NCP1337
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7
APPLICATION INFORMATION
INTRODUCTION
The NCP1337 implements a standard current mode
architecture where the switch−off time is dictated by the
peak current setpoint, whereas the core−reset detection
triggers the turn−on event. This component represents
the ideal candidate where low part−count is the key
parameter, particularly in low−cost AC−DC adapters,
consumer electronics, auxiliary supplies, etc. Due to
its high−performance, high−voltage technology, the
NCP1337 incorporates all the necessary features needed to
build a rugged and reliable Switch−Mode Power Supply
(SMPS):
• Quasi−Resonant Operation: Valley−switching
operation is ensured whatever the operating conditions
are, due to the internal soxyless circuitry. As a result,
there are virtually no primary switch turn−on losses,
and no secondary diode recovery losses, and EMI and
video noise perturbations are reduced. The converter
also stays a first−order system and accordingly eases
the feedback loop design.
• Dynamic Self−Supply (DSS): Due to its Very High
Voltage Integrated Circuit (VHVIC) technology,
ON Semiconductor’s NCP1337 allows for a direct pin
connection to the high−voltage DC rail. A dynamic
current source charges up a capacitor and thus
provides a fully independent V
CC
level. As a result,
low power applications will not require any auxiliary
winding to supply the controller. In applications
where this winding is anyway required (see “Power
Dissipation” section in the application note), the DSS
will simplify the V
CC
capacitor selection.
• Overcurrent Protection (OCP): When the feedback
current is below minimum value, a fault is detected. If
this fault is present for more than 80 ms, NCP1337
enters an auto−recovery soft burst mode. All pulses are
stopped and the V
CC
capacitor discharges down to
5.0 V. Then, by monitoring the V
CC
level, the startup
current source is activated ON and OFF to create a
burst mode. After the current source being activated
twice, the controller tries to restart, with a 4.0 ms
soft−start. If the fault has gone, the SMPS resumes
operation. If the fault is still there, the burst sequence
starts again. The soft−start, together with a minimum
frequency clamp, allow to reduce the noise generated
in the transformer in short−circuit conditions.
• Overvoltage Protection (OVP): By continuously
monitoring the V
CC
voltage level, the NCP1337 stops
switching whenever an overvoltage condition is
detected.
• Brown−Out Detection (BO): By monitoring the level
on Pin 1 during normal operation, the controller
protects the SMPS against low mains condition. When
Pin 1 level falls below 500 mV, the controller stops
switching until this level goes back and resumes
operation, unless the fault timer has timed out. By
adjusting the resistor divider connected between the
high input voltage and this pin, start and stop levels
are programmable.
• Over Power Compensation (OPC): An internal
current source injects out of Pin 3 (CS pin) a current
proportional to the voltage applied on Pin 1. As this
voltage is an image of the input voltage, by inserting a
resistor in series with Pin 3, it is possible to create an
offset on the current sense signal that will compensate
the effect of the input voltage variation.
• External Latch Trip Point: By externally forcing a
level on Pin 1 (e.g., with a signal coming from a
temperature sensor) greater than 3.0 V (but below
5.0 V), it is possible to disable the output of the
controller. If the voltage is forced over 5.0 V, the
controller is permanently latched−off: to resume
normal operation, the V
CC
voltage should go below
4.0 V, which implies to unplug the SMPS from the
mains.
• Standby Ability: Under low load conditions,
NCP1337 enters a Soft−Skip mode: when the CS
setpoint becomes lower than 20% of the maximum
peak current, output pulses are stopped, then switching
is starting again when FB loop forces a setpoint higher
than 25%. As this occurs at low peak current, with
Soft−Skip activated, and as the T
OFF
is clamped,
noise−free operation is guaranteed, even with a cheap
transformer.
