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MAX6615AEE+
Dual-Channel Temperature Monitors and Fan-Speed Controllers with Thermistor Inputs
General DescriptionThe MAX6615/MAX6616 monitor two temperature chan-
nels, either the internal die temperature and the temper-
ature of an external thermistor, or the temperatures of
two external thermistors. The temperature data controls
a PWM output signal to adjust the speed of a cooling
fan, thereby minimizing noise when the system is run-
ning cool, but providing maximum cooling when power
dissipation increases. The fans’ tachometer output sig-
nals are monitored by the MAX6615/MAX6616 to detect
fan failure. If a fan failure is detected, the FAN_FAIL
output is asserted.
The 2-wire serial interface accepts standard system
management bus (SMBusTM) write byte, read byte,
send byte, and receive byte commands to read the
temperature data and program the alarm thresholds.
The programmable alarm output can be used to gener-
ate interrupts, throttle signals, or overtemperature shut-
down signals.
The MAX6616 features six GPIOs to provide additional
flexibility. All of the GPIOs power-up as inputs, with the
exception of GPIO0, which powers up as either an input
or an output as determined by connecting the PRESET
pin to ground or VCC.
The MAX6616 is available in a 24-pin QSOP package,
while the MAX6615 is available in a 16-pin QSOP pack-
age. Both devices operate from a single-supply voltage
range of 3.0V to 5.5V, have operating temperature
ranges of -40°C to +125°C, and consume just 500µA of
supply current.
ApplicationsDesktop Computers
Servers
Power Supplies
Networking Equipment
Workstations
FeaturesTwo Thermistor InputsTwo Open-Drain PWM Outputs for Fan-Speed
ControlLocal Temperature SensorSix GPIOs (MAX6616)Programmable Fan-Control CharacteristicsControlled PWM Rate-of-Change Ensures
Unobtrusive Fan-Speed AdjustmentsFail-Safe System ProtectionOT
Output for Throttling or ShutdownNine Different Pin-Programmable SMBus
Addresses16-Pin and 24-Pin QSOP Packages
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs19-3713; Rev 2; 10/08
Ordering Information
PARTTEMP RANGEPIN-PACKAGE
MAX6615AEE-40°C to +125°C16 QSOP
MAX6616AEG-40°C to +125°C24 QSOP
SMBus is a trademark of Intel Corp.
Typical Application Circuits and Pin Configurations appear at
end of data sheet.THERMISTORS
AND LOCAL
TEMP SENSOR
PWM
GENERATOR
AND TACH
COUNTER
SMBus
INTERFACE
AND
REGISTERSLOGIC
ADD0ADD1
*MAX6616 ONLY
FAN_FAIL
MAX6615
MAX6616
GND
VCC
SDA
SCL
REF
TH1
TH2
PWM1
PWM2
TACH1
TACH2
GPIO0*
GPIO5*
PRESET*
Functional Diagram
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
ABSOLUTE MAXIMUM RATINGSStresses 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 Are Referenced to GND
Supply Voltage (VCC)...............................................-0.3V to +6V
PWM_, TACH_, OT, FAN_FAIL............................-0.3V to +13.5V
ADD0, ADD1, SDA, SCL..........................................-0.3V to +6V
All Other Pins..............................................-0.3V to (VCC+ 0.3V)
SDA, OT, FAN_FAIL, PWM_, GPIO_ Current....................±50mA
TH_ Current........................................................................±1mA
REF Current......................................................................±20mA
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derated at 8.3mW/°C
above +70°C)............................................................666.7mW
24-Pin QSOP (derated at 9.5mW/°C
above +70°C)...........................................................761.9 mW
ESD Protection (all pins, Human Body Model)....................±2kV
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
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= +25°C.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSOperating Supply VoltageVCC3.05.5V
Standby CurrentInterface inactive, ADC in idle state10µA
Operating CurrentISInterface inactive, ADC active0.51mA
External Temperature ErrorVCC = +3.3V, 0.15V ≤ VTH_ ≤ +0.71V (excludes
thermistor errors, thermistor nonlinearity) (Note1)±1°C
VCC = +3.3V, 0°C ≤ TA ≤ +85°C,±2.5Internal Temperature ErrorVCC = +3.3V, 0°C ≤ TA ≤ +125°C±4°C
Temperature Resolution0.125°C
Conversion Time250ms
Conversion Rate Timing Error-20+20%
PWM Frequency Error-20+20%
INPUT/OUTPUTOutput Low VoltageVOLVCC = +3V, IOUT = 6mA0.4V
Output High Leakage CurrentIOH1µA
Logic Low Input VoltageVIL0.8V
Logic High Input VoltageVIH2.1V
Input Leakage Current1µA
Input CapacitanceCIN5pF
SMBus TIMING (Figures 2, 3) (Note 2)Serial Clock FrequencyfSCLK10400kHz
Clock Low PeriodtLOW10% to 10%4µs
Clock High PeriodtHIGH90% to 90%4.7µs
Bus Free Time Between STOP
and START ConditionstBUF4.7µs
SMBus START Condition
Setup TimetSU:STA90% of SCL to 90% of SDA4.7µs
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
Note 1:1°C of error corresponds to an ADC error of 7.76mV when VREF= 1V.
