7

LTC3427

3427fa

APPLICATIO S I FOR ATIO

+

SW

LTC3427

1

GND

MINIMIZE

TRACE ON FB

2

MULTIPLE VIAS

TO GROUND PLANE

FB

SHDN

3427 F01

3

6

5

4

Close to the LTC3427 as Possible. A Multilayer Board with a Separate Ground Plane is Ideal, but not Absolutely Necessary

COMPONENT SELECTION

Inductor Selection

The LTC3427 can utilize small surface mount and chip

inductors due to its fast 1.25MHz switching frequency. A

minimum inductance value of 3.3

µH is necessary for 3.6V

and lower voltage applications and a 4.7

µH for output

voltages greater than 3.6V. Larger values of inductance

will allow greater output current capability by reducing the

inductor ripple current. Increasing the inductance above

10

µH will increase size while providing little improvement

in output current capability.

The approximate output current capability of the LTC3427

vs Inductance value is given below in Equation 1 and

illustrated graphically in Figure 2.

In

I

VD

fL

D

OUT MAX

P

IN

()

•–

•

••

•–

=

⎛

⎝⎜

⎞

2

1

(1)

where:

n = estimated efficiency

f = switching frequency (1.25MHz typical)

L = inductance value

The inductor current ripple is typically set for 20% to 40%

ferrite core inductor materials reduce frequency depen-

dent power losses compared to cheaper powdered iron

types, improving efficiency. The inductor should have low

power losses, and must be able to handle the peak

inductor current without saturating. Molded chokes and

some chip inductors usually do not have enough core to

support the peak inductor currents of greater than 500mA

seen on the LTC3427. To minimize radiated noise, use a

toroid, pot core or shielded bobbin inductor. See Table 1

for suggested suppliers.

Output and Input Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used to minimize the output voltage ripple. Multilayer

ceramic capacitors are an excellent choice as they have

extremely low ESR and are available in small footprints. A

2.2

µF to 10µF output capacitor is sufficient for most

applications. Larger values up to 22

µF may be used to

obtain extremely low output voltage ripple and improve

transient response. An additional phase lead capacitor

may be required with output capacitors larger than 10

µFto

maintain acceptable phase margin. X5R and X7R dielec-

tric materials are preferred for their ability to maintain

capacitance over wide voltage and temperature ranges.