T
HIGH
, the duty cycle may increase just a few percent
above the minimum duty cycle. If the power dissipation or
ambient temperature increases to a high-enough value,
the duty cycle may eventually need to increase to 100%.
If the ambient temperature or the power dissipation
reduces to the point that the measured temperature is
less than T
LOW
, the duty cycle begins slowly decre-
menting until either the duty cycle reaches its minimum
value or the temperature rises above T
LOW
.
The small duty-cycle increments and slow rate-of-
change of duty cycle (1.5% maximum per 4s) reduce
the likelihood that the process of fan-speed control is
acoustically objectionable. The “dead band” between
T
LOW
and T
HIGH
keeps the fan speed constant when
the temperature is undergoing small changes, thus
making the fan-control process even less audible.
Fan-Fail Sensing
The MAX6643/MAX6644/MAX6645 feature a FANFAIL
output. The FANFAIL output is an active-low, open-
drain alarm. The MAX6643/MAX6644/MAX6645 detect
fan failure either by measuring the fan’s speed and rec-
ognizing when it is too low, or by detecting a locked-
rotor logic signal from the fan. Fan-failure detection is
enabled only when the duty cycle of the PWM drive sig-
nal is equal to 100%. This happens during the spin-up
period when the fan first turns on and whenever the
temperature is above T
HIGH
long enough that the duty
cycle reaches 100%.
Many fans have open-drain tachometer outputs that
produce periodic pulses (usually two pulses per revolu-
tion) as the fan spins. These tachometer pulses are
monitored by the FAN_IN_ inputs to detect fan failures.
If a 2-wire fan with no tachometer output is used, the
fan’s speed can be monitored by using an external
sense resistor at the source of the driving FET (see
Figure 3). In this manner, the variation in the current
flowing through the fan develops a periodic voltage
waveform across the sense resistor. This periodic
waveform is then highpass filtered and AC-coupled to
the FAN_IN_ input. Any variations in the waveform that
have an amplitude of more than ±150mV are converted
to digital pulses. The frequency of these digital pulses
is directly related to the speed of the rotation of the fan
and can be used to detect fan failure.
Note that the value of the sense resistor must be
matched to the characteristics of the fan’s current
waveform. Choose a resistor that produces voltage
variations of at least ±200mV to ensure that the fan’s
operation can be reliably detected. Note that while
most fans have current waveforms that can be used
with this detection method, there may be some that do
not produce reliable tachometer signals. If a 2-wire fan
is to be used with fault detection, be sure that the fan is
compatible with this technique.
To detect fan failure, the analog sense-conditioned
pulses or the tachometer pulses are deglitched and
counted for 2s while the duty cycle is 100% (either dur-
ing spin-up or when the duty cycle rises to 100% due to
measured temperature). If more than 32 pulses are
counted (corresponding to 480rpm for a fan that pro-
duces two pulses per revolution), the fan is assumed to
be functioning normally. If fewer than 32 pulses are
received, the FANFAIL output is enabled and the PWM
duty cycle to the FET transistor is either shut down in
case of a single-fan (MAX6643) configuration or contin-
ues normal operation in case of a dual-fan configuration
(MAX6644/MAX6645).
Some fans have a locked-rotor logic output instead of a
tachometer output. If a locked-rotor signal is to be used
to detect fan failure, that signal is monitored for 2s while
the duty cycle is 100%. If a locked-rotor signal remains
active (low) for more than 2s, the fan is assumed to
have failed.
The MAX6643/MAX6644/MAX6645 have two channels
for monitoring fan-failure signals, FAN_IN1 and
FAN_IN2. For the MAX6643, the FAN_IN_ channels
monitor a tachometer. The MAX6643’s fault sensing can
also be turned off by floating the TACHSET input.
For the MAX6644 and MAX6645, the FAN_IN1 and
FAN_IN2 channels can be configured to monitor either
a logic-level tachometer signal, the voltage waveform
on a current-sense resistor, or a locked-rotor logic sig-
nal. The TACHSET input selects which type of signal is
to be monitored (see Table 3). To disable fan-fault
sensing, TACHSET should be unconnected and
FAN_IN1 and FAN_IN2 should be connected to V
DD
.
OT
Output
The MAX6643/MAX6644/MAX6645 include an over-
temperature output that can be used as an alarm or a
system-shutdown signal. Whenever the measured tem-
perature exceeds the value selected using the OT pro-
gramming inputs OT1 and OT2 (see Table 4), OT is
asserted. OT deasserts only after the temperature
drops below the threshold.
FULLSPD Input
The MAX6643 features a FULLSPD input. Pulling FULL-
SPD high forces PWM_OUT to 100% duty cycle. The
FULLSPD input allows a microcontroller to force the fan
to full speed when necessary. By connecting FANFAIL
to an inverter, the MAX6643 can force other fans to
100% in multifan systems, or for an over-temperature
condition (by connecting OT inverter to FULLSPD).
MAX6643/MAX6644/MAX6645
Automatic PWM Fan-Speed Controllers with
Overtemperature Output
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