ADT7468 Analog Devices, Inc., ADT7468 Datasheet - Page 61

• Manufacturer Part Number: ADT7468
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

Approaches to System Acoustic Enhancement
There are two different approaches to implementing system
acoustic enhancement: temperature-centric and fan-centric.
The ADT7468 uses the fan-centric approach.
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)
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,
although both fans are controlled by Remote 1 temperature.
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
MIN
and T
RANGE
settings have
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period, T, the PWM period is subdivided into 255 equal time
slots. One time slot corresponds to the smallest possible incre-
ment 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.
The ramp rates in the enhanced acoustics mode are selectable
from the values 1, 2, 3, 5, 8, 12, 24, and 48. The ramp rates are
discrete time slots. For example, if the ramp rate is 8, then eight
time slots are added to the PWM high duty cycle each time
the PWM duty cycle needs to be increased. If the PWM duty
cycle value needs to be decreased, it is decreased by eight time
slots. Figure 84 shows how the enhanced acoustics mode
algorithm operates.
The enhanced acoustics mode algorithm calculates a new PWM
duty cycle based on the temperature measured. If the new
PWM duty cycle value is greater than the previous PWM value,
then the previous PWM duty cycle value is incremented by
either 1, 2, 3, 5, 8, 12, 24, or 48 time slots, depending on the
settings of the enhance acoustics registers. If the new PWM
duty cycle value is less than the previous PWM value, then the
previous PWM duty cycle is decremented by 1, 2, 3, 5, 8, 12, 24,
or 48 time slots. Each time the PWM duty cycle is incremented
or decremented, its value is stored as the previous PWM duty
cycle for the next comparison.
A ramp rate of 1 corresponds to one time slot, which is 1/255 of
the PWM period. In enhanced acoustics mode, incrementing or
decrementing by 1 changes the PWM output by 1/255 × 100%.
PWM_OUT
33% DUTY
CYCLE
Figure 83. 33% PWM Duty Cycle Represented in Time Slots
TIME SLOTS
Figure 84. Enhanced Acoustics Algorithm
TEMPERATURE
BY RAMP RATE
85
DUTY CYCLE
IS NEW PWM
CALCULATE
INCREMENT
PWM VALUE
PREVIOUS
NEW PWM
PREVIOUS
VALUE >
VALUE?
READ
= 255 TIME SLOTS
YES
PWM OUTPUT
(ONE PERIOD)
NO
TIME SLOTS
170
BY RAMP RATE
DECREMENT
PWM VALUE
PREVIOUS
ADT7468