Datasheet
x
=
¨
¨
©
§
+
s
1
Z
1P
¸
¸
¹
·
0U
T
U
T
¨
¨
©
§
-
s
1
Z
1Z
¸
¸
¹
·
x
=
¨
¨
©
§
+
s
1
Z
1P
¸
¸
¹
·
0U
T
U
T
1
SLP
R
=
SNSTO
RRV xx
1Lx
13
e5.1
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
füir xx
LED
DI x
O
C =
PPL
i
-
'
PPLEDDSW
irf8
-
'xxx
LM3424
www.ti.com
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
6. LED RIPPLE CURRENT
Set the nominal LED ripple current (Δi
LED-PP
), by solving for the output capacitance (C
O
):
Buck
(50)
Boost and Buck-boost
(51)
To set the worst case LED ripple current, use D
MAX
when solving for C
O
. Remember, when PWM dimming it is
recommended to use a minimum of 40 µF of output capacitance to improve performance.
The minimum allowable RMS output capacitor current rating (I
CO-RMS
) can be approximated:
Buck
(52)
Boost and Buck-boost
(53)
7. PEAK CURRENT LIMIT
Set the peak current limit (I
LIM
) by solving for the transistor path sense resistor (R
LIM
):
(54)
8. SLOPE COMPENSATION
For all topologies, the preferred method to set slope compensation is to ensure any duty cycle is attainable for
the nominal V
O
and chosen L by solving for R
SLP
:
(55)
9. 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
(56)
Boost and Buck-boost
(57)
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