Datasheet

V
RD-MAX
= V
IN-MAX
DSON
2
RMSTT
RIP
x=
-
I
RMST
=
-
D
x
I
LED
D
c
DI
T-
I
LEDRMS
x=
I
T-MAX
= x I
LED
1 - D
MAX
D
MAX
I
T-MAX
= D
MAX
x I
LED
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
LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574E –JULY 2008–REVISED MAY 2013 www.ti.com
Boost
(73)
Buck-boost
(74)
9. NFET
The NFET voltage rating should be at least 15% higher than the maximum NFET drain-to-source voltage (V
T-
MAX
):
Buck
(75)
Boost
(76)
Buck-boost
(77)
The current rating should be at least 10% higher than the maximum average NFET current (I
T-MAX
):
Buck
(78)
Boost and Buck-boost
(79)
Approximate the nominal RMS transistor current (I
T-RMS
) :
Buck
(80)
Boost and Buck-boost
(81)
Given an NFET with on-resistance (R
DS-ON
), solve for the nominal power dissipation (P
T
):
(82)
10. DIODE
The Schottky diode voltage rating should be at least 15% higher than the maximum blocking voltage (V
RD-MAX
):
Buck
(83)
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