LTC3545 datasheet(12/20 Pages) LINER

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LTC3545/LTC3545-1

35451fb

APPLICATIONS INFORMATION

The basic LTC3545/LTC3545-1 application circuit is shown

on the ﬁrst page of this data sheet. External component

selection is driven by the load requirement and begins

with the selection of L followed by CIN and COUT.

Inductor Selection

For most applications, the value of the inductor will fall in

the range of 1μH to 10μH. Its value is chosen based on the

desired ripple current. Large inductor values lower ripple

current and small inductor values result in higher ripple

currents. Higher VIN or VOUT also increases the ripple

current as shown in Equation 1. A reasonable starting

point for setting ripple current for an 800mA regulator is

ΔIL = 320mA (40% of 800mA).

ΔI

OUT

= ()( )

⎛

⎝⎜

⎞

⎠⎟

–

(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 960mA rated inductor

should be enough for most applications (800mA + 160mA).

For better efﬁciency, choose a low DCR inductor.

Inductor Core Selection

Different core materials and shapes will change the

size/current and price/current relationship of an induc-

tor. 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 electrical characteristics. The choice of which

style inductor to use often depends more on the price vs

size requirements and any radiated ﬁeld/EMI requirements

than on what the LTC3545/LTC3545-1 require to operate.

Table 1 shows typical surface mount inductors that work

well in LTC3545/LTC3545-1 applications.

CIN and COUT Selection

In continuous mode, a worst-case estimate for the input

current ripple can be determined by assuming that the

source current of the top MOSFET is a square wave of

duty cycle VOUT/VIN, and amplitude IOUT(MAX). 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:

RMS

OUT MAX

OUT

≅

()

–

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

= IOUT/2. This simple worst-case condition is commonly

used for design. Note that the capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of

life (non-ceramic capacitors). This makes it advisable to

further de-rate the capacitor, or choose a capacitor rated

at a higher temperature than required. Always consult the

manufacturer if there is any question.

Table 1. Representative Surface Mount Inductors

PART

NUMBER

VALUE

(μH)

DCR

(

Ω MAX)

MAX DC

CURRENT (A)

× L × H (mm3)

Wurth WE-

TPC 744031

1.5

2.5

3.6

0.035

0.045

0.065

1.75

1.45

1.38

3.8

× 3.8 × 1.65

CoilCraft

LPS4012

1.5

2.2

3.3

0.06

0.07

0.1

2.5

2.1

1.5

4.0

× 4.0 × 1.1

Sumida

CDH38D11/

SLD

1.4

2.4

3.6

0.055

0.094

0.13

1.8

1.3

1.1

4.0

× 4.0 × 1.2

Sumida

CDRH3D16

1.5

2.2

3.3

0.043

0.075

0.11

1.55

1.2

1.1

3.8

× 3.8 × 1.8

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