Datasheet

Fan 1 and 2 Configuration 1 (10h and 14h)
The following registers control the modes of operation
of the MAX6639:
• D7: PWM Mode. D7 = 1 sets the fan into manual
PWM duty-cycle control mode. Write the target duty
cycle in the fan duty-cycle register. D7 = 0 puts the
fan into RPM control mode. To set RPM manually, set
both fan-control temperature channels (bits D2 and
D3) to zero and write the desired tachometer count
into the TACH count register.
• D[6:4]: Fan Duty-Cycle Rate-of-Change. D[6:4]
sets the time between increments of the duty cycle.
Each increment is 1/120 of the duty cycle. By adjust-
ing the rate-of-change, audibility of fan-speed
changes can be traded for response time. Table 5
shows the effect of D[6:4] and, for reference, the time
required for the fan speed to change from 33% to
100% duty cycle as a function of the rate-of-change
bits.
• D[3:2]: Temperature Channel(s) for Fan Control.
Selects the temperature channel(s) that control the
PWM output when the MAX6639 is in automatic RPM
control mode (PWM mode bit is zero). If two chan-
nels are selected, the fan goes to the higher of the
two possible speeds. If neither channel is selected,
then the fan is in manual RPM mode and the speed
is forced to the value written to the target tach count
register 22h or 23h.
• D[1:0]: RPM Range. Scales the tachometer counter
by setting the maximum (full-scale) value of the RPM
range to 2000, 4000, 8000, or 16,000. (Table 3
shows the internal clock frequency as a function of
the range.)
2-Channel Temperature Monitor with Dual,
Automatic, PWM Fan-Speed Controller
14 Maxim Integrated
MAX6639/MAX6639F
A C T U A L R A T E O F CH A N G E AT SPEC IF I C PW M FREQUENCIES
REGISTER 10h
OR 14h D[6:4]
NOMINAL RATE
OF CHANGE (s)
100Hz (s) 50Hz (s) 33.3Hz (s) 20Hz (s)
NOMINAL TIME FROM
33% TO 100% (s)
000 0 0000 0
001 0.0625 0.06 0.06 0.06 0.05 5
010 0.125 0.13 0.12 0.12 0.15 10
011 0.25 0.25 0.26 0.24 0.25 20
100 0.5 0.5 0.5 0.51 0.5 40
101 1 1 1 0.99 1 80
110 2 2 2 1.98 2 160
111 4 4 4 3.96 4 320
Table 5. Fan Duty-Cycle Rate-of-Change
REGISTER 10h OR 14h FAN MAXIMUM RPM VALUE
00 2000
01 4000
10 8000
11 16,000
Table 6. Fan RPM Speed
MAXIMUM RPM VALUE ACTUAL RPM
SELECTED NUMBER
OF PULSES PER
REVOLUTION
ACTUAL FAN PULSES
PER REVOLUTION
TACHOMETER COUNT
VALUE*
2000 1000 2 2 3Ch
4000 1000 2 2 78h
4000 3000 2 2 28h
4000 3000 2 4 14h
16,000 8000 4 4 3Ch
16,000 8000 4 2 78h
Table 7. RPM-to-Tachometer Count Relationship Examples
* Tachometer count value = ((internal clock frequency) x 60) / actual RPM) (selected number of pulses per revolution / actual fan pulses)