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V
OUT
x t
D
L
+
0.20V
k x R
ON
2L
-
R
SNS
I
F
=
V
IN
- V
OUT
2L
V
OUT
x t
D
L
+
0.20V
k x R
ON
V
IN
- V
OUT
-
R
SNS
I
F
=
R
SNS
=
V
OUT
x t
D
L
(I
F
) -
0.20V
k x R
ON
2L
+
www.ti.com Modified Application Circuit Design Example 4
(35)
Typical Application:
• V
OUT
= 13.8V
• I
F
= 500 mA
• R
ON
= 113 kΩ
• L = 68 µH
• t
D
= 220 ns
R
SNS
= 462 mΩ
This equation shows that only variations in V
OUT
will affect the average current over the entire application
range. These variations should be very minor even with large variations in output voltage.
Calculate Average Current through LED
Modified application circuit average forward current equation.
(36)
Simplified:
(37)
Table 9. Example 3 Average LED Current
V
IN
(V) V
OUT
(V) I
F
(A)
Three Series LEDs
36 10.4 0.511
48 10.4 0.511
60 10.4 0.511
Four Series LEDs
36 13.8 0.500
48 13.8 0.500
60 13.8 0.500
Five Series LEDs
36 17.2 0.489
48 17.2 0.489
60 17.2 0.489
In this application you can see that there is a difference of 22 mA between the low and high of the
average LED current.
11 Modified Application Circuit Design Example 4
• V
IN
= 48V (±20%)
• Driving 3, 4, or 5 HB LEDs with V
F
= 3.4V
• I
F
= 500 mA (typical application)
• Estimated efficiency = 82%
• f
SW
= 500 kHz (typ app)
The inductor, R
ON
resistor, and the R
SNS
resistor are calculated for a typical or average design.
• V
OUT
= 3 x 3.4V + 200 mV = 10.4V
15
SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current
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