General Description
The MAX6642 precise, two-channel digital temperature
sensor accurately measures the temperature of its own
die and a remote PN junction and reports the temperature data over a 2-wire serial interface. The remote PN
junction is typically a substrate PNP transistor on the
die of a CPU, ASIC, GPU, or FPGA. The remote PN
junction can also be a discrete diode-connected smallsignal transistor.
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 MAX6642 includes an
SMBus timeout. The temperature data format is 10 bit
with the least significant bit (LSB) corresponding to
+0.25°C. The ALERT output asserts when the local or
remote overtemperature thresholds are violated. A fault
queue may be used to prevent the ALERT output from
setting until two consecutive faults have been detected.
Measurements can be done autonomously or in a single-shot mode.
Remote accuracy is ±1°C maximum error between
+60°C and +100°C. The MAX6642 operates from -40°C
to +125°C, and measures remote temperatures
between 0°C and +150°C. The MAX6642 is available in
a 6-pin TDFN package with an exposed pad.
Applications
Desktop Computers
Notebook Computers
Servers
Thin Clients
Test and Measurement
Workstations
Graphic Cards
Features
o Dual Channel: Measures Remote and Local
Temperature
o +0.25°C Resolution
o High Accuracy ±1°C (max) (Remote) and
±2°C (Local) from +60°C to +100°C
o Measures Remote Temperature Up to +150°C
o Programmable Overtemperature Alarm
Temperature Thresholds
o SMBus/I
2
C-Compatible Interface
o Tiny TDFN Package with Exposed Pad
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
19-2920; Rev 3; 10/09
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.
EVALUATION KIT
AVAILABLE
Typical Operating Circuit
PART
MEASURED TEMP RANGE
TOP
MARK
MAX6642ATT90-T 0°C to +150°C
AFC
MAX6642ATT92-T 0°C to +150°C
AFD
MAX6642ATT94-T 0°C to +150°C
AFE
MAX6642ATT96-T 0°C to +150°C
AFF
MAX6642ATT98-T 0°C to +150°C
AEW
MAX6642ATT9A-T 0°C to +150°C
AFG
MAX6642ATT9C-T 0°C to +150°C
AFH
MAX6642ATT9E-T 0°C to +150°C AFI
Selector Guide
T = Tape and reel.
*
EP = Exposed pad.
Pin Configuration and Functional Diagram appear at end of
data sheet.
SMBus is a trademark of Intel Corp.
PART TEMP RANGE PIN-PACKAGE
MAX6642ATT90-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT92-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT94-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT96-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT98-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT9A-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT9C-T -40°C to +125°C 6 TDFN-EP*
MAX6642ATT9E-T -40°C to +125°C 6 TDFN-EP*
0.1μF
2200pF
V
DXP
CC
MAX6642
47Ω
SDA
3.3V
10kΩ EACH
DATA
SCLK
ALERT
μP
GND
CLOCK
INTERRUPT TO μP
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V
CC
= +3.0V to +5.5V, TA = -40°C to +125°C, unless otherwise specified. Typical values are at V
CC
= +3.3V and TA = +25°C.) (Note 1)
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)
SCLK, SDA, ALERT ..................................................-0.3V to +6V
SDA, ALERT Current ...........................................-1mA to +50mA
Continuous Power Dissipation (T
A
= +70°C)
6-Pin TDFN (derate 24.4mW/°C above +70°C) .........1951mW
ESD Protection (all pins, Human Body Model) ................±2000V
Junction Temperature......................................................+150°C
Operating Temperature Range .........................-40°C to +125°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage V
Temperature Resolution
Remote Temperature Error V
Local Temperature Error V
Supply Sensitivity of Temperature
Error
Undervoltage Lockout Threshold UVLO Falling edge of VCC disables ADC 2.4 2.7 2.95 V
Undervoltage Lockout Hysteresis 90 mV
Power-On-Reset (POR) Threshold VCC falling edge 1.5 2.0 2.4 V
POR Threshold Hysteresis 90 mV
Standby Supply Current SMBus static 3 10 µA
Operating Current During conversion 0.5 1.0 mA
Average Operating Current 260 µA
Conversion Time t
Conversion Rate f
Remote-Diode Source Current I
ALERT
Output-Low Sink Current
Output-High Leakage Current VOH = V
CC
CONV
CONV
RJ
= 3.3V
CC
= 3.3V
CC
From stop bit to conversion completion 106 125 143 ms
High level 80 100 120
Low level 8 10 12
VOL = 0.4V 1
V
= 0.6V 4
OL
CC
3.0 5.5 V
0.25 °C
10 Bits
TRJ = +60°C to +100°C,
= +25°C to +85°C
T
A
TRJ = 0°C to +125°C -3.0 +3.0
T
= +125°C to +150°C -3.5 +3.5
RJ
TA = +60°C to +100°C -2.0 +2.0
= 0°C to +125°C -3.0 +3.0
T
A
-1.0 +1.0
±0.2 °C/V
8Hz
1µA
°C
°C
µA
mA
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
_______________________________________________________________________________________ 3
Note 1: All parameters tested at TA= +25°C. 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.
