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MAX6640AEE
2-Channel Temperature Monitor with Dual Automatic PWM Fan-Speed Controller
General DescriptionThe MAX6640 monitors its own temperature and one
external diode-connected transistor or the temperatures
of two external diode-connected transistors, typically
available in CPUs, FPGAs, or GPUs. The 2-wire serial
interface accepts standard System Management Bus
(SMBus)TMwrite byte, read byte, send byte, and
receive byte commands to read the temperature data
and program the alarm thresholds. Temperature data
can be read at any time over the SMBus, and three pro-
grammable alarm outputs can be used to generate
interrupts, throttle signals, or overtemperature shut-
down signals.
The temperature data is also used by the internal dual
PWM fan-speed controller to adjust the speed of up to
two cooling fans, thereby minimizing noise when the
system is running cool, but providing maximum cooling
when power dissipation increases. Speed control is
accomplished by tachometer feedback from the fan, so
that the speed of the fan is controlled, not just the PWM
duty cycle. Accuracy of speed measurement is ±4%.
The MAX6640 is available in 16-pin QSOP and 16-pin
TQFN 5mm x 5mm packages. It operates from 3.0V to
5.5V and consumes just 500µA of supply current.
ApplicationsDesktop Computers
Notebook Computers
Workstations
Servers
Networking Equipment
FeaturesTwo Thermal-Diode InputsLocal Temperature Sensor1°C Remote Temperature Accuracy (+60°C to
+100°C)Two PWM Outputs for Fan Drive (Open Drain; can
be Pulled Up to +13.5V)Programmable Fan-Control CharacteristicsAutomatic Fan Spin-Up Ensures Fan StartControlled Rate-of-Change Ensures Unobtrusive
Fan-Speed Adjustments±4% Fan-Speed Measurement AccuracyTemperature Monitoring Begins at POR for Fail-
Safe System ProtectionOT
and THERMOutputs for Throttling or
ShutdownMeasures Temperatures Up to +150°CTiny 5mm x 5mm 16-Pin TQFN and QSOP
Packages
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
Ordering Information19-3344; Rev 0; 8/04
Pin ConfigurationsSMBus is a trademark of Intel Corp.
Typical Application Circuit appears at end of data sheet.
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= +3.0V to +5.5V, TA= 0°C to +125°C, unless otherwise noted. Typical values are at VCC= +3.3V,TA= +85°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.
VCCto GND..............................................................-0.3V to +6V
PWM1, PWM2, TACH1, and TACH2 to GND......-0.3V to +13.5V
DXP1 and DXP2 to GND..........................-0.3V to +(VCC+ 0.3V)
DXN to GND..........................................................-0.3V to +0.8V
SCL, SDA, THERM, OT, FANFAIL,
and ALERTto GND..............................................-0.3V to +6V
SDA, OT, THERM, ALERT, FANFAIL,
PWM1, and PWM2 Current.............................-1mA to +50mA
DXN Current.......................................................................±1mA
ESD Protection (all pins, Human Body Model)..................2000V
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derated 8.3mW/°C above +70°C).......667mW
16-Pin TQFN 5mm x 5mm
(derated at 33.3mW/°C above +70°C)................2666.7mW
Operating Temperature Range.........................-40°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range ............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
ELECTRICAL CHARACTERISTICS (continued)(VCC= +3.0V to +5.5V, TA= 0°C to +125°C, unless otherwise noted. Typical values are at VCC= +3.3V,TA= +85°C.) (Note 1)
Note 2:Timing specifications guaranteed by design.
Note 3:The serial interface resets when SCL is low for more than tTIMEOUT.
Note 4:A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SCL's falling edge.
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
Typical Operating Characteristics(VCC= 3.3V, TA= +25°C)
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controllerypical Operating Characteristics (continued)(VCC= 3.3V, TA= +25°C)
MAX6640
Detailed DescriptionThe MAX6640 monitors its own temperature and a
remote diode-connected transistor or the temperatures
of two external diode-connected transistors, which typi-
cally reside on the die of a CPU or other integrated cir-
cuit. The 2-wire serial interface accepts standard
SMBus write byte, read byte, send byte, and receive
byte commands to read the temperature data and pro-
gram the alarm thresholds. Temperature data can be
read at any time over the SMBus, and a programmable
alarm output can be used to generate interrupts, throt-
tle signals, or overtemperature shutdown signals.
