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MAX6640 Datasheet(PDF) 9 Page - Maxim Integrated Products

Part No. MAX6640
Description  2-Channel Temperature Monitor with Dual Automatic PWM Fan-Speed Controller
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com

MAX6640 Datasheet(HTML) 9 Page - Maxim Integrated Products

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the FANFAIL status bits and the output. The MAX6640
measures the fan speed again after 2s. The MAX6640
asserts FANFAIL if 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 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 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
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 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
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
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 Mode
In 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
stored in register 22h or 23h. Bit 0 of register 11h (fan
1) and register 15h (fan 2) selects the behavior below
Initial duty cycle
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller

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