U2008B_07 Datasheet, PDF (4/13 Pages) ATMEL Corporation – Low-cost Phase-control IC with Soft Start

2.2 Voltage Monitoring
When the voltage is built up, uncontrolled output pulses are avoided by internal voltage monitor-
ing. Apart from that, all latches of the circuit (phase control, load limit regulation) are reset and
the soft start capacitor is short circuited. This guarantees a specified start-up behavior each time
the supply voltage is switched on or after short interruptions of the mains supply. Soft start is ini-
tiated after the supply voltage has been built up. This behavior guarantees a gentle start-up for
the motor and automatically ensures the optimum run-up time.
2.3 Phase Control, Pin 6
The function of the phase control is identical to that of the well-known IC U211B. The phase
angle of the trigger pulse is derived by comparing the ramp voltage V₂ at Pin 2 with the set value
on the control input, Pin 3. The slope of the ramp is determined by C₃ and its charging current I
ϕ.
The charging current can be regulated, changed or altered using R₈ at Pin 6.
The maximum phase angle, α_max, (minimum current flow angle ϕ_min) can also be adjusted by
using R₈ (see Figure 5-1 on page 7).
When the potential on Pin 2 reaches the set point level of Pin 3, a trigger pulse is generated
whose pulse width, t_p, is determined from the value of C₃ (t_p = 9 µs/nF, Figure 5-3 on page 8). At
the same time, a latch is set with the output pulse, as long as the automatic retriggering has not
been activated, then no more pulses can be generated in that half cycle. Control input at Pin 3
(with respect to Pin 4) has an active range from -9 V to -2 V. When V₃ = -9 V the phase angle is
at its maximum amax, i.e., the current flow angle is minimum. The minimum phase angle amin is
set with V₃ ≥ -1 V.
2.4 Automatic Retriggering
The current-detector circuit monitors the state of the triac after triggering by measuring the volt-
age drop at the triac gate. A current flow through the triac is recognized when the voltage drop
exceeds a threshold level of typically 40 mV.
If the triac is quenched within the relevant half wave after triggering (for example owing to low
load currents before or after the zero crossing of current wave, or for commutator motors, owing
to brush lifters), the automatic retriggering circuit ensures immediate retriggering, if necessary
with a high repetition rate, t_pp/t_p, until the triac remains reliably triggered.
2.5 Current Synchronization, Pin 8
Current synchronization fulfils two functions:
• Monitoring the current flow after triggering. In case the triac extinguishes again or it does not
switch on, automatic triggering is activated as long as triggering is successful.
• Avoiding triggering due to inductive load. In the case of inductive load operation, the current
synchronization ensures that in the new half wave no pulse is enabled as long as there is a
current available from the previous half wave, which flows from the opposite polarity to the
actual supply voltage.
A special feature of the IC is the realization of current synchronization. The device evaluates the
voltage at the pulse output between the gate and reference electrode of the triac. This results in
saving the separate current synchronization input with specified series resistance.
4 U2008B
4712C–AUTO–07/07