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I
F
=
R
SNS
0.20V
+
V
IN
- V
OUT
2L
(t
ON
) -
V
OUT
x t
D
L
R
SNS
=
V
IN
- V
OUT
2L
x t
ON
+
V
OUT
x t
D
L
(I
F
) -
0.20V
www.ti.com Application Circuit Calculations
Table 5. Example 2 Ripple Current
V
IN
(V) V
OUT
(V) Δi
L
(A)
Three Series LEDs
36 10.4 0.192
48 10.4 0.211
60 10.4 0.223
Four Series LEDs
36 13.8 0.166
48 13.8 0.192
60 13.8 0.208
Four Series LEDs
36 17.2 0.141
48 17.2 0.173
60 17.2 0.193
Calculate R
SNS
Calculate R
SNS
at V
IN
typical (48V), with four series LEDs (13.8V = V
OUT
), and average LED current (I
F
) set
to 500 mA.
• I
F
= 500 mA
• V
IN
= 48V
• V
OUT
= 13.8V
• L = 68 µH
• t
D
= 220 ns
• t
ON
= 382 ns
(25)
R
SNS
= 446 mΩ
Calculate Average Current through LED
All combinations of V
IN
, V
OUT
with R
SNS
= 446 mΩ
(26)
Table 6. Example 2 Average LED Current
V
IN
(V) V
OUT
(V) I
F
(A)
Three Series LEDs
36 10.4 0.511
48 10.4 0.521
60 10.4 0.526
Four Series LEDs
36 13.8 0.487
48 13.8 0.500
60 13.8 0.508
Five Series LEDs
36 17.2 0.463
48 17.2 0.479
60 17.2 0.489
11
SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current
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