:
N R
AV CC1 CC2
N R N R
N 1 2 N
I I I
N 2 N 1 N 2 N 1
+ ×
= + ×
+ × + + × +
where:
NN = the number of remote channels that are operating
in normal mode.
NR = the number of remote channels that are in RC
mode.
IAV = the average operating power-supply current over a
complete series of conversions.
ICC1 = the average operating power-supply current
during a conversion in normal mode.
ICC2 = the average operating power-supply current
during a conversion in RC mode.
Pin Description (continued)
PIN NAME FUNCTION
21 GND Ground
23 DXP1
Combined Current Source and ADC Positive Input for Channel 1 Remote Diode. Connect DXP1 to
the anode of a remote-diode-connected, temperature-sensing transistor. Leave DXP1 unconnected
or connect to DXN1 if a remote diode is not used. Connect a 100pF capacitor between DXP1 and
DXN1 for noise filtering.
24 DXN1
Cathode Input for Channel 1 Remote Diode. Connect the cathode of the channel 1 remote-diode-
connected transistor to DXN1. If the channel 1 remote transistor is a substrate pnp (e.g., on a CPU
die), connect the base of the pnp to DXN1. Leave DXN1 unconnected or connect to DXP1 if a
remote diode is not used. Connect a 100pF capacitor between DXP1 and DXN1 for noise filtering.
EP Exposed Pad. Connect EP to GND.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
_______________________________________________________________________________________ 9
Figure 1. Internal Block Diagram
VCC
DXN2
DXP2
DXN1
DXP1
DXN3
DXP3
DXN4
DXP4
DXN5
DXP5
DXN6
DXP6
LOCAL
TRANSISTOR
SMBCLK SMBDATA
INPUT
BUFFER
REF
COUNTER
SMBus INTERFACE
COUNT
+
-
DXN7
DXP7
IRJ
REGISTER BANK
COMMAND BYTE
REMOTE TEMPERATURES
LOCAL TEMPERATURES
STBY
OVERT
ALERT THRESHOLD
OVERT THRESHOLD
ALERT RESPONSE ADDRESS
ALERT
ALARM
ALU
MAX6581
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
10 _____________________________________________________________________________________
SMBus Digital Interface
From a software perspective, the MAX6581 appears
as a series of 8-bit registers that contain temperature-
measurement data, alarm threshold values, and control
bits. A standard SMBus-compatible, 2-wire serial inter-
face is used to read temperature data and write control
bits and alarm threshold data. The same SMBus slave
address also provides access to all functions.
The MAX6581 employs four standard SMBus proto-
cols: write byte, read byte, send byte, and receive byte
(Figure 2). The shorter receive-byte protocol allows
quicker transfers, provided that the correct data regis-
ter 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 without informing the first master. Figure
3 is the SMBus write timing diagram and Figure 4 is the
SMBus read timing diagram.
The remote-diode-measurement channels provide
11 bits of data (1 LSB = 0.125NC). The eight most
significant bits (MSBs) can be read from the local tem-
perature and remote temperature registers. The remain-
ing 3 bits for remote can be read from the extended
temperature register. If extended resolution is desired,
the extended-resolution register should be read first.
This prevents the MSBs from being overwritten by new
conversion results until they have been read. If the MSBs
have not been read within a SMBus timeout period (nom-
inally 37ms), normal updating continues. Table 1 shows
the main temperature register (high-byte) data format
and Table 2 shows the extended-resolution register (low-
byte) data format.
