Datasheet

x
=
¨
¨
©
§
+
s
1
Z
1P
¸
¸
¹
·
0U
T
U
T
1
LIM
R
=
LIM
I
245 mV
1-D
MAX
D
MAX
I
CO-RMS
= I
LED
x
I
RMSCO
=
-
12
PP-LED
iü
O
C =
SWPP-LEDD
f
ü
i
r xx
LED
DI x
O
C =
PPL
i
-
'
PPLEDDSW
irf8
-
'xxx
I
L-RMS
=
1 +
1
12
x
I
LED
'
I
L-PP
x D'
¸
¹
·
¨
©
§
2
D'
I
LED
x
LM3429, LM3429-Q1
SNVS616G –APRIL 2009–REVISED MAY 2013 www.ti.com
Boost and Buck-boost
(40)
5. LED RIPPLE CURRENT
Set the nominal LED ripple current (Δi
LED-PP
), by solving for the output capacitance (C
O
):
Buck
(41)
Boost and Buck-boost
(42)
To set the worst case LED ripple current, use D
MAX
when solving for C
O
.
The minimum allowable RMS output capacitor current rating (I
CO-RMS
) can be approximated:
Buck
(43)
Boost and Buck-boost
(44)
6. PEAK CURRENT LIMIT
Set the peak current limit (I
LIM
) by solving for the transistor path sense resistor (R
LIM
):
(45)
7. LOOP COMPENSATION
Using a simple first order peak current mode control model, neglecting any output capacitor ESR dynamics, the
necessary loop compensation can be determined.
First, the uncompensated loop gain (T
U
) of the regulator can be approximated:
Buck
(46)
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