ELECTRICAL CHARACTERISTICS (continued)
(V
CC
= +3.0V to +5.5V, TA = -40°C to +125°C, unless otherwise specified. Typical values are at V
CC
= +3.3V and TA = +25°C.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
SMBus-COMPATIBLE INTERFACE (SCLK and SDA)
Logic Input Low Voltage V
Logic Input High Voltage V
Input Leakage Current I
Output Low Sink Current I
Input Capacitance C
SMBus TIMING (Note 2)
Serial Clock Frequency f
Bus Free Time Between STOP
and START Condition
START Condition Setup Time 4.7 µs
Repeat START Condition Setup
Time
START Condition Hold Time t
STOP Condition Setup Time t
Clock Low Period t
Clock High Period t
Data Setup Time t
Receive SCLK/SDA Rise Time t
Receive SCLK/SDA Fall Time t
Pulse Width of Spike Suppressed t
SMBus Timeout t
IL
IH
LEAK
OL
IN
SCLK
t
BUF
t
SU:STA
HD:STA
SU:STO
LOW
HIGH
HD:DAT
R
F
SP
TIMEOUT
VCC = 3.0V 2.2 V
VIN = GND or 5.5V -1 +1 µA
VOL = 0.6V 6 mA
(Note 3) 100 kHz
90% to 90% 50 ns
10% of SDA to 90% of SCLK 4 µs
90% of SCLK to 90% of SDA 4 µs
10% to 10% 4.7 µs
90% to 90% 4 µs
(Note 4) 250 µs
SDA low period for interface reset 20 28 40 ms
0.8 V
5pF
4.7 µs
1µs
300 ns
050ns
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC= 3.3V, TA= +25°C, unless otherwise noted.)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.01 0.1 1 10 100
STANDBY SUPPLY CURRENT
vs. CLOCK FREQUENCY
MAX6642 toc01
CLOCK FREQUENCY (kHz)
SUPPLY CURRENT (μA)
-4
-2
-3
0
-1
1
2
REMOTE TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
MAX6642 toc02
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
0507525 100 125
2N3906
-3
-1
-2
1
0
2
3
0 125
LOCAL TEMPERATURE ERROR
vs. DIE TEMPERATURE
MAX 6642 toc03
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
5025 75 100
-1.5
-0.5
-1.0
0.5
0
1.5
1.0
2.0
0.0001 0.01 0.10.001 1 10 100
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6642 toc04
FREQUENCY (kHz)
TEMPERATURE ERROR (°C)
VIN = 100mV
P-P
SQUARE WAVE
APPLIED TO V
CC
WITH NO BYPASS CAPACITOR
LOCAL ERROR
REMOTE ERROR
0
30
20
10
40
50
60
70
80
90
100
0.001 0.10.01 1 10 100
TEMPERATURE ERROR
vs. DXP NOISE FREQUENCY
MAX6642 toc05
FREQUENCY (kHz)
TEMPERATURE ERROR (°C)
LOCAL ERROR
REMOTE ERROR
VIN = AC-COUPLED TO DXP
V
IN
= 100mV
P-P
SQUARE WAVE
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0
1.0
2.0
0.1 1 10 100
TEMPERATURE ERROR
vs. DXP-GND CAPACITANCE
MAX6642 toc06
DXP-GND CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
Detailed Description
The MAX6642 is a temperature sensor for local
and remote temperature-monitoring applications.
Communication with the MAX6642 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 limit.
The MAX6642 converts temperatures to digital data
either at a programmed rate of eight conversions per
second or in single conversions. Temperature data is
represented by 8 data bits (at addresses 00h and 01h),
with the LSB equal to +1°C and the MSB equal to
+128°C. Two additional bits of remote temperature data
are available in the “extended” register at address 10h
and 11h (Table 2) providing resolution of +0.25°C.