Figure 2. SMBus Protocols
S ADDRESS WR ACK ACK PDATA ACKCOMMAND
7 BITS 18 BITS8 BITS
SLAVE ADDRESS: EQUIVALENT
TO CHIP-SELECT LINE OF
A 3-WIRE INTERFACE
DATA BYTE: DATA GOES INTO THE REGISTER
SET BY THE COMMAND BYTE (TO SET
THRESHOLDS, CONFIGURATION MASKS, AND
SAMPLING RATE)
WRITE-BYTE FORMAT
S ADDRESSADDRESS WR ACK ACK PS RD ACK ///DATACOMMAND
7 BITS 7 BITS 8 BITS8 BITS
READ-BYTE FORMAT
SLAVE ADDRESS: EQUIVALENT
TO CHIP SELECT LINE
COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
READING FROM
S PADDRESS WR ACK ACKCOMMAND
7 BITS 8 BITS
SEND-BYTE FORMAT
COMMAND BYTE: SENDS COMMAND
WITH NO DATA, USUALLY
USED FOR ONE-SHOT COMMAND
S PADDRESS RD ACK ///DATA
7 BITS 8 BITS
RECEIVE-BYTE FORMAT
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
SLAVE ADDRESS: REPEATED
DUE TO CHANGE IN DATA-
FLOW DIRECTION
DATA BYTE: READS FROM
THE REGISTER SET BY THE
COMMAND BYTE
S = START CONDITION
P = STOP CONDITION
SHADED = SLAVE TRANSMISSION
/// = NOT ACKNOWLEDGED
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 11
Figure 3. SMBus Write Timing Diagram
Figure 4. Read-Timing Diagram
Table 1. Main Temperature Register (High-Byte) Data Format
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
A B C D EF G H I J
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tSU:STO tBUF
L MK
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 = SLAVE PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
SMBCLK
A B C D EF G H I
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
JK
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
TEMPERATURE (NC) DIGITAL OUTPUT
NORMAL FORMAT EXTENDED FORMAT
Diode fault (open or short) 1111 1111 1111 1111
> +191 1111 1111 1111 1111
+191 1111 1111 1111 1111
+150 1101 0110 1100 1100
+127 1011 1111 1011 1111
+25 0101 1001 1001 1001
0 0100 0000 0100 0000
-39 0001 1001 0101 1001
-64 0000 0000 0000 0000
< -64 0000 0000 0000 0000
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
12 _____________________________________________________________________________________
Table 2. Extended-Resolution Temperature Register (Low-Byte) Data Format
Table 3. Command Byte Register Bit Assignment
X = Don’t care.
TEMPERATURE (NC) DIGITAL OUTPUT
0 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
REGISTER ADDRESS
(HEX)
POR
VALUE
(HEX)
READ/
WRITE DESCRIPTION
Remote 1 01 00 R Read channel 1 remote temperature
Remote 2 02 00 R Read channel 2 remote temperature
Remote 3 03 00 R Read channel 3 remote temperature
Remote 4 04 00 R Read channel 4 remote temperature
Remote 5 05 00 R Read channel 5 remote temperature
Remote 6 06 00 R Read channel 6 remote temperature
Local 07 00 R Read local temperature
Remote 7 08 00 R Read channel 7 remote temperature
Remote 1 Extended
Bits* 09 00 R Read channel 1 remote-diode extended temperature
Manufacturer ID 0A 4D R Read manufacturer ID
Revision ID 0F 00 R Read revision ID
Remote 1 ALERT High
Limit 11 7F R/W Read/write channel 1 remote-diode alert high-temperature
threshold limit
Remote 2 ALERT High
Limit 12 7F R/W Read/write channel 2 remote-diode alert high-temperature
threshold limit
Remote 3 ALERT High
Limit 13 64 R/W Read/write channel 3 remote-diode alert high-temperature
threshold limit
Remote 4 ALERT High
Limit 14 64 R/W Read/write channel 4 remote-diode alert high-temperature
threshold limit
Remote 5 ALERT High
Limit 15 64 R/W Read/write channel 5 remote-diode alert high-temperature
threshold limit
Remote 6 ALERT High
Limit 16 64 R/W Read/write channel 6 remote-diode alert high-temperature
threshold limit
Local ALERT High Limit 17 5A R/W Read/write local-diode alert high-temperature threshold limit
Remote 7 ALERT High
Limit 18 64 R/W Read/write channel 7 remote-diode alert high-temperature
threshold limit
Local OVERT High Limit 20 50 R/W Read/write channel local-diode overtemperature threshold limit
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 13
Table 3. Command Byte Register Bit Assignment (continued)
*Duplicate entries.
