LTC1877 Просмотр технического описания (PDF) - Linear Technology

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LTC1877

High Efficiency Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC1877

APPLICATIONS INFORMATION

∆IL

=

1

(f)(L)

VOUT





1−

VOUT

VIN





(1)

Accepting larger values of ∆IL allows the use of low

inductance, 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

250mA. 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 Mµ® 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 pre-

ferred 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!

Kool Mµ (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

Kool Mµ is a registered trademark of Magnetics, Inc.

wound on bobbins are generally easier to surface mount.

New designs for surface mount inductors are available

from Coiltronics, Coilcraft, Dale 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:

[ ( )] CIN required IRMS ≅ IOMAX

VOUT VIN − VOUT

VIN

1/ 2

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 the 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 size or height requirements in the design. Always

consult the manufacturer if there is any question.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically, once the ESR require-

ment is satisfied, the capacitance is adequate for filtering.

The output ripple ∆VOUT is determined by:

∆VOUT

≅



∆IL ESR +

1

8fC OUT





where f = operating frequency, COUT = output capacitance

and ∆IL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since ∆IL increases

with input voltage. For the LTC1877, the general rule for

proper operation is:

COUT required ESR < 0.25Ω

The choice of using a smaller output capacitance

increases the output ripple voltage due to the frequency

dependent term but can be compensated for by using

capacitor(s) of very low ESR to maintain low ripple volt-

age. The ITH pin compensation components can be opti-

9

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