ADT7470 Datasheet, PDF (22/40 Pages) Analog Devices – Temperature Sensor Hub and Fan Controller

ADT7470
FAN SPEED CONTROL
PWM LOGIC STATE
The PWM outputs can be programmed to be high for 100%
duty cycle (non-inverted) or low for 100% duty cycle (inverted).
Table 18. PWM1/PWM2 Configuration (Register 0x68)
Bit
Mnemonic Description
<5> INV1
0 = Logic high for 100% PWM1 duty cycle.
1 = Logic low for 100% PWM1 duty cycle.
<4> INV2
0 = Logic high for 100% PWM2 duty cycle.
1 = Logic low for 100% PWM2 duty cycle.
Table 19. PWM3/PWM4 Configuration (Register 69H)
Bit Mnemonic Description
<5> INV3
0 = Logic high for 100% PWM3 duty cycle.
1 = Logic low for 100% PWM3 duty cycle.
<4> INV4
0 = Logic high for 100% PWM4 duty cycle.
1 = Logic low for 100% PWM4 duty cycle.
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 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.
Programming the PWM Current Duty Cycle Registers
The PWM current duty cycle registers are 8-bit registers that
allow the PWM duty cycle for each output to be set anywhere
from 0% to 100%. 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
Value (decimal) = PWMMIN/0.39
Preliminary Technical Data
Example 1: For a PWM Duty Cycle of 50%
Value (decimal) = 50/0.39 = 128 decimal
Value = 128 decimal or 80 hex.
Example 2: For a PWM Duty Cycle of 33%
Value (decimal) = 33/0.39 = 85 decimal
Value = 85 decimal or 54 hex.
Table 20. PWM Duty Cycle Registers
Register Address
Description
0x32
PWM1 Duty Cycle
0x33
PWM2 Duty Cycle
0x34
PWM3 Duty Cycle
0x35
PWM4 Duty Cycle
Default
0xFF (100%)
0xFF (100%)
0xFF (100%)
0xFF (100%)
By reading the PWMx current duty cycle registers you can keep
track of the current duty cycle on each PWM output, even when
the fans are running in automatic fan speed control mode.
VARY PWM DUTY
CYCLE WITH 8-BIT
RESOLUTION
Figure 30. Control PWM Duty Cycle Manually with a Resolution of 0.39%
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 of this is that when
a system hangs, the user is guaranteed that the system is protected
from overheating. The automatic fan speed control incorporates
a feature called dynamic TMIN calibration. This feature reduces
the design effort required to program the automatic fan speed
control loop. For more information and how to program the
automatic fan speed control loop and dynamic TMIN calibration,
see Application Note AN-613, Programming the Automatic Fan
Speed Control Loop.
Rev. PrA | Page 22 of 40