REGISTER ADDRESS
(HEX)
POR
VALUE
(HEX)
READ/
WRITE DESCRIPTION
Remote 1 OVERT High
Limit 21 6E R/W Read/write channel 1 remote-diode overtemperature threshold limit
Remote 2 OVERT High
Limit 22 6E R/W Read/write channel 2 remote-diode overtemperature threshold limit
Remote 3 OVERT High
Limit 23 6E R/W Read/write channel 3 remote-diode overtemperature threshold limit
Remote 4 OVERT High
Limit 24 7F R/W Read/write channel 4 remote-diode overtemperature threshold limit
Remote 5 OVERT High
Limit 25 5A R/W Read/write channel 5 remote-diode overtemperature threshold limit
Remote 6 OVERT High
Limit 26 5A R/W Read/write channel 6 remote-diode overtemperature threshold limit
Remote 7 OVERT High
Limit 27 5A R/W Read/write channel 7 remote-diode overtemperature threshold limit
ALERT Low Limits (all
channels) 30 00 R/W Read/write all channels alert low-temperature threshold limit
Configuration 41 00 R/W Read/write configuration
ALERT Mask 42 00 R/W Read/write ALERT mask
OVERT Mask 43 00 R/W Read/write OVERT mask
ALERT High Status 44 00 R Read ALERT high status
OVERT Status 45 00 R Read OVERT status
Diode Fault Status 46 00 R Read diode fault status
ALERT Low Status 47 00 R Read ALERT low status
ALERT Low Disable 48 FF R/W Read/write ALERT low disable
Resistance Cancellation 4A 00 R/W Read/write resistance cancellation enable bits (1 = On, 0 = Off)
Transistor Ideality 4B 00 R/W Read/write ideality value for remote-sense transistor
Ideality Select 4C 00 R/W Read/write ideality value selection bits (1 = selected transistor
ideality, 0 = 1.008)
Offset 4D 00 R/W Read/write temperature offset value
Offset Select 4E 00 R/W Read/write offset value selection bits (1 = value in Offset Select
register, 0 = 0)
Remote 1 Extended
Bits* 51 00 R Read channel 1 remote extended temperature
Remote 2 Extended Bits 52 00 R Read channel 2 remote extended temperature
Remote 3 Extended Bits 53 00 R Read channel 3 remote extended temperature
Remote 4 Extended Bits 54 00 R Read channel 4 remote extended temperature
Remote 5 Extended Bits 55 00 R Read channel 5 remote extended temperature
Remote 6 Extended Bits 56 00 R Read channel 6 remote extended temperature
Local Extended Bits 57 00 R Read local channel extended temperature
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
14 _____________________________________________________________________________________
Diode Fault Detection
If a channel’s input DXP_ and DXN_ are left open or are
shorted, the MAX6581 detects a diode fault. An open
diode fault does not cause either ALERT or OVERT to
assert. A bit in the status register for the correspond-
ing channel is set to 1 and the temperature data for the
channel is stored as all 1s (FFh). It takes approximately
4ms for the MAX6581 to detect a diode fault. Once a
diode fault is detected, the MAX6581 goes to the next
channel in the conversion sequence.
Alarm Threshold Registers
There are 17 alarm threshold registers that store over-
temperature and undertemperature ALERT and OVERT
threshold values. Nine of these registers are dedicated
to storing one local alert overtemperature threshold limit,
seven remote alert overtemperature threshold limits, and
one shared alert undertemperature temperature thresh-
old limit (see the ALERT Interrupt Mode section). The
remaining eight registers are dedicated to storing one
local overtemperature threshold limit and seven remote
channels to store overtemperature threshold limits (see
the OVERT Overtemperature Alarms section). Access to
these registers is provided through the SMBus interface.
ALERT Interrupt Mode
ALERT interrupts occur when the internal or external
temperature reading exceeds a high-temperature limit
(user programmable) or a low-temperature limit. The
ALERT interrupt output signal can be cleared by reading
the status register(s) associated with the fault(s) or by
successfully responding to an alert response address
transmission by the master. In both cases, the alert is
cleared but is reasserted at the end of the next conver-
sion if the fault condition still exists. The interrupt does
not halt automatic conversions. The ALERT output is
open-drain so that multiple devices can share a common
interrupt line. All ALERT interrupts can be masked using
the ALERT Mask register (42h). The POR state of these
registers is shown in Table 3.
ALERT Responses Address
The SMBus alert response interrupt pointer provides
quick fault identification for simple slave devices that
lack the complex logic necessary to be a bus master.
Upon receiving an interrupt signal, the host master
can broadcast a receive-byte transmission to the alert
response slave address (19h). Then, 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 arbitra-
tion rules apply, and the device with the lower address
code wins. The losing device does not generate an
acknowledgment 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 alert response protocol clears the out-
put latch. If the condition that caused the alert still exists,
the MAX6581 reasserts the ALERT interrupt at the end of
the next conversion.