Note 2:Guaranteed by design and characterization.
Note 3:Production tested.
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= +25°C.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSTART Condition Hold TimetHD:STO10% of SDA to 10% of SCL4µs
STOP Condition Setup TimetSU:STO90% of SCL to 10% of SDA4µs
Data Setup TimetSU:DAT10% of SDA to 10% of SCL250ns
Data Hold TimetHD:DAT10% of SCL to 10% of SDA300ns
SMBus Fall TimetF300ns
SMBus Rise TimetR1000ns
SMBus Timeout(Note 3)293755ms
Typical Operating Characteristics (VCC = +3.3V, TA= +25°C, unless otherwise noted.)
SUPPLY CURRENT
vs. SUPPLY VOLTAGEMAX6615/6 toc01
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
LOCAL
REMOTE
SHUTDOWN
THERMISTOR TEMPERATURE DATA
vs. THERMISTOR TEMPERATURE
MAX6615/6 toc02
THERMISTOR TEMPERATURE (°C)
THERMISTOR TEMPERATURE DATA (
LOCAL TEMPERATURE ERROR
vs. DIE TEMPERATURE
MAX6615/6 toc03
DIE TEMPERATURE (°C)
TEMPERATURE ERROR (5025
0100
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
Typical Operating Characteristics (continued)(VCC = +3.3V, TA= +25°C, unless otherwise noted.)
GPIO SINK CURRENT
vs. SUPPLY VOLTAGEMAX6615/6 toc04
VCC (V)
IGPIO_
(mA)
VGPIO_ = 0.4V
GPIO OUTPUT VOLTAGE
vs. GPIO SINK CURRENTMAX6615/6 toc05
IGPIO_ (mA)
GPIO_
(V)604050203010
VCC = 3V
VCC = 5V
PWM FREQUENCY
vs. DIE TEMPERATURE
MAX6615/6 toc06
DIE TEMPERATURE (°C)
FREQUENCY SHIFT (Hz)
NORMALIZED AT TA = +25°C
PWM FREQUENCY
vs. SUPPLY VOLTAGE
MAX6615/6 toc07
VCC (V)
FREQUENCY SHIFT (Hz)
NORMALIZED AT VCC = 5.0V
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
PIN
MAX6616MAX6615NAMEFUNCTION1, 2, 5, 20,
23, 24—GPIO0–
GPIO5Active-Low, Open-Drain GPIOs. Can be pulled up to 5.5V regardless of VCC.1PWM1Fan Driver Output 1. The pullup resistor can be connected to a supply voltage as high as
12V, regardless of the supply voltage. See the PWM Output section for configuration.2TACH1Fan Tachometer Input. Accepts logic-level signal from fan’s tachometer output. Can be
connected to a supply voltage as high as 12V, regardless of the supply voltage.3ADD0SMBus Slave Address Selection4ADD1SMBus Slave Address Selection5, 10GNDGround. Must be connected together for MAX6615.6TH1External Thermistor Input 1. Connect a thermistor in series with a fixed resistor between
REF and ground.
