Datasheet
OMAXINMAXT
VVV +
=
--
O
V
=
MAXT
V
-
MAXINMAXT
VV
--
=
1-D
MAX
D
MAX
I
CIN-RMS
= I
LED
x
12
I
CIN-RMS
=
'
i
L-PP
(1-D
MID
)DII
LEDRMSCIN
xx=
-
MID
C
IN
=
'V
IN-PP
x f
SW
I
LED
x D
C
IN
=
8 x 'V
IN-PP
x f
SW
'
i
L-PP
C
IN
=
I
LED
x (1 - D) x D
'V
IN-PP
x f
SW
x=
0U
TT
-1
¸
¸
¹
·
¨
¨
©
§
s
Z
1
Z
xx +1
¸
¸
¹
·
¨
¨
©
§
s
Z
3P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
2P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
1P
x=
0U
TT
1
xx +1
¸
¸
¹
·
¨
¨
©
§
s
Z
3P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
2P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
1P
LM3424
www.ti.com
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
The total system loop gain (T) can then be written as:
Buck
(70)
Boost and Buck-boost
(71)
10. INPUT CAPACITANCE
Set the nominal input voltage ripple (Δv
IN-PP
) by solving for the required capacitance (C
IN
):
Buck
(72)
Boost
(73)
Buck-boost
(74)
Use D
MAX
to set the worst case input voltage ripple, when solving for C
IN
in a buck-boost regulator and D
MID
= 0.5
when solving for C
IN
in a buck regulator.
The minimum allowable RMS input current rating (I
CIN-RMS
) can be approximated:
Buck
(75)
Boost
(76)
Buck-boost
(77)
11. NFET
The NFET voltage rating should be at least 15% higher than the maximum NFET drain-to-source voltage (V
T-
MAX
):
Buck
(78)
Boost
(79)
Buck-boost
(80)
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