ADT7470 Analog Devices, Inc., ADT7470 Datasheet - Page 23

• Manufacturer Part Number: ADT7470
• Description: Temperature Sensor Hub And Fan Controller
• Manufacturer: Analog Devices, Inc.

FAN SPEED CONTROL
PWM LOGIC STATE
The PWM outputs can be programmed high for PWMMAX
duty cycle (noninverted) or low for PWMMAX duty cycle
(inverted).
Table 18. PWM1/PWM2 Configuration (Register 0x68)
Bit No.
<5>
<4>
Table 19. PWM3/PWM4 Configuration (Register 0x69)
Bit No.
<5>
<4>
PWM Drive Frequency
The PWM drive frequency is variable on the ADT7470. The
PWM drive frequency is a high frequency signal greater than
20 kHz. This is most suitable for use with 4-wire fans. It is also
possible to use low frequency PWM drive, such as 1.4 kHz.
MANUAL FAN SPEED CONTROL
The ADT7470 allows the duty cycle of any PWM output to be
manually adjusted. This can be useful if users want to change
fan speed in software or want to adjust PWM duty cycle output
for test purposes. The PWM current duty cycle registers
(Register 0x32 to Register 0x35) can be written with 8-bit
values in manual fan speed control mode to manually adjust
the speeds of the cooling fans.
PWM Configuration (Register 0x68, 0x69)
These registers control the behavior of the fans under certain
conditions as well as define whether the fans are being used in
manual or automatic fan speed control mode.
Mnemonic
INV1
INV2
Mnemonic
INV3
INV4
Description
0 = Logic high for PWMMAX PWM1
duty cycle.
1 = Logic low for PWMMAX PWM1
duty cycle.
0 = Logic high for PWMMAX PWM2
duty cycle.
1 = Logic low for PWMMAX PWM2
duty cycle.
Description
0 = Logic high for PWMMAX PWM3
duty cycle.
1 = Logic low for PWMMAX PWM3
duty cycle.
0 = Logic high for PWMMAX PWM4
duty cycle.
1 = Logic low for PWMMAX PWM4
duty cycle.
Rev. B | Page 23 of 40
Programming the PWM Current Duty Cycle Registers
The PWM current duty cycle registers are 8-bit registers, which
allow the PWM duty cycle for each output to be set anywhere
from 0% to PWMMAX. This allows PWM duty cycle to be set
in steps of 0.39%.
The value to be programmed into the PWMMIN register
is given by
Example 1: For a PWM Duty Cycle of 50%
Example 2: For a PWM Duty Cycle of 33%
Table 20. PWM Duty Cycle Registers
Register Address
0x32
0x33
0x34
0x35
By reading the PWM current duty cycle registers the user can
keep track of the current duty cycle on each PWM output, even
when the fans are running in automatic fan speed control mode.
AUTOMATIC FAN SPEED CONTROL
In automatic fan speed control mode, fan speed automatically
varies with temperature and without CPU intervention, once
initial parameters are set up. The advantage is that when a
system hangs, the user is guaranteed that the system is protected
from overheating.
Figure 31. Control PWM Duty Cycle Manually with a Resolution of 0.39%
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
Description
PWM1 Duty Cycle
PWM2 Duty Cycle
PWM3 Duty Cycle
PWM4 Duty Cycle
MIN
CYCLE WITH 8-BIT
VARY PWM DUTY
RESOLUTION
/0.39
Default
0xFF (PWMMAX)
0xFF (PWMMAX)
0xFF (PWMMAX)
0xFF (PWMMAX)
ADT7470