10, 15—N.C.No Connection7REFReference Voltage Output. Provides 1V during measurements. High impedance when not
measuring.8TH2External Thermistor Input 2. Connect a thermistor in series with a fixed resistor between
REF and ground.9FAN_FAILFan-Failure Output. Asserts low when either fan fails. Can be pulled up as high as 5.5V
regardless of VCC. High impedance when VCC = 0V.—PRESETConnect to GND or VCC to set POR state of the GPIO0.11OT
Overtemperature Output. Active low, open drain. Typically used for system shutdown or
clock throttling. Can be pulled up as high as 5.5V regardless of VCC. High impedance
when VCC = 0V.12VCCPower Supply. 3.3V nominal. Bypass with a 0.1µF capacitor to GND.13SDASMBus Serial-Data Input/Output. Pull up with a 10kΩ resistor. Can be pulled up as high
as 5.5V regardless of VCC. High impedance when VCC = 0V.14SCLSMBus Serial-Clock Input. Pull up with a 10kΩ resistor. Can be pulled up as high as 5.5V
regardless of VCC. High impedance when VCC = 0V.15TACH2Fan Tachometer Input. Accepts logic-level signal from fan’s tachometer output. Can be
connected to a supply voltage as high as 12V, regardless of the supply voltage.16PWM2Fan Driver Output 2. The pullup resistor can be connected to a supply voltage as high as
12V, regardless of the supply voltage. See the PWM Output section for configuration.
Pin Description
MAX6615/MAX6616
Detailed DescriptionThe MAX6615/MAX6616 accurately monitor two tem-
perature channels, either the internal die temperature
and the temperature of an external thermistor, or the
temperatures of two external thermistors. They report
temperature values in digital form using a 2-wire
SMBus/I2C-compatible serial interface. The MAX6615/
MAX6616 operate from a supply voltage range of 3.0V
to 5.5V and consume 500µA (typ) of supply current.
The temperature data controls the duty cycles of two
PWM output signals that are used to adjust the speed
of a cooling fan. They also feature an overtemperature
alarm output to generate interrupts, throttle signals, or
shutdown signals.
The MAX6616 also includes six GPIO input/outputs to
provide additional flexibility. The GPIO0 power-up state
is set by connecting the GPIO PRESET input to ground
or VCC.
SMBus Digital InterfaceFrom a software perspective, the MAX6615/MAX6616
appear as a set of byte-wide registers. Their devices use
a standard SMBus 2-wire/I2C-compatible serial interface
to access the internal registers. The MAX6615/MAX6616
have nine different slave addresses available; therefore, a
maximum of nine MAX6615/MAX6616 devices can share
the same bus.
The MAX6615/MAX6616employ four standard SMBus
protocols: write byte, read byte, send byte, and receive
byte (Figures 1, 2, and 3). The shorter receive byte proto-
col allows quicker transfers, provided that the correct
data register was previously selected by a read byte
instruction. Use caution with the shorter protocols in mul-
timaster systems, since a second master could overwrite
the command byte without informing the first master.
Temperature 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.
All values below 0°C clip to 00h.
Table 3 details the register address and function, whether
they can be read or written to, and the power-on reset
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
WRITE BYTE FORMAT
READ BYTE FORMAT
SEND BYTE FORMATRECEIVE BYTE FORMATSLAVE ADDRESS: EQUIVA-
LENT TO CHIP-SELECT LINE
OF A 3-WIRE INTERFACE
COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
WRITING TO
DATA BYTE: DATA GOES INTO THE REG-
ISTER SET BY THE COMMAND BYTE (TO
SET THRESHOLDS, CONFIGURATION
MASKS, AND SAMPLING RATE)
SLAVE ADDRESS:
EQUIVALENT TO CHIP-
SELECT LINE
COMMAND BYTE:
SELECTS WHICH
REGISTER YOU ARE
READING FROM
SLAVE ADDRESS: REPEAT-
ED DUE TO CHANGE IN
DATA- FLOW DIRECTION
DATA BYTE: READS
FROM THE REGISTER
SET BY THE COMMAND
BYTE
COMMAND BYTE: SENDS COM-
MAND WITH NO DATA, USUALLY
USED FOR ONE-SHOT COMMAND
DATA BYTE: READS DATA FROM
THE REGISTER COMMANDED BY
THE LAST READ BYTE OR WRITE
BYTE TRANSMISSION; ALSO
USED FOR SMBUS ALERT
RESPONSE RETURN ADDRESSS = START CONDITIONSHADED = SLAVE TRANSMISSION
P = STOP CONDITION/// = NOT ACKNOWLEDGED
Figure 1. SMBus Protocols
ADDRESSRDACKDATA///P7 BITS——8 BITS——
SACKCOMMANDACKP——8 BITS——
ADDRESS7 BITS
ACK
DATA8 BITS
ACK
COMMAND8 BITS
ACK
ADDRESS7 BITS
ADDRESSWRACKCOMMANDACKSADDRESS7 BITS——8 BITS——7 BITS—
ACK
DATA8 BITS
///(POR) state. See Tables 3–7 for all other register functions
and the Register Descriptionssection.
