NCP1351 ON Semiconductor, NCP1351 Datasheet - Page 14

• Manufacturer Part Number: NCP1351
• Description: Variable Off Time Controller Featuring Short-circuit Protection
• Manufacturer: ON Semiconductor

by the optocoupler current. The feedback current, I
Figure 15, enter an internal 45 kW resistor which develops
a voltage. This voltage becomes the variable threshold point
for the capacitor charge, as indicated by Figure 10. Thus, in
lack of feedback current (start-up or short-circuit), there is
no voltage across the 45 kW and the series offset of 500 mV
clamps the capacitor swing. If a 270 pF capacitor is used, the
maximum switching frequency is 65 kHz.
can obviously generate audible noise. For this reason, the
NCP1351 uses a patented current compression technique
which reduces the peak current in lighter load conditions. By
design, the peak current changes from 100% of its full load
value, to 30% of this value in light load conditions. This is
the block placed on the lower left corner of Figure 15. In full
The FB pin can actually be seen as a diode, forward biased
Folding the frequency back at a rather high peak current
to R
sense
22pF
C1
100n
R
3.9k
C
270p
C3
offset
t
V
CC
R1
2.5k
C
t
CS
FB
V
Figure 15. The Feedback Section Inside the NCP1351
CC
I
V
FB
CC
IC
10m
I
diff
t
ICS
= ICS
Reset
min
max
D
- ICS
FB
http://onsemi.com
FB
min
NCP1351
on
14
I
diff
I CS + I CS_min ) I dif + I CS_max * I CS_min ) I CS_min
load conditions, the feedback current is weak and all the
current flowing through the external offset resistor is:
As the load goes lighter, the feedback current increases and
starts to steal current away from the generators. Equation 12
can thus be updated by:
Equation 13 testifies for the current reduction on the offset
generator, k represents an internal coefficient. When the
feedback current equals I
I CS + I CS_max * kI FB
I CS + I CS_min
+ I CS_max
I
FB
I
FB
R
45k
FB
500mV
V
offset
I
diff
+
dif
f
(IFB)
, the offset becomes:
-
+
Clock
(eq. 13)
(eq. 14)
(eq. 15)