NCL30088
www.onsemi.com
18
Application Information
The NCL30088 is a driver for power−factor corrected
flyback and non−isolated buck−boost and SEPIC
converters. Its current−mode, quasi−resonant architecture
optimizes the efficiency by turning on the MOSFET when
its drain−source voltage is minimal (valley). At high line, the
circuit delays the MOSFET turn on until the second valley
is detected to reduce the switching losses. A proprietary
circuitry ensures both accurate regulation of the output
current (without the need for a secondary−side feedback)
and near−unity power factor correction. The circuit contains
a suite of powerful protections to ensure a robust LED driver
design without the need for extra components or overdesign.
• Quasi−Resonance Current−Mode Operation:
implementing quasi−resonance operation in peak
current−mode control, the NCL30088 optimizes the
efficiency by turning on the MOSFET when its
drain−source voltage is minimal (valley). In light−load
conditions, the circuit changes valleys to reduce the
switching losses. For stable operation, the valley at
which the MOSFET switches on remains locked until
the input voltage or the output current set−point
significantly changes.
• Primary−Side Constant−Current Control with
Power Factor Correction: a proprietary circuitry
allows the LED driver to achieve both near−unity
power factor correction and accurate regulation of the
output current without requiring any secondary−side
feedback (no optocoupler needed). A power factor as
high as 0.99 and an output current deviation below ±2%
are typically obtained.
• Main protection features:
♦ Over Temperature Thermal Fold−back /
Shutdown/ Over Voltage Protection: the
NCL30088 features a gradual current foldback to
protect the driver from excessive temperature down
to 50% of the programmed current. This represents a
power reduction of the LED by more than 50%. If
the temperature continues to rise after this point to a
second level, the controller stops operating. This
mode would only be expected to be reached if there
is a severe fault. The first and second temperature
thresholds depend on the value of the NTC
connected to the SD pin. Note, the SD pin can also
be used to shutdown the device by pulling this pin
below the V
OTP(off)
min level. A Zener diode can
also be used to pull−up the pin and stop the
controller for adjustable OVP protection. Both
protections are latching−off (A and C versions) or
auto−recovery (the circuit can recover operation
after 4−s delay has elapsed − B and D versions).
♦ Cycle−by−cycle peak current limit: when the
current sense voltage exceeds the internal threshold
V
ILIM
, the MOSFET is immediately turned off
(cycle−by−cycle current limitation).
♦ Winding or Output Diode Short−Circuit
Protection: an additional comparator senses the CS
signal and stops the controller if it exceeds 150% x
VILIM for 4 consecutive cycles. This feature can
protect the converter if a winding is shorted or if the
output diode is shorted or simply if the transformer
saturates. This protection is latching−off (A and C
versions) or auto−recovery (B and D versions).
♦ Output Short−circuit protection: if the ZCD pin
voltage remains low for a 90−ms time interval, the
controller detects that the output or the ZCD pin is
grounded and hence, stops operation. This protection
is latching−off (A and C versions) or auto−recovery
(B and D versions).
♦ Open LED protection: if the V
CC
pin voltage
exceeds the OVP threshold, the controller shuts
down and waits 4 seconds before restarting
switching operation.
♦ Floating or Short Pin Detection: the circuit can
detect most of these situations which helps pass
safety tests.
Power Factor and Constant Current Control
The NCL30088 embeds an analog/digital block to control
the power factor and regulate the output current by
monitoring the ZCD, V
S
and CS pin voltages (signals ZCD,
V
S
and V
CS
of Figure 59). This circuitry generates the
current setpoint (V
CONTROL
/4) and compares it to the
current sense signal (V
CS
) to dictate the MOSFET turning
off event when V
CS
exceeds V
CONTROL
/4.
Power Factor and
Constant−Current
Control
PWM Latch reset
STOP
V
VS
REFX
V
COMP
ZCD
C1
CS
V
Figure 59. Power Factor and Constant−Current Control
