FL7760
www.onsemi.com
8
APPLICATION INFORMATION
General
The FL7760 is a step down hysteretic LED current
controller that is easily configured in varies input voltage
range from 8 V to 70 V. The converter employs a high side
current sensing resistor to detect and regulate the LED
current. Analog, PWM and hybrid dimming can be easily
implemented with single DIM pin. In addition, the time
delay control operation can realize analog dimming less than
5%.
Continuous Conduction Mode Regulation
The FL7760 employs hysteretic reference architecture
that accurately regulates LED current by detecting an
external high−side current−sense resistor voltage. The
voltage across the current sensing resistor is kept measured
and regulated in 200 mV±15% range. This control scheme
performs stable LED current regulation at input voltage and
load transient conditions..
Figure 15. CCM Operation with Hysteresis
V
VIN
−V
SEN
High Reference
Low Reference
Gate
VIN biasing at startup
Internal VIN biasing circuit quickly charges external
VCC capacitor to begin IC operation. During the initial
start−up, the VCC pin voltage gradually increases, and when
the voltage reaches 4.5 V, the IC starts operating by VCC
good signal.
Figure 16. Start Up and Regulation
V
VIN
−V
SEN
High Reference
Low Reference
Gate
7.5V
7V
V
in
V
DIM
Cross Over
Distortion
Cross Over
Distortion
LED V
F
VCC
4.5V
4V
Thereafter, the internal current source in the DIM pin pulls
up the DIM voltage and internal hysteresis reference is
enabled with gate switching. Although the gate signal is
generated and the MOSFET is turned on, the LED current is
still close to zero in the crossover distortion area where the
input voltage is lower than the LED forward voltage.
Soft Start
The hysteric reference voltage to regulate LED current is
proportional to DIM voltage. Internal current source [6 uA]
charges an external capacitor connected at DIM pin and soft
start time can be programmable with capacitances. Soft start
time can be calculated as below equation.
T
SoftStart
+
C
DIM
3V
6uA
(eq. 1)
V
in
V
DIM
V
VIN
−V
SEN
High Reference
Low Reference
Figure 17. Soft Start with DIM pin Resistor
V
in
V
DIM
V
VIN
−V
SEN
High Reference
Low Reference
Figure 18. Soft Start with DIM pin Capacitor
Although soft start is not preferred, small filtering
capacitor (~ hundreds pF) at DIM pin is recommended for
noise immunity. PWM dimming signal delivered from an
external PWM generator can be filtered by the capacitor, so
the capacitor value needs to be carefully selected by
considering an output impedance of PWM signal generator.
Analog Dimming
When DIM voltage is higher than 3 V, hysteretic reference
voltage is set to 200 mV±30 mV. This hysteretic reference
condition limits LED current ripple spec of ±15% without
storage capacitor in parallel with the LED string.
The control range of the DIM pin in analog dimming is
from 3 V to 0.5 V. As DIM voltage decreases, hysteretic