ADC and Multiplexer
The averaging ADC integrates over a 60ms period
(each channel, typ), 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 GND rather than leaving DXP open.
The conversion time per channel (remote and internal)
is 125ms. If both channels are being used, then each
channel is converted four times per second. If the
external conversion-only option is selected, then the
remote temperature is measured eight times per second. 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 4). 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 the
Typical Operating
Characteristics
). In standby mode, the MAX6642 can
be forced to perform ADC 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). In standby mode, supply current
drops to 3µA (typ).
SMBus Digital Interface
From a software perspective, the MAX6642 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.
The MAX6642 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
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
_______________________________________________________________________________________ 5
Pin Description
PIN NAME FUNCTION
1V
2 GND Ground
3 DXP
4 SCLK SMBus Serial-Clock Input. May be pulled up to +5.5V regardless of VCC.
5 SDA SMBus Serial-Data Input/Output, Open Drain. May be pulled up to +5.5V regardless of VCC.
6 ALERT
—EP
Supply Voltage Input, +3V to +5.5V. Bypass V
CC
recommended but not required for additional noise filtering.
Combined Remote-Diode Current Source and ADC Input for Remote-Diode Channel. Place a 2200pF
capacitor between DXP and GND for noise filtering.
SMBus Alert (Interrupt) Output, Open Drain. ALERT asserts when temperature exceeds user-set limits. See
the
ALERT
Interrupts section.
Exposed Pad. Internally connected to GND. Connect to a PCB ground pad for optimal performance. Not
intended as an electrical connection point.
to GND with a 0.1µF capacitor. A 47Ω series resistor is
CC
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
6 _______________________________________________________________________________________
Figure 1. SMBus Protocols
Figure 2. SMBus Write Timing Diagram
WRITE BYTE FORMAT
S ADDRESS WR ACK ACK PDATA ACKCOMMAND
7 BITS 18 BITS8 BITS
SLAVE ADDRESS: EQUIVALENT TO CHIP-SELECT LINE OF
A 3-WIRE INTERFACE
READ BYTE FORMAT
S ADDRESSADDRESS WR ACK ACK PS RD ACK ///DATACOMMAND
7 BITS 7 BITS 8 BITS8 BITS
SLAVE ADDRESS: EQUIVALENT TO CHIP SELECT LINE
SEND BYTE FORMAT
SPADDRESS WR ACK ACKCOMMAND
7 BITS 8 BITS
S = START CONDITION
P = STOP CONDITION
COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
REDING FROM
COMMAND BYTE: SENDS COMMAND WITH NO DATA, USUALLY
USED FOR ONE-SHOT COMMAND
SHADED = SLAVE TRANSMISSION
/// = NOT ACKNOWLEDGED
DATA BYTE: DATA GOES INTO THE REGISTER
SET BY THE COMMAND BYTE (TO SET
THRESHOLDS, CONFIGURATION MASKS, AND
SAMPLING RATE)
SLAVE ADDRESS: REPEATED
DUE TO CHANGE IN DATAFLOW DIRECTION
DATA BYTE: READS FROM
THE REGISTER SET BY THE
COMMAND BYTE
RECEIVE BYTE FORMAT
SPADDRESS RD ACK ///DATA
7 BITS 8 BITS
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
AB CDEFG
t
LOW
SMBCLK
SMBDATA
t
SU:STAtHD:STA
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
t
HIGH
t
SU:DAT
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
HIJ
I = SLAVE PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
LMK
t
SU:STOtBUF
register was previously selected by a Write 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.
Read temperature data 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).
Read the additional bits from the read extended temperature byte register (10h, 11h), which extends the
data to 10 bits and the resolution to +0.25°C per LSB
(Table 2).
When a conversion is complete, the main temperature
register and the extended temperature register are
updated.
Alarm Threshold Registers
Two registers store ALERT threshold values—one each
for the local and remote channels. If either measured
temperature equals or exceeds the corresponding
ALERT threshold value, the ALERT interrupt asserts
unless the ALERT bit is masked.
The power-on-reset (POR) state of the local ALERT
T
HIGH
register is +70°C (0100 0110). The POR state of
the remote ALERT T
HIGH
register is +120°C (0111 1000).
Diode Fault Detection
A continuity fault detector at DXP detects an open circuit on DXP, or a DXP short to VCCor GND. If an open
or short circuit exists, the external temperature register
is loaded with 1111 1111 and status bit 2 (OPEN) of the
status byte is set to 1. 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. Diode faults do not set
the ALERT output.
