2

0.2 x L

2

LM3402, LM3402HV

SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

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Inductor datasheets should contain three specifications which are used to select the inductor. The first of these is

current rating is given by a specified temperature rise in the inductor, normally 40°C. For this example, the

average current rating should be greater than 350 mA to ensure that heat from the inductor does not reduce the

lifetime of the LED or cause the LM3402 to enter thermal shutdown.

The second specification is the tolerance of the inductance itself, typically ±10% to ±30% of the rated inductance.

In this example an inductor with a tolerance of ±20% will be used. With this tolerance the typical, minimum, and

maximum inductor current ripples can be calculated:

P-P

(23)

P-P

(24)

P-P

(25)

The third specification for an inductor is the peak current rating, normally given as the point at which the

inductance drops off by a given percentage due to saturation of the core. The worst-case peak current occurs at

maximum input voltage and at minimum inductance, and can be determined with the equation from the DESIGN

CONSIDERATIONS section:

(26)

(27)

For this example the peak current rating of the inductor should be greater than 479 mA. In the case of a short

circuit across the LED array, the LM3402 will continue to deliver rated current through the short but will reduce

the output voltage to equal the CS pin voltage of 200 mV. Worst-case peak current in this condition is equal to:

(28)

In the case of a short at the switch node, the output, or from the CS pin to ground the short circuit current limit

will engage at a typical peak current of 735 mA. In order to prevent inductor saturation during these short circuits

the inductor’s peak current rating must be above 735 mA. The device selected is an off-the-shelf inductor rated

33 µH ±20% with a DCR of 96 m

Ω and a peak current rating of 0.82A. The physical dimensions of this inductor

are 7.0 x 7.0 x 4.5 mm.

The current sensing resistor value can be determined by re-arranging the expression for average LED current

from the LED Current Accuracy section:

(29)

(30)

Sub-1

Ω resistors are available in both 1% and 5% tolerance. A 1%, 0.75Ω resistor will give the best accuracy of

the average LED current. To determine the resistor size the power dissipation can be calculated as:

(31)

Standard 0805 size resistors are rated to 125 mW and will be suitable for this application.

To select the proper output capacitor the equation from Buck Regulators with Output Capacitors is re-arranged to

yield the following:

(32)

(33)

A ceramic capacitor will be used and the required capacitance is selected based on the impedance at 468 kHz:

24

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