OVERT Overtemperature Alarms
The MAX6581 has eight overtemperature registers that
store alarm threshold data for the OVERT output. OVERT
is asserted when a channel’s measured temperature
is greater than the value stored in the corresponding
threshold register. OVERT remains asserted until the
temperature drops below the programmed threshold
minus 4NC hysteresis. An overtemperature output can be
used to activate a cooling fan, send a warning, initiate
clock throttling, or trigger a system shutdown to prevent
component damage. See Table 3 for the POR state of the
overtemperature threshold registers.
Command Byte Register Functions
The 8-bit Command Byte register (Table 3) is the master
index that points to the various other registers within the
MAX6581. This register’s POR state is 0000 0000 (00h).
Configuration Register (41h)
The Configuration register (Table 4) has several
functions. Bit 7 (MSB) is used to put the MAX6581
either in software-standby mode (STOP) or continuous-
conversion mode. Bit 6 resets all registers to their POR
conditions and then clears itself. Bit 5 disables the
SMBus timeout. Bit 1 sets the extended range of the
remote temperature diodes. The remaining bits of the
Configuration register are not used. The POR state of this
register is 0000 0000 (00h).
ALERT Mask Register (42h)
The ALERT Mask register functions are described
in Table 5. Bits [7:0] are used to mask the ALERT
interrupt output. Bit 6 masks the local alert interrupt and
the remaining bits mask the remote alert interrupts. The
power-up state of this register is 0000 0000 (00h).
OVERT Mask Register (43h)
Table 6 describes the OVERT Mask register. Bit 6 and
the remaining bits mask the OVERT interrupt output for
all channels. The power-up state of this register is 0000
0000 (00h).
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 15
Table 4. Configuration Register (41h)
Table 5. ALERT Mask Register (42h)
Table 6. OVERT Mask Register (43h)
BIT NAME POR
VALUE FUNCTION
7 (MSB) STOP 0 Standby-Mode Control Bit. If STOP is set to logic 1, the MAX6581 stops converting
and enters standby mode.
6 POR 0 Reset Bit. Set to logic 1 to put the device into its power-on state. This bit is self-
clearing.
5TIMEOUT 0 Timeout Enable Bit. Set to logic 0 to enable SMBus timeout.
4 RESERVED 0 Reserved. Must be set to 0.
3 RESERVED 0 Reserved. Must be set to 0.
2 RESERVED 0 Reserved. Must be set to 0.
1 EXTRANGE 0 Extended-Range Enable Bit. Set bit 1 to logic 1 to set the temperature and limit data
range to -64NC to +191NC. Set bit 1 to logic 0 to set the range to 0NC to +255NC.
0 RESERVED 0 Reserved. Must be set to 0.
BIT NAME POR
VALUE FUNCTION
7 (MSB) Mask ALERT 7 0Channel 7 Alert Mask. Set to logic 1 to mask channel 7 ALERT.
6Mask Local
ALERT 0Local Alert Mask. Set to logic 1 to mask local channel ALERT.
5Mask ALERT 6 0Channel 6 Alert Mask. Set to logic 1 to mask channel 6 ALERT.
4Mask ALERT 5 0Channel 5 Alert Mask. Set to logic 1 to mask channel 5 ALERT.
3Mask ALERT 4 0Channel 4 Alert Mask. Set to logic 1 to mask channel 4 ALERT.
2Mask ALERT 3 0Channel 3 Alert Mask. Set to logic 1 to mask channel 3 ALERT.
1Mask ALERT 2 0Channel 2 Alert Mask. Set to logic 1 to mask channel 2 ALERT.
0Mask ALERT 1 0Channel 1 Alert Mask. Set to logic 1 to mask channel 1 ALERT.
BIT NAME POR
VALUE FUNCTION
7 (MSB) Mask OVERT 7 0Channel 7 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 7 OVERT.
6Mask Local
OVERT 0Local Overt Mask. Set to logic 1 to mask local channel OVERT.
5Mask OVERT 6 0Channel 6 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 6 OVERT.
4Mask OVERT 5 0Channel 5 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 5 OVERT.
3Mask OVERT 4 0Channel 4 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 4 OVERT.
2Mask OVERT 3 0Channel 3 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 3 OVERT.
1Mask OVERT 2 0Channel 2 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 2 OVERT.
