Datasheet

18
MSL2021 [DATASHEET]
42062A–LED–02/2013
Figure 11-2. Color-adjust string LED on-current details.
The color-adjust string LED on-current regulates by monitoring the voltage at CS, the color-adjust string FET source
resistor connection. The reference voltage V
CSFB
for CS is 200mV (V
CSFB
is 200mV by default, and is adjustable through
the serial interface; see the register definitions for details about changing V
CSFB
). Choose the current sense resistor R
CS
using
Determine V
BUCK
, the voltage across the color-adjust LEDs, using
V
where N is the number of LEDs in the string and V
F
is the forward voltage drop of the LEDs at I
PEAK
.
The duty ratio of MOSFET Q is
where V
LED
is the main string voltage, Figure 11-1 on page 17. The constant off-time of the MOSFET is t
off
and calculated
in seconds using
s
where f
S
is the selected switching frequency in Hz. Use 100kHz to 1MHz for f
S
. Set t
off
with resistor R
TOFF
from TOFF to
GND (Figure 11-1 on page 17), whose value is
Choose the inductor value using
H
Use a ferrite inductor with a saturation current at least 50% higher than the peak current flowing in it:
A
Note here a particular advantage of constant off-time operation of the buck converter is that ripple current is independent
of the input voltage. The circuit provides a constant average LED current, I
AVE
, but the buck converter actually regulates
the peak inductor current, I
PEAK
(Figure 11-1 on page 17 and Figure 11-2 on page 18). From the equation for the inductor
value L
0
above, we see that because t
off
is constant, and V
BUCK
is relatively constant, the ripple current ∆i
L
is also
constant, so that I
AVE
is a constant, as desired. If the main string voltage changes, the switching frequency changes to
keep the on-time constant, thus the ripple current is independent of the input voltage.
I
PEAK
I
AVE
INDUCTOR CURRENT
LED CURRENT
(WHEN USING C
O
)t
OFF
t
I
?i
L
R
CS
V
CSFB
I
PEAK
----------------
=
V
BUCK
NV
f
=
D
V
BUCK
V
LED
-----------------=
t
off
1 D–
f
s
-------------=
R
t
off
t
off
90.9 10
9
=
L
O
V
BUCK
t
off

i
L
----------------------------- -=
I
L
SAT
1.5 I
PEAK
