LTC3418 datasheet(9/20 Pages) LINER

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LTC3418

3418f

APPLICATIO S I FOR ATIO

The basic LTC3418 application circuit is shown on the

front page of this data sheet. External component selec-

tion is determined by the maximum 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 LTC3418 is determined by

an external resistor that is connected between the RT pin

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

that is used to charge and discharge an internal timing

capacitor within the oscillator and can be calculated by

using the following equation:

OSC =

Ω

[]

Ω

73 10

.•

–.

Although frequencies as high as 4MHz are possible, the

minimum on-time of the LTC3418 imposes a minimum

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

typically 80ns. Therefore, the minimum duty cycle is equal

to:

100 • 80ns • f(Hz)

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current ∆IL increases with higher VIN or VOUT and

decreases with higher inductance:

∆ =

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

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

is ∆IL = 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 from 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

increase.

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!

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