0Mask OVERT 1 0Channel 1 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 1 OVERT.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
16 _____________________________________________________________________________________
Status Register Functions
There are four status registers (see Tables 7–10). The
ALERT High Status register indicates whether a mea-
sured local or remote temperature has exceeded the
associated threshold limit set in an ALERT High Limit
register. The OVERT Status register indicates whether
a measured temperature has exceeded the associated
threshold limit set in an OVERT High Limit register. The
Diode Fault Status register indicates whether there is a
diode fault (open or short) in any of the remote-sensing
channels. The ALERT Low Status register indicates
whether the measured temperature has fallen below the
threshold limit set in the ALERT Low Limits register for
the local or remote-sensing diodes.
Bits in the alert status registers are cleared by a success-
ful read, but set again after the next conversion unless
the fault is corrected, either by a drop in the measured
temperature or a change in the threshold temperature.
The ALERT interrupt output follows the status flag bit.
Once the ALERT output is asserted, it can be deasserted
by either reading the ALERT High Status register or by
successfully responding to an alert response address. In
both cases, the alert is cleared even if the fault condition
exists, but the ALERT output reasserts at the end of the
next conversion.
The bits indicating OVERT faults clear only when the
measured temperature drops below the temperature
threshold minus the hysteresis value (4NC), or when the
trip temperature is set to a value at least 4NC above the
current temperature.
Table 7. ALERT High Status Register (44h)
BIT NAME POR
STATE FUNCTION
7 (MSB) Remote ALERT
High 7 0
Channel 7 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 7
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 7 ALERT High Limit register.
6Local ALERT
High 0Local Channel High-Alert Bit. This bit is set to logic 1 when the local temperature
exceeds the temperature threshold limit in the Local ALERT High Limit register.
5Remote ALERT
High 6 0
Channel 6 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 6
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 6 ALERT High Limit register.
4Remote ALERT
High 5 0
Channel 5 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 5
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 5 ALERT High Limit register.
3Remote ALERT
High 4 0
Channel 4 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 4
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 4 ALERT High Limit register.
2Remote ALERT
High 3 0
Channel 3 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 3
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 3 ALERT High Limit register.
1Remote ALERT
High 2 0
Channel 2 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 2
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 2 ALERT High Limit register.
0Remote ALERT
High 1 0
Channel 1 Remote-Diode High-Alert Bit. This bit is set to logic 1 when the channel 1
remote-diode temperature exceeds the programmed temperature threshold limit in the
Remote 1 ALERT High Limit register.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 17
Table 8. OVERT Status Register (45h)
Table 9. Diode Fault Status Register (46h)
BIT NAME POR
STATE FUNCTION
7 (MSB) Remote OVERT 7 0
Channel 7 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 7 remote-diode temperature exceeds the temperature threshold limit in
the Remote 7 OVERT High Limit register.
6Local OVERT 0
Local Channel Overtemperature Status Bit. This bit is set to logic 1 when the local
temperature exceeds the temperature threshold limit in the Local OVERT High Limit
register.
5Remote OVERT 6 0
Channel 6 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 6 remote-diode temperature exceeds the temperature threshold limit in
the Remote 6 OVERT High Limit register.
4Remote OVERT 5 0
Channel 5 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 5 remote-diode temperature exceeds the temperature threshold limit in
the Remote 5 OVERT High Limit register.
3Remote OVERT 4 0
Channel 4 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 4 remote-diode temperature exceeds the temperature threshold limit in
the Remote 4 OVERT High Limit register.
2Remote OVERT 3 0
Channel 3 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 3 remote-diode temperature exceeds the temperature threshold limit in
the Remote 3 OVERT High Limit register.
1Remote OVERT 2 0
Channel 2 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 2 remote-diode temperature exceeds the temperature threshold limit in
the Remote 2 OVERT High Limit register.
0Remote OVERT 1 0
Channel 1 Remote-Diode Overtemperature Status Bit. This bit is set to logic 1 when
the channel 1 remote-diode temperature exceeds the temperature threshold limit in
the Remote 1 OVERT High Limit register.
BIT NAME POR
STATE FUNCTION
7 (MSB) RESERVED 0
6 Diode Fault 7 0 Channel 7 Remote-Diode Fault Bit. This bit is set to 1 when DXP7 and DXN7 are open
circuit or when DXP7 is connected to VCC.
5 Diode Fault 6 0 Channel 6 Remote-Diode Fault Bit. This bit is set to 1 when DXP6 and DXN6 are open
circuit or when DXP6 is connected to VCC.
