Datasheet

V91V21V70VVV
OMAXINMAXT
=+=+=
--
C
IN
= 4 x 4.7 PF
x
A1
=
I
LED
I
RMSIN-
=
1- 0.677
677.0
1.45A
x
1- D
MAX
D
MAX
=
C
IN
==
kHz504mV100 x
467.0A1 x
F27.9 P=
f
'v
SWPPIN-
x
DI
LED
x
F27.0C
FS
P=
F0.33C
CMP
P=
:10R
FS
=
1
= F28.0
1
P==C
FS
10:
x
sec
rad
k360
10:
3P
Zx
Z
P3
= (max Z
P1
, Z
Z1
) x 10 = Z
Z1
x 10
Z
P3
= 36k
sec
rad
x 10 = 360k
sec
rad
F30.0
1
1
C
CMP
P===
675.0
e5
sec
rad
6
x :
e5
6
2P
xZ
:
= =
sec
rad
675.0=
sec
rad
k19
min(Z
P1
, Z
Z1
)
Z
P2
=
5 x T
U0
Z
P1
5 x 5630 5 x 5630
=T
0U
= 5630=
04.0A1467.1 :
xx
V620533.0
x
( )
D1
+
RI
LIMLED
xx
V620D
x
c
LM3421, LM3421-Q1
LM3423, LM3423-Q1
www.ti.com SNVS574E –JULY 2008–REVISED MAY 2013
(121)
To ensure stability, calculate ω
P2
:
(122)
Solve for C
CMP
:
(123)
To attenuate switching noise, calculate ω
P3
:
(124)
Assume R
FS
= 10Ω and solve for C
FS
:
(125)
The chosen components from step 7 are:
(126)
8. INPUT CAPACITANCE
Solve for the minimum C
IN
:
(127)
To minimize power supply interaction a 200% larger capacitance of approximately 20 µF is used, therefore the
actual Δv
IN-PP
is much lower. Since high voltage ceramic capacitor selection is limited, four 4.7 µF X7R capacitors
are chosen.
Determine minimum allowable RMS current rating:
(128)
The chosen components from step 8 are:
(129)
9. NFET
Determine minimum Q1 voltage rating and current rating:
(130)
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