7

LTC3413

sn3413 3413fs

APPLICATIO S I FOR ATIO

The basic LTC3413 application circuit is shown in Figure

1a. External component selection is determined by the

maximum load current and begins with the selection of the

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 LTC3413 is determined by

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.

R

f

k

()

Ω

323 10

10

11

.•

–

Although frequencies as high as 2MHz are possible, the

minimum on-time of the LTC3413 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

decreases with higher inductance.

∆=

















I

V

f

V

V

L

OUT

L

OUT

IN

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

is

below a specified maximum, the inductor value should be

chosen according to the following equation:

L

V

fI

V

V

OUT

L MAX

OUT

IN MAX

=

∆

























()

()

–

1

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 used

often at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

inductance collapses abruptly when the peak design cur-

rent 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

materials are small and don’t radiate much energy, but

generally cost more than powdered iron core inductors

with similar characteristics. The choice of which style

inductor to use mainly depends on the price versus size

requirements and any radiated field/EMI requirements.