NCP1234
www.onsemi.com
12
APPLICATION INFORMATION
Introduction
The NCP1234 includes all 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.
• 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 stays a first order if in DCM, which eases the
design of the FB loop. The controller can be also used
in CCM applications with a wide input voltage range
thanks to its fixed ramp compensation that prevents the
appearance of sub−harmonic oscillations in most
applications.
• Fixed−Frequency Oscillator with Jittering: The
NCP1234 is available in different frequency options to
fit any application. The internal oscillator features a
low−frequency jittering that helps passing the EMI
limits by spreading out the energy content of frequency
peaks in quasi−peak and average mode of
measurement.
• Latched Timer−Based Overload Protection: The
overload protection depends only on the FB signal,
making it able to work with any transformer, even with
very poor coupling or high leakage inductance. The
protection is fully latched on the A version (the power
supply has to be stopped then restarted in order to
resume operation, even if the overload condition
disapears), and autorecovery on the B version. The
timer duration is fixed. The controller also enters the
same protection mode if the voltage on the CS pin
reaches 1.5 times the maximum internal setpoint
(allows to detect winding short-circuits).
• High Voltage Start−Up Current Source: Thanks to
ON Semiconductor’s Very High Voltage technology,
the NCP1234 can directly be connected to the high
input voltage. The start-up current source ensures a
clean start-up while ensuring low losses when it is off,
and the Dynamic Self-Supply (DSS) restarts the
start-up current source to supply the controller if the
V
CC
supply transiently drops.
• Adjustable Overpower Compensation: The high
input voltage sensed on the HV pin is converted into a
current to build on 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 skip mode for light load
operation: In order to ensure a high efficiency under all
load conditions, the NCP1234 implements a frequency
foldback for light load condition and a skip mode for
extremely low load condition. The switching frequency
is decreased down to 27 kHz to reduce switching
losses.
• Extended VCC range: The NCP1234 accepts a supply
voltage as high as 28 V, with an overvoltage threshold
V
CC(ovp)
(typically 26.5 V) that latches the controller
off.
• Clamped Driver Stage: Despite the high maximum
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 NCP1234 can be latched
off by 2 ways: The voltage increase applied to its Latch
pin (typically an overvoltage) or by a decrease this
voltage. Thanks to the internal precise pull−up current
source a NTC can be directly connected to the latch pin.
This NTC will provide an overtemperature protection
by decreasing its resistance and consequently the
voltage at Latch pin,
• Soft−Start: At every start−up the peak current is
gradually increased during 4.0 ms to minimize the
stress on power components.
• Temperature Shutdown: The NCP1234 is internally
protected against self−overheating: if the die
temperature is too high, the controller shuts all
circuitries down (including the HV start−up current
source), allowing the silicon to cool down before
attempting to restart. This ensures a safe behavior in
case of failure.
Typical Operation
• Start−up: The HV start−up current source ensures the
charging of the V
CC
capacitor up to the start−up
threshold V
CC(on)
, until the input voltage is high
enough (above V
HV(start)
) to allow the switching to
start. The controller then delivers pulses, starting with a
soft−start period t
SSTART
during which the peak current
linearly 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 Latch pin decoupling capacitor.
• Normal operation: As long as the feedback voltage is
within the regulation range and V
CC
is maintained
above V
CC(min)
, the NCP1234 runs at a fixed frequency
(with jittering) in current−mode control. The peak
current (sensed on the CS pin) is set by the voltage on
the FB pin. Fixed ramp compensation is applied
internally to prevent sub−harmonic oscillations from
occurring.
• Light load operation: When the FB voltage decreases
below V
FB(foldS)
, typically corresponding to a load of
