LT1432CS8 datasheet(19/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

average, so a ten year life is only 15,000 hours. The

manufacturer should be consulted for a final blessing. See

Application Note 46 for specific formulas for calculating

the life time or allowed ripple current in capacitors.

The reason for all this attention to ripple rating is that

everyone is in a size squeeze, and the temptation is to use

the smallest possible components. Do not cheat here

folks, or you may be faced with costly field failures.

ESR on the output capacitor determines output voltage

ripple, so this is also of much concern. Mother Nature has

decreed that for a given capacitor technology, ESR is a

direct function of the

volume of the capacitor. In other

words, if you want low ESR you must consume

space. This

is quickly confirmed by scanning the ESR numbers for a

wide range of capacitor values and voltage ratings within

a given family of capacitors. It is immediately obvious that

can size determines ESR, not capacitance, or voltage

rating. The only way to cheat on this limitation is to find the

best family of capacitors. Manufacturers such as Nichicon,

Chemicon, and Sprague should be checked. Sanyo makes

a very low ESR capacitor type know as OSCON, utilizing a

semiconductor dielectric. Its major disadvantage is some-

what higher price, and a tendency to make regulator

feedback loops unstable because of its extremely low ESR.

Most switching regulator loops depend to some extent on

the output capacitor ESR for a phase lead!

Output Filters

Output ripple voltage at the switching frequency is a fact of

life with switching regulators. Everyone knows that this

ripple must be held below some level to guarantee that it

does not affect system performance. The question is, what

is that level? For sensitive analog systems with wide

bandwidths, supply ripple may have to be a 1mV or less.

Digital systems can often tolerate 400mVp-p ripple with no

effect on performance. In most of these digital applica-

tions of the LT1432 as a buck converter, an output filter is

not needed because output ripple is normally in the 25mV

to 100mVp-p range without a filter. Note that burst mode

ripple is at low frequencies where small output filters are

not effective. The decision to add an output filter does

allow the main filter capacitor to get smaller, so the overall

board space may not increase prohibitively. See the dis-

cussion of waveforms for load transient response implica-

tions when adding a filter.

If modest reductions in output ripple are required, one can

increase the size of the main inductor and/or the output

capacitor. Buck converters are easier than other types

because the main inductor acts as a filter element. The

square wave voltage is converted to a triangular current

before being fed to the output capacitor. Actually, at

switching frequencies, the output capacitor is resistive

and output ripple voltage is determined not by the capaci-

tor value in

µF, but rather by the capacitor effective series

resistance (ESR). This parameter is determined by capaci-

tor volume within any given family, so to get ESR down,

one must still use a “bigger” capacitor. The problem is that

often the main inductor/capacitor becomes physically too

large if low output ripple is needed. Inverters, such as the

positive to negative converter, tend to have much higher

output ripple voltage because the main inductor is not a

filter element – it simply acts as an energy storage device

for shuttling essentially square wave currents from input

to output. Unlike the buck converter, these currents can be

much higher in amplitude than the output current.

An output filter of very modest size can reduce normal

mode output ripple voltage by a factor of ten or more. The

formula for filter attenuation in buck converters and invert-

ers is shown below.

Attenuation

ESR

8L f

= ()( ) (BUCKCONVERTER)

(INVERTER)

(The factor “4” is an

approximation

assuming worst case

duty cycle of 50%)

A 10

µH, 100µF (ESR = 0.4Ω) filter on a buck converter

using a 60kHz LT1271 will give an attenuation of:

Attenuation

ESR

4L f

= ()

()( )

0.4

8 10E

60E

0.083

–6

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