NCP1239
www.onsemi.com
14
DEFINITION
General
The NCP1239 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. The NCP1239 packs all the
necessary components normally needed in today modern
power supply designs, bringing several enhancements such
as a non-dissipative over power protection (OPP),
a brown-out protection or HV start-up current source.
Current-Mode Operation with Internal Ramp
Compensation
Implementing peak current mode control operating at a 65
or 100-kHz switching frequency, the NCP1239 offers
a fixed internal compensation ramp that can easily by
summed up to the sensed current. The controller can be used
in CCM applications with wide input voltage range thanks
to its fixed ramp compensation that prevents the appearance
of sub-harmonic oscillations
Internal Brown-Out Protection
A portion of the bulk voltage is internally sensed via the
high-voltage pin monitoring (pin 8). When the voltage on
this pin is too low, the part stops pulsing. No re-start attempt
is made until the controller senses that the voltage is back
within its normal range. When the brown-out comparator
senses the voltage is acceptable, de-latch occurs and the
controller authorizes a re-start synchronized with V
CC(on)
.
Adjustable Overpower Compensation
The high input voltage sensed on the HV pin is converted
into a current. This current builds an offset superimposed on
the current sense voltage which is proportional to the input
voltage. By choosing the resistance value in series with the
CS pin, the amount of compensation can be adjusted to the
application.
High-Voltage Start-Up
Low standby power results cannot be obtained with the
classical resistive start-up network. In this part,
a high-voltage current-source provides the necessary
current at start-up and turns off afterwards. An option is
available to activate the Dynamic Self−Supply (DSS). The
start−up current source is turned on to supply the controller
if the Vcc voltage drops below a certain level in light load.
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 1.9 V, the oscillator starts to reduce
its switching frequency as the feedback level continues to
decrease. When the feedback level reaches 1.5 V, the
frequency hits its lower stop at 26 kHz. When the feedback
pin goes further down and reaches 1.0 V, the peak current
setpoint is internally frozen. Below this point, if the power
continues to drop, the controller enters classical skip-cycle
mode at a 31% frozen peak current.
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 8 ms. Soft-start is activated when a new
start-up sequence occurs or during an auto-recovery hiccup.
Fault Input
The NCP1239 includes a dedicated fault input accessible
via its fault pin (pin 1). It can be used to sense an
over-voltage condition on the adapter. The circuit can be
latched off by pulling the pin above the upper fault threshold,
V
FAULT(OVP)
, typically 3.0 V. The controller is also disabled
if the fault pin voltage, V
FAULT
, is pulled below the lower
fault threshold, V
FAULT(OTP)
, typically 0.4 V. The lower
threshold is normally used for detecting an over-temperature
fault (by the means of an NTC).
OVP Protection on V
CC
It is sometimes interesting to implement a circuit
protection by sensing the V
CC
level. This is what this
controller does by monitoring its V
CC
pin. When the voltage
on this pin exceeds V
cc(ovp)
threshold, the pulses are
immediately stopped and the part enters in an endless hiccup
or auto-recovery mode depending on controller options.
Short-Circuit/Overload Protection
Short-circuit and especially overload 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 the
64-ms 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. An internal timer keeps the pulses off for 1 s typically
which, associated to the 64-ms pulsing re-try period, ensures
a duty-cycle in fault mode less than 10%, independent from
the line level. As soon as the fault disappears, the SMPS
resumes operation. Please note that some version offers an
auto-recovery mode (B, C, D and E versions) as we just
described, some do not and latch off in case of a short-circuit
(A, F, G, H, I and J versions).
