ADM1026
Automatic Fan Speed Control
output jumps to full scale. To ensure that the maximum cooling
capacity is always available, the fan drive is always set by the
sensor channel demanding the highest fan speed.
The ADM1026 offers a simple method of controlling fan speed
according to temperature without intervention from the host
processor. Monitoring must be enabled by setting Bit 0 of
Configuration Register 1 (Address 00h), to enable automatic fan
speed control. Automatic fan speed control can be applied to the
DAC output, the PWM output, or both, by setting Bit 5 and/or
Bit 6 of Configuration Register 1.
If the temperature falls, the fan does not turn off until the
temperature measured by all three temperature sensors has
fallen to their corresponding TMIN − 4°C. This prevents the fan
from cycling on and off continuously when the temperature is
close to TMIN
.
The TMIN registers (Addresses 10h to 12h) contain minimum
temperature values for the three temperature channels (on-chip
sensor and two remote diodes). This is the temperature at which
a fan starts to operate when the temperature sensed by the
controlling sensor exceeds TMIN. TMIN can be the same or
different for all three channels. TMIN is set by writing a twos
complement temperature value to the TMIN registers. If any
sensor channel is not required for automatic fan speed control,
TMIN for that channel should be set to 127°C (01111111).
Whenever a fan starts or stops during automatic fan speed
control, a one-off interrupt is generated at the output. This
INT
is described in more detail in the section on the ADM1026
Interrupt Structure.
SPIN UP FOR 2 SECONDS
100%
PWM
OUTPUT
In automatic fan speed control mode, (as shown Figure 42 and
Figure 43) the four MSBs of the DAC control register (Address
04h) and PWM control register (Address 05h) set the minimum
values for the DAC and PWM outputs. Note that, if both DAC
control and PWM control are enabled (Bits 5 and 6 of
MIN
Configuration Register 1 = 1), the four MSBs of the DAC
control register (Address 04h) define the minimum fan speed
values for both the DAC and PWM outputs. The value in the
PWM control register (Address 05h) has no effect.
T
– 4°C
T
T
+ 20°C
MIN
MIN
MIN
TEMPERATURE
Minimum DAC Code DACMIN = 16 × D
Figure 42. Automatic PWM Fan Control Transfer Function
Code
256
DAC Output Voltage = 2.5×
SPIN UP FOR 2 SECONDS
255
Minimum PWM Duty Cycle PWMMIN = 6.67 × D
where D is the decimal equivalent of Bits 7 to 4 of the register.
240
DAC
OUTPUT
When the temperature measured by any of the sensors exceeds
the corresponding TMIN, the fan is spun up for 2 seconds with
the fan drive set to maximum (full scale from the DAC or 100%
PWM duty cycle). The fan speed is then set to the minimum as
previously defined. As the temperature increases, the fan drive
increases until the temperature reaches TMIN + 20°C.
MIN
The fan drive at any temperature up to 20°C above TMIN is
given by
T
– 4°C
T
T
+ 20°C
MIN
MIN
MIN
TEMPERATURE
Figure 43. Automatic DAC Fan Control Transfer Function
TACTUAL −TMIN
PWM = PWMMIN
+
(
100 − PWMMIN ×
)
20
Fan Inputs
or
Pins 3 to 6 and 9 to 12 may be configured as fan speed
measuring inputs by clearing the corresponding bit(s) of
Configuration Register 2 (Address 01h), or as general-purpose
logic inputs/outputs by setting bits in this register. The power-
on default value for this register is 00h, which means all the
inputs are set for fan speed measurement.
TACTUAL −TMIN
DAC = DACMIN
+
(
240 − DACMIN ×
)
20
For simplicity of the automatic fan speed algorithm, the DAC
code increases linearly up to 240, not its full scale of 255.
However, when the temperature exceeds TMIN +20°C, the DAC
Rev. A | Page 24 of 56
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