Datasheet

© 2012 Fairchild Semiconductor Corporation 11 FEBFL7701_L30U003A • Rev. 1.0.2
8.2. Operating Frequency & Minimum Duty
The programmable switching frequency is between 20kHz ~ 250kHz, determined by
selecting the RT resistor value. If no RT resistor is used (RT pin OPEN), the FL7701
default switching frequency is set to 45kHz. The maximum duty ratio is fixed below 50%
and has a fixed minimum typical on-time of 400ns. There are two crucial points to design
properly. The first is consideration of the minimum duty ratio at minimum input voltage
because the FL7701 is limited to 50% duty ratio. The second consideration is minimum
on-time at maximum input voltage condition. The FL7701 cannot control output power
when the operating conditions are such that the required on-time is less than the 400ns
minimum on-time.
CH1: V
CC
, CH2: V
DRAIN
, CH3: V
LED
, CH4: I
LED
Figure 12. Operating Frequency & Minimum Duty
Minimum on time:
1.12µs
Switching frequency:
69.91kHz