LM3402 datasheet(6/18 Pages) NSC

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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, VOUT etc).

•

IN = 48V (±20%)

•

Driving 3, 4, or 5 HB LEDs with V

F = 3.4V

•

F = 500 mA (typical application)

•

Estimated efficiency = 82%

•

SW = fast as possible

•

Design for typical application within t

ON and tOFF limitations

The inductor, R

ON resistor, and the RSNS resistor is calculated

for a typical or average design.

•

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

•

OUT = 4 x 3.4V + 200 mV = 13.8V

•

OUT = 5 x 3.4V + 200 mV = 17.2V

Calculate t

ON, tOFF & RON

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 switch-

ing 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 VIN = 60V, VOUT = 10.4V, and set tON = 300

ns:

ON = 137 kΩ, tON = 306 ns

Check to see if t

OFF minimum is satisfied:

OFF minimum occurs when VIN is at its lowest value, and

OUT is at its maximum value.

At V

IN = 36V, VOUT = 17.2V, and RON = 137 kΩ calculate tON

from the above equation:

ON = 510 ns

Rearrange the above equation and solve for t

OFF with tON set

to 510 ns

OFF = 365 ns (satisfied)

Example 2 On & Off Time

Three Series LEDs

IN (V)

OUT (V)

OFF

10.4

137 k

Ω

5.10E-07

9.38E-07

10.4

137 k

Ω

3.82E-07

1.06E-06

10.4

137 k

Ω

3.06E-07

1.14E-06

Four Series LEDs

13.8

137 k

Ω

5.10E-07

5.81E-07

13.8

137 k

Ω

3.82E-07

7.08E-07

13.8

137 k

Ω

3.06E-07

7.85E-07

Five Series LEDs

17.2

137 k

Ω

5.10E-07

3.65E-07

17.2

137 k

Ω

3.82E-07

4.93E-07

17.2

137 k

Ω

3.06E-07

5.69E-07

Calculate Switching Frequency

The switching frequency will only change with output voltage.

Substituting equations:

Or:

• f

SW = 691 kHz (VOUT = 10.4V)

• f

SW = 916 kHz (VOUT = 13.8V)

• f

SW = 1.14 MHz (VOUT = 17.2V)

Calculate Inductor Value

With 50% ripple at V

IN = 48V, and VOUT = 10.4V

• I

AVG = 500 mA

• Δi

L = 250 mA (target)

• L = 53 µH (68 uH standard value)

Calculate

Δi for V

IN = 36V, 48V, & 60V with L = 68 µH.

LM3402 Datasheet(PDF) 6 Page - National Semiconductor (TI)

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