The temperature data is also used by the internal dual
PWM fan-speed controller to adjust the speed of up to
two cooling fans, thereby minimizing noise when the
system is running cool, but providing maximum cooling
when power dissipation increases. RPM feedback
allows the MAX6640 to control the fan’s actual speed.
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
Block DiagramFigure 1. SMBus Protocols
SMBus Digital InterfaceFrom a software perspective, the MAX6640 appears as
a set of byte-wide registers. This device uses a stan-
dard SMBus 2-wire/I2C™-compatible serial interface to
access the internal registers.
The MAX6640 employs four standard SMBus protocols:
write byte, read byte, send byte, and receive byte
(Figures 1, 2, and 3). The shorter receive byte protocol
allows quicker transfers, provided that the correct data
register was previously selected by a read byte instruc-
tion. Use caution with the shorter protocols in multimas-
ter systems, since a second master could overwrite the
command byte without informing the first master.
Table 3 details the register addresses and functions,
whether they can be read or written to, and the power-
on reset (POR) state. See Tables 4–8 for all other regis-
ter functions and the Register Descriptionssection.
Temperature ReadingTemperature data can be read from registers 00h and
01h. The temperature data format for these registers is
8 bits, with the LSB representing 1°C (Table 1) and the
MSB representing +128°C. The MSB is transmitted first.
Three additional temperature bits provide resolution
down to 0.125°C and are in the channel 1 extended
temperature (05h) and channel 2 extended tempera-
ture (06h) registers. All values below 0°C clip to 00h.
The MAX6640 employs a register lock mechanism to
avoid getting temperature results from the temperature
register and the extended temperature register sam-
pled at two different time points. Reading the extended
register stops the MAX6640 from updating the temper-
ature register for at least 0.25s, unless there is a tem-
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed ControllerFigure 2. SMBus Write Timing Diagram
Figure 3. SMBus Read Timing Diagram
MAX6640perature register read before the scheduled update.
This allows enough time to read the main register
before it is updated, thereby preventing reading the
temperature register data from one conversion and the
extended temperature register data from a different
conversion.
The MAX6640 measures the temperature at a fixed rate
of 4Hz immediately after it is powered on. Setting bit 7
of the configuration register (04h) shuts down the tem-
perature measurement cycle.
OutputWhen a measured temperature exceeds the corre-
sponding OTtemperature threshold and OTis not
masked, the associated OTstatus register bit sets and
the OToutput asserts. If OTfor the respective channel
is masked, the OTstatus register sets, but the OTout-
put does not assert. To deassert the OToutput and the
associated status register bit, either the measured tem-
perature must fall at least 5°C below the trip threshold
or the trip threshold must be increased to at least 5°C
above the current measured temperature.
THERMWhen a measured temperature exceeds the corre-
sponding THERMtemperature threshold and THERMis
not masked, the associated THERMstatus register bit
is set and the THERMoutput asserts. If THERMfor the
respective channel is masked, the THERMstatus regis-
ter is set, but the THERMoutput does not assert. To
deassert the THERMoutput and the associated status
register bit, either the measured temperature must fall
at least 5°C below the trip threshold or the trip thresh-
old must be increased to at least 5°C above the current
measured temperature. Asserting THERMinternally or
externally forces both PWM outputs to 100% duty cycle
when bit 6 in address 13h (fan 1) or bit 6 in address
17h (fan 2) is set.
ALERTThe ALERToutput asserts to indicate that a measured
temperature exceeds the ALERTtrip threshold for that
temperature channel. The status bit and the ALERTout-
put clear by reading the ALERTstatus register. If the
ALERTstatus bit is cleared, but the temperature still
exceeds the ALERTtemperature threshold, ALERT
reasserts on the next conversion, and the status bit sets
again. A successful alert response protocol clearsALERT, but does not affect the ALERTstatus bit.
TACH1 and TACH2 InputsTo measure the fan speed, the MAX6640 has two
tachometers. Each tachometer has an accurate internal
clock to count the time elapsed in one revolution.
Therefore, it is counting the time between two tachome-
ter pulses for a fan with four poles. When the PWM sig-
nal is used to directly modulate the fan’s power supply,
the PWM frequency is normally in the 20Hz to 100Hz
range. In this case, the time required for one revolution
may be longer than the PWM on-time. For this reason,
the PWM pulses are periodically stretched to allow
tachometer measurement over a full revolution. Turn off
pulse stretching by setting bit 5 of register 13h or regis-
ter 17h when using a 4-wire fan.
