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

menting until either the duty cycle reaches its minimum

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

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

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

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).

Automatic PWM Fan-Speed Controllers with

Overtemperature Output

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