ADT7473 Analog Devices, Inc., ADT7473 Datasheet - Page 54

• Manufacturer Part Number: ADT7473
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7473
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
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)
, Remote 1 temperature). The temperature values
AMBIENT TEMP
REMOTE 2 =
REMOTE 1 =
VRM TEMP
CPU TEMP
LOCAL =
Figure 71. Acoustic Enhancement Smoothes Fan Speed Variations Under Automatic Fan Speed Control
THERMAL CALIBRATION
THERMAL CALIBRATION
THERMAL CALIBRATION
T
T
T
MIN
MIN
MIN
T
T
T
RANGE
RANGE
RANGE
100%
100%
100%
0%
0%
0%
MUX
used
Rev. A | Page 54 of 76
PW
PW
PWM
MIN
M
MIN
IN
using Remote 1 temperature. The T
already
is, thermal characterization of the control loop has been
optimized. Here, 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
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 (0x62)
Bit 3 = 1, enables acoustic enhancement on PWM1 output
Enhance Acoustics Register 2 (0x63)
Bit 7 = 1, enables acoustic enhancement on PWM2 output
Bit 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.
ENHANCEMENT
M
M
ACOUSTIC
ENHANCEMENT)
ENHANCEMENT)
TACHOMETER 1
MEASUREMENT
TACHOMETER 2
MEASUREMENT
ENHANCEMENT)
TACHOMETER 3
MEASUREMENT
(ACOUSTIC
(ACOUSTIC
(ACOUSTIC
C
CONTROL
CONTROL
ONTROL
been defined in automatic fan speed control mode, t
RAMP
RAMP
AND 4
RAMP
GENERATOR
GENERATOR
GENERATOR
CONFIG
CONFIG
CONFIG
PWM
PWM
PWM
PWM
PWM
PWM
MIN
PWM1
TACH1
PWM2
TACH2
PWM3
TACH3
and T
FRONT CHASSIS
REAR CHASSIS
CPU FAN SINK
RANGE
settings ha e
without
hat
v