ncp1239f ON Semiconductor, ncp1239f Datasheet - Page 16

• Manufacturer Part Number: ncp1239f
• Description: Lowstandby High Performance Pwm Controller
• Manufacturer: ON Semiconductor

building a rugged and safe switch−mode power supply. The
following details the major benefits brought by
implementing the NCP1239 controller:
compensation: implementing peak current mode control,
the NCP1239 offers an internal ramp compensation signal
that can easily be summed up to the sensed current.
Subharmonic oscillations can thus be fought via the
inclusion of a simple resistor,
(NCP1239F): the NCP1239 operating in current mode, the
circuit Pin 10 monitors the current to modulate its level
according to the power demand. Due to the ramp
compensation, one must generally note that the Pin 10
voltage is not the exact image of the inductor current. A
precise current limitation being essential, the NCP1239
features a separate current sense pin (Pin 9) for an accurate
overcurrent detection. The low threshold of this protection
(500 mV) avoids excessive losses in the current sense
resistor and improves the efficiency. In addition, Pin 9
sources a current that proportional to the high−voltage rail,
compensates the current−sense and turn off delays at high
line. A resistor inserted between Pin 9 and the sensing
resistor offsets the Pin 9 current−sense information to build
a precise overload protection, independent of the mains
input.
V
or Forward applications.
levels of standby power represents a difficult exercise when
the controller requires an external, lossy, resistor connected
to the bulk capacitor. Due to an internal logic, the controller
disables the high−voltage current source after startup which
no longer hampers the consumption in no−load situations.
compatible with no−load standby power requirements.
Slicing the switching pattern in bunch of pulses drastically
reduces overall losses but can, in certain cases, bring
acoustic noise in the transformer. Due to a skip operation
taking place at low peak currents only, no mechanical noise
appears in the transformer. Furthermore, the skip threshold
is made programmable to allow the best trade−off between
noise and efficiency.
NCP1239 incorporates an internal logic that is able to detect
a standby situation. Pin1 state changes in accordance to the
CC
The NCP1239 includes all necessary features to help
Current−mode
500 mV Current Sense threshold for Over Power Limit
Large V
Internal high−voltage startup switch: reaching low
Skip−cycle capability: a continuous flow of pulses is not
Standby Detect/Shutdown of the PFC front−stage: The
range up to 36 V, bringing greater flexibility in Flyback
CC
operation: the NCP1239 offers an extended
operation
with
internal
APPLICATION INFORMATION
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ramp
NCP1239
16
detected mode (standby or normal mode). Simply connect a
pnp transistor between the NCP1239 V
controller one and drive it using Pin 1, to enable the PFC
stage in normal mode and disable it in standby.
soft−start sequence that precludes the main power switch
from being stressed upon startup. The same voltage is also
used to perform frequency jittering and timing for the fault
condition detection.
voltage exceeds 2.4 V. When this occurs, the NCP1239
considers that a major fault is present and as a consequence,
the circuit gets permanently latched−off. In this mode, the
circuit needs the V
instance when the user un−plugs the SMPS. This capability
is mainly intended to detect an overvoltage condition or/and
an over−heating of the application that would be sensed by
a thermistor.
during normal operation, the controller protects the SMPS
against low mains conditions. When the Pin 5 voltage falls
below 250 mV, the controllers stops pulsing until this level
goes back to 500 mV to prevent any instability.
overload protections are difficult to implement when a
strong leakage inductance affects the transformer (the
auxiliary winding level does not properly collapse…). Here,
every time the feedback pin is at its maximum (higher than
5.0 V practically), an error flag is asserted and the circuit
activates a timer that is programmed by the Pin 6 capacitor.
If Pin 6 reaches 4.3 V while the error flag is still present, the
controller stops the pulses and goes into a latch−off phase,
operating in a low−frequency burst−mode. As soon as the
fault disappears, the SMPS resumes its operation. The
latch−off phase can also be initiated, more classically, when
V
improved EMI signature: Pin 4 offers a means to precisely
adjust the switching frequency through a simple resistor to
ground. Frequency operation is allowed up to 250 kHz. By
modulating the internal switching frequency with the Pin 6
saw−tooth (100 Hz with 390 nF), natural energy spread
appears and softens the controller’s EMI signature.
help biasing any external circuitry in the vicinity of the
controller. This reference voltage can typically supply up to
10 mA.
CC
Soft−Start: the capacitor connected to Pin 6 provides a
Major Fault Detection: the circuit detects when Pin 3
Brown−out detection: by monitoring the level on Pin 5
Short−circuit protection: short−circuit and especially
Adjustable frequency and Internal dithering for
5.0 V reference voltage: a 5.0 V regulator is provided to
drops below UVLO (11.2 V typical).
CC
to go down below 4.0 V to reset, for
CC
and the PFC