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LTC1778/LTC1778-1

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ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a tradeoff between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). The largest ripple current

occurs at the highest VIN. To guarantee that ripple current

does not exceed a specified maximum, the inductance

should be chosen according to:

OUT

LMAX

OUT

IN MAX

⎛

⎝

⎜

⎞

⎠

⎟

−

⎛

⎝

⎜

⎞

⎠

⎟

∆ ()

()

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

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron

cores, forcing the use of more expensive ferrite, molyper-

malloy or Kool Mµ

® cores. A variety of inductors designed

for high current, low voltage applications are available

from manufacturers such as Sumida, Panasonic, Coil-

tronics, Coilcraft and Toko.

Schottky Diode D1 Selection

The Schottky diode D1 shown in Figure 1 conducts during

the dead time between the conduction of the power

MOSFET switches. It is intended to prevent the body diode

of the bottom MOSFET from turning on and storing charge

during the dead time, which can cause a modest (about

1%) efficiency loss. The diode can be rated for about one

half to one fifth of the full load current since it is on for only

a fraction of the duty cycle. In order for the diode to be

effective, the inductance between it and the bottom MOS-

FET must be as small as possible, mandating that these

components be placed adjacently. The diode can be omit-

ted if the efficiency loss is tolerable.

CIN and COUT Selection

The input capacitance CIN is required to filter the square

wave current at the drain of the top MOSFET. Use a low

ESR capacitor sized to handle the maximum RMS current.

RMS

OUT MAX

OUT

≅

()

–1

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

IRMS = IOUT(MAX)/ 2. This simple worst-case condition is

commonly used for design because even significant

deviations do not offer much relief. Note that ripple

current ratings from capacitor manufacturers are often

based on only 2000 hours of life which makes it advisable

to derate the capacitor.

The selection of COUT is primarily determined by the ESR

required to minimize voltage ripple and load step

transients. The output ripple ∆VOUT is approximately

bounded by:

∆≤ ∆

⎛

⎝

⎜

⎞

⎠

⎟

I ESR

OUT

Since ∆IL increases with input voltage, the output ripple is

highest at maximum input voltage. Typically, once the ESR

requirement is satisfied, the capacitance is adequate for

filtering and has the necessary RMS current rating.

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount

packages. Special polymer capacitors offer very low ESR

but have lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density

but it is important to only use types that have been surge

tested for use in switching power supplies. Aluminum

electrolytic capacitors have significantly higher ESR, but

can be used in cost-sensitive applications providing that

consideration is given to ripple current ratings and long

term reliability. Ceramic capacitors have excellent low

ESR characteristics but can have a high voltage coefficient

and audible piezoelectric effects. The high Q of ceramic

capacitors with trace inductance can also lead to signifi-

cant ringing. When used as input capacitors, care must be

taken to ensure that ringing from inrush currents and

switching does not pose an overvoltage hazard to the

power switches and controller. To dampen input voltage

transients, add a small 5µF to 50µF aluminum electrolytic

capacitor with an ESR in the range of 0.5Ω to 2Ω. High

performance through-hole capacitors may also be used,

APPLICATIO S I FOR ATIO

Kool Mµ is a registered trademark of Magnetics, Inc.

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