LTC3414MPFE datasheet(8/16 Pages) LINER

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LTC3414

3414fb

The basic LTC3414 application circuit is shown in Figure 1.

External component selection is determined by the maxi-

mum load current and begins with the selection of the

operating frequency and inductor value followed by CIN

and COUT.

Operating Frequency

Selection of the operating frequency is a tradeoff between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low

output ripple voltage.

The operating frequency of the LTC3414 is determined by

an external resistor that is connected between pin RT and

ground. The value of the resistor sets the ramp current that

is used to charge and discharge an internal timing capaci-

tor within the oscillator and can be calculated by using the

following equation:

OSC =

() Ω

308 10

.•

–

Although frequencies as high as 4MHz are possible, the

minimum on-time of the LTC3414 imposes a minimum

limit on the operating duty cycle. The minimum on-time is

typically 110ns; therefore, the minimum duty cycle is

equal to 100 • 110ns • f(Hz).

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

ΔIL increases with higher VIN or VOUT and

decreases with higher inductance.

Δ =

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

f • L

OUT

1–

Having a lower ripple current reduces the core losses in

the inductor, the ESR losses in the output capacitors, and

the output voltage ripple. Highest efficiency operation is

achieved at low frequency with small ripple current. This,

however, requires a large inductor.

A reasonable starting point for selecting the ripple current

ΔIL = 0.4(IMAX). The largest ripple current occurs at the

highest VIN. To guarantee that the ripple current stays

below a specified maximum, the inductor value should be

chosen according to the following equation:

OUT

L MAX

OUT

IN MAX

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

()

–

The inductor value will also have an effect on Burst Mode

operation. The transition to low current operation begins

when the peak inductor current falls below a level set by

the burst clamp. Lower inductor values result in higher

ripple current which causes this to occur at lower load

currents. This causes a dip in efficiency in the upper range

of low current operation. In Burst Mode operation, lower

inductance values will cause the burst frequency to in-

crease.

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. Actual core loss is independent of core size for a

fixed inductor value, but it is very dependent on the

inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance re-

quires more turns of wire and therefore copper losses will

increase.

Ferrite designs have very low core losses and are preferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy mate-

rials are small and don’t radiate much energy, but gener-

ally cost more than powdered iron core inductors with

similar characteristics. The choice of which style inductor

to use mainly depends on the price verus size require-

ments and any radiated field/EMI requirements. New

designs for surface mount inductors are available from

Coiltronics, Coilcraft, Toko, and Sumida.

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