NCP1203
APPLICATION INFORMATION
Introduction
The NCP1203 implements a standard current mode
architecture where the switch−off time is dictated by the
peak current setpoint. This component represents the ideal
candidate where low part−count is the key parameter,
particularly in low−cost AC−DC adapters, auxiliary
supplies etc. Due to its high−performance SMARTMOS
High−V
oltage technology, the NCP1203 incorporates all the
necessary components normally needed in UC384X based
supplies: timing components, feedback devices, low−pass
filter and startup device. This later point emphasizes the fact
that ON Semiconductor’s NCP1203 does not need an
external startup resistance but supplies the startup current
directly from the high−voltage rail. On the other hand, more
and more applications are requiring low no−load standby
power, e.g. for AC−DC adapters, VCRs etc. UC384X series
have a lot of difficulty to reduce the switching losses at low
power levels. NCP1203 elegantly solves this problem by
skipping unwanted switching cycles at a user−adjustable
power level. By ensuring that skip cycles take place at low
peak current, the device ensures quiet, noise free operation.
Finally, an auto−recovery output short−circuit protection
(OCP) prevents from any lethal thermal runaway in
overload conditions.
Startup Sequence
When the power supply is first powered from the mains
outlet, the internal current source (typically 6.0 mA) is
biased and charges up the V
CC
capacitor. When the voltage
on this V
CC
capacitor reaches the V
CC(on)
level (typically
12.8 V), the current source turns off and no longer wastes
any power. At this time, the V
CC
capacitor only supplies the
controller and the auxiliary supply is supposed to take over
before V
CC
collapses below V
CC(min)
. Figure 13 shows the
internal arrangement of this structure:
8
12.8 V/4.9 V
+
−
6 mA or 0
6
CV
CC
4
HV
Aux
Figure 13. The Current Source Brings V
CC
Above 12.8 V and then Turns Off
Once the power supply has started, the V
CC
shall be
constrained below 16 V, which is the maximum rating on
pin 6. Figure 14 portrays a typical startup sequence with a
V
CC
regulated at 12.5 V:
13.5
12.8 V
REGULATION
12.5
11.5
10.5
9.5
3.00 M
8.00 M
13.0 M
t, TIME (sec)
18.0 M
23.0 M
Figure 14. A Typical Startup Sequence for
the NCP1203
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