LM3402_1 Datasheet, PDF (4/18 Pages) National Semiconductor (TI) – LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board

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 orig-
inal 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 exam-
ple 3, but the switching frequency will be reduced to improve
efficiency. The reduced switching frequency can further re-
duce 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. Elimi-
nating 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 & 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 shown below in the two
graphs (figures 6 & 7).
FIGURE 6. V_OUT-MAX vs f_SW
30061615
FIGURE 7. V_OUT-MIN vs f_SW
30061616
R_ON = 135 kΩ (use standard value of 137 kΩ)
t_ON = 306 ns
Check to see if t_OFF minimum is satisfied. This occurs when
V_IN is at its minimum value.
At V_IN = 36V, and R_ON = 137 kΩ calculate t_ON from previous
equation.
t_ON = 510 ns
We know that:
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