AN-9744 Datasheet, PDF (1/7 Pages) Fairchild Semiconductor – Smart LED Lamp Driver IC with PFC Function

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AN-9744
Smart LED Lamp Driver IC with PFC Function
Introduction
The FL7701 is a PWM peak current controller for a buck
converter topology operating in Continuous Conduction
Mode (CCM) with an intelligent PFC function using a
digital control algorithm. The FL7701 has an internal self-
biasing circuit that is a current source using a high-voltage
switching device. When the input voltage is applied to the
HV pin is over 25 V to 500 V, the FL7701 maintains a
15.5 V_DC at the VCC pin. The FL7701 also has a UVLO
block for stable operation. When the V_CC voltage reaches
higher than V_CCST+, the UVLO block starts operation.hen
the V_CC drops below the V_CCST-, IC operation stops.
Hysteresis is provided for stable operation of the IC when
input the voltage is in noisy circumstances or unstable
conditions. The FL7701 has a “smart” internal block for AC
input condition. If an AC source with 50 Hz or 60 Hz is
applied, the IC automatically changes the internal reference
to adjust to input conditions with an internal fixed transient
time. When a DC source connects to the IC, the internal
reference immediately changes to DC waveform.
The internal DAC_OUT reference signal is dependent on
the V_CC voltage. Using the DAC_OUT signal and internal
clock, CLK_GEN; the FL7701 automatically makes a
digital reference signal, DAC_OUT. If the FL7701 cannot
detect the ZCD_OUT signal, the IC has an abnormal
internal reference signal. In this situation, this phenomenon
causes a lighting flicker.
Iline
FL7701
VCC
DAC
C
ZCD_OUT
DAC_OUT
Reference
Vsup
LED Load
D1
L ^IL
VSUP_SEN
HV
HV
Device
DAC: Digital to Analog
HV Device : High-Voltage Device
S
Q
R
Driver _OUT
Isw
CS
GND
Figure 1. Basic Block of FL7701
Figure 2. FL7701 Operation
Soft-Start Function
The FL7701 has an internal soft-start to reduce inrush
current at IC startup. When the IC starts operation, the
internal reference of the IC slowly increases up to a fixed
level for around seven cycles. After settling down this
transient period, the internal reference is fixed at a certain
DC level. In this time, the IC continually tries to find input
phase information from the VCC pin. If the IC succeeds in
getting phase information from the VCC, the IC
automatically follows a similar shape reference, which it
made during the transient times, seven periods. If not, the
IC has a DC reference level.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
www.fairchildsemi.com