U211B Datasheet, PDF (5/30 Pages) ATMEL Corporation – Phase Control IC with Overload Limitation for Tacho Applications

U211B
5. Voltage Monitoring
As the voltage is built up, uncontrolled output pulses are avoided by internal voltage surveil-
lance. At the same time, all latches in the circuit (phase control, load limit regulation, soft start)
are reset and the soft-start capacitor is short circuited. Used with a switching hysteresis of
300 mV, this system guarantees defined start-up behavior each time the supply voltage is
switched on or after short interruptions of the mains supply.
6. Soft Start
As soon as the supply voltage builds up (t₁), the integrated soft start is initiated. Figure 6-1
shows the behavior of the voltage across the soft-start capacitor, which is identical with the volt-
age on the phase-control input on pin 12. This behavior guarantees a gentle start-up for the
motor and automatically ensures the optimum run-up time.
Figure 6-1. Soft Start
V_C3
V₁₂
4752B–INDCO–09/05
V₀
t
t₁
t₃
t₂
t_tot
t₁ = Build-up of supply voltage
t₂ = Charging of C₃ to starting voltage
t₁ + t₂ = Dead time
t₃ = Run-up time
t_tot = Total start-up time to required speed
C₃ is first charged up to the starting voltage V₀ with a current of typically 45 µA (t₂). By reducing
the charging current to approximately 4 µA, the slope of the charging function is also substan-
tially reduced, so that the rotational speed of the motor only slowly increases. The charging
current then increases as the voltage across C₃ increases, resulting in a progressively rising
charging function which accelerates the motor more and more with increasing rotational speed.
The charging function determines the acceleration up to the set point. The charging current can
have a maximum value of 55 µA.
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