4 Diode Fault 5 0 Channel 5 Remote-Diode Fault Bit. This bit is set to 1 when DXP5 and DXN5 are open
circuit or when DXP5 is connected to VCC.
3 Diode Fault 4 0 Channel 4 Remote-Diode Fault Bit. This bit is set to 1 when DXP4 and DXN4 are open
circuit or when DXP4 is connected to VCC.
2 Diode Fault 3 0 Channel 3 Remote-Diode Fault Bit. This bit is set to 1 when DXP3 and DXN3 are open
circuit or when DXP3 is connected to VCC.
1 Diode Fault 2 0 Channel 2 Remote-Diode Fault Bit. This bit is set to 1 when DXP2 and DXN2 are open
circuit or when DXP2 is connected to VCC.
0 Diode Fault 1 0 Channel 1 Remote-Diode Fault Bit. This bit is set to 1 when DXP1 and DXN1 are open
circuit or when DXP1 is connected to VCC.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
18 _____________________________________________________________________________________
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 default value for the MAX6581
is n = 1.008 (channels 1–7). A thermal diode on the
substrate of an IC is normally a pnp with the base and
emitter brought out and the collector (diode connection)
grounded. DXP_ must be connected to the anode (emit-
ter) and DXN_ must be connected to the cathode (base)
of this pnp. If a sense transistor with an ideality factor
other than 1.008 is used, the output data is different from
the data obtained with the optimum ideality factor. If
necessary, a different ideality factor value can be chosen
using the Transistor Ideality register (see Table 11). The
Ideality Select register allows each channel to have the
default ideality of 1.008 or the value programmed in the
Transistor Ideality register.
Table 10. ALERT Low Status Register (47h)
BIT NAME POR
STATE FUNCTION
7 (MSB) Remote ALERT
Low 7 0
Channel 7 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 7
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 7 ALERT Low Limit register.
6Local ALERT Low 0
Local Channel Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the local
channel remote-diode temperature falls below the programmed temperature threshold
limit in the Local ALERT Low Limit register.
5Remote ALERT
Low 6 0
Channel 6 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 6
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 6 ALERT Low Limit register.
4Remote ALERT
Low 5 0
Channel 5 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 5
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 5 ALERT Low Limit register.
3Remote ALERT
Low 4 0
Channel 4 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 4
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 4 ALERT Low Limit register.
2Remote ALERT
Low 3 0
Channel 3 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 3
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 3 ALERT Low Limit register.
1Remote ALERT
Low 2 0
Channel 2 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 2
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 2 ALERT Low Limit register.
0Remote ALERT
Low 1 0
Channel 1 Remote-Diode Low-Alert Bit. This bit is set to logic 1 when the channel 1
remote-diode temperature falls below the programmed temperature threshold limit in
the Remote 1 ALERT Low Limit register.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 19
Table 11. Transistor Ideality Register
X = Don’t care.
REGISTER B7 B6 B5 B4 B3 B2 B1 B0 IDEALITY
FACTOR HEX
0x4B
X X X 0 0 0 0 0 .999 0x00
X X X 0 0 0 0 1 1.000 0x01
X X X 0 0 0 1 0 1.001 0x02
X X X 0 0 0 1 1 1.002 0x03
X X X 0 0 1 0 0 1.003 0x04
X X X 0 0 1 0 1 1.004 0x05
X X X 0 0 1 1 0 1.005 0x06
X X X 0 0 1 1 1 1.006 0x07
X X X 0 1 0 0 0 1.007 0x08
X X X 0 1 0 0 1 1.008 0x09
X X X 0 1 0 1 0 1.009 0x0A
X X X 0 1 0 1 1 1.010 0x0B
X X X 0 1 1 0 0 1.011 0x0C
X X X 0 1 1 0 1 1.012 0x0D
X X X 0 1 1 1 0 1.013 0x0E
X X X 0 1 1 1 1 1.014 0x0F
X X X 1 0 0 0 0 1.015 0x10
X X X 1 0 0 0 1 1.016 0x11
X X X 1 0 0 1 0 1.017 0x12
X X X 1 0 0 1 1 1.018 0x13
X X X 1 0 1 0 0 1.019 0x14
X X X 1 0 1 0 1 1.020 0x15
X X X 1 0 1 1 0 1.021 0x16
X X X 1 0 1 1 1 1.022 0x17
X X X 1 1 0 0 0 1.023 0x18
X X X 1 1 0 0 1 1.024 0x19
X X X 1 1 0 1 0 1.025 0x1A
X X X 1 1 0 1 1 1.026 0x1B
X X X 1 1 1 0 0 1.027 0x1C
X X X 1 1 1 0 1 1.028 0x1D
X X X 1 1 1 1 0 1.029 0x1E
X X X 1 1 1 1 1 1.030 0x1F
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
20 _____________________________________________________________________________________
Series-Resistance Cancellation
Some thermal diodes on high-power ICs have exces-
sive series resistance that can cause temperature-mea-
surement errors when used with conventional remote-
temperature sensors. Channels 1–7 of the MAX6581
have a series-resistance cancellation feature (enabled
by bits [7:0] of the Resistance Cancellation register) that
eliminates the effect of diode series resistance and inter-
connection resistance. Set these bits to 1 if the series
resistance is large enough to affect the accuracy of the
channels. The series-resistance cancellation function
increases the conversion time for the remote channels by
125ms (typ). This feature cancels the bulk resistance of
the sensor and any other resistance in series (e.g., wire,
contact resistance, etc.). The cancellation range is from
0I to 100I.
