User manual

'i
V
IN
- V
OUT
L =
x t
ON
f
SW
=
1
t
ON
+ t
OFF
f
SW
=
V
OUT
K x k x R
ON
f
SW
=
V
OUT
V
IN
x K x t
ON
V
OUT
V
IN
x K
t
OFF
= t
ON
- 1
V
OUT
V
IN
x K
=
t
ON
t
ON
+ t
OFF
Application Circuit Calculations www.ti.com
t
ON
= 510 ns
(19)
Rearrange the above equation and solve for t
OFF
with t
ON
set to 510 ns
(20)
t
OFF
= 365 ns (satisfied)
Table 4. Example 2 On and Off Time
Three Series LEDs
V
IN
(V) V
OUT
(V) R
ON
t
ON
t
OFF
36 10.4 137 kΩ 5.10E-07 9.38E-07
48 10.4 137 kΩ 3.82E-07 1.06E-06
60 10.4 137 kΩ 3.06E-07 1.14E-06
Four Series LEDs
36 13.8 137 kΩ 5.10E-07 5.81E-07
48 13.8 137 kΩ 3.82E-07 7.08E-07
60 13.8 137 kΩ 3.06E-07 7.85E-07
Five Series LEDs
36 17.2 137 kΩ 5.10E-07 3.65E-07
48 17.2 137 kΩ 3.82E-07 4.93E-07
60 17.2 137 kΩ 3.06E-07 5.69E-07
Calculate Switching Frequency
The switching frequency will only change with output voltage.
(21)
Substituting equations:
(22)
Or:
(23)
• f
SW
= 691 kHz (V
OUT
= 10.4V)
• f
SW
= 916 kHz (V
OUT
= 13.8V)
• f
SW
= 1.14 MHz (V
OUT
= 17.2V)
Calculate Inductor Value
(24)
With 50% ripple at V
IN
= 48V, and V
OUT
= 10.4V
• I
AVG
= 500 mA
• Δi
L
= 250 mA (target)
• L = 53 µH (68 uH standard value)
Calculate Δi for V
IN
= 36V, 48V, and 60V with L = 68 µH.
10
AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
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