© 2012 Fairchild Semiconductor Corporation
13
FEBFL7701_L31H008A • Rev.1.0.2
8.2.
Operating Frequency & Minimum Duty
The programmable switching frequency is between 20 kHz ~ 250 kHz, determined by
selecting the RT resistor value. If no RT resistor is used (RT pin OPEN), the FL7701
default switching frequency is set to 45 kHz. The maximum duty ratio is fixed below
50% and has a fixed minimum typical on-time of 400 ns. 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 400 ns minimum on-time.
CH1: V
CC
,
CH2: V
DRAIN
,
CH3: V
LED
,
CH4: I
LED
Figure 13.
Operating Frequency & Minimum Duty
Minimum On Time:
1.91µs
Switching Frequency:
44.64kHz