ADT7466
Rev. 2 | Page 31 of 48 | www.onsemi.com
AFC Loop Operation
The automatic fan speed control loop operates as follows.
Once the temperature exceeds T_MIN, the ADT7466 outputs
the voltage V_FAN_ON on its DRIVE pin. For Fan 1, FAN1 ON
is also asserted. When the fan starts rotating reliably, the drive
voltage is reduced to V_FAN_MIN. Reliable startup is
determined when two tachometer pulses are sensed on the tach
input. As the measured temperature increases, the voltage
output by the ADT7466 also increases linearly. The rate with
which the voltage output (fan speed) increases is controlled by
the T_RANGE parameter.
Once the measured fan speed reaches a programmable
maximum limit, the fan speed does not increase further. This is
to maintain low acoustics. If, however, the
THERM
fail safe
limit is breached, the fans immediately run to full speed (0xFF).
They continue to run at full speed until the temperature falls by
a programmable hysteresis value below the
THERM
limit. Then
the fan speed reduces to its value before the
THERM
limit is
exceeded.
As the temperature decreases, the fan speed decreases along the
same curve. Once the temperature falls below T_MIN, the fan
runs at V_FAN_MIN. If the temperature continues to decrease,
the fan can continue to run at V_FAN_MIN, or if the
temperature drops below a hysteresis value, the fan can be
switched off completely. This is controlled by Bits 4:5 of
Configuration Register 4. Setting these bits ensures that the fans
never go below minimum speed. FAN 1 ON is also deasserted
when the fan drive is set to 0 V.
The fan speed is updated every 250 ms to 500 ms in the
automatic fan speed control loop.
04711-040
FULL SPEED
2.25V
(0xFF)
FAN DRIVE V
MAX FAN
SPEED
FAN SPIN UP
FOR 2 TACH
PULSES
TEMPERATURE
T_THERM
TMIN
TMIN_HYS
TRANGE
TMAX
V_FAN_ON
V_FAN_MIN
FAN OFF
0V
(0x00)
T_THERM_HYS
Figure 41. Operation of AFC Loop
STARTING THE FAN
Under normal conditions, the V_FAN_ON register sets DRIVE
at a voltage sufficient to start the fan rotating. Fan startup is
confirmed after two tach pulses are generated.
1.
Set the initial V_FAN_ON by BIOS.
2.
Wait for two tach pulses (up to 2 seconds maximum).
3.
If successful, set the drive to V_FAN_MIN and follow the
automatic slope.
If not successful, increase the V_FAN_ON voltage on
DRIVE by a programmed value (set in step size register)
and return to Step 1. This sequence can be repeated five
times or until DRIVE is set at full scale. If the fan still fails
to start, the
FANLOCK
pin is asserted.
4.
Set the drive at 0 V (to avoid high power dissipation).
5.
Wait 1 minute and repeat the entire sequence. (This
sequence recovers the situation if the fan is temporarily
stalled due a mechanical reason such as jammed with a
stick.)
04711-041
FULL SPEED
2.25V
(0xFF)
FAN DRIVE
TMIN
TACHO
V_FAN_ON
V_FAN_MIN
FAN OFF
0V
(0x00)
Figure 42. Normal Fan Starting Timing Diagram
04711-042
FULL SPEED
2.25V
(0xFF)
FAN DRIVE V
FAN1_ON
FAN_LOCK
V_FAN_ON
V_FAN_ON_STEP
V_FAN_MIN
FAN OFF
0V
(0x00)
2SEC
1MIN
Figure 43. Abnormal Fan Starting (Fan Stalled)
