LTC3407-3

12

34073fa

with a load current of 800mA and an ambient temperature

of 70

°C. From the Typical Performance Characteristics

main switch is 0.425

Ω. Therefore, power dissipated by

each channel is:

The DFN package junction-to-ambient thermal resistance,

regulator operating in a 70

°C ambient temperature is

approximately:

which is below the absolute maximum junction tempera-

ture of 125

°C.

Design Example

As a design example, consider using the LTC3407-3 in an

portable application with a Li-Ion battery. The battery

mum of 800mA in active mode and 2mA in standby mode.

needs power in standby, Burst Mode operation is selected

for good low load efficiency.

First, calculate the inductor value for about 30% ripple

L

V

MHz

mA

V

V

H

=

⎛

⎝⎜

⎞

⎠⎟

=µ

18

2 25

300

1

18

42

15

.

.•

•–

.

.

.

Choosing a vendor’s closest inductor value of 2.2

µH,

results in a maximum ripple current of:

∆ =

µ

−

⎛

⎝⎜

⎞

⎠⎟

=

I

V

MHz

H

V

V

m

L

18

225

22

1

18

42

208

.

.• .

•

.

.

A

A

selection is then based on load step droop instead of ESR

requirements. For a 5% output droop:

C

mA

MHz

V

F

=µ

18

800

225

5

18

71

.

.•( % • .

)

.

A good standard value is 10

µF. Since the output imped-

The PGOOD pin is a common drain output and requires a

pull-up resistor. A 100k resistor is used for adequate speed.

Figure 1 shows the complete schematic for this design

example.

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC3407-3. These items are also illustrated graphically in

the layout diagram of Figure 3. Check the following in your

layout:

and GND (exposed pad) as close as possible? This capaci-

tor provides the AC current to the internal power MOSFETs

and their drivers.

APPLICATIO S I FOR ATIO

RUN2

RUN1

POR

SW1

GND

SW2

MODE/SYNC

LTC3407-3

L1

L2

34073 F03

BOLD LINES INDICATE

HIGH CURRENT PATH

Figure 3. LTC3407-3 Layout Diagram (See Board Layout Checklist)