11

LTC3408

3408f

(0.0143 + 0.0425)(43) = 71.1

°C.

Reductions in power dissipation occur at higher supply

voltages, where the junction temperature is lower due to

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

overshoot or ringing that would indicate a stability prob-

lem. For a detailed explanation of switching control loop

theory, see Application Note 76.

A second, more severe transient is caused by switching in

loads with large (>1

µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

deliver enough current to prevent this problem if the load

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so that

Thus, a 10

µF capacitor charging to 3.3V would require a

250

µs rise time, limiting the charging current to about

130mA.

APPLICATIO S I FOR ATIO

Figure 6. Suggested Layout

Figure 5. Layout Diagram

REF

RUN

1

2

3

4

8

7

6

5

LTC3408

3403 F05

SW

GND

DAC

BOLD LINES INDICATE HIGH CURRENT PATHS

8

7

6

5

1

2

3

4

REF

RUN

GND

SW

L1

LTC3408

TO DAC

VIA TO REF

VIA TO PIN 8

VIA TO PIN 7

VIA TO GND

VIA TO PIN 1

3408 F06

VIA TO PIN 2

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC3408. These items are also illustrated graphically in

Figures 5 and 6. Check the following in your layout:

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

possible? This capacitor provides the AC drive to the

internal power MOSFETs.

Design Example

As a design example, assume the LTC3408 is used in a

single lithium-ion battery-powered cellular phone applica-

down to about 2.7V. The load current requirement is a

maximum of 0.6A 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 2.5V. With

this information we can calculate L using Equation (1),

L

fI

V

V

V

L

OUT

OUT

IN

=

∆









1

1

()(

)

–

(2)

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.