Temperature MeasurementsThe averaging ADC integrates over a 120ms period
(each channel, typically), with excellent noise rejection.
For internal temperature measurements, the ADC and
associated circuitry measure the forward voltage of the
internal sensing diode at low- and high-current levels
and compute the temperature based on this voltage.
For thermistor measurements, the reference voltage
and the thermistor voltage are measured and offset is
applied to yield a value that correlates well to thermistor
temperature within a wide temperature range. Both
channels are automatically converted once the conver-
sion process has started. If one of the two channels is
not used, the circuit still performs both measurements,
and the data from the unused channel may be ignored.
0°, the value in the corresponding temperature register
is clipped to zero when a negative offset is pro-
grammed into the thermistor offset register (17h).
Local (internal) temperature data is expressed directly
in degrees Celsius. Two registers contain the tempera-
ture data for the local channel. The high-byte register
has an MSB of 128°C and an LSB of 1°C. The low- byte
register contains 3 bits, with an MSB of 0.5°C and an
LSB of 0.125°C. The data format is shown in Table 1.
Thermistors allow measurements of external tempera-
tures. Connect a thermistor in series with a resistor,
REXT. The thermistor should be connected between the
TH_ input and ground, and REXT should be connected
between the reference output, REF, and the TH_ input,
as shown in the Typical Application Circuit.
The voltage across REXTis measured by the ADC,
resulting in a value that is directly related to tempera-
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor InputsSMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVECDEFGHIJ
SMBDATA
tSU:STAtHD:STA
tLOWtHIGH
tSU:DATtSU:STOtBUFK
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 2. SMBus Write Timing Diagram
SMBCLKCDEFGHIJK
SMBDATA
tSU:STAtHD:STA
tLOWtHIGH
tSU:DATtHD:DATtSU:STOtBUF
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 M
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 3. SMBus Read Timing Diagram
MAX6615/MAX6616ture. The thermistor data in the temperature register(s)
gives the voltage across REXTas a fraction of the refer-
ence voltage. The LSB of the high byte has a nominal
weight of 7.68mV.OTT
OutputThe OToutput asserts when a thermal fault occurs, and
can therefore be used as a warning flag to initiate sys-
tem shutdown, or to throttle clock frequency. When
temperature exceeds the OTtemperature threshold
and OTis not masked, the OTstatus register indicates
a fault and OToutput becomes asserted. If OTfor the
respective channel is masked off, the OTstatus register
continues to be set, but the OToutput does not
become asserted.
The fault flag and the output can be cleared by reading
the OTstatus register. The OToutput can also be
cleared by masking the affected channel. If the OTsta-
tus bit is cleared, OTreasserts on the next conversion if
the temperature still exceeds the OTtemperature
threshold.
PWM OutputThe PWM_ signals are normally used in one of three
ways to control the fan’s speed: PWM_ drives the gate of a MOSFET or the base of a
bipolar transistor in series with the fan’s power sup-
ply. The Typical Application Circuitshows the PWM_
driving an n-channel MOSFET. In this case, the PWM
invert bit (D4 in register 02h) is set to 1. Figure 4
shows PWM_ driving a p-channel MOSFET and the
PWM invert bit must be set to zero.PWM_ is converted (using an external circuit) into a
DC voltage that is proportional to duty cycle. This
duty-cycle-controlled voltage becomes the power
supply for the fan. This approach is less efficient
than (1), but can result in quieter fan operation.
