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LTC3541EDD

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541

APPLICATIONS INFORMATION

The basic LTC3541 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 feedback resistor values

for the buck and the selection of the output capacitor and

feedback values for the VLDO and linear regulator.

BUCK REGULATOR

Inductor Selection

For most applications, the appropriate inductor value will

be in the range of 1.5μH to 3.3μH with 2.2μH the most

commonly used. The exact inductor value is chosen

largely based on the desired ripple current and burst rip-

ple 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).

ΔIL

=

(

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 requirement

and any radiated field/EMI requirements rather than what

the LTC3541 requires to operate. Table 2 shows some

typical surface mount inductors that work well in LTC3541

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

2.0

3.9 × 3.9 × 2.4

CDRH3D23

1.5 0.029

1.5

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

1.0

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

Sumida

1.5

0.06

1.00

3.2 × 3.2 × 1.2

CDRH2D11/HP 2.2

0.10

0.72

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 common-

ly used for design. Note that the capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of

life. This makes it advisable to further derate the capac-

itor or choose a capacitor rated at a higher temperature

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 effective

series resistance (ESR) and burst ripple performance.

The LTC3541 minimizes the required number of external

components by providing internal loop compensation

for the buck regulator loop. Loop stability, transient re-

sponse and burst performance can be tailored by choice

of output capacitance. For many applications, desirable

stability, transient response and ripple performance can

12

For more information www.linear.com/LTC3541

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