NCP1237 ONSEMI [ON Semiconductor], NCP1237 Datasheet - Page 8

• Manufacturer Part Number: NCP1237
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Introduction
The NCP1237 includes all of the necessary features to build
a safe and efficient power supply based on a
fixed−frequency flyback converter. It is particularly well
suited for applications where low part count is a key
parameter, without sacrificing safety.
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Current−Mode Operation with slope compensation:
The primary peak current is permanently controlled by
the FB voltage, ensuring maximum safety: the DRV
turn−off event is dictated by the peak current setpoint.
It also ensures that the frequency response of the
system remains a first order if in DCM, which eases the
design of the feedback loop. The controller can also be
used in CCM with a wide input voltage range due to its
fixed ramp compensation that prevents the appearance
of sub−harmonic oscillations in most of the
applications.
Fixed−Frequency Oscillator with Jittering: The
NCP1237 is available in various frequency options to
fit any application. The internal oscillator features a
low−frequency jittering that helps to pass the EMI
requirements by spreading out the energy content of
frequency peaks in quasi−peak and average mode.
Latched / Autorecovery Timer−Based dual−level
Overcurrent Protection: The overcurrent protection
has 2 different levels. At the low level the controller
can still regulate but starts a long timer. The high level
corresponds to the loss of regulation and starts the usual
overload timer. This allows a power supply to
transiently deliver a higher power for a limited time.
The overcurrent protection depends only on the FB
signal, enabling it to work with any transformer, even
with very poor coupling or high leakage inductance.
Both protections are fully latched on the A version (the
power supply has to be unplugged then restarted in
order to resume operation, even if the overload
condition disappears), and autorecovery on the B
version. The timers’ durations are fixed. The controller
also enters the same protection mode if the voltage on
the CS pin reaches 1.5 times the maximum internal
setpoint, which enables to detect winding short circuits.
High Voltage Startup Current Source with
Brown−Out and Line Overvoltage Detections: Due
to On Semiconductor’s Very High Voltage technology,
the NCP1237 can directly be connected to the high
input voltage. The startup current source ensures a
clean startup while ensuring low losses when it is off,
and the Dynamic Self−Supply (DSS) restarts the startup
current source to supply the controller if the V
supply transiently drops. The high voltage pin also
features a high−voltage sensing circuitry, which is able
to turn the controller off if the input voltage is too low
(brown−out condition) or too high (line overvoltage).
APPLICATION INFORMATION
CC
http://onsemi.com
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Typical Operation
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This protection works either with a DC input voltage or
a rectified AC input voltage, and is independent of the
high voltage ripple. It uses a peak detector
synchronized with line frequency, or with the internal
watchdog timer if the HV pin is tied to a dc voltage.
Adjustable Overpower Compensation: The high
voltage sensed on the HV pin is converted into a
current to add to the current sense voltage an offset
proportional to the input voltage. By choosing the value
of the resistor in series with the CS pin, the amount of
compensation can be adjusted to the application.
Frequency foldback then Soft−Skip mode for light
load operation: In order to ensure a high efficiency in
all load conditions, the NCP1237 implements a
frequency foldback (the switching frequency is lowered
to reduce switching losses) for light load condition; and
a Soft−Skip (disabled in case of fast load transients) for
extremely low load condition.
Extended V
voltage as high as 28 V, making the design of the power
supply easier.
Clamped Driver Stage: Despite the high supply
voltage, the voltage on DRV pin is safely clamped
below 16 V; allowing the use of any standard MOSFET,
and reducing the current consumption of the controller.
Dual Latch−off Input: The NCP1237 can be latched
off by an increasing voltage applied to its Latch pin
(typically an overvoltage) or by a decreasing one, and
an NTC can be directly connected to the latch pin
thanks to the precise internal current source.
Soft−Start: At every startup the peak current is
gradually increased during 4 ms to minimize the stress
on power components.
Temperature Shutdown: The NCP1237 is internally
protected against self−heating: if the die temperature is
too high, the controller shuts all circuitries down
(including the HV startup current source), allowing the
silicon to cool down before attempting to restart. This
ensures a safe behavior in case of failure.
Startup: The HV startup current source ensures the
charging of the V
threshold V
enough (above V
controller then delivers pulses, starting with a soft−start
period t
increases before the current−mode control takes over.
During the soft−start period, the low level latch is
ignored, and the latch current is double, to ensure a fast
pre−charge of the decoupling capacitor on the Latch
pin.
SSTART
CC(on)
CC
during which the peak current linearly
range: The NCP1237 accepts a supply
HV(start)
CC
, until the input voltage is high
capacitor up to the startup
) to enable the switching. The