Datasheet

17
MSL2021 [DATASHEET]
42062A–LED–02/2013
11.6 Selecting the Color-Adjust String Floating Buck Components
Figure 11-1. Floating buck LED driver
The MSL2021 includes a driver for a constant off-time floating buck topology, shown in Figure 11-1, to convert the main
string voltage to a value appropriate for the color-adjust LED string. The buck is operated in continuous conduction
mode.
Continuous conduction operation is assured when the peak-to-peak ripple current in the inductor, ∆i
L
, is less than twice
the average LED current. A peak-to-peak ripple current magnitude of 15% of the average LED on-current is suggested,
i.e.
A
where I
AVE
is the average color-adjust LED string on-current. Choose I
AVE
appropriate for the color-adjust LEDs (Figure
11-1 on page 17 and Figure 11-2 on page 18) and calculate the peak string on-current using
A
DRV
CS
R
CS
COLOR LEDS
(COLOR-ADJUST STRING)
L
o
V
LED
D
1
TOFF
R
TOFF
C
o
C
i
Q
PGND
V
BUCK
+
-
WHITE LEDS
(MAIN STRING)
I
AVE
MSL2021
LED Driver
i
L
0.15I
AVE

I
PEAK
I
AVE
i
L
2
--------+=