The tachometer count is inversely proportional to the
fan’s RPM. The tachometer count data is stored in regis-
ter 20h (for TACH1) and register 21h (for TACH2).
Reading a value of 255 from the TACH count register
means the fan’s RPM is zero or too slow for the range.
Reading a value of zero in the TACH count register
means the fan’s RPM is higher than the range selected.
Table 2 shows the fan’s available RPM ranges. Use reg-
isters 10h or 14h to select the appropriate RPM range for
the fan being used.
FANFAILThe FANFAILoutput asserts to indicate that one of the
fans has failed or is spinning slower than the required
speed. The MAX6640 detects fan fault depending on
the fan control mode. In PWM mode, the MAX6640 pro-
duces a square wave with a duty cycle set by the value
written to the duty-cycle registers (26h and 27h). In this
mode, the MAX6640 signals a fan fault when the
tachometer count is greater than the maximum
tachometer count value stored in the appropriate regis-
ter (22h and 23h). After the MAX6640 asserts FAN
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
Table 2. Tachometer Setting
FAIL, the fan with a tachometer fault goes to full speed for
2s in an attempt to restart the fan and then returns to the
original duty-cycle settings. Reading the status register
clears the FANFAILstatus bits and the output. The
MAX6640 measures the fan speed again after 2s. The
MAX6640 asserts FANFAILif it detects the fan fault again.
In RPM mode (either automatic or manual), the
MAX6640 checks for fan failure only when the duty
cycle reaches 100%. It asserts FANFAILwhen the
tachometer count is greater than twice the target
tachometer count. In manual RPM mode, registers 22h
and 23h store the target tachometer count value. In
automatic RPM mode, these registers store the maxi-
mum tachometer count.
Fan-Speed ControlThe MAX6640 adjusts fan speed by controlling the duty
cycle of a PWM signal. This PWM signal then either
modulates the DC brushless fan’s power supply or dri-
ves a speed-control input on a fan that is equipped with
one. There are three speed-control modes: PWM, in
which the PWM duty cycle is directly programmed over
the SMBus; manual RPM, in which the desired
tachometer count is programmed into a register and
the MAX6640 adjusts its duty cycle to achieve the
desired tachometer count; and automatic RPM, in
which the tachometer count is adjusted based on a
programmed temperature profile.
The MAX6640 divides each PWM cycle into 120 time
slots. Registers 26h and 27h contain the current values
of the duty cycles for PWM1 and PWM2, expressed as
the effective time slot length. For example, the PWM1
output duty cycle is 25% when register 26h reads 1Eh
(30/120).
PWM Control ModeEnter PWM mode by setting bit 7 of the fan 1 or 2 con-
figuration 1 register (10h and 14h) to 1. In PWM control
mode, the MAX6640 generates PWM signals whose
duty cycles are specified by writing the desired values
to fan duty-cycle registers 26h and 27h. When a new
duty-cycle value is written into one of the fan duty-cycle
registers, the duty cycle changes to the new value at a
rate determined by the rate-of-change bits [6:4] in the
fan 1 or 2 configuration 1 register. The rate-of-change
of the duty cycle ranges from 000 (immediately
changes to the new programmed value) to 111
(changes by 1/120 every 4s). See Table 4 and the Fan
1 and 2 Configuration 1 (10h and 14h)section.
Manual RPM Control ModeEnter manual RPM control mode by setting bits 2, 3,
and 7 of the fan 1 or 2 configuration 1 register (10h and
14h) to zero. In the manual RPM control mode, the
MAX6640 adjusts the duty cycle and measures the fan
speed. Enter the target tachometer count in register
22h for fan 1 and register 23h for fan 2. The MAX6640
compares the target tachometer count with the mea-
sured tachometer count and adjusts the duty cycle so
that the fan speed gradually approaches the target
tachometer count.
The first time manual RPM control mode is entered, the
initial PWM duty cycle is determined by the target
tachometer count:
where targetTACH is the value of the target tachometer
count in the target tach count register (22h or 23h).
If the initial duty cycle value is over 120, the duty cycle
is 100%. If spin-up is enabled (bit 7 in registers 13h
and 17h) and the fan is not already spinning, the duty
cycle first goes to 100% and then goes to the initial
duty-cycle value. Every 2s, the MAX6640 counts the
fan’s period by counting the number of pulses stored in
registers 24h and 25h. If the count is different from the
target count, the duty cycle is adjusted.
