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LTC3541EDD-2

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541-2

APPLICATIONS INFORMATION

As an example, consider the LTC3541-2 with an input

voltage VIN of 2.9V, an LVIN voltage of 1.875V, an LVOUT

voltage of 1.5V, a load current of 300mA for the VLDO

regulator, a load current of 200mA for the buck (total load

for buck = 500mA), and an ambient temperature of 85°C.

From the typical performance graph of switch resistance,

the RDS(ON) of the P-channel switch at 85°C is approxi-

mately 0.25Ω.The RDS(ON) of the N-channel switch is

approximately 0.4Ω. Therefore, power dissipated by the

part is approximately:

PD = (ILOADBUCK)2 • RSW + (ILOADVLDO) •

(LVIN – LVOUT) = 188mW

For the 3mm × 3mm DFN package, the qJA is 43°C/W.

Thus, the junction temperature of the regulator is:

TJ = 85°C + (0.188)(43) = 93°C

which is well below the maximum junction temperature

of 125°C.

Note that at higher supply voltages, the junction tempera-

ture is lower due to reduced switch resistance RDS(ON).

PC BOARD LAYOUT CHECKLIST

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC3541-2. 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 (+) plate of CIN connect to VIN as closely as

possible? This capacitor provides the AC current to the

internal power MOSFETs.

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

LFB node.

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

possible.

DESIGN EXAMPLE

As a design example, assume the LTC3541-2 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 2.9V. 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 1.875V. The require-

ment for the output of the VLDO regulator is 1.5V output

voltage while providing up to 0.3A of current. With this

information we can calculate L using Equation 2:

L

=

(

f

)

1

(∆IL

)

VOUT



1−

VOUT

VIN



(2)

Substituting VOUT = 1.875V, VIN = 3.55V (typ), ΔIL =

200mA and f = 2.25MHz in Equation 3 gives:

L

=

1.8V

2.25MHz(200mA)



1−

1.8V

3.45V



= 1.91µH

(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

to have a value of 22µF and an ESR of less than 0.25Ω. In

most cases, a ceramic capacitor will satisfy this require-

ment.

COUT for the VLDO regulator is chosen as 2.2µF.

For more information www.linear.com/LTC3541-2

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