MAX31785 MAXIM [Maxim Integrated Products], MAX31785 Datasheet - Page 24

• Manufacturer Part Number: MAX31785
• Description: 6-Channel Intelligent Fan Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

6-Channel Intelligent Fan Controller
At temperature sample 2, the temperature has increased
to greater than temperature level 4 so again the PWM
duty cycle must be increased. At temperature sample
3, the temperature has dropped but not below the ther-
mal hysteresis level (which is set by the HYS bits in the
MFR_FAN_CONFIG command code), so the fan speed
remains at level 4. At temperature sample 4, the tem-
perature has dropped below the hysteresis point so the
PWM duty cycle is decreased.
In some 3-wire fan applications, when the supply power
is interrupted to control fan speed, the tachometer sig-
nal is not available. Some fan controllers periodically
stretch the PWM signal to allow the tachometer to be
accurately detected. Pulse stretching can create audible
noise. This device does not implement pulse stretch-
ing. Hence, the RPM fan modes that require a reliable
tachometer signal to be available at all times should
Figure 3. Fan Spin-Up
24
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0
AUTOMATIC
SPIN-UP
ENABLED
AUTOMATIC
SPIN-UP
DISABLED
0.2
0.4
Pulse Stretching
SPIN-UP
CRITERIA MET
0.6
CYCLE AFTER AUTOMATIC SPIN-UP
BEFORE BEGINNING PWM RAMP
ALWAYS RETURN TO 40% DUTY
(EVEN IF BEYOND 1 SECOND)
TIME (SECONDS)
not be used in applications that switch power to the fan
to control the speed of the fan. For example, low-side
switching of a 3-wire fan should not use either of the RPM
fan modes.
Figure 3 shows the fan spin-up process. When the fan is
spinning up, the number of revolutions is checked every
200ms for up to 2s. When the number of cumulated revo-
lutions is greater than or equal to the spin-up relaxation
criteria, the fan passes spin-up. If the fan has a locked
rotor output, fan spin-up passes when the locked rotor
signal is no longer asserted.
When spin-up passes, the device forces the fan PWM
with a 40% duty cycle. The 40% duty cycle is maintained
until the next temperature conversion is completed,
which occurs once a second. After the temperature con-
version, the device enters either manual mode operation
or automatic operation using MFR_FAN_LUT.
0.8
1.0
RAMP PWM TO TARGET PWM OR RPM
(CONSTRAINED BY THE RAMP BITS)
INITIAL TARGET PWM OR
RPM DETERMINED AFTER
TEMPERATURE CONVERSION
1.2
1.4
Fan Spin-Up