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NCP1253 Datasheet(PDF) 10 Page - ON Semiconductor

Part No. NCP1253
Description  Current-Mode PWM Current-Mode PWM Power Supplies
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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NCP1253 Datasheet(HTML) 10 Page - ON Semiconductor

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NCP1253
http://onsemi.com
10
APPLICATION INFORMATION
Introduction
The NCP1253 implements a standard current mode
architecture where the switch−off event is dictated by the
peak current setpoint. This component represents the ideal
candidate where low part−count and cost effectiveness are
the key parameters, particularly in low−cost ac−dc adapters,
open−frame power supplies etc. Capitalizing on the
NCP1200 series success, the NCP1253 brings all the
necessary components normally needed in today modern
power supply designs, bringing several enhancements such
as a VCC OVP or an adjustable slope compensation signal.
Current−mode operation with internal ramp
compensation: implementing peak current mode control
at a fixed 65 kHz or 100 kHz frequency, the NCP1253
offers an internal ramp compensation signal that can
easily by summed up to the sensed current. Sub harmonic
oscillations can thus be compensated via the inclusion of
a simple resistor in series with the current−sense
information.
Low startup current: reaching a low no−load standby
power always represents a difficult exercise when the
controller draws a significant amount of current during
start−up. Thanks to its proprietary architecture, the
NCP1253 is guaranteed to draw less than 15
mA
maximum, easing the design of low standby power
adapters.
EMI jittering: an internal low−frequency modulation
signal varies the pace at which the oscillator frequency is
modulated. This helps spreading out energy in conducted
noise analysis. To improve the EMI signature at low
power levels, the jittering will not be disabled in
frequency foldback mode (light load conditions).
Frequency foldback capability: a continuous flow of
pulses is not compatible with no−load/light−load standby
power requirements. To excel in this domain, the
controller observes the feedback pin and when it reaches
a level of 1.5 V, the oscillator then starts to reduce its
switching frequency as the feedback level continues to
decrease. When the feedback pin reaches 1.05 V, the peak
current setpoint is internally frozen and the frequency
continues to decrease. It can go down to 26 kHz (typical)
reached for a feedback level of 350 mV roughly. At this
point, if the power continues to drop, the controller enters
classical skip−cycle mode.
Internal soft−start: a soft−start precludes the main power
switch from being stressed upon start−up. In this
controller, the soft−start is internally fixed to 4 ms.
Soft−start is activated when a new startup sequence
occurs or during an auto−recovery hiccup.
Latched OVP on Vcc: it is sometimes interesting to
implement a circuit protection by sensing the VCC level.
This is what NCP1253 does by monitoring its VCC pin.
When the voltage on this pin exceeds 25.5 V typical, the
pulses are immediately stopped and the part latches off.
When the user cycles the VCC down or the converter
recovers from a brown−out event, the circuit is reset and
the part enters a new start−up sequence.
Short−circuit protection: short−circuit and especially
over−load protections are difficult to implement when a
strong leakage inductance between auxiliary and power
windings affects the transformer (the aux winding level
does not properly collapse in presence of an output short).
Here, every time the internal 0.8 V maximum peak
current limit is activated, an error flag is asserted and a
time period starts, thanks to an internal timer. When the
fault is validated, all pulses are stopped and the controller
enters an auto−recovery burst mode, with a soft−start
sequence at the beginning of each cycle. As soon as the
fault disappears, the SMPS resumes operation. Please
note that some version offers an auto−recovery mode as
we just described, some do not and latch off in case of a
short circuit.
Start−up Sequence
The NCP1253 start−up voltage is made purposely high to
permit large energy storage in a small VCC capacitor value.
This helps to operate with a small start−up current which,
together with a small Vcc capacitor, will not hamper the
start−up time. To further reduce the standby power, the
start−up current of the controller is extremely low, below
15
mA. The start−up resistor can therefore be connected to
the bulk capacitor or directly to the mains input voltage if
you wish to save a few more mW.


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