U217B
Pin Description
Ramp 1
8 Vsync
U217B
CRamp 2
OP+ 3
U217B
7 GND
6 Output
OP– 4
5 VS
1
–VS
Ramp
control
2 C2
Figure 2. Pinning
Figure 3. Pin 1 internal network
Pin Symbol
Function
1 Ramp Ramp output
2 CRAmp Ramp capacitor
3 OP+ OP non-inverting input
4 OP– OP inverting input
5 VS Supply voltage
6 Output Trigger pulse output
7 GND Ground
8 Vsync Voltage synchronization
General Description
V1
1.4 V
t
Final voltage
Vmin
7.3 V
Initial voltage
T Vmax
–VS(Pin5)
Figure 4. Threshold voltage of the ramp
The integrated circuit U217B is a triac controller for zero-
crossing mode. It is designed to control power in
switching resistive loads of mains supplies.
Information regarding supply sync. is provided at Pin 8
via resistor RSync.
To avoid DC load on the mains, the full-wave logic
guarantees that complete mains cycles are used for load
switching.
A fire pulse is released when the inverted input of the
comparator is negative (Pin 4) with respect to the
non–inverted input (Pin 3) and internal reference voltage.
A ramp generator with free selectable duration can be
performed by capacitor C2 at Pin 2. The ramp function is
used for open-loop control (figure 4), but also for applica-
tion with proportional band regulation (figure 11). Ramp
voltage available at capacitor C2 is decoupled across the
emitter follower at Pin l. To maintain the lamp flicker
specification, ramp duration is adjusted according to the
controlling load. In practice, interference should be
avoided (temperature control). Therefore, a two-point
control is preferred to proportional control. One can use
internal reference voltage for simple applications. In that
case, Pin 3 is inactive and connected to Pin 7 (GND), see
figure 13.
Triac Firing Current (Pulse)
This depends on the triac requirement. It can be limited
with gate series resistance which is calculated as follows:
RGmax
7.5 V – VGmax
IGmax
– 36 W
IP = IGmax
T
tp
where:
VG = Gate voltage
IGmax = Maximum gate current
Ip = Average gate current
tp = Firing pulse width
T = Mains period duration
Firing Pulse Width tp (Figure 5)
This depends on the latching current of the triac and its
load current. The firing pulse width is determined by the
zero-crossing identification which can be influenced with
the help of sync. resistance, Rsync, (figure 6).
tp =
2
arc. sin
IL
VM
w P Ǹ2
2 (11)
Rev. A4, 23-Feb-01
