L =

x (1-D)

r =

D =

D =

LM3405

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SNVS429B – OCTOBER 2006 – REVISED MAY 2013

DESIGN GUIDE

INDUCTOR (L1)

(11)

The catch diode (D1) forward voltage drop and the voltage drop across the internal NMOS must be included to

calculate a more accurate duty cycle. Calculate D by using the following formula:

(12)

(13)

is, the higher the operating efficiency of the converter.

The inductor value determines the output ripple current (

decrease the size of the inductor, but increases the output ripple current. An increase in the inductor value will

decrease the output ripple current. The ratio of ripple current to LED current is optimized when it is set between

0.3 and 0.4 at 1A LED current. This ratio r is defined as:

(14)

One must also ensure that the minimum current limit (1.2A) is not exceeded, so the peak current in the inductor

(15)

When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.2A, r can

be made as high as 0.7. The ripple ratio can be increased at lighter loads because the net ripple is actually quite

low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for

the maximum ripple ratio at any current below 2A is:

-0.3667

(16)

Note that this is just a guideline.

The LM3405 operates at a high frequency allowing the use of ceramic output capacitors without compromising

transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing LED current

ripple. See the output capacitor and feed-forward capacitor sections for more details on LED current ripple.

Now that the ripple current or ripple ratio is determined, the inductance is calculated by:

(17)

capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden

reduction in inductance and prevent the regulator from operating correctly. Because of the operating frequency of

LM3405, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the

variety of ferrite based inductors is huge. Lastly, inductors with lower series resistance (DCR) will provide better

operating efficiency. For recommended inductor selection, refer to Circuit Examples and Recommended

Inductance Range in Table 1. Note that it is a good practice to use small inductance value at light load (for

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