LM3502 datasheet(15/18 Pages) NSC

15 / 18 page

Application Information (Continued)

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Duty Cycle for DCM Operation.

OUT: Output Voltage.

IN:

Input Voltage.

OUT:

White LED Current/Load Current.

Fs:

Switching Frequency.

Inductor Value/Inductance Magnitude.

INDUCTOR SELECTION

In order to maintain inductance, an inductor used with the

LM3502 should have a saturation current rating larger than

the peak inductor current of the particular application. Induc-

tors with low DCR values contribute decreased power losses

and increased efficiency. The peak inductor current can be

computed for both modes of operation: CCM and DCM.

The cycle-by-cycle peak inductor current for CCM operation

can be computed with:

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20131746

IN:

Input Voltage.

Eff:

Efficiency of the LM3502.

Fs:

Switching Frequency.

OUT:

White LED Current/Load Current.

Inductance Magnitude/Inductor Value.

Duty Cycle for CCM Operation.

PEAK:

Peak Inductor Current.

∆i

Inductor Ripple Current.

L(avg): Average Inductor Current.

The cycle-by-cycle peak inductor current for DCM operation

can be computed with:

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IN:

Input Voltage.

Fs:

Switching Frequency.

Inductance Magnitude/Inductor Value.

Duty Cycle for DCM Operation.

PEAK: Peak Inductor Current.

The minimum inductance magnitude/inductor value for the

LM3502 can be calculated using the following, which is only

valid when the duty cycle is > 0.5:

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Duty Cycle.

1–D.

DS(ON): NMOS Power Switch ON Resistance.

IN:

Input Voltage.

Inductance Magnitude/Inductor Value.

This equation gives the value required to prevent subhar-

monic oscillations. The result of this equation and the induc-

tor average and ripple current should be accounted for when

choosing an inductor value.

Some recommended inductor manufacturers included but

are not limited to:

CoilCraft

DO1608C-223

www.coilcraft.com

DT1608C-223

CAPACITOR SELECTION

Multilayer ceramic capacitors are the best choice for use

with the LM3502. Multilayer ceramic capacitors have the

lowest equivalent series resistance (ESR). Applied voltage

or DC bias, temperature, dielectric material type (X7R, X5R,

Y5V, etc), and manufacturer component tolerance have an

affect on the true or effective capacitance of a ceramic

capacitor. Be aware of how your application will affect a

particular ceramic capacitor by analyzing the aforemen-

tioned factors of your application. Before selecting a capaci-

tor always consult the capacitor manufacturer’s data curves

to verify the effective or true capacitance in your application.

INPUT CAPACITOR SELECTION

The input capacitor serves as an energy reservoir for the

inductor. In addition to acting as an energy reservoir for the

inductor the input capacitor is necessary for the reduction in

input voltage ripple and noise experienced by the LM3502.

The reduction in input voltage ripple and noise helps ensure

the LM3502’s proper operation, and reduces the effect of the

LM3502 on other devices sharing the same supply voltage.

To ensure low input voltage ripple, the input capacitor must

have an extremely low ESR. As a result of the low input

voltage ripple requirement multilayer ceramic capacitors are

the best choice. A minimum capacitance of 2.0 µF is required

for normal operation, so consult the capacitor manufactur-

er’s data curves to verify whether the minimum capacitance

requirement is going to be achieved for a particular applica-

tion.

OUTPUT CAPACITOR SELECTION

The output capacitor serves as an energy reservoir for the

white LED load when the internal power FET switch (Figure

2: N1) is on or conducting current. The requirements for the

output capacitor must include worst case operation such as

when the load opens up and the LM3502 operates in over-

voltage protection (OVP) mode operation. A minimum ca-

pacitance of 0.5µF is required to ensure normal operation.

Consult the capacitor manufacturer’s data curves to verify

whether the minimum capacitance requirement is going to

be achieved for a particular application.

Some recommended capacitor manufacturers included but

are not limited to:

LM3502 Datasheet(PDF) 15 Page - National Semiconductor (TI)

LM3502 Datasheet(HTML) 15 Page - National Semiconductor (TI)

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