2

90

2

x SIN(135

= 45V

1 -

LM3444

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SNVS682C – NOVEMBER 2010 – REVISED MAY 2013

OUTPUT CAPACITOR

A capacitor placed in parallel with the LED or array of LEDs can be used to reduce the LED current ripple while

keeping the same average current through both the inductor and the LED array. With a buck topology the output

inductance (L2) can now be lowered, making the magnetics smaller and less expensive. With a well designed

converter, you can assume that all of the ripple will be seen by the capacitor, and not the LEDs. One must

ensure that the capacitor you choose can handle the RMS current of the inductor. Refer to manufacture’s

datasheets to ensure compliance. Usually an X5R or X7R capacitor between 1 µF and 10 µF of the proper

voltage rating will be sufficient.

SWITCHING MOSFET

The main switching MOSFET should be chosen with efficiency and robustness in mind. The maximum voltage

across the switching MOSFET will equal:

(30)

The average current rating should be greater than:

(31)

RE-CIRCULATING DIODE

The LM3444 Buck converter requires a re-circulating diode D10 (see the Typical Application circuit Figure 11) to

carry the inductor current during the MOSFET Q2 off-time. The most efficient choice for D10 is a diode with a low

forward drop and near-zero reverse recovery time that can withstand a reverse voltage of the maximum voltage

(32)

The current rating must be at least:

(33)

Or:

(34)

Design Example

The following design example illustrates the process of calculating external component values.

Known:

2. Number of LEDs in series = 7

3. Forward voltage drop of a single LED = 3.6V

4. LED stack voltage = (7 x 3.6V) = 25.2V

Choose:

3.

4. Valley fill stages (1,2, or 3) = 2

5. Assumed minimum efficiency = 80%

Calculate:

(35)

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