NCP1255_12 Datasheet, PDF (19/22 Pages) ON Semiconductor – Current-Mode PWM Controller for Off-line Power Supplies featuring Peak Power Excursion

V_FB (V)
NCP1255
max
4.5
Open loop voltage
4.0
25% timer − short−circuit
130 kHz
3.2
1.9
1.5
1
Frozen current
0.4
100% timer − overload
Fixed−frequency variable peak current
Frequency foldback variable peak current
Fixed frequency variable peak current
Frozen peak current
Skip cycle
Max I_peak
31% max I_peak
Figure 18. Depending on the feedback level, the timer will take two different values:
it will authorize a transient overload, but will reduce a short−circuit duration.
65 kHz
65 kHz
26 kHz
t
Please note that the overload situation (OVL) is detected
when the maximum peak current limit is hit. It can be 3.2 V
as indicated in the graph in case of no Over Power Protection
(OPP). If you have programmed an OPP level of −200 mV
for instance, the OVL threshold becomes (0.8 − 0.2) x 4 =
V_FB
2.4 V. When the maximum peak current situation is lifted,
the converter returns to a normal situation, the timer is reset.
The short circuit situation is detected by sensing a feedback
voltage beyond 4 V. For the sake of the explanation, we have
gathered two different events in Figure 19:
V_FB
4.0 V
3.2 V
130 kHz
65 kHz
4.0 V
3.2 V
130 kHz
65 kHz
V_Ct
fault
fault
SC
OVL
SC
OVL
OVL ^SC
Figure 19.
When the feedback voltage exits a fault region before time
completion, the timer is reset. On the contrary, if the timer
elapses, the part enters an auto−recovery hiccup or latches
off depending on the operated version.
In the first case, the feedback is pushed to the maximum
upon start−up. The timer starts with a charging slope of the
short−circuit condition (SC). If the timer would be
externally set to 200 ms, the timer duration in this start−up
sequence would be 50 ms. As soon as regulation occurs, the
timer gets reset. An overload occurs shortly after (OVL).
The internal timer immediately starts to count when the
3.2−V level is crossed (V_FB with no OPP). As the overload
lasts less than 200 ms (in this example the timer is set to
200 ms), the feedback returns to its regulation level and
resets the timer.
In the second case, the overload occurs after regulation but
the feedback voltage quickly jumps into the short−circuit
area. At this point, the countdown is accelerated as the
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