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In high frequency mode, the PWM drive frequency is
always 22.5 kHz. When high frequency mode is enabled, the
dc bits are automatically asserted internally and do not need
to be changed.
Fan Speed Control
The ADT7476A controls fan speed using automatic and
manual modes:
•
In automatic fan speed control mode, fan speed is
automatically varied with temperature and without CPU
intervention once initial parameters are set up. The
advantage is that if the system hangs, the user is
guaranteed that the system is protected from
overheating.
•
In manual fan speed control mode, the ADT7476A
allows the duty cycle of any PWM output to be
adjusted manually. This can be useful if the user wants
to change fan speed in software or adjust PWM duty
cycle output for test purposes. Bits [7:5] of
Register 0x5C to Register 0x5E (PWM Configuration)
control the behavior of each PWM output.
Table 42. PWM CONFIGURATION REGISTERS
(REG. 0x5C TO 0x5E)
Bit
Mnemonic
[7:5]
BHVR
Once under manual control, each PWM output can be
manually updated by writing to Register 0x30 to
Register 0x32 (PWM current duty cycle registers).
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% in steps of 0.39%. The value to
be programmed into the PWM
Value (decimal) = PWM
Example 1:
For a PWM duty cycle of 50%,
Value (decimal) = 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 0x80 (hex)
Example 2:
For a PWM duty cycle of 33%,
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal) or 0x54 (hex)
Table 43. PWM CURRENT DUTY CYCLE REGISTERS
Register
0x30
0x31
0x32
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Description
111 = Manual Mode
register is given by:
MIN
/0.39
MIN
Description
PWM1 Current Duty Cycle
0xFF (100%)
PWM2 Current Duty Cycle
0xFF (100%)
PWM3 Current Duty Cycle
0xFF (100%)
ADT7476A
By reading the PWMx 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 or acoustic enhancement mode.
Programming T
T
RANGE
PWM. For the ADT7467, ADT7468 and ADT7473,
T
is effectively a slope. For the ADT7475
RANGE
andADT7476A, T
the temperature region where the PWM output linearly
ramps from PWM
PWM = 100%
PWM
PWM
PWM = 0%
Programming the Automatic Fan Speed Control Loop
To understand the automatic fan speed control loop more
efficiently, it is recommended to use the ADT7476A
evaluation board and software while reading this section.
This section provides the system designer with an
understanding of the automatic fan control loop and
provides step-by-step guidance on effectively evaluating
and selecting critical system parameters. To optimize the
system characteristics, the designer needs to give some
thought to system configuration, including the number of
fans, where they are located, and what temperatures are
being measured in the particular system.
The mechanical or thermal engineer who is tasked with
the system thermal characterization should also be involved
at the beginning of the system development process.
Manual Fan Control Overview
In unusual circumstances, it can be necessary to manually
control the speed of the fans. Because the ADT7476A has an
SMBus interface, a system can read back all necessary
voltage, fan speed, and temperature information, and use
this information to control the speed of the fans by writing
to the PWM current duty cycle register (0x30, 0x31, and
0x32) of the appropriate fan. Bits [7:5] of the PWMx
configuration registers (0x5C, 0x5D, 0x5E) are used to set
Default
fans up for manual control.
www.onsemi.com
33
RANGE
defines the distance between T
is no longer a slope, but defines
RANGE
to 100% PWM.
MIN
MAX
MIN
T
RANGE
T
MIN
Figure 46. T
RANGE
and 100%
MIN
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