9

LTC1430

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

(3 months) lifetime; further derating of the input capacitor

ripple current beyond the manufacturer’s specification is

recommended to extend the useful life of the circuit.

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 a fraction 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 LTC1430 can adjust the

inductor current to the new value. ESR in the output

capacitor results in a step in the output voltage equal to the

ESR value multiplied by the change in load current. A 5A

load step with a 0.05

Ω ESR output capacitor will result in

a 250mV output voltage shift; this is a 7.6% output voltage

shift for a 3.3V 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

to control steady-state output ripple.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC1430 applications. OS-CON

electrolytic capacitors from Sanyo give excellent perfor-

mance and have a very high performance/size ratio for an

electrolytic capacitor. 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 tantalum capaci-

tors that work well in LTC1430 applications. A common

way to lower ESR and raise ripple current capability is to

parallel several capacitors. A typical LTC1430 application

might require an input capacitor with a 5A ripple current

capacity and 2% output shift with a 10A output load step,

which requires a 0.007

Ω output capacitor ESR. Sanyo

inductor value, the input and output voltage and the

operating frequency. If the efficiency is high and can be

approximately equal to 1, the ripple current is approxi-

mately equal to:

∆=

−

()

•

•

=

I

VV

fL

DC

DC

V

V

IN

OUT

OSC

OUT

IN

L = inductor value

Solving this equation with our typical 5V to 3.3V applica-

tion, we get:

17 066

200

2

28

..

.

•

•µ

=

−

kHz

H

Peak inductor current at 10A load:

10

28

2

11 4

A

A

A

+=

.

.

The inductor core must be adequate 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 the current may rise above

this maximum level in circuits under current limit or under

fault conditions in unlimited circuits; the inductor should

be sized to withstand this additional current.

Input and Output Capacitors

A typical LTC1430 design puts significant demands on

both the input and output capacitors. Under normal steady

load operation, a buck converter like the LTC1430 draws

square waves of current from the input supply at the

switching frequency, with the peak value equal to the

output current and the minimum value near zero. Most of

this current must come from the input bypass capacitor,

since few raw supplies can provide the current slew rate to

feed such a load directly. 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

APPLICATIO S I FOR ATIO