LTC1553_15 datasheet(16/24 Pages) LINER

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LTC1553

Solving this equation with our typical 5V to 2.8V applica-

tion with a 2

µH inductor, we get:

22 056

300

()( )

kHz

P-P

Peak inductor current at 11.2A load:

11 2

12 2

The ripple current should generally be between 10% and

40% of the output current. The inductor must be able to

withstand this peak current without saturating, and the

copper resistance in the winding should be kept as low as

possible to minimize resistive power loss. Note that in

circuits not employing the current limit function, the

current in the inductor may rise above this maximum

under short circuit or fault conditions; the inductor should

be sized accordingly to withstand this additional current.

Inductors with gradual saturation characteristics are often

the best choice.

Input and Output Capacitors

A typical LTC1553 design puts significant demands on

both the input and the output capacitors. During constant

load operation, a buck converter like the LTC1553 draws

square waves of current from the input supply at the

switching frequency. The peak current value is equal to the

output load current plus 1/2 peak-to-peak ripple current,

and the minimum value is zero. Most of this current is

supplied by the input bypass capacitor. The resulting RMS

current flow in the input capacitor will heat it up, causing

premature capacitor failure in extreme cases. Maximum

RMS current occurs with 50% PWM duty cycle, giving an

RMS current value equal to IOUT/2. A low ESR input

capacitor with an adequate ripple current rating must be

used to ensure reliable operation.

Note that capacitor manufacturers’ ripple current ratings

are often based on only 2000 hours (three months)

APPLICATIONS INFORMATION

lifetime at rated temperature. Further derating of the input

capacitor ripple current beyond the manufacturer’s speci-

fication is recommended to extend the useful life of the

circuit. Lower operating temperature will have the largest

effect on capacitor longevity.

The output capacitor in a buck converter sees much less

ripple current under steady-state conditions than the input

capacitor. Peak-to-peak current is equal to that in the

inductor, usually 10% to 40% of the total load current.

Output capacitor duty places a premium not on power

dissipation but on ESR. During an output load transient,

the output capacitor must supply all of the additional load

current demanded by the load until the LTC1553 can

adjust the inductor current to the new value. Output

capacitor ESR results in a step in the output voltage equal

to the ESR value multiplied by the change in load current.

An 11A load step with a 0.05

Ω ESR output capacitor will

result in a 550mV output voltage shift; this is 19.6% of the

output voltage for a 2.8V supply! Because of the strong

relationship between output capacitor ESR and output

load transient response, the output capacitor is usually

chosen for ESR, not for capacitance value; a capacitor with

suitable ESR will usually have a larger capacitance value

than is needed for energy storage.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC1553 applications. OS-CON

electrolytic capacitors from SANYO and other manufac-

turers give excellent performance and have a very high

performance/size ratio for electrolytic capacitors. Surface

mount applications can use either electrolytic or dry

tantalum capacitors. Tantalum capacitors must be surge

tested and specified for use in switching power supplies.

Low cost, generic tantalums are known to have very short

lives followed by explosive deaths in switching power

supply applications. AVX TPS series surface mount

devices are popular surge tested tantalum capacitors that

work well in LTC1553 applications.

A common way to lower ESR and raise ripple current

capability is to parallel several capacitors. A typical LTC1553

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