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Application Circuit Calculations www.ti.com
8 Application Circuit Calculations
To better explain the improvements made to the COT LM3402/04 demonstration board, a comparison is
shown between the unmodified average output LED current circuit to the improved circuit. Design
Examples 1 and 2 use two original LM3402 / 04 circuits. The switching frequencies will be maximized to
provide a small solution size.
Design Example 3 is an improved average current application. Example 3 will be compared against
example 2 to illustrate the improvements.
Example 4 will use the same conditions and circuit as example 3, but the switching frequency will be
reduced to improve efficiency. The reduced switching frequency can further reduce any variations in
average LED current with a wide operating range of series LEDs and input voltages.
Design Example 1
• V
IN
= 48V (±20%)
• Driving three HB LEDs with V
F
= 3.4V
• V
OUT
= (3 x 3.4V +200 mV) = 10.4V
• 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
LED (inductor) ripple current of 10% to 60% is acceptable when driving LEDs. With this much allowed
ripple current, you can see that there is no need for an output capacitor. Eliminating the output capacitor is
actually desirable. An LED connected to an inductor without a capacitor creates a near perfect current
source, and this is what we are trying to create.
In this design we will choose 50% ripple current.
Δi
L
= 500 mA x 0.50 = 250 mA
I
PEAK
= 500 mA + 125 mA = 625 mA
Calculate t
ON
, t
OFF
and R
ON
From the datasheet there are minimum control MOSFET ON and OFF times that need to be met.
t
OFF
minimum = 300 ns
t
ON
minimum = 300 ns
The minimum ON time will occur when V
IN
is at its maximum value. Therefore calculate R
ON
at V
IN
= 60V,
and set t
ON
= 300 ns.
A quick guideline for maximum switching frequency allowed versus input and output voltages are in
Figure 6 and Figure 7.
6
AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
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