LTC3400/LTC3400B

6

3400f

PCB LAYOUT GUIDELINES

The high speed operation of the LTC3400/LTC3400B

demands careful attention to board layout. You will not get

advertised performance with careless layout. Figure 2

shows the recommended component placement. A large

ground pin copper area will help to lower the chip tempera-

ture. A multilayer board with a separate ground plane is

ideal, but not absolutely necessary.

APPLICATIO S I FOR ATIO

II

VD

fL

D

OUT MAX

P

IN

()

•–

•

••

•–

=







η

2

1

where:

η = estimated efficiency

f = switching frequency (1.2MHz 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 850mA seen on the

LTC3400/LTC3400B. To minimize radiated noise, use a

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

for some suggested components and suppliers.

SW

GND

FB

1

2

3

6

5

4

SHDN

SHDN

(OPTIONAL)

3400 F02

RECOMMENDED COMPONENT PLACEMENT. TRACES

CARRYING HIGH CURRENT ARE DIRECT. TRACE AREA AT

FB PIN IS SMALL. LEAD LENGTH TO BATTERY IS SHORT

Figure 2. Recommended Component Placement

for Single Layer Board

COMPONENT SELECTION

Inductor Selection

The LTC3400/LTC3400B can utilize small surface mount

and chip inductors due to their fast 1.2MHz switching

frequency. A minimum inductance value of 3.3

µH is

necessary for 3.6V and lower voltage applications and

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 LTC3400/

LTC3400B versus inductance value is given in the equa-

tion below and illustrated graphically in Figure 3.

INDUCTANCE (

µH)

3

60

80

110

120

160

7

11 13

21

180

140

59

15 17 19

23

3400 F03

Figure 3. Maximum Output Current vs

Inductance Based On 90% Efficiency