Analog Devices ADT7473 Manual page 52

ADT7473
REMOTE 2 =
CPU TEMP
LOCAL =
VRM TEMP
REMOTE 1 =
AMBIENT TEMP
Figure 71. Acoustic Enhancement Smoothes Fan Speed Variations Under Automatic Fan Speed Control
Approaches to System Acoustic Enhancement
There are two different approaches to implementing system
acoustic enhancement: temperature-centric and fan-centric.
The temperature-centric approach involves smoothing transient
temperatures as they are measured by a temperature source (for
example, Remote 1 temperature). The temperature values used
to calculate the PWM duty cycle values are smoothed, reducing
fan speed variation. However, this approach causes an inherent
delay in updating fan speed and causes the thermal characteris-
tics of the system to change. It also causes the system fans to
stay on longer than necessary because the fan's reaction is
merely delayed. The user has no control over noise from
different fans driven by the same temperature source. Consider,
for example, a system in which control of a CPU cooler fan (on
PWM1) and a chassis fan (on PWM2) use Remote 1 tempera-
ture. Because the Remote 1 temperature is smoothed, both fans
are updated at exactly the same rate. If the chassis fan is much
louder than the CPU fan, there is no way to improve its
acoustics without changing the thermal solution of the CPU
cooling fan.
The fan-centric approach to system acoustic enhancement
controls the PWM duty cycle, driving the fan at a fixed rate (for
example, 6%). Each time the PWM duty cycle is updated, it is
incremented by a fixed 6%. As a result, the fan ramps smoothly
to its newly calculated speed. If the temperature starts to drop,
the PWM duty cycle immediately decreases by 6% at every
update. Therefore, the fan ramps smoothly up or down without
inherent system delay. Consider, for example, controlling the
same CPU cooler fan (on PWM1) and chassis fan (on PWM2)
THERMAL CALIBRATION
100%
0%
T T
MIN RANGE
THERMAL CALIBRATION
100%
0%
T T
MIN RANGE
THERMAL CALIBRATION
100%
0%
T T
MIN RANGE
ACOUSTIC
ENHANCEMENT
PWM
MIN
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 1
MEASUREMENT
PWM
MIN
RAMP
CONTROL
MUX
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 2
MEASUREMENT
PWM
MIN
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 3
AND 4
MEASUREMENT
using Remote 1 temperature. The T
already been defined in automatic fan speed control mode, that
is, thermal characterization of the control loop has been
optimized. Now the chassis fan is noisier than the CPU cooling
fan. Using the fan-centric approach, PWM2 can be placed into
acoustic enhancement mode independently of PWM1. The
acoustics of the chassis fan can, therefore, be adjusted without
affecting the acoustic behavior of the CPU cooling fan, even
though both fans are controlled by Remote 1 temperature. The
fan-centric approach is how acoustic enhancement works on
the ADT7473.
Enabling Acoustic Enhancement for Each PWM Output
Enhance Acoustics Register 1 (Reg. 0x62)
<3> = 1, enables acoustic enhancement on PWM1 output.
Enhance Acoustics Register 2 (Reg. 0x63)
<7> = 1, enables acoustic enhancement on PWM2 output.
<3> = 1, enables acoustic enhancement on PWM3 output.
Effect of Ramp Rate on Enhanced Acoustics Mode
The PWM signal driving the fan has a period, T, given by the
PWM drive frequency, f, because T = 1/f. For a given PWM
period, T, the PWM period is subdivided into 255 equal time
slots. One time slot corresponds to the smallest possible
increment in the PWM duty cycle. A PWM signal of 33% duty
cycle is, therefore, high for 1/3 × 255 time slots and low for 2/3
× 255 time slots. Therefore, a 33% PWM duty cycle corresponds
to a signal that is high for 85 time slots and low for 170 time slots.
Rev. 0 | Page 52 of 76
PWM
CONFIG
PWM
PWM1
GENERATOR
TACH1
PWM
CPU FAN SINK
CONFIG
PWM
PWM2
GENERATOR
TACH2
PWM
CONFIG
FRONT CHASSIS
PWM
PWM3
GENERATOR
TACH3
REAR CHASSIS
and T
MIN RANGE
settings have