Applications Information
Remote-Diode Selection
The MAX6581 directly measures the die temperature of CPUs
and other ICs that have on-chip temperature-sensing diodes
(see the Typical Application Circuit), or it can measure the
temperature of a discrete diode-connected transistor.
Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
its collector and base must be connected together. Table
13 lists examples of discrete transistors that are appro-
priate for use with the MAX6581. 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 10FA, and at the lowest
expected temperature the forward voltage must be less
than 0.95V at 100FA. Large power transistors must not
be used. Also, ensure that the base resistance is less
than 100I. Tight specifications for forward-current gain
(e.g., 50 < A < 150) indicate that the manufacturer has
good process controls and that the devices have con-
sistent VBE characteristics. Manufacturers of discrete
transistors do not normally specify or guarantee ideality
factor. This normally is not a problem since good-quality
discrete transistors tend to have ideality factors that fall
within a relatively narrow range. Variations in remote
temperature readings of less than Q2NC with a variety of
discrete transistors have been observed. However, it is
good design practice to verify good consistency of tem-
perature readings with several discrete transistors from
any supplier under consideration.
Table 12. Resistance Cancellation Register (4Ah)
X = Don’t care.
BIT NAME POR
STATE FUNCTION
7 (MSB) X 0
6RESISTANCE
CANCELLATION 7 0Channel 7 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
5RESISTANCE
CANCELLATION 6 0Channel 6 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
4RESISTANCE
CANCELLATION 5 0Channel 5 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
3RESISTANCE
CANCELLATION 4 0Channel 4 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
2RESISTANCE
CANCELLATION 3 0Channel 3 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
1RESISTANCE
CANCELLATION 2 0Channel 2 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
0RESISTANCE
CANCELLATION 1 0Channel 1 Resistance Cancellation Enable Bit. Set this bit to logic 1 to enable
resistance cancellation. Set this bit to logic 0 to disable resistance cancellation.
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 21
Unused Diode Channels
If one or more of the remote-diode channels is not
needed, disconnect the DXP_ and DXN_ inputs for
that channel, or connect the DXP_ to the correspond-
ing DXN_. The status register indicates a diode “fault”
for this channel and the channel is ignored during the
temperature-measurement sequence. It is also good
practice to mask any unused channels immediately upon
power-up by setting the appropriate bits in the ALERT
Mask and OVERT Mask registers. This prevents unused
channels from causing ALERT or OVERT to assert.
Thermal Mass and Self-Heating
When sensing local temperature, the MAX6581 mea-
sures the temperature of the PCB to which it is soldered.
The leads provide a good thermal path between the PCB
traces and the die. As with all IC temperature sensors,
thermal conductivity between the die and the ambient
air is poor by comparison, making air-temperature mea-
surements impractical. Since the thermal mass of the
PCB is far greater than that of the MAX6581, the device
follows 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 virtually no effect; the measured temperature
of the junction tracks the actual temperature within a
conversion cycle. When measuring temperature with
discrete remote transistors, the best thermal-response
times are obtained with transistors in small packages
(i.e., SOT23 or SC70). 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.
