NCP1072, NCP1075
Figure 31. Ipk Set−point is Frozen at Lower Power Demand.
Feedback and Skip
Figure 32 depicts the relationship between feedback
voltage and current. The feedback pin operates linearly as
the absolute value of feedback current (IFB) is above 40 mA.
In this linear operating range, the dynamic resistance is
19.5 kW typically (RFB(up)) and the effective pull up voltage
is 3.3 V typically (VFB(REF)). When IFB is below 40 mA, the
FB voltage will jump to close to 4.5 V.
Figure 32. Feedback Voltage vs. Current
Figure 33 depicts the skip mode block diagram. When the
FB current information reaches IFBskip, the internal clock to
set the flip−flop is blanked and the internal consumption of
the controller is decreased. The hysteresis of internal skip
comparator is minimized to lower the ripple of the auxiliary
voltage for VCC pin and VOUT of power supply during skip
mode. It easies the design of VCC over load range.
http://onsemi.com
18