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To Current Sense Comparator
17 k
FB
Current
Mirror
4:3
Figure 15. Feedback Loop − Current Sense Control
The resulting current sense regulation characteristic can
be seen from Figure 16.
50 mA
12.5 mA
0 mA
50 mA
FB Pin Sink Current
Figure 16. Current Sense Regulation Characteristic
When the load goes light, the compression circuitry
decreases the peak current. This has the effect of slightly
increasing the switching frequency but the compression
ratio is selected to not hamper the standby power.
OFF Time Control
The loop signal together with the internal current source,
via an external capacitor, controls the switch−off time. This
is portrayed in Figure 17.
NCP1215
CS
37.5 m
12.5 m
A
A
100 mA
140 mA
http://onsemi.com
From Feedback Loop Block
Voffset
CT
10 mA
CT
Figure 17. OFF Time Control
During the switch−ON time, the CT capacitor is kept
discharged by a MOSFET switch. As soon as the latch
output changes to a low state, the voltage across CT created
by the internal current source, starts to ramp−up until its
value reaches the threshold given by the feedback loop
demand.
V
V
DD
P
Goes Down
OUT
CT Pin
P
Goes Up
OUT
Voltage
V
offset
P1
t
off−min
Figure 18. CT Pin Voltage (P
The voltage that can be observed on CT pin is shown in
Figure 18. The bold waveform shows the maximum output
power when the OFF time is at its minimum. The IC allows
an OFF time of several seconds.
8
−
+
V
to V
offset
DD
−
To Latch's Set Input
+
To Latch's Output
GND
P3
P2
1 u P
2 u P
3)
out
out
out
t
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