NCL30081
www.onsemi.com
23
Zero Crossing Detection Block
The ZCD pin allows detecting when the drain−source
voltage of the power MOSFET reaches a valley.
A valley is detected when the voltage on pin 1 crosses
below the V
ZCD(THD)
internal threshold.
At startup or in case of extremely damped free
oscillations, the ZCD comparator may not be able to detect
the valleys. To avoid such a situation, the NCL30081
features a Time−Out circuit that generates pulses if the
voltage on ZCD pin stays below the V
ZCD(THD)
threshold
for 6.5 ms.
The time−out also acts as a substitute clock for the valley
detection and simulates a missing valley in case of too
damped free oscillations.
Figure 50. Time−out Chronograms
4
3
14
12
15
16
17
low
high
Clk
TimeOut
low
high
low
high
low
high
ZCD comp
2nd,
V
ZCD
The 3rd valley is not detected
by the ZCD comp
Time−out circuit adds a pulse to
account for the missing 3rd valley
The 2nd valley is detected
By the ZCD comparator
V
ZCD(THD)
The 3rd valley
is validated
3rd
Normally with this type of time−out function, in the event
the ZCD pin or the auxiliary winding is shorted, the
controller could continue switching leading to improper
regulation of the LED current. Moreover during an output
short circuit, the controller will strive to maintain constant
current operation.
To avoid these scenarios, a protection circuit consisting of
a comparator and secondary timer starts counting when the
ZCD voltage is below the V
ZCD(short)
threshold. If this timer
reaches 90 ms, the controller detects a fault and shutdown.
The auto−restart version (B suffix) waits 4 seconds, then the
controller restarts switching. In the latched version
(A suffix), the controller is latched as long as V
CC
stays
above the V
CC(reset)
threshold.
