User manual

R
SNS
=
V
IN
- V
OUT
2L
V
OUT
x t
D
L
(I
F
) -
0.20V
R
ON
V
IN
- V
OUT
+
k x
R
SNS
=
V
IN
- V
OUT
2L
x t
ON
+
V
OUT
x t
D
L
(I
F
) -
0.20V
'i
R
ON
L =
x k
V
IN
- V
OUT
R
ON
t
ON
= k x
'i
V
IN
- V
OUT
L =
x t
ON
f
SW
=
V
OUT
V
IN
x K x t
ON
Or:
f
SW
=
1
t
ON
+ t
OFF
Modified Application Circuit Design Example 3 www.ti.com
Calculate Switching Frequency
(30)
Table 8. Example 3 Switching Frequency
V
IN
(V) V
OUT
(V) f
SW
(kHz)
Three Series LEDs
36 10.4 595
48 10.4 656
60 10.4 692
Four Series LEDs
36 13.8 685
48 13.8 791
60 13.8 855
Five Series LEDs
36 17.2 723
48 17.2 888
60 17.2 987
Calculate Inductor Value
(31)
Therefore:
(32)
You can quickly see one benefit of the modified circuit. The improved circuit eliminates the input and
output voltage variation on RMS current.
• I
F
= 500 mA (typical application)
• Δi
L
= 250 mA (target)
• R
ON
= 113 kΩ
• L = 59 µH (68 µH standard value)
• Δi
L
= 223 mA (L = 68 µH all combinations)
Calculate R
SNS
Original R
SNS
equation:
(33)
Substitute improved circuit t
ON
calculation:
(34)
Simplified:
14
AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
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