U209B_07 Datasheet, PDF (6/19 Pages) ATMEL Corporation – Phase Control IC for Tacho Applications

3.3 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, 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.
3.4 Soft Start
As soon as the supply voltage builds up (t₁), the integrated soft start is initiated. Figure 3-2
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 11. This behavior guarantees a gentle start-up for the
motor and automatically ensures the optimum run-up time.
C₃ is first charged up to the starting voltage V_o with typically 30 µA current (t₂). By reducing the
charging current to approximately 4 µA, the slope of the charging function is also substantially
reduced, so that the rotational speed of the motor only slowly increases. The charging current
then increases as the voltage across C₃ increases giving a progressively rising charging function
which accelerates the motor with increasing rotational speed. The charging function determines
the acceleration up to the set-point. The charging current can have a maximum value of 50 mA.
Figure 3-2. Soft Start
V_C3
V₁₂
V₀
t
t₁
t₃
t2
t_tot
t₁
t₂
t₁ + t₂
t₃
t_tot
= build-up of supply voltage
= charging of C₃ to starting voltage
= dead time
= run-up time
= total start-up time to required speed
6 U209B
4765C–INDCO–02/07