Datasheet

17
MEAS_TEMP
25°C
(tamb)
T
OFF
T
ON
T
OT
GPO output
1
Fan Speed
Off
MaxSpeed
sec 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
t = 0
sec 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0
sec 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Temp increase
above T
ON
: Assert
GPO to turn on Fan
Temp drops below
T
ON
: GPO and Fan
stays on (hysteresis)
Temp drops below above
T
OFF
: De-assert GPO to
turn off Fan
t = 0
t = 0
UCD90124A
SLVSAN8 –JANUARY 2012 www.ti.com
Inputs: T
ON
, T
OFF
, T
OT
, Update
Interval, Rail where MEAS_TEMP
is monitored, GPOx pin
• System starts up at t = 0
seconds
• MEAS_TEMP = 25°C →
ambient temp
• GPO/PWM is low and Fan is off
• Check MEAS_TEMP every 1
second (or 250 msec)
• When MEAS_TEMP = T
ON
, set
GPO/PWM = 1 → turn fan on
• Leave GPO/PWM = 1 unless
MEAS_TEMP < T
OFF
• If MEAS_TEMP is > T
ON
,
declare a fault and take the
prescribed action.
Figure 26. Hysteretic Temperature Control for 2- or 3-wire Fans
Set Point Fan Control
The second algorithm (Figure 27) uses five control set points that each have a temperature and a fan speed.
When the monitored temperature increases above one of the set point temperatures, the fan speed is increased
to the corresponding set point value. When the monitored temperature drops below a set point temperature, the
fan speed is reduced to the corresponding set point value. The ramp rate for speed can be selected, allowing the
user to optimize fan performance and minimize audible noise.
The fan speed is varied by changing the duty cycle of a PWM output. For two- and three-wire fans, as the fan is
turned on and off, the inertia of the fan smoothes out the fan speed changes, resulting in variable speed
operation. This approach can be taken with any fan, but would most likely be used with two- or three-wire fans at
a PWM frequency in the 40-Hz to 80-Hz range. Four-wire fans would use the PWM input as described earlier in
this section.
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