LM3410_08 Datasheet, PDF(10/32 Page) National Semiconductor (TI) – PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

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LM3410_08 Datasheet, PDF (10/32 Pages) National Semiconductor (TI) – PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

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From the previous equations, the inductor value is then ob-

tained.

FIGURE 5. Inductor Current

30038519

The Duty Cycle (D) for a Boost converter can be approximat-

ed by using the ratio of output voltage (VOUT) to input voltage

(VIN).

Therefore:

Power losses due to the diode (D1) forward voltage drop, the

voltage drop across the internal NMOS switch, the voltage

drop across the inductor resistance (RDCR) and switching

losses must be included to calculate a more accurate duty

cycle (See Calculating Efficiency and Junction Tempera-

ture for a detailed explanation). A more accurate formula for

calculating the conversion ratio is:

Where Î· equals the efficiency of the LM3410 application.

Or:

Therefore:

Inductor ripple in a LED driver circuit can be greater than what

would normally be allowed in a voltage regulator Boost &

Sepic design. A good design practice is to allow the inductor

to produce 20% to 50% ripple of maximum load. The in-

creased ripple shouldnât be a problem when illuminating

LEDs.

Where

1/TS = fSW

One must also ensure that the minimum current limit (2.1A)

is not exceeded, so the peak current in the inductor must be

calculated. The peak current (Lpk I) in the inductor is calcu-

lated by:

ILpk = IIN + ÎIL or ILpk = IOUT /D' + ÎiL

When selecting an inductor, make sure that it is capable of

supporting the peak input current without saturating. Inductor

saturation will result in a sudden reduction in inductance and

prevent the regulator from operating correctly. Because of the

speed of the internal current limit, the peak current of the in-

ductor need only be specified for the required maximum input

current. For example, if the designed maximum input current

is 1.5A and the peak current is 1.75A, then the inductor should

be specified with a saturation current limit of >1.75A. There is

no need to specify the saturation or peak current of the in-

ductor at the 2.8A typical switch current limit.

Because of the operating frequency of the LM3410, ferrite

based inductors are preferred to minimize core losses. This

presents little restriction since the variety of ferrite-based in-

ductors is huge. Lastly, inductors with lower series resistance

(DCR) will provide better operating efficiency. For recom-

mended inductors see Example Circuits.

INPUT CAPACITOR

An input capacitor is necessary to ensure that VIN does not

drop excessively during switching transients. The primary

specifications of the input capacitor are capacitance, voltage,

RMS current rating, and ESL (Equivalent Series Inductance).

The recommended input capacitance is 2.2 ÂµF to 22 ÂµF de-

pending on the application. The capacitor manufacturer

specifically states the input voltage rating. Make sure to check

any recommended deratings and also verify if there is any

significant change in capacitance at the operating input volt-

age and the operating temperature. The ESL of an input

capacitor is usually determined by the effective cross sec-

tional area of the current path. At the operating frequencies

of the LM3410, certain capacitors may have an ESL so large

that the resulting impedance (2ÏfL) will be higher than that

required to provide stable operation. As a result, surface

mount capacitors are strongly recommended. Multilayer ce-

ramic capacitors (MLCC) are good choices for both input and

output capacitors and have very low ESL. For MLCCs it is

recommended to use X7R or X5R dielectrics. Consult capac-

itor manufacturer datasheet to see how rated capacitance

varies over operating conditions.

OUTPUT CAPACITOR

The LM3410 operates at frequencies allowing the use of ce-

ramic output capacitors without compromising transient re-

sponse. Ceramic capacitors allow higher inductor ripple

without significantly increasing output ripple. The output ca-

pacitor is selected based upon the desired output ripple and

transient response. The initial current of a load transient is

provided mainly by the output capacitor. The output

impedance will therefore determine the maximum voltage

perturbation. The output ripple of the converter is a function

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