LTC7149 datasheet(13/26 Pages) LINER

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LTC7149

7149fa

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Using Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now be-

coming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When only a ceramic capacitor is used at the input

and the power is supplied by a wall adapter through long

wires, a load step at the output can induce ringing at the

input. At best, this ringing can couple to the output and

be mistaken as loop instability. At worst, a sudden inrush

of current through the long wires can potentially cause

a voltage spike at VIN large enough to damage the part.

When choosing the input and output ceramic capacitors,

use X5R or X7R dielectric formulations. These dielectrics

have the best temperature and voltage characteristics of

all the ceramics for a given value and size.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement.Duringaloadstep,theoutputcapacitormust

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

to support the load.

More capacitance may be required depending on the duty

cycle and load step requirements. In most applications,

the input capacitor is merely required to supply high fre-

quency bypassing, since the impedance to the supply is

very low. A 22µF ceramic capacitor is usually enough for

these conditions. Place this input capacitor as physically

close to the VIN pin as possible.

Inductor Selection

Given the desired input and output voltages, the inductor

valueandoperatingfrequencydeterminetheripplecurrent:

∆IL =

–VIN(MAX)

fSW • L

| VOUT

– |

VIN(MAX)+ | VOUT

– |

















Lower ripple current reduces core losses in the inductor

and reduces output voltage ripple. However, at extremes,

low ripple causes inductor current sensing issues. High-

est efficiency operation is obtained at low frequency with

reasonably small ripple current. However, achieving this

requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 2A. To guarantee that ripple current does not

exceed specified inductor saturation current ratings, the

inductance should be chosen according to:

L =

VIN(MAX)

fSW • ∆IL(MAX)

| VOUT

– |

VIN(MAX)+ | VOUT

– |

















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

be selected. Core loss is very dependent on the material,

frequency and inductance selected. Higher inductance

reduces ripple. Unfortunately, increased inductance re-

quires more turns of wire and therefore copper losses

will increase.

Ferrite materials have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

minimizecopperlossandpreventingsaturation.However,

ferrite core material saturates “hard”, which means that

inductancecollapsesabruptlywhenthepeakdesigncurrent

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/

currentandprice/currentrelationshipofaninductor.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. New designs for

surface mount inductors are available from Toko, Vishay,

NEC/Tokin, Cooper, TDK and Wurth Elektronik. Refer to

Table 1 for more details.

APPLICATIONS INFORMATION

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