10

LTC3405A-1.375

3405a1375f

APPLICATIO S I FOR ATIO

is the thermal resistance from the junction of the die to the

ambient temperature.

As an example, consider the LTC3405A-1.375 with an

input voltage of 2.7V, a load current of 300mA and an

ambient temperature of 70°C. From the typical perfor-

channel switch at 70°C is approximately 0.94Ω and the

mately 0.75Ω.

The series resistance looking into the SW pin is:

Therefore, power dissipated by the part is:

junction temperature of the regulator is:

which is well below the maximum junction temperature of

125°C.

Note that at higher supply voltages, the junction tempera-

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

tored for overshoot or ringing that would indicate a stability

problem. For a detailed explanation of switching control

loop theory, see Application Note 76.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC3405A-1.375. These items are also illustrated graphi-

cally in Figures 3 and 4. Check the following in your layout:

1. The power traces, consisting of the GND trace, the SW

wide.

possible? This capacitor provides the AC current to the

internal power MOSFETs.

Figure 3. LTC3405A-1.375 Layout Diagram

RUN

LTC3405A-1.375

GND

SW

6

L1

BOLD LINES INDICATE HIGH CURRENT PATHS

3405A1375 F03

4

5

1

3

+

–

2

MODE

Design Example

As a design example, assume the LTC3405A-1.375 is

used in a single lithium-ion battery-powered cellular phone

4.2V down to about 2.7V. The load current requirement

is a maximum of 0.15A but most of the time it will be in

standby mode, requiring only 2mA. Efficiency at both low

and high load currents is important. Output voltage is

1.375V. With this information we can calculate L using

equation (1),

L

fI

V

V

V

L

OUT

OUT

IN

−

⎛

⎝

⎜

⎞

⎠

⎟

1

1

(3)