ADC Noise Filtering
The integrating ADC has good noise rejection for low-
frequency signals, such as power-supply hum. In envi-
ronments with significant high-frequency EMI, connect
an external 100pF capacitor between DXP_ and DXN_.
Larger capacitor values can be used for added filter-
ing; however, it can introduce errors due to the rise time
of the switched current source. High-frequency noise
reduction is needed for high-accuracy remote measure-
ments. Noise can be reduced with careful PCB layout as
discussed in the PCB Layout section.
Slave Address
The slave address for the MAX6581 is shown in Table 14.
Table 13. Remote Sensors Transistor Suppliers (for Channels 1–7)
Note: Discrete transistors must be diode connected (base shorted to collector).
Table 14. Slave Address
DEVICE ADDRESS
A7 A6 A5 A4 A3 A2 A1 A0
1001101R/W
SUPPLIER MODEL NO.
PNP NPN
Central Semiconductor Corp. (USA) CMPT3906
2N3906
CMPT3904
2N3904
Fairchild Semiconductor (USA) MMBT3906
2N3906 2N3904
Infineon (Germany) SMBT3906
ON Semiconductor (USA) MMBT3906
2N3906 2N3904
ROHM Semiconductor (USA) SST3906 SST3904
Samsung (Korea) KST3906-TF KST3904-TF
Siemens (Germany) SMBT3906 SMBT3904
Zetex (England) FMMT3906CT-ND FMMT3904CT-ND
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
22 _____________________________________________________________________________________
PCB Layout
Follow the guidelines below to reduce the measurement
error when measuring remote temperature:
1) Place the MAX6581 as close as possible to the
remote diode. In noisy environments, such as a com-
puter motherboard, this distance is typically 4in to
8in. This length can be increased if the worst-noise
sources are avoided. Noise sources include displays,
clock generators, memory buses, and 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
+30NC error, even with good filtering.
3) Route the DXP_ and DXN_ traces in parallel and in
close proximity to each other. Each parallel pair of
traces should go to a remote diode. Route these
traces away from any higher voltage traces, such as
+12VDC. Leakage currents from PCB contamination
must be dealt with carefully since a 20MI leakage
path from DXP_ to ground causes approximately
+1NC error. If high-voltage traces are unavoidable,
connect guard traces to GND on either side of the
DXP_–DXN_ traces (Figure 5).
4) Route through as few vias and crossunders as possi-
ble to minimize copper/solder thermocouple effects.
5) Use wide traces when possible (5-mil to 10-mil traces
are typical). Be aware of the effect of trace resistance
on temperature readings when using long, narrow
traces.
6) When the power supply is noisy, add a resistor (up to
47I) in series with VCC.
Twisted-Pair and Shielded Cables
Use 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 shield-
ed twisted pair such as those 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 cables to DXP_ and DXN_ and
the shielded cable to GND. Leave the shielded cable
unconnected at the remote sensor. For very long cable
runs, the cable’s parasitic capacitance often provides
noise filtering; therefore the 100pF capacitor can often
be removed or reduced in value. Cable resistance also
affects remote-sensor accuracy. For every 1I of series
resistance, the error is approximately +0.5NC.
Figure 5. Recommended DXP_–DXN_ PCB Traces. The two
outer guard traces are recommended if high-voltage traces
are near the DXN_ and DXP_ traces.
5–10 mils
5–10 mils
5–10 mils
MINIMUM
5–10 mils
GND
DXP_
DXN_
GND
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
______________________________________________________________________________________ 23
Typical Application Circuit
Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns,
go to www.maxim-ic.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package
drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
DXP2 ALERT
VCC
OVERT
STBY
DXP7
I.C.
24 23 22 21 20 19
18
17
16
15
14
13
12
DXN2
1
2
DXP1 N.C. GND
+3.3V
SMBCLK SMBDATA
CPU
FPGA
ASIC
100pF
100pF
DXP3
DXN3
DXN1
DXN7
3
4
5
6
100pF
100pF
100pF100pF
10 11
DXN6 DXP6
87 9
DXP5DXN4 DXN5
4.7kI4.7kI4.7kI4.7kI
TO µP
TO µP
TO µP
TO µP
0.1µF
DXP4
N.C.
MAX6581
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
24 TQFN-EP T2444+4 21-0139 90-0022
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
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.
24 Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 8/10 Initial release