User manual

t
ON
= k x
R
ON
V
IN
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
R
SNS
=
V
IN
- V
OUT
2L
V
OUT
x t
D
L
(I
F
) -
0.20V
Or:
k x R
ON
V
IN
+
www.ti.com Application Circuit Calculations
(16)
Therefore: R
SNS
= 467 mΩ
Calculate Average LED current (I
F
)
Calculate average current through the LEDs for V
IN
= 36V and 60V.
(17)
Table 3. Example 1 Average LED Current
V
IN
(V) V
OUT
(V) I
F
(A)
36 10.4 0.490
48 10.4 0.500
60 10.4 0.506
Design Example 2
Design example 2 demonstrates a design if a single Bill of Materials (Bom) is desired over many different
applications (number of series LEDs, V
IN
, V
OUT
etc).
• 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
= fast as possible
• Design for typical application within t
ON
and t
OFF
limitations
The inductor, R
ON
resistor, and the R
SNS
resistor is calculated for a typical or average design.
• V
OUT
= 3 x 3.4V + 200 mV = 10.4V
• V
OUT
= 4 x 3.4V + 200 mV = 13.8V
• V
OUT
= 5 x 3.4V + 200 mV = 17.2V
Calculate t
ON
, t
OFF
and R
ON
In this design we will maximize the switching frequency so that we can reduce the overall size of the
design. In a later design, a slower switching frequency is utilized to maximize efficiency. If the design is to
use the highest possible switching frequency, you must ensure that the minimum on and off times are
adhered to.
Minimum on time occurs when V
IN
is at its maximum value, and V
OUT
is at its lowest value.
Calculate R
ON
at V
IN
= 60V, V
OUT
= 10.4V, and set t
ON
= 300 ns:
(18)
R
ON
= 137 kΩ, t
ON
= 306 ns
Check to see if t
OFF
minimum is satisfied:
t
OFF
minimum occurs when V
IN
is at its lowest value, and V
OUT
is at its maximum value.
At V
IN
= 36V, V
OUT
= 17.2V, and R
ON
= 137 kΩ calculate t
ON
from the above equation:
9
SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current
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