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LTC3541EDD

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541

APPLICATIONS INFORMATION

PC BOARD LAYOUT CHECKLIST

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3541. 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 LFB pin connect directly to the feedback re-

sistors? The resistive divider R1/R2 must be connected

between the (+) plate of COUT and ground.

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

LFB node.

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

DESIGN EXAMPLE

As a design example, assume the LTC3541 is used in a sin-

gle lithium-ion battery powered cellular phone application.

The VIN will be operating from a maximum of 4.2V down to

about 3V. The load current requirement is a maximum of

0.5A for the buck output but most of the time it will be in

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

and high load currents is important. The output voltage for

the buck is 1.8V. The requirement for the output voltage

of the VLDO is 1.5V while providing up to 0.3A of current.

With this information we can calculate L using Equation 2:

L

=

(

f

)

1

(ΔIL

)

VOUT



1−

VOUT

VIN



(2)

Substituting VOUT = 1.8V, VIN = 3.6V (typ), ΔIL = 0.2A and

f = 2.25MHz in Equation 3 gives:

L

=

1.8V

2.25MHz(200mA)



1−

1.8V

3.6V



=

2µH

(3)

A 2.2μH inductor works well for this application. For best

efficiency choose a 600mA or greater inductor with less

than 0.2Ω series resistance.

CIN will require an RMS current rating of at least 0.25A =

ILOAD(MAX)/2 at temperature . COUT for the buck is chosen

as 22μF with an ESR of less than 0.2Ω. In most cases, a

ceramic capacitor will satisfy this requirement.

For the feedback resistors of the buck, choose R1 = 80k.

R2 can then be calculated from Equation 4 to be:

R2 = 

VOUT

0.8

− 1

R1= 100k

(4)

For the feedback resistors of the VLDO, choose R1 = 200k.

R2 can then be calculated from Equation 5 to be:

R2 = 

VOUT

0.4

− 1

R1=

550k

COUT for the VLDO is chosen as 2.2μF.

For more information www.linear.com/LTC3541

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