Fan 1 and 2 Configuration 2b (12h and 16h)
The following registers select the tachometer step sizes
and number of steps for step-size A to step-size B
slope changes (see Figure 1):
• D[7:4]: RPM (Tachometer) Step Size B. Selects
number of tachometer counts the target value
decreases for each temperature step increase after
the number of steps selected by D[3:0]. Value = n +
1 (1 through 16) where n is the value of D[7:4].
• D[3:0]: Selects the number of temperature/tachome-
ter steps above the fan-start temperature at which
step-size B begins.
Fan 1 and Fan 2 Configuration 3 (13h and 17h)
The following registers control fan spin-up, PWM output
frequency, pulse stretching, and THERM to fan full-
speed enable:
• D7: Fan Spin-Up Disable. Set to zero to enable fan
spin-up. Whenever the fan starts up from zero drive,
it is driven with 100% duty cycle for 2s to ensure that
it starts. Set to 1 to disable the spin-up function.
• D6: THERM to Full-Speed Enable. When this bit is
1, THERM going low (either by being pulled low
externally or by the measured temperature exceed-
ing the THERM limit) forces the fan to full speed. In
all modes, this happens at the rate determined by
the rate-of-change selection. When THERM is
deasserted (even if fan has not reached full speed),
the speed falls at the selected rate-of-change to the
target speed.
• D5: Disable Pulse Stretching. Pulse stretching is
enabled when this bit is set to zero. When modulat-
ing the fan’s power supply with the PWM signal, the
PWM pulses are periodically stretched to keep the
tachometer signal available for one full revolution.
Setting this bit to 1 disables pulse stretching. The
MAX6640 still measures the fan speed but does not
stretch the pulses for measurements, so the fan’s
power supply must not be pulse modulated.
• D[1:0]: PWM Output Frequency. These bits control
the PWM output frequency as shown in Table 8.
Fan Tach Count 1 and 2 (20h and 21h)
These registers have the latest tachometer measure-
ment of the corresponding channel. This is inversely
proportional to the fan’s speed. The fan RPM range
should be set so this count falls in the 30 to 160 range
for normal fan operation.
Fan Start Tach Count/Target Tach Count
(22h and 23h)
D[7:0]: This sets the starting tachometer count for the
fan in automatic RPM mode. Depending on the setting
of the minimum duty-cycle bit, the tachometer count
has this value either at all temperatures below the fan-
start temperature or the count is zero below the fan-
start temperature and has this value when the fan-start
temperature is reached. These registers are the target
tach count when in manual RPM mode.
Fan 1 and 2 Pulses and Min RPM (24h and 25h)
D[7:6]: This sets the number of tachometer pulses per
revolution for the fan. When set properly, a 2000RPM fan
with two pulses per revolution has the same tachometer
count as a 2000RPM fan with four pulses per revolution.
Table 9 lists tachometer pulses per revolution.
D[5:0]: This sets the minimum allowable fan tachometer
count (maximum speed). This limits the maximum
speed of the fan to reduce noise at high temperatures.
For reasonable speed resolution, the fan RPM range
should be set so this value is between about 30 and 60.
If a maximum RPM limit is unnecessary, this value can
be set to the full-speed tachometer count.
Fan 1 and 2 Duty Cycle (26h and 27h)
These registers contain the present value of the PWM
duty cycle. In PWM fan-control mode, the desired (tar-
get) value of the PWM duty cycle can be written directly
into this register.
Channel 1 and Channel 2 Fan-Start Temperature
(28h and 29h)
These registers contain the temperatures at which fan
control begins (in automatic RPM mode).
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
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