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LTC3541EDD-2

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541-2

APPLICATIONS INFORMATION

The basic LTC3541-2 application circuit is shown on the

first page of this data sheet. External component selection

is driven by the load requirement and requires the selection

of L, followed by CIN, COUT and the selection of the output

capacitor for the VLDO and linear regulator.

BUCK REGULATOR

Inductor Selection

For most applications, the appropriate inductor value will be

2.2µH. Its value is chosen largely based on the desired ripple

current and burst ripple performance. Generally, large value

inductors reduce ripple current, and conversely, small value

inductors produce higher ripple current. Higher VIN or VOUT

may also increase the ripple current as shown in Equation

1. A reasonable starting point for setting ripple current is

ΔIL = 200mA (40% of 500mA).

∆IL

=

(

1

f)(L)

VOUT



1−

VOUT

VIN



(1)

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 600mA rated

inductor should be enough for most applications (500mA

+ 100mA). For better efficiency, choose a low DC resis-

tance inductor.

Inductor Core Selection

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar elec-

trical characteristics. The choice of which style inductor

to use often depends more on the price vs size require-

ment and any radiated field/EMI requirements rather than

what the LTC3541-2 requires to operate. Table 2 shows

some typical surface mount inductors that work well in

LTC3541-2 applications.

Table 2. Representative Surface Mount Inductors

PART

NUMBER

VALUE DCR

MAX DC

SIZE

(µH) (Ω MAX) CURRENT (A) W × L × H (mm3)

Sumida

1.0

0.025

2

3.9 × 3.9 × 2.4

CDRH3D23

1.5

0.029

1.65

2.2

0.038

1.3

3.3

0.048

1.1

Sumida

CMD4D06

2.2

0.116

3.3

0.174

0.950

0.770

3.5 × 4.3 × 0.8

Coilcraft

ME3220

1.0

0.058

2.7

2.5 × 3.2 × 2.0

1.5

0.068

2.2

2.2

0.104

1.8

3.3

0.138

1.3

Murata

LQH3C

1.0

0.060

1.00

2.5 × 3.2 × 2.0

2.2

0.097

0.79

CIN and COUT Selection

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle VOUT/VIN. To prevent large

voltage transients, a low ESR input capacitor sized for the

maximum RMS current must be used. The maximum RMS

capacitor current is given by:

( ) CIN required IRMS ≅ IOMAX  VOUT

VIN − VOUT

VIN

 1/2

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple, worst-case condition is com-

monly used for design. Note that the capacitor manu-

facturer’s ripple current ratings are often based on 2000

hours of life. This makes it advisable to further derate the

capacitor or choose a capacitor rated at a higher tempera-

ture than required. Always consult the manufacturer with

any question regarding proper capacitor choice.

The selection of COUT for the buck regulator is driven by

the desired buck loop transient response, required effec-

tive series resistance (ESR) and burst ripple performance.

The LTC3541-2 minimizes the required number of external

components by providing internal loop compensation for

the buck regulator loop. Loop stability, transient response

and burst ripple performance can be tailored by choice

of output capacitance. For many applications, desirable

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

35412fc

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