FANFAIL

The FANFAIL output asserts to indicate that one of the

fans has failed or is spinning slower than the required

speed. The MAX6639 detects fan fault depending on the

fan-control mode. In PWM mode, the MAX6639 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 MAX6639 signals a fan fault when the

tachometer count is greater than the maximum tachome-

ter count value stored in the appropriate register (22h

and 23h). After the MAX6639 asserts FANFAIL, 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 FANFAIL status bits and the output. The MAX6639

measures the fan speed again after 2s. The MAX6639

asserts FANFAIL if it detects the fan fault again.

In RPM mode (either automatic or manual), the

MAX6639 checks for fan failure only when the duty

cycle reaches 100%. It asserts FANFAIL when 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 Control

The MAX6639 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 MAX6639 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 MAX6639 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 Mode

Enter 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 MAX6639 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 5 and the Fan

1 and 2 Configuration 1 (10h and 14h) section.

Manual RPM Control Mode

Enter 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

MAX6639 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 MAX6639

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 MAX6639 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 MAX6639 gets into a

locked state and updates the duty cycle every 2s.

Automatic RPM Control Mode

In the automatic RPM control mode, the MAX6639 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 RPM modes (automatic and manual), the

MAX6639 implements a low limit for the tachometer

Initial duty cycle

t

etTACH

arg

=

−

255

2

2-Channel Temperature Monitor with Dual,

Automatic, PWM Fan-Speed Controller

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