© 2014 Fairchild Semiconductor Corporation www.fairchildsemi.com
FL7733AMX • Rev. 1.2 9
FL7733A — Primary-Side-Regulated LED Driver with Power Factor Correction
Functional Description
FL7733A is AC-DC PWM controller for LED lighting
applications. TRUECURRENT® technology regulate
accurate constant LED current independent of input
voltage, output voltage, and magnetizing inductance
variations. The DCM control in the oscillator reduces
conduction loss and maintains DCM operation over a
wide range of output voltage, which implements high
power factor correction in a single-stage flyback or
buck-boost topology. A variety of protections, such as
LED short / open protection, sensing resistor short /
open protection, over-current protection, over-
temperature protection, and cycle-by-cycle current
limitation stabilize system operation and protect external
components.
Startup
At startup, an internal high-voltage JFET supplies
startup current and VDD capacitor charging current, as
shown in Figure 17. When VDD reaches 16 V, switching
begins and the internal high-voltage JFET continues to
supply VDD operating current for an initial 250 ms to
maintain VDD voltage higher than VDD-OFF. As the output
voltage increases, the auxiliary winding becomes the
dominant VDD supply current source.
250 ms
Timer
16 V /
7.75 V
8
HV 4
VDD
Internal
Bias
VS
VDC
5
CVDD
RVS1
RVS2
VDD Good
Figure 17. Startup Block
Switching is controlled by current-mode for 20 ms after
VDD-ON. During current-mode switching with the flyback
or buck-boost topology, output current is only
determined by output voltage. Therefore, the output
voltage increases with constant slope, regardless of line
voltage variation. Short-LED Protection (SLP) is enabled
after the 15 ms SLP blanking time so that the output
voltage is higher than SLP threshold voltage and
successful startup is guaranteed without SLP in normal
condition.
During current-mode switching, COMI voltage, which
determines turn-on time in voltage mode, is adjusted
close to the steady state level. The COMI capacitor is
charged to 1.2 V for 15 ms and adjusted to a modulated
level inversely proportional to VIN peak value for 5 ms.
Turn-on time right after 20 ms startup time can be
controlled close to steady state on time so that voltage
mode is smoothly entered without LED current
overshoot or undershoot.
ILED
Time
VCOMI
15 ms
Startup Time 20 ms
1.0 V
VCS
0.2 V
VIN VDD = VDD_ON
Low line
High Line
Low line
High Line
Current Mode Voltage Mode
Figure 18. Startup Sequence
PFC and THD
In the flyback or the buck-boost topology, constant turn-
on time and constant frequency in Discontinuous
Conduction Mode (DCM) operation can achieve high PF
and low THD, as shown in Figure 19. Constant turn-on
time is maintained by the internal error amplifier and a
large external COMI capacitor (typically over 1 µF) at
COMI pin. Constant frequency and DCM operation are
managed by DCM control.
Constant tON
Average
input current
Secondary current
peak envelope
Constant tOFF
Primary current
peak envelope
Figure 19. Power Factor Correction
Constant-Current Regulation
The output current can be estimated using the peak
drain current and inductor current discharge time
because output current is the same as the average of
the diode current in steady state. The peak value of the
drain current is determined by the CS peak voltage
detector. The inductor current discharge time (tDIS) is
sensed by a tDIS detector. With peak drain current,
inductor current discharging time and operating
switching period information, the TRUECURRENT®
calculation block estimates output current as follows: