AN1427 Datasheet, PDF (7/12 Pages) Microchip Technology – High-Efficiency Solutions for Portable LED Lighting

AN1427
FIGURE 11: OUTPUT CURRENT WAVEFORM WITH FIXED OFF-TIME CONTROL
The off time is fixed and is user-configurable, and the
on time depends on the input voltage. Since the induc-
tor current drop caused by the off time is always the
same (LED voltage constant), we have:
EQUATION 1:
-^V----^I--^N----^–-----^V----^O----^U---^T-
L
TON
=
-^V----^O---^U----^T-
L
TOF
F
The inductor value, LED forward voltage and “off time”
are known values, so the “on time” can be calculated
using the input voltage value. The inductor current
increase during the “on time” is equal to the current
decrease during the “off time”.
One very interesting observation is that the fixed “off
time” essentially sets the inductor ripple current.
 2012 Microchip Technology Inc.
DS01427A-page 7