LTC3429/LTC3429B

8

3429fa

APPLICATIO S I FOR ATIO

is achieved by terminating the switching of the synchro-

ensures that the slope of the inductor current will reverse

during the time current is flowing to the output. Since the

PMOS no longer acts as a low impedance switch in this

mode, there will be more power dissipation within the IC.

This will cause a sharp drop in the efficiency (see Typical

mum output current should be limited in order to maintain

an acceptable junction temperature.

OPERATIO

PCB LAYOUT GUIDELINES

The high speed operation of the LTC3429/LTC3429B

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.

inductor ripple current. Increasing the inductance above

10µH will increase size while providing little improvement

in output current capability.

The approximate output current capability of the LTC3429

versus inductance value is given in the equation below and

illustrated graphically in Figure 3.

II

VD

fL

D

OUT MAX

P

IN

()

•–

•

••

•–

=

⎛

⎝

⎜

⎞

⎠

⎟

()

η

2

1

where:

η = estimated efficiency

f = switching frequency (500kHz typical)

L = inductance value

INDUCTANCE (µH)

3

120

160

200

19

3429 F03

80

40

100

140

180

60

20

0

7

5

11

9

15 17

21

13

23

Figure 3. Maximum Output Current vs

Inductance Based on 90% Efficiency

SW

GND

FB

1

2

3

6

5

4

SHDN

SHDN

3429 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 LTC3429/LTC3429B can utilize small surface mount

and chip inductors due to its fast 500kHz switching

frequency. Typically, a 4.7µH inductor is recommended

for most applications. Larger values of inductance will

allow greater output current capability by reducing the