Figure 5 shows an example of a circuit that converts
the PWM signal to a DC voltage. Because this circuit
produces a full-scale output voltage when PWM =
0V, bit D4 in register 02h should be set to zero.PWM_ directly drives the logic-level PWM speed-
control input on a fan that has this type of input. This
approach requires fewer external components and
combines the efficiency of (1) with the low noise of
(2). An example of PWM_ driving a fan with a speed-
control input is shown in Figure 6. Bit D4 in register
02h should be set to 1 when this configuration is
used.
Whenever the fan has to start turning from a motionless
state, PWM_ is forced high for 2s. After this spin-up
period, the PWM_ duty cycle settles to the predeter-
mined value. Whenever spin-up is disabled (bit 2 in the
configuration byte = 1) and the fan is off, the duty cycle
changes immediately from zero to the nominal value,
ignoring the duty-cycle rate-of-change setting.
The frequency-select register controls the frequency of
the PWM signal. When the PWM signal modulates the
power supply of the fan, a low PWM frequency (usually
33Hz) should be used to ensure the circuitry of the
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
Table 1. Temperature Data Format (High Byte and Low Byte)
HIGH BYTELOW BYTETEMPERATURE (°C)BINARY VALUEHEX VALUEBINARY VALUEHEX VALUE140.01000 11008Ch0000 000000h
127.00111 11117Fh0000 000000h
25.3750001 100119h0110 000060h
25.00001 100119h0000 000000h
0.50000 000000h1000 000080h
0.00000 000000h0000 000000h0000 000000h0000 000000h
VCC
PWM
10kΩ
Figure 4. Driving a p-Channel MOSFET for Top-Side PWM Fan
Drive
brushless DC motor has enough time to operate. When
driving a fan with a PWM-to-DC circuit as shown in
Figure 5, the highest available frequency (35kHz) should
be used to minimize the size of the filter capacitors.
When using a fan with a PWM control input, the frequen-
cy normally should be high as well, although some fans
have PWM inputs that accept low-frequency drive.
The duty cycle of the PWM can be controlled in two ways:Manual PWM control: setting the duty cycle of the fan
directly through the fan target duty-cycle registers
(0Bh and 0Ch).Automatic PWM control: setting the duty cycle based
on temperature.
Manual PWM Duty-Cycle ControlClearing the bits that select the temperature channels for
fan control (D5 and D4 for PWM1 and D3 and D2 for
PWM2) in the fan-configuration register (11h) enables
manual fan control. In this mode, the duty cycle written to
the fan target duty-cycle register directly controls the
corresponding fan. The value is clipped to a maximum of
240. Any value entered above that is changed to 240
automatically. In this control mode, the value in the maxi-
mum duty-cycle register is ignored and does not affect
the duty cycle used to control the fan.
Automatic PWM Duty-Cycle ControlIn the automatic control mode, the duty cycle is con-
trolled by the local or remote temperature according to
the settings in the control registers. Below the fan-start
temperature, the duty cycle is either 0% or is equal to
the fan-start duty cycle, depending on the value of bit
D3 in the configuration byte register. Above the fan-
start temperature, the duty cycle increases by one
duty-cycle step each time the temperature increases by
one temperature step. The target duty cycle is calculat-
ed based on the following formula; for temperature >
FanStartTemperature:
where:
DC = DutyCycle
FSDC = FanStartDutyCycle
T = Temperature
FST = FanStartTemperature
DCSS = DutyCycleStepSize
TS = TempStep
Duty cycle is recalculated after each temperature con-
version if temperature is increasing. If the temperature
begins to decrease, the duty cycle is not recalculated
until the temperature drops by 5°C from the last peak
temperature. The duty cycle remains the same until the
temperature drops 5°C from the last peak temperature or
the temperature rises above the last peak temperature.
For example, if the temperature goes up to +85°C and
starts decreasing, duty cycle is not recalculated until the
temperature reaches +80°C or the temperature rises
above +85°C. If the temperature decreases further, the
duty cycle is not updated until it reaches +75°C.
