Datasheet

TEMP1, SPD1
TEMP2, SPD2
TEMP3, SPD3
TEMP4, SPD4
TEMP5, SPD5
MEAS_TEMP
0
sec
5 10 15 20 25 30 35 40 45 50 55 60t =
0
sec
5 10 15 20 25 30 35 40 45 50 55 60t =
0
sec
5 10 15 20 25 30 35 40 45 50 55 60t =
SPD1
SPD2
SPD3
SPD4
SPD5
100%
Target and
Ramp Speed
T
OT
Off (SPD0)
Temp rises above
TEMP1 à Target Speed
increases to SPD1
Fan Speed ramps up to
Target Speed by
increasing PWM Duty
Cycle
Fan Speed ramps
down to Target Speed
by reducing
PWM Duty Cycle
Temp falls below
TEMP2 à Target Speed
decreases to SPD1
Max Speed
PWM duty cycle
0%
25°C (T
amb
)
UCD90124A
www.ti.com SLVSAN8 –JANUARY 2012
Inputs: T
OT
, Updates Interval, Rail that
MEAS_TEMP is being monitored on, PWM
pin, PWM freq, PWM temp rate, FANTAC
pin, 5x (TEMPn, SPEEDn) setpoints.
• System starts up at t = 0 seconds
• MEAS_TEMP = 25°C at ambient temp
• PWM DUTY_CYCLE = 0% and fan is
off
• Check MEAS_TEMP every 250 ms (or 1
s)
• When MEAS_TEMP > TEMP1:
– set SPEED_TARGET = SPEED1
– increase DUTY_CYCLE to
DUTY_CYCLE_ON
– increase DUTY_CYCLE by ramp
rate (10%/second) until SPEED =
SPEED_TARGET
When MEAS_TEMP > TEMP2:
– set SPEED_TARGET = SPEED2
– increase DUTY_CYCLE by ramp
rate until SPEED =
SPEED_TARGET
• Repeat as temperature is increased for
each new setpoint
• If MEAS_TEMP > T
OT
, declare a fault
Figure 27. Temperature and Speed Set Point PWM Control for
and take the prescribed action Four-Wire Fans
• If temperature drops - above TEMP4 to below TEMP3 for example
– when MEAS_TEMP drops below TEMP4, maintain SPEED4 → do not change the DUTY_CYCLE
– when MEAS_TEMP drops below TEMP3, set SPEED_TARGET = SPEED3
– decrease DUTY_CYCLE by ramp rate (10%/second) until SPEED = SPEED_TARGET
• To turm the fan off when MEAS_TEMP < TEMP1, set SPEED1 = 0 RPM
EXAMPLE: MEAS_TEMP = 25°C at ambient temp:
• t = 0 to 5 sec: MEAS_TEMP increases from ambient to TEMP1 → increases SPEED_TARGET from SPD0
(Off) to SPD1 → increases DUTY_CYCLE from 0% to DUTYON (30%) → ACTUAL fan speed ramps up
from 0 RPM to SPD1.
• t = 5 to 10 sec: MEAS_TEMP increases > TEMP2 → increases SPEED_TARGET from SPD1 to SPD2 →
increases DUTY_CYCLE → ACTUAL fan speed ramps up from SPD1 to SPD2.
• t = 10 to 25 sec: MEAS_TEMP increases to > TEMP5 → SPEED_TARGET increases from SPD2 to SPD5
→ DUTY_CYCLE ramps to DUTYMAX → ACTUAL fan speed increases SPD5.
• t = 25 to 30 sec: MEAS_TEMP stays > TEMP5 → SPEED_TARGET and DUTY_CYCLE do not change →
ACTUAL fan speed stays at SPD5.
• t = 30 to 35 sec: MEAS_TEMP decreases to < TEMP4 → SPEED_TARGET drops to SPD4 and then to
SPD3 → decreases DUTY_CYCLE → ACTUAL fan speed ramps down from SPD5 to SPD3.
• t = 35 to 60 sec: MEAS_TEMP decreases to < TEMP1 → SPEED_TARGET drops to SPD0 → decreases
DUTY_CYCLE to DUTYOFF → ACTUAL fan speed ramps down from SPD3 to SPD0 (Off).
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