LTC1707 datasheet(8/16 Pages) LINER

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LTC1707

with the oscillator synchronized at its minimum frequency,

i.e., to a clock just above the oscillator free-running

frequency. The actual reduction in average current is less

than for peak current.

The basic LTC1707 application circuit is shown in Figure 1a.

External component selection is driven by the load re-

quirement and begins with the selection of L followed by

CIN and COUT.

Inductor Value Calculation

The inductor selection will depend on the operating fre-

quency of the LTC1707. The internal preset frequency is

350kHz, but can be externally synchronized up to 550kHz.

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. However, oper-

ating at a higher frequency generally results in lower

efficiency because of increased internal gate charge losses.

The inductor value has a direct effect on ripple current. The

ripple current

∆IL decreases with higher inductance or

frequency and increases with higher VIN or VOUT.

∆I

OUT

= ()( )

−









(1)

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆IL = 0.4(IMAX).

The inductor value also has an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor current peaks fall to approximately

200mA. Lower inductor values (higher

∆IL) will cause this

to occur at lower load currents, which can cause 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. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy,

or Kool M

µ® cores. Actual core loss is independent of core

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requires

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!

Kool M

µ (from Magnetics, Inc.) is a very good, low loss

core material for toroids with a “soft” saturation character-

istic. Molypermalloy is slightly more efficient at high

(>200kHz) switching frequencies but quite a bit more

expensive. Toroids are very space efficient, especially

when you can use several layers of wire, while inductors

wound on bobbins are generally easier to surface mount.

New designs for surface mount are available from

Coiltronics, Coilcraft and Sumida.

CIN and COUT Selection

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle VOUT/VIN. To prevent large

voltage transients, a low ESR input capacitor sized for the

maximum RMS current must be used. The maximum

RMS capacitor current is given by:

MAX

OUT

required IRMS ≅

−

()

[]12/

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of life.

This makes it advisable to further derate the capacitor, or

choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

Kool M

µ is a registered trademark of Magnetics, Inc.

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