NCP1379 Datasheet, PDF (12/22 Pages) ON Semiconductor – Quasi-Resonant Current-Mode Controller for High-Power Universal Off-line Supplies

NCP1379
VDD
VDD
Rpullup
FB
Vdd
ICt
Ct
+
−
Ct setpoint
VFB
VFBth
LOGIC BLOCK
S
Q
Q
DRV
R
ZCD
La ux
10 V
ES D
DRV
Ct
Discharge
+
−
Vth
leakage
blanking
3 us puls e
Time Out CS comparator
de m a g
Figure 23. Valley Detection Circuit
As the output load decreases (FB voltage decreases the
valleys are incremented from the first to the fourth. When
the fourth valley is reached, if FB voltage further decreases
below 0.8 V, the controller enters VCO mode.
During VCO operation, the peak current continues to
decrease until it reaches 17.5% of the maximum peak
current: the switching frequency expands to deliver the
necessary output power. This allows achieving very low
standby power consumption.
The Figure 24 shows a simulation case where the output
current of a 19 V / 60 W decreases from 2.8 A to 0.1 A. No
instability is seen during the valley transitions
(Figures 25, 26, 27 and 28)
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