ADT7463 Analog Devices, ADT7463 Datasheet - Page 8

• Manufacturer Part Number: ADT7463
• Description: Dbcool Thermal Management Controller And Voltage Monitor
• Manufacturer: Analog Devices

AN-613
STEP 4: DETERMINING PWM
OUTPUT
PWM
fan in the system will run. It is also the “start” speed for
each fan under automatic fan control once the tempera-
ture rises above T
benefit, PWM
the fans at higher speeds than necessary will merely
make the system louder than necessary. Depending on
the fan used, the PWM
33% duty cycle range. This value can be found through
fan validation.
It is important to note that more than one PWM
output can be controlled from a single temperature
measurement channel. For example, Remote 1 Temp
can control PWM1 and PWM2 outputs. If two different
fans are used on PWM and PWM2, then the fan charac-
teristics can be set up differently. As a result, Fan 1
driven by PWM1 can have a different PWM
that of Fan 2 connected to PWM2. Figure 9 illustrates
this as PWM1
duty cycle of 20%, whereas PWM2
on at a minimum of 40% duty cycle. Note, however,
that both fans turn on at exactly the same tempera-
ture, defined by T
Figure 8. PWM
Duty Cycle
MIN
PWM
is the minimum PWM duty cycle at which each
100%
MIN
0%
MIN
MIN
should be as low as possible. Starting
(front fan) is turned on at a minimum
MIN
MIN
MIN
T
.
MIN
Determines Minimum PWM
. For maximum system acoustic
MIN
setting should be in the 20% to
MIN
FOR EACH PWM (FAN)
TEMPERATURE
MIN
(rear fan) turns
MIN
value than
–8–
PROGRAMMING THE PWM
The PWM
minimum PWM duty cycle for each output to be config-
ured anywhere from 0% to 100%. This allows minimum
PWM duty cycle to be set in steps of 0.39%.
The value to be programmed into the PWM
given by:
Example 1: For a minimum PWM duty cycle of 50%,
Example 2: For a minimum PWM duty cycle of 33%,
PWM
Reg. 0x64 PWM1 Min Duty Cycle = 0x80 (50% default)
Reg. 0x65 PWM2 Min Duty Cycle = 0x80 (50% default)
Reg. 0x66 PWM3 Min Duty Cycle = 0x80 (50% default)
FAN SPEED AND PWM DUTY CYCLE
It should be noted that PWM duty cycle does not
directly correlate to fan speed in RPM. Running a fan at
33% PWM duty cycle does not equate to running the fan
at 33% speed. Driving a fan at 33% PWM duty cycle
actually runs the fan at closer to 50% of its full speed.
This is because fan speed in %RPM relates to the square
root of PWM duty cycle. Given a PWM square wave as
the drive signal, fan speed in RPM equates to:
Figure 9. Operating Two Different Fans from a Single
Temperature Channel
MIN
Value (decimal) = PWM
Value (decimal) = 50/0.39 = 128 decimal
Value = 128 decimal or 80 hex
Value (decimal) = 33/0.39 = 85 decimal
Value = 85 decimal or 54 hex
PWM2
PWM1
REGISTERS
% fan speed
100%
MIN
MIN
MIN
0%
registers are 8-bit registers that allow the
T
MIN
=
PWM duty cycle 10
MIN
REGISTERS
MIN
TEMPERATURE
/0.39
MIN
×
register is
REV. 0