If a nonzero rate-of-change is selected, the duty cycle
changes at the specified rate until the tachometer count
is within ±5 of the target. Then the MAX6640 gets into a
locked state and updates the duty cycle every 2s.
Automatic RPM Control ModeIn the automatic RPM control mode, the MAX6640 mea-
sures temperature, sets a target tachometer count
based on the measured temperature, and then adjusts
the duty cycle so the fan spins at the desired speed.
Enter this mode by setting bit 7 of the fan 1 or 2 config-
uration 1 register (10h and 14h) to zero and selecting
the temperature channel that controls the fan speed
using bits 2 and 3 of the configuration register.
In both the RPM modes (automatic and manual), the
MAX6640 implements a low limit for the tachometer
counts. This limits the maximum speed of the fan by
ensuring that the fan’s tachometer count does not go
lower than the tachometer count specified by bits 5
through 0 of register 24h for fan 1 and register 25h for
fan 2. Typical values for the minimum tachometer count
are 30h to 60h. Set the value to correspond to the full-
rated RPM of the fan. See Figure 4.
Figure 5 shows how the MAX6640 calculates the target
tachometer value based on the measured temperature.
At TMIN, the fan spins at a minimum speed value corre-
sponding to the maximum tachometer count value
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
MAX6640stored in register 22h or 23h. Bit 0 of register 11h (fan
1) and register 15h (fan 2) selects the behavior below
TMIN. If bit 0 is equal to zero, the fan will be completely
off below TMIN. When the temperature is falling, it must
drop 5°C below TMINbefore the fan turns off. If bit 0 is
set to 1, the fan does not turn off below TMIN, but
instead stays at the maximum tachometer count in reg-
ister 22h or 23h.
When the measured temperature is higher than TMIN,
the MAX6640 calculates the target tachometer count
value based on two linear equations. The target
tachometer count decreases by the tach step size
value stored in bits 7 through 4 of registers 11h and
15h each time the measured temperature increases by
the temperature step size value stored in bits 2 and 3 of
registers 11h and 15h. As the measured temperature
continues to increase, a second tachometer step size
goes into effect. Bits 3 through 0 of register 12h and
16h select the number temperature/PWM steps after
which the new step size takes effect. The new step size
is selected by bits 7 to 4 of registers 12h and 16h.
Register Descriptions
Channel 1 and Channel 2 Temperature Registers
(00h and 01h)These registers contain the results of temperature mea-
surements. The MSB has a weight of +128°C and the
LSB +1°C. Temperature data for remote diode 1 is in
the channel 1 temperature register. Temperature data
for remote diode 2 or the local sensor (selectable by bit
4 in the global configuration register) is in the channel 2
temperature register. Three additional temperature bits
provide resolution down to 0.125°C and are in the
channel 1 extended temperature (05h) and channel 2
extended temperature (06h) registers. The channel 1
and channel 2 temperature registers do not update
until at least 250ms after the access of the associated
extended temperature registers. All values below 0°C
return 00h.
Status Register (02h)A 1 indicates that an ALERT, THERM, OT, or fan fault has
occurred. Reading this register clears bits 7, 6, 1, and 0.
Reading the register also clears the ALERTand
FANFAILoutputs, but not the THERMand OToutputs. If
the fault is still present on the next temperature measure-
ment cycle, any cleared bits and outputs are set again.
A successful alert response clears the values on the out-
puts but does not clear the status register bits. The
ALERTbits assert when the measured temperature is
higher than the respective thresholds. The THERMandoutputs behave like comparators with 5°C hysteresis.
Mask Register (03h)This register masks the ALERT, OT, THERM, and
FANFAILoutputs. A 1 prevents the corresponding fail-
ures from being asserted on these outputs. The mask
bits do not affect the status register.
Global Configuration Register (04h)The global configuration register controls the shutdown
mode, power-on reset, SMBus timeout, and tempera-
ture channel 2 source select:
D7: Run/Standby.Normal operation is run(0).
Setting this bit to 1 suspends conversions and puts
the MAX6640 into low-power sleep mode.
D6: Software POR.Writing a 1 resets all registers to
their default values.
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed ControllerFigure 4. Tachometer Target Calculation
Figure 5. RPM Target Calculation
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
