ADT7476
Rev. B | Page 44 of 72
05382-
054
0%
100%
P
W
M
D
U
T
Y
C
Y
C
L
E
T
MIN
REAR CHASSIS
FRONT CHASSIS
CPU FAN SINK
LOCAL =
VRM TEMP
PWM1
PWM2
TACH1
TACH2
TACH3
PWM3
REMOTE 1 =
AMBIENT TEMP
REMOTE 2 =
CPU TEMP
MUX
THERMAL CALIBRATION
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
PWM
MIN
PWM
MIN
PWM
MIN
100%
PWM
GENERATOR
PWM
GENERATOR
TACHOMETER 3
AND 4
MEASUREMENT
TACHOMETER 1
MEASUREMENT
TACHOMETER 2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
GENERATOR
PWM
CONFIG
PWM
CONFIG
PWM
CONFIG
Figure 53. Understanding the T
MIN
Parameter
STEP 4: PWM
MIN
FOR EACH PWM (FAN) OUTPUT
PWM
MIN
is the minimum PWM duty cycle at which each fan in
the system runs. It is also the start speed for each fan under
automatic fan control once the temperature rises above T
MIN
.
For maximum system acoustic benefit, PWM
MIN
should be as
low as possible. Depending on the fan used, the PWM
MIN
set-
ting is usually in the 20% to 33% duty cycle range. This value
can be found through fan validation.
TEMPERATURE
T
MIN
100%
PWM
MIN
0%
P
W
M DUTY
CY
CLE
05382-055
Figure 54. PWM
MIN
Determines Minimum PWM Duty Cycle
More than one PWM output can be controlled from a single
temperature measurement channel. For example, Remote 1
Temperature can control PWM1 and PWM2 outputs. If two
different fans are used on PWM1 and PWM2, the fan charac-
teristics can be set up differently. As a result, Fan 1 driven by
PWM1 can have a different PWM
MIN
value than that of Fan 2
connected to PWM2. Figure 55 illustrates this as follows:
PWM1
MIN
(front fan) is turned on at a minimum duty cycle of
20%, while PWM2
MIN
(rear fan) turns on at a minimum of 40%
duty cycle. Note that both fans turn on at exactly the same
temperature, defined by T
MIN
.
TEMPERATURE
T
MIN
100%
PWM1
MIN
0%
P
W
M DUTY
CY
CLE
PWM
1
PW
M2
PWM2
MIN
05382-056
Figure 55. Operating Two Different Fans from a Single Temperature Channel
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