LM3402 datasheet(4/18 Pages) NSC

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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

•

IN = 48V (±20%)

•

Driving three HB LEDs with V

F = 3.4V

•

OUT = (3 x 3.4V +200 mV) = 10.4V

•

F = 500 mA (typical application)

•

Estimated efficiency = 82%

•

SW = fast as possible

•

Design for typical application within t

ON and tOFF 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

PEAK = 500 mA + 125 mA = 625 mA

Calculate t

ON, tOFF & RON

From the datasheet there are minimum control MOSFET ON

and OFF times that need to be met.

OFF minimum = 300 ns

ON minimum = 300 ns

The minimum ON time will occur when V

IN is at its maximum

value. Therefore calculate R

ON at VIN = 60V, and set tON = 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).

30061615

FIGURE 6. V

OUT-MAX vs fSW

30061616

FIGURE 7. V

OUT-MIN vs fSW

ON = 135 kΩ (use standard value of 137 kΩ)

ON = 306 ns

Check to see if t

OFF minimum is satisfied. This occurs when

IN is at its minimum value.

At V

IN = 36V, and RON = 137 kΩ calculate tON from previous

equation.

ON = 510 ns

We know that:

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