User manual

I
LED(t)
i
PEAK
t
ON
I
F
'i
L
i
LED-MIN
t
OFF
t
Application Circuit Calculations www.ti.com
Table 1. Example 1 ON and OFF Times
V
IN
(V) V
OUT
(V) t
ON
t
OFF
36 10.4 5.10E-07 9.38E-07
48 10.4 3.82E-07 1.06E-06
60 10.4 3.06E-07 1.14E-06
Calculate Switching Frequency
V
IN
= 36V, 48 and 60V.
Substituting equations:
f
SW
= 691kHz (V
IN
= 36V, 48V, and 60V)
Calculate Inductor Value
With 50% ripple at V
IN
= 48V
• I
F
= 500 mA
• Δi
L
= 250 mA (target)
• L = 57 µH (68 µH standard value)
Calculate Δi for V
IN
= 36V, 48V, and 60V with L = 68 µH
Table 2. Example 1 Ripple Current
V
IN
(V) V
OUT
(V) Δi
L
(A)
36 10.4 0.192
48 10.4 0.211
60 10.4 0.223
Calculate R
SNS
Calculate R
SNS
at V
IN
typical (48V), and average LED current (I
F
) set to 500 mA.
Figure 8. Inductor Current Waveform
• I
F
= 500 mA
• V
IN
= 48V
• V
OUT
= 10.4V
• L = 68 µH
• t
D
= 220 ns
• t
ON
= 382 ns
Using equations from the COT Overview section, calculate R
SNS
.
8
AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
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