NCL30186
www.onsemi.com
19
Application Information
The NCL30186 is a driver for power−factor corrected
flyback and non−isolated buck−boost/ SEPIC converters. It
implements a current−mode, quasi−resonant architecture
including valley lockout and frequency fold−back
capabilities for maintaining high−efficiency performance
over a wide load range. 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 of extra external components or overdesign
• Quasi−Resonance Current−Mode Operation:
implementing quasi−resonance operation in peak
current−mode control, the NCL30186 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 a 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.
• Linear or PWM dimming: the DIM pin allows
implementing both analog and PWM dimming.
• Main protection features:
♦ Over Temperature Thermal Fold−back/
Shutdown/Over Voltage Protection: the
NCL30186 features a gradual current foldback to
protect the driver from excessive temperature down
to 50% of the programmed current. 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 under
normal conditions if there is a severe fault. The first
and second temperature thresholds depend on the
NTC connected to the circuit SD pin. 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.
♦ Winding or Output Diode Short−Circuit
Protection: an additional comparator senses the CS
signal and stops the controller if it exceeds 150% x
V
ILIM
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: NCL30186
protections aid in pass safety tests. For instance, the
circuit stops operating when the CS pin is grounded
or open.
Power Factor and Constant Current Control
The NCL30186 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
REF
V
COMP
ZCD
C1
CS
V
DIM_disable
Figure 59. Power Factor and Constant−Current Control