
© 2015 Fairchild Semiconductor Corporation www.fairchildsemi.com
FL6632 • Rev. 1.0 9
FL6632 — Primary-Side-Regulated LED Driver with Power Factor Correction
Functional Description
FL6632 is AC-DC PWM controller for LED lighting
applications. TRUECURRENT™ techniques regulate
accurate LED current independent of input voltage,
output voltage, and magnetizing inductance variations.
The linear frequency control in the oscillator reduces
conduction loss and maintains DCM operation in the
wide range of output voltage, which implements high
power factor correction in a single-stage flyback
topology. A variety of protections, such as short/open-
LED protection, over-temperature protection, and cycle-
by-cycle current limitation stabilize system operation
and protect external components.
Startup
Powering at startup is slow due to the low feedback loop
bandwidth in PFC converter. To boost powering during
startup, an internal oscillator counts 12ms to define
Startup Mode. During Startup Mode, turn-on time is
determined by Current-Mode control with a 0.2 VCS
voltage limit and transconductance becomes 14 times
larger, as shown in Figure 15. After startup, turn-on time
is controlled by Voltage Mode using COMI voltage and
error amplifier transconductance is reduced to 85 mho.
0.2 V
VCS
VCOMI
VIN
14gm gm
Startup Mode: 12 ms
ILED
Time
VDD = VDD_ON
Figure 15. Startup Sequence
Constant-Current Regulation
The output current can be estimated using the peak
drain current and inductor current discharge time since
output current is same as the average of the diode
current in steady state. The peak value of the drain
current is determined by the CS pin and the inductor
discharge time (tdis) is sensed by tdis detector. By using
three points of information (peak drain current, inductor
discharging time, and operating switching period); the
TRUECURRENT™ calculation block estimates output
current. The output of the calculation is compared with
an internal precise reference to generate an error
voltage (VCOMI), which determines turn-on time in
Voltage-Mode control. With Fairchild’s innovative
TRUECURRENT™ technique, constant-current output
can be precisely controlled.
PFC and THD
In a conventional boost converter, Boundary Conduction
Mode (BCM) is generally used to keep input current in-
phase with input voltage for PF and THD. In
flyback/buck boost topology, constant turn-on time and
constant frequency in Discontinuous Conduction Mode
(DCM) can implement high PF and low THD, as shown
in Figure 16. Constant turn-on time is maintained by
the internal error amplifier and a large external
capacitor (typically over 1 µF) at the COMI pin.
Constant frequency and DCM operation are managed
by linear frequency control.
IIN
IIN_AVG
GATE
Constant Frequency
Figure 16. Input Current and Switching
DCM Control
As mentioned above, DCM should be guaranteed for
high power factor in flyback topology. To maintain DCM
across a wide range of output voltage, the switching
frequency is linearly adjusted by the output voltage in
linear frequency control in the whole Vs range. Output
voltage is detected by the auxiliary winding and the
resistive divider connected to the VS pin, as shown in
Figure 17. When the output voltage decreases,
secondary diode conduction time is increased and the
DCM control lengthens the switching period, which
retains DCM operation over the wide output voltage
range, as shown in Figure 18. The frequency control
lowers the primary rms current with better power
efficiency in full-load condition.
OSC
Gate
Driver 2GATE
CC
Control
5VS
VOUT
S/H
tDIS
Detector
DCM
Controller
Figure 17. DCM and BCM Control