ALERT
Interrupts
The ALERT interrupt occurs when the internal or external
temperature reading exceeds a high temperature limit
(user programmed). The ALERT interrupt output signal is
latched and can be cleared only by reading the status
register after the fault condition no longer exists or by
successfully responding to the alert response address. If
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
_______________________________________________________________________________________ 7
Figure 3. SMBus Read Timing Diagram
Table 1. Main Temperature Register
(High Byte) Data Format
Table 2. Extended Resolution
Temperature Register (Low Byte) Data
Format
TEMP (°C) DIGITAL OUTPUT
130.00 1 000 0010
127.00 0 111 1111
126.00 0 111 1110
25 0 001 1001
0.00 0 000 0000
<0.00 0 000 0000
Diode fault (short or open) 1 111 1111
FRACTIONAL TEMP (°C) DIGITAL OUTPUT
0.000 00XX XXXX
0.250 01XX XXXX
0.500 10XX XXXX
0.750 11XX XXXX
AB CDEFG H
t
LOWtHIGH
SMBCLK
SMBDATA
t
t
HD:STA
SU:STA
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
t
SU:DAT
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
t
HD:DAT
I
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
t
SU:STO
K
J
t
BUF
MAX6642
the ALERT is cleared by responding to the alert
response address and the temperature fault condition
still exists, ALERT is reasserted after the next temperature-monitoring cycle. To clear ALERT while the temperature is above the trip threshold, write a new high limit that
is higher than the current temperature. The ALERT out-
put is open drain, allowing multiple devices to share a
common interrupt line.
Alert Response Address
The SMBus alert response interrupt pointer provides
quick fault identification for simple slave devices like
temperature sensors. Upon receiving an ALERT inter-
rupt 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 arbitra-
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
8 _______________________________________________________________________________________
Table 3. Command-Byte Assignments
Table 4. Configuration-Byte Bit Assignments
Table 5. Status-Byte Bit Assignments
A D D R ESS PO R ST A T EF U N C T IO N
00h 00h ( 0000 0000) Read l ocal tem p er atur e
01h 00h ( 0000 0000) Read r em ote tem p er atur e
02h N /A Read status b yte
03h 10h ( 0001 0000) Read confi g ur ati on b yt e
05h 46h ( 0100 0110) + 70° C Read l ocal hi g h l i m i t
07h 78h ( 0111 1000) + 120° C Read r em ote hi g h l i m i t
09h N /A W r i te confi g ur ati on b yte
0Bh N /A W r i te l ocal hi g h l i m i t
0D hN /A W r i te r em ote hi g h l i m i t
0Fh N /A S i ng l e shot
10h 0000 0000
11h 0000 0000
FE h4D h ( 0100 1101) Read m anufactur er ID
Read r em ote extend ed
tem p er atur e
Read i nter nal extend ed
tem p er atur e
BIT NAME POR STATE FUNCTION
7 (MSB) MASK1 0 A 1 masks off (disables) the ALERT interrupts.
6 STOP/RUN 0 A 1 puts the MAX6642 into standby mode.
5 External only 0
4
3 to 0 — 0000 Reserved.
Fault
queue
1
A 1 disables local temperature measurements so that only
remote temperature is measured. The measurement rate
becomes 8Hz.
0: ALERT is set by a single fault. 1: Two consecutive faults
are required to set ALERT.
BIT NAME POR STATE FUNCTION
7 (MSB) BUSY 0 A 1 indicates the MAX6642 is busy converting.
6 LHIGH 0
5 — 0 Reserved.
4 RHIGH 0
3 — 0 Reserved.
2 OPEN 0
1 to 0 — 0 Reserved.
A 1 indicates an internal high-temperature fault. Clear
LHIGH with a POR or by reading the status byte.
A 1 indicates an external high-temperature fault. Clear
RHIGH with a POR or by reading the status byte.
A 1 indicates a diode open condition. Clear OPEN with a
POR or by reading the status byte when the condition no
longer exists.
tion 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 alert response protocol
clears the interrupt latch. If the condition still exists, the
device reasserts the ALERT interrupt at the end of the
next conversion.
Command Byte Functions
The 8-bit command byte register (Table 3) is the master
index that points to the various other registers within the
MAX6642. 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.
Single-Shot
The single-shot command immediately forces a new
conversion cycle to begin. If the single-shot command
is received while the MAX6642 is in standby mode
(RUN bit = 1), a new conversion begins, after which the
device returns to standby mode. If a single-shot conversion is in progress when a single-shot command is
received, the command is ignored. If a single-shot
command is received in autonomous mode (RUN bit =
0), the command is ignored.