For temperature < FanStartTemperature and D2 of
configuration register = 0:
DutyCycle = 0
For temperature < FanStartTemperature and D2 of
configuration register = 1:
DutyCycle = FanStartDutyCycle
Once the temperature crosses the fan-start temperature
threshold, the temperature has to drop below the fan-
start temperature threshold minus the hysteresis beforeFSDCTFSTDCSS=+× ( ) -
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs+3.3V
PWM
18kΩ
27kΩ
10kΩ120kΩ
+3.3V
+12V
500kΩ
VOUT
TO FAN
1μF1μF
0.01μF
0.1μF
Figure 5. Driving a Fan with a PWM-to-DC Circuit
VCC
PWM
4.7kΩ
Figure 6. Controlling a PWM Input Fan with the MAX6615/
MAX6616s’ PWM Output (Typically, the 35kHz PWM
Frequency Is Used)
MAX6615/MAX6616the duty cycle returns to either 0% or the fan-start duty
cycle. The value of the hysteresis is set by D7 of the
fan-configuration register.
The duty cycle is limited to the value in the fan maximum
duty-cycle register. If the duty-cycle value is larger than
the maximum fan duty cycle, it is set to the maximum
fan-duty cycle as in the fan maximum duty-cycle register.
The temperature step is bit D6 of the fan-configuration
register (0Dh).
Notice if temperature crosses FanStartTemperature
going up with an initial DutyCycle of zero, a spin-up of
2s applies before the duty-cycle calculation controls
the value of the fan’s duty cycle.
FanStartTemperature for a particular channel follows the
channel, not the fan. If DutyCycle is an odd number, it is
automatically rounded down to the closest even number.
Duty-Cycle Rate-of-Change ControlTo reduce the audibility of changes in fan speed, the
rate of change of the duty cycle is limited by the values
set in the duty-cycle rate-of-change register. Whenever
the target duty cycle is different from the instantaneous
duty cycle, the duty cycle increases or decreases at
the rate determined by the duty-cycle rate-of-change
byte until it reaches the target duty cycle. By setting the
rate of change to the appropriate value, the thermal
requirements of the system can be balanced against
good acoustic performance. Slower rates of change
are less noticeable to the user, while faster rates of
change can help minimize temperature variations.
Remember that the fan controller is part of a complex
control system. Because several of the parameters are
generally not known, some experimentation may be
necessary to arrive at the best settings.
Fan-FailWhen the fan tachometer count is larger than the fan
tachometer limit, the fan is considered failing. The
MAX6615/MAX6616 PWM_ drives the fan with 100%
duty cycle for about 2s immediately after detecting a
fan-fail. At the end of that period, another measurement
is initiated. If the fan fails both measurements, the
FAN_FAILbit, as well as the FAN_FAILoutput, assert if
the pin is not masked. If the fan fails only the first mea-
surement, the fan goes back to normal settings.
If one fan fails, it can be useful to drive the other fan
with 100% duty cycle. This can be enabled with bit D0
of the fan-status register (1Ch).
Slave AddressesThe MAX6615/MAX6616 appear to the SMBus as one
device having a common address for both ADC chan-
nels. The devices’ address can be set to one of nine
different values by pinstrapping ADD0 and ADD1 so
that more than one MAX6615/MAX6616 can reside on
the same bus without address conflicts (see Table 2).
The address input states are checked regularly, and
the address data stays latched to reduce quiescent
supply current due to the bias current needed for high-
impedance state detection.
Power-On DefaultsAt power-on, or when the POR bit in the configuration
byte register is set, the MAX6615/MAX6616 have the
default settings indicated in Table 3. Some of these set-
tings are summarized below:Temperature conversions are active.Channel 1 and channel 2 are set to report the
remote temperature channel measurements.Channel 1 OTlimit = +110°C.Channel 2 OTlimit = +80°C.Manual fan mode.Fan-start duty cycle = 0.PWM invert bit = 1.
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor InputsFAN-START
DUTY CYCLE
TEMPERATURE
DUTY CYCLE
REGISTER 02h,
BIT D3 = 1
DUTY-CYCLE
STEP SIZE
FAN-START
TEMPERATURE
TEMP
STEP
REGISTER 02h,
BIT D3 = 0
Figure 7. Automatic PWM Duty Control
