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LTC3630

3630fb

APPLICATIONS INFORMATION

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and do not radiate energy but generally cost more

than powdered iron core inductors with similar charac-

teristics. 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 Coiltronics, Coilcraft,

TDK, Toko, and Sumida.

CIN and COUT Selection

The input capacitor, CIN, is needed to filter the trapezoidal

current at the source of the top high side MOSFET. CIN

should be sized to provide the energy required to charge

the inductor without causing a large decrease in input

voltage (ΔVIN). The relationship between CIN and ΔVIN

is given by:

CIN >

L•IPEAK2

2• VIN • ΔVIN

It is recommended to use a larger value for CIN than

calculated by the above equation since capacitance de-

creases with applied voltage. In general, a 4.7μF X7R

ceramic capacitor is a good choice for CIN in most LTC3630

applications.

To minimize large ripple voltage, a low ESR input capaci-

tor sized for the maximum RMS current should be used.

RMS current is given by:

IRMS = IOUT(MAX) •

VOUT

VIN

•

VIN

VOUT

–1

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT/2.Thissimpleworst-caseconditioniscommonlyused

for design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based only on 2000 hours of life

which 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.

The output capacitor, COUT, filters the inductor’s ripple

current and stores energy to satisfy the load current when

the LTC3630 is in sleep. The output ripple has a lower limit

of VOUT/160 due to the 5mV typical hysteresis of the feed-

back comparator. The time delay of the comparator adds

an additional ripple voltage that is a function of the load

current. During this delay time, the LTC3630 continues to

switch and supply current to the output. The output ripple

can be approximated by:

ΔVOUT ≈

IPEAK

–ILOAD

⎛

⎝

⎜

⎞

⎠

⎟ •

4• 10–6

COUT

VOUT

160

The output ripple is a maximum at no load and approaches

lower limit of VOUT/160 at full load. Choose the output

capacitor COUT to limit the output voltage ripple ΔVOUT

using the following equation:

COUT ≥

IPEAK •2 • 10

–6

ΔVOUT –

VOUT

160

The value of the output capacitor must be large enough

to accept the energy stored in the inductor without a large

change in output voltage during a single switching cycle.

Setting this voltage step equal to 1% of the output voltage,

the output capacitor must be:

COUT > 50 • L •

IPEAK

VOUT

⎛

⎝

⎜

⎞

⎠

⎟

Typically, a capacitor that satisfies the voltage ripple re-

quirement is adequate to filter the inductor ripple. To avoid

overheating, the output capacitor must also be sized to

handle the ripple current generated by the inductor. The

worst-case ripple current in the output capacitor is given

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