=
pF
470
C7
:
=
k
5
.
16
R6
t
OFF
f
SW
=
=
ns
242
=
kHz
404
1
-
¸
¹
·
¨
©
§
V
48
97
.
0
x
V
42
¸¸
¹
·
¨¨
©
§
V
O
V
IN
x
K
1
-
1
ln
k
5
.
16
pF
490
t
OFF
-
x
:
x
-
=
¨¨
©
§
ns
242
=
¸¸
¹
·
V
24
.
1
V
42
(C7+20pF)
t
OFF
-
=
x R6 x
1
1
ln -
¸
¸
¹
·
¨
¨
©
§
V
O
V
24
.
=
R6
-
-
1
¸¸
¹
·
¨¨
©
§
V
O
x
K
V
IN
=
R6
:
=
k
7
.
16
x
ln
pF
490
x
kHz
400
¨
©
§
-
1
V
42
¸
¹
·
V
24
.
1
-
-
1
¨
©
§
¸
¹
·
V
42
x
V
48
97
.
0
-
x
x
1
ln
f
(C7 + 20 pF)
SW
¸¸
¹
·
¨¨
©
§
V
O
V
24
.
1
www.ti.com
Design Procedure
6
Design Procedure
6.1
Specifications
V
IN
= 48V; V
IN-MAX
= 75V
V
O
= 42V
f
SW
= 400kHz
I
LED
= 1.5A
Δ
i
LED-PP
=
Δ
i
L-PP
= 300mA
Δ
v
IN-PP
= 1.44V
V
TURN-ON
= 10V; V
HYS
= 1.1V
η
= 0.97
6.2
Nominal Switching Frequency
Assume C7 = 470pF and
η
= 0.97. Solve for R6:
(1)
The closest 1% tolerance resistor is 16.5 k
Ω
therefore the actual t
OFF
and target f
SW
are:
(2)
(3)
The chosen components from step 1 are:
(4)
5
SNVA390D – May 2009 – Revised May 2013
AN-1953 LM3409HV Evaluation Board
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