Datasheet

NCL30081
http://onsemi.com
21
V
CC
Over Voltage Protection (Open LED Protection)
If no output load is connected to the LED power supply,
the controller must be able to safely limit the output voltage
excursion.
In the NCL30081, when the V
CC
voltage reaches the
V
CC(OVP)
threshold, the controller stops the DRV pulses and
the 4−s timer starts counting. The IC re−start switching after
the 4−s timer has elapsed as long as V
CC
≥ V
CC(on)
. This is
illustrated in Figure 48.
Figure 48. Open LED Protection Chronograms
0
10.0
20.0
30.0
40.0
1
0
10.0
20.0
30.0
40.0
2
0
200m
400m
600m
800m
3
1.38 3.96 6.54 9.11 11.7
time in seconds
0
2.00
4.00
6.00
8.00
4
V
CC(on)
V
CC(OVP)
V
CC(off)
V
out
I
out
V
CC
OVP
(V)(A)(V) (V)
Valley Lockout
Quasi−Square wave resonant systems have a wide
switching frequency excursion. The switching frequency
increases when the output load decreases or when the input
voltage increases. The switching frequency of such systems
must be limited.
The NCL30081 changes valley as the input voltage
increases and as the output current set−point is varied
(thermal fold−back and step dimming). This limits the
switching frequency excursion. Once a valley is selected,
the controller stays locked in the valley until the input
voltage or the output current set−point varies significantly.
This avoids valley jumping and the inherent noise caused by
this phenomenon.
The input voltage is sensed by the VIN pin. The internal
logic selects the operating valley according to VIN pin
voltage (Figure 49) and the dimming state imposed by the
Step Dimming feature.
By default, when the output current is not dimmed, the
controller operates in the first valley at low line and in the
second valley at high line.