Configuration Byte Functions
The configuration byte register (Table 4) is a read-write
register with several functions. Bit 7 is used to mask
(disable) interrupts. Bit 6 puts the MAX6642 into standby mode (STOP) or autonomous (RUN) mode. Bit 5 disables local temperature conversions for faster (8Hz)
remote temperature monitoring. Bit 4 prevents setting
the ALERT output until two consecutive measurements
result in fault conditions.
Status Byte Functions
The status byte register (Table 5) 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 on 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 the overtemperature fault
condition no longer exists.
Slave Addresses
The MAX6642 has eight fixed addresses available.
These are shown in Table 6.
The MAX6642 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 2.1 (typ). When
power is first applied and VCCrises above 2.1 (typ),
the logic blocks begin operating, although reads and
writes at VCClevels below 3V are not recommended. A
second VCCcomparator, the ADC undervoltage lockout
(UVLO) comparator prevents the ADC from converting
until there is sufficient headroom (VCC= +2.7V typ).
Power-Up Defaults
Power-up defaults include:
• ALERT output is cleared.
• ADC begins autoconverting at a 4Hz rate.
• Command byte is set to 00h to facilitate quick
local Receive Byte queries.
• Local (internal) T
HIGH
limit set to +70°C.
• Remote (external) T
HIGH
limit set to +120°C.
Applications Information
Remote-Diode Selection
The MAX6642 can directly measure the die temperature
of CPUs and other ICs that have on-board temperaturesensing diodes (see the
Typical Operating Circuit
) or
they 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 MAX6642 is optimized for n
= 1.008, which is the typical value for the Intel Pentium
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
_______________________________________________________________________________________ 9
Table 6. Slave Address
PART NO. SUFFIX ADDRESS
MAX6642ATT90 1001 000
MAX6642ATT92 1001 001
MAX6642ATT94 1001 010
MAX6642ATT96 1001 011
MAX6642ATT98 1001 100
MAX6642ATT9A 1001 101
MAX6642ATT9C 1001 110
MAX6642ATT9E 1001 111
MAX6642
III. A thermal diode on the substrate of an IC is normally
a PNP with its collector grounded. Connect the anode
(emitter) to DXP and the cathode to GND of the
MAX6642.
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. Fortunately,
the difference is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor n
NOMINAL
is used to measure the temperature of a diode with a different ideality factor n1.
The measured temperature T
M
can be corrected using:
where temperature is measured in Kelvin and
n
NOMIMAL
for the MAX6642 is 1.008.
As an example, assume you want to use the MAX6642
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 (358.15K), the measured temperature is +82.91°C (356.02K), 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, the change in the measured voltage due to
series resistance is:
ΔVM= RS (100µA - 10µA) = 90µA R
S
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 calculated 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.
Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
connect its collector and base together. Table 7 lists
examples of discrete transistors that are appropriate for
use with the MAX6642.
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
V
BE
characteristics.
Manufacturers of discrete transistors do not normally
specify or guarantee ideality factor. This is normally not
a problem since good-quality discrete transistors tend
to have ideality factors that fall within a relatively narrow
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
10 ______________________________________________________________________________________
Table 7. Remote-Sensor Transistor
Manufacturers
Note: Discrete transistors must be diode connected (base shorted to collector).
TT
M ACTUAL
TT
ACTUAL M
( . )
=
=
T
⎛
n
⎜
⎝
1 00599
M
⎛
⎜
n
⎝
NOMINAL
NOMINAL
n
1
⎞
n
1
⎟
⎠
⎞
T
=
M
⎟
⎠
1 008
.
⎛
⎜
⎝
1 002
.
μ
V
90
Ω
=
μ
V
.
198 6
°
C
.
0 453
C
°
Ω
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3906
Rohm Semiconductor (USA) SST3906
Samsung (Korea) KST3906-TF
Siemens (Germany) SMBT3906
Zetex (England) FMMT3906CT-ND
°
3 0 453 1 36Ω×
⎞
=
⎟
⎠
C
=+ ° ..
Ω
C
range. We have observed variations in remote temperature readings of less than ±2°C with a variety of discrete transistors. Still, it is good design practice to
verify good consistency of temperature readings with
several discrete transistors from any manufacturer
under consideration.
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 PCB
layout and proper external noise filtering.
