NCP1379
http://onsemi.com
15
Figure 27. Zoom 3: 3
rd
to 4
th
Valley Transition
Figure 28. Zoom 4: 4
th
Valley to VCO Mode Transition
Time Out
In case of extremely damped free oscillations, the ZCD
comparator can be unable to detect the valleys. To avoid
such situation, NCP1379 integrates a Time Out function that
acts as a substitute clock for the decimal counter inside the
logic bloc. The controller thus continues its normal
operation. To avoid having a too big step in frequency, the
time out duration is set to 5.9 ms. Figures 30 and 31 detail the
time out operation.
The NCP1379 also features an extended time out during
the soft−start.
Indeed, at startup, the output voltage reflected on the
auxiliary winding is low. Because of the voltage drop
introduced by the Over Power Compensation diode
(Figure 35), the voltage on the ZCD pin is very low and the
ZCD comparator might be unable to detect the valleys. In
this condition, setting the DRV Latch with the 5.9 ms
time−out leads to a continuous conduction mode operation
(CCM) at the beginning of the soft−start. This CCM
operation only last a few cycles until the voltage on ZCD pin
becomes high enough to be detected by the ZCD
comparator.
To avoid this, the time−out duration is extended to 40 ms
during the soft−start in order to ensure that the transformer
is fully demagnetized before the MOSFET is turned−on.
