LTC3543 datasheet(13/16 Pages) LINER

13 / 16 page

LTC3543

3543f

APPLICATIO S I FOR ATIO

problem. For a detailed explanation of the switching control

loop theory, see Application Note 76.

A second, more severe transient is caused by switching

in loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT. No regulator

can 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 the load rise time is limited to approximately

25 • CLOAD. Thus, a 10µF capacitor charging to 3.3V would

require a 250µs rise time, limiting the charging current

to about 130mA.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3543. 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

trace, and the VIN trace should be kept short, direct

and wide.

2. Does the VFB pin connect directly to the feedback voltage

reference? Ensure that there is no load current running

from the feedback reference voltage and the VFB pin.

3. Does the (+) plate of CIN connect to VIN as closely as

possible? This capacitor provides the AC current to the

internal power MOSFETs.

4. Keep the switching node, SW, away from the sensitive

VFB node.

5. Keep the (–) plates of CIN and COUT as close as pos-

sible.

Design Example

As a design example, assume the LTC3543 is used in

a single lithium-ion battery-powered cellular phone ap-

plication. The VIN will be operating from a maximum of

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

is a maximum of 600mA, 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 1.5V. With this information we can calculate L using

Equation 1,

OUT

=−

⎛

⎝⎜

⎞

⎠⎟

•

∆

A 3.3µH inductor works well for this application, yielding

a ripple current of only 130mA. For best efﬁciency choose

a 665mA or greater inductor with less than 0.05Ω series

resistance.

In Equation 2, CIN will require an RMS current rating of at

least 0.3A at ILOAD(MAX)/2 at temperature.

Using Equation 3, Selecting a 4.7µF capacitor with an

ESR of 0.05Ω for COUT yields an 8mV voltage ripple on

the regulated output voltage.

For feedback resistors, choose R1 = 402k. R2 can then

be calculated using Equation 4.

OUT

1 604

=−

⎛

⎝⎜

⎞

⎠⎟

•

Figure 6 shows the complete circuit along with its ef-

ﬁciency curve.

VIN

VFB

MODE

RUN

CFWD

CIN

GND

VIN

CAP

VOUT

COUT

3543 F07

+–

–

+–

Figure 5. LTC3543 Layout Diagram

LTC3543 Datasheet(PDF) 13 Page - Linear Technology

LTC3543 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL