ON Semiconductor NCP1239FDR2G Reference Manual page 37

When the load current falls to a low level (CH4), the FB pin voltage diminishes to take into account the decay of the power
demand. As a consequence, the FB pin voltage goes below the "Vskip" threshold and the soft start timer counts about 100 ms
(if C = 330 nF). When the 100 ms time has elapsed, the PFC V
pin6
When the load current increases from 1A to 5A, the FB pin increases too so that the supplied power matches the new demand.
The normal mode is recovered without delay
NCP1239
Figure 62. Transition Normal to Standby
.
Figure 63. Transition Standby to Normal
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stops being fed.
CC