NCP1028
www.onsemi.com
10
APPLICATION INFORMATION
Introduction
The NCP1028 offers a complete current−mode control
solution and enhances the NCP101X series. The
component integrates everything needed to build a rugged
and low−cost Switch−Mode Power Supply (SMPS)
featuring low standby power.
• Current−Mode Operation: The controller uses a
current−mode control architecture, which, together
with an adjustable ramp compensation circuitry,
ensures efficient and stable continuous or
discontinuous conduction designs.
• 700 V–5.8 W Power Switch Circuit: Due to
ON Semiconductor Very High Voltage Integrated
Circuit technology, the circuit hosts a high−voltage
power switch circuit featuring a 5.8 W R
DS(on)
– T
J
=
25°C. This value lets the designer build a 15 W
power supply operated on universal mains as
long as sufficient copper area exists to lower the
junction−to−ambient thermal resistance. An internal
current source delivers the startup current, necessary
to crank the power supply.
• Short−Circuit Protection: By permanently
monitoring the feedback line activity, the circuit is
able to detect the presence of a short−circuit,
immediately reducing the output power for a total
system protection. A 55 ms timer is started as soon as
the feedback pin asks for the maximum peak current.
At the end of this timer, if the fault is still present,
then the device enters a safe, auto−recovery burst
mode, affected by a fixed 440 ms recurrence. Once the
short has disappeared, the controller resumes and goes
back to normal operation. The timer duration is fully
independent from the V
CC
capacitor value.
• Over Power Protection: A possibility exists to reduce
the maximum output power capability in high line
conditions. A simple two resistor network wired to the
bulk capacitor will program the maximum current
reduction for a given input voltage (down to 20% of
the maximum peak current).
• Brown−Out Input: A fraction of the input voltage
appears on pin 3, due to a resistive divider. If the
mains drops below a level adjusted by this resistive
divider, the circuit does not switch. As soon as the
mains goes back within its normal range, the device
resumes operation and operates normally. By adjusting
the bridge resistors, it becomes possible to set the
brown−out levels (on and off) independently.
• Latchoff: Pin 3 also welcomes a comparator who
offers a way to fully latch the controller. If an external
event (e.g. an overtemperature) brings the brown−out
pin above 3.5 V, the circuit stays permanently off
until the user cycles its V
CC
down, for instance by
unplugging the converter from the mains outlet.
• Frequency Jittering: The internal clock receives a
low frequency modulation which helps smoothing the
power supply EMI signature.
• Soft−Start: A 1.0 ms soft−start ensures a smooth
startup sequence, reducing output overshoots.
• Skip Cycle: If SMPS naturally exhibit a good
efficiency at nominal load, they begin to be less
efficient when the output power demand diminishes.
By skipping unneeded switching cycles, the NCP1028
drastically reduces the power wasted during light load
conditions. Experiments carried over the 5.0 V/2.0 A
demonstration board reveal a standby power at
no−load and 265 Vac of 85 mW and an efficiency for
500 mW output power of 64% at 230 Vac.