High-frequency EMI is best filtered at DXP with an
external 2200pF capacitor. Larger capacitor values can
be used for added filtering, but do not exceed 3300pF
because excessive capacitance can introduce errors
due to the rise time of the switched current source.
Nearly all noise sources tested cause the temperature
conversion results to be higher than the actual temperature, typically by +1°C to +10°C, depending on the
frequency and amplitude (see the
Typical Operating
Characteristics
).
PCB Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Connect the thermal-sense diode to the MAX6642
using two traces—one between DXP and the
anode, the other between the MAX6642’s GND and
the cathode. Do not connect the cathode to GND at
the sense diode.
2) Place the MAX6642 as close as is practical to the
remote thermal 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.
3) Do not route the thermal diode lines next to the
deflection coils of a CRT. Also, do not route the
traces across fast digital signals, which can easily
introduce a 30°C error, even with good filtering.
4) Route the thermal diode traces in parallel and in
close proximity to each other, away from any higher
voltage traces, such as +12VDC. Leakage currents
from PCB 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 trace (Figure 4).
5) Route through as few vias and crossunders as possible to minimize copper/solder thermocouple
effects.
6) When introducing a thermocouple, make sure that
both the thermal diode paths have matching thermocouples. A copper-solder thermocouple exhibits
3µV/°C, and it takes about 200µV of voltage error at
DXP to cause a +1°C measurement error. Adding a
few thermocouples causes a negligible error.
7) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10-mil
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.
8) Add a 47Ω resistor in series with VCCfor best noise
filtering (see the
Typical Operating Circuit
).
9) 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 distances longer than 8in or in very
noisy environments. Twisted-pair cable lengths can be
between 6ft and 12ft before noise introduces excessive
errors. For longer distances, the best solution is a
shielded twisted pair like that used for audio 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 GND and
the shield to GND. Leave the shield unconnected at the
remote diode.
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.
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
______________________________________________________________________________________ 11
Figure 4. Recommended DXP PC Traces
GND
10 mils
10 mils
10 mils
THERMAL DIODE ANODE/DXP
MINIMUM
THERMAL DIODE CATHODE/GND
10 mils
GND
MAX6642
Cable resistance also affects remote-sensor accuracy.
For every 1Ω of series resistance, the error is approximately 1/2°C.
Thermal Mass and Self-Heating
When sensing local temperature, this device is intended to measure the temperature of the PCB to which it is
soldered. The leads provide a good thermal path
between the PCB traces and the die. Thermal conductivity between the die and the ambient air is poor by
comparison, making air temperature measurements
impractical. Because the thermal mass of the PCB is far
greater than that of the MAX6642, the device follows
temperature changes on the PCB with little or no perceivable delay.
When measuring 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 an 8Hz conversion rate and with ALERT sink-
ing 1mA, the typical power dissipation is:
5.0V x 450µA + 0.4V x 1mA = 2.65mW
ø
J-A
for the 6-pin TDFN package is about +41°C/W, so
assuming no copper PCB heat sinking, the resulting
temperature rise is:
ΔT = 2.65mW x 41°C/W = +0.11°C
Even under nearly worst-case conditions, it is difficult to
introduce a significant self-heating error.
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
12 ______________________________________________________________________________________
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
______________________________________________________________________________________ 13
Functional Diagram
Pin Configuration
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
6 TDFN-EP T633-2
21-0137
Chip Information
PROCESS: BiCMOS
DXP
ALERT
V
CC
2
MUX
Q
MAX6642
REMOTE
ADC
LOCAL
DIODE
FAULT
S
R
REGISTER BANK
COMMAND BYTE
REMOTE TEMPERATURE
LOCAL TEMPERATURE
ALERT THRESHOLD
ALERT RESPONSE
ADDRESS
CONTROL
LOGIC
SMBus
8
READ
8
WRITE
7
ADDRESS
DECODER
SDA
SCLK
TOP VIEW
MAX6642
1
V
CC
2GND
3
TDFN-EP
*EXPOSED PAD CONNECTED TO GND.
EP*
ALERT
6
SDA
5
4DXP
SCLK
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
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.
14
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
0 8/03 Initial release —
1 10/08
2 7/09 Corrected errors in Figures 2 and 3 6, 7
3 10/09 Corrected error in Package Information table 13
REVISION
DATE
DESCRIPTION
Added missing EP description to Ordering Information and Pin Description,
removed the transistor count on page 12, and corrected some minor style issues
PAGES
CHANGED
1, 5, 9, 10, 12
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