NCP1254 View Datasheet(PDF) - ON Semiconductor
Part Name
Description
Manufacturer
NCP1254
ON Semiconductor
NCP1254 Datasheet PDF : 25 Pages
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D3 15 V
NTC
11
RoppU
841 k
OPP
10
4
ROPPL
2.5 k
5
Vlatch
NCP1254
additional filter
D2 1N4148
2
1
OVP
C1
330 pF
R3
220
Vcc
9
3
aux.
winding
OPP
Figure 41. A small RC filter avoids the fast rising spikes from reaching the protection pin of
the NCP1254 in presence of energetic perturbations superimposed on the input line.
Latching Off with the Vcc Pin
The NCP1254 hosts a dedicated comparator on the Vcc
pin. When the voltage on this pin exceeds 25.5 V typically
for more than 20 ms, a signal is sent to the internal latch and
the controller immediately stops the driving pulses while
remaining in a lockout state. The part can be reset by cycling
down its Vcc, for instance by pulling off the power. This
technique offers a simple and cheaper means to protect the
converter against optocoupler failures without using the
OPP pin and a Zener diode.
Peak Power Excursions
There are applications where the load profile heavily
changes from a nominal to a peak value. For instance, it is
possible that a 30−W ac−dc adapter accepts power
excursions up to 60 W in certain conditions. Inkjet printers
typically fall in that category of peak power adapters.
However, to avoid growing the transformer size, an existing
technique consists in freezing the peak current to a
maximum value (0.8/Rsense in our case) but authorizes
frequency increase to a certain point. This point is internally
fixed at 130 kHz.
VFB (V)
4.5
Maximum frequency is FOSC,max
4.0
Fsw increases
3.2
Fsw is fixed
65 kHz
1.9
Fsw decreases
1.5
65 kHz
26 kHz
Peak current
is clamped
Ipeak max
Peak current
can change
1
0.4
0 duty−ratio
Ipeak min
Peak current
is frozen
t
Figure 42. The feedback pin modulates the frequency up to 130 kHz (short−circuit,
maximum power) or down to 26 kHz in frequency foldback.
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