LTC3446 datasheet(13/20 Pages) LINER

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LTC3446

3446fd

APPLICATIONS INFORMATION

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 LTC3446 requires to operate. Table 1

shows some typical surface mount inductors that work

well in LTC3446 applications.

Table 1. Representative Surface Mount Inductors

MANU-

FACTURER PART NUMBER

VALUE

MAX DC

CURRENT

DCR

HEIGHT

Toko

A914BYW-2R2M-

D52LC

2.2μH

2.05A

49mΩ

2mm

Toko

A915AY-2ROM-

D53LC

2μH

3.3A

22mΩ

3mm

Coilcraft

D01608C-222

2.2μH

2.3A

70mΩ

3mm

Coilcraft

LP01704-222M

2.2μH

2.4A

120mΩ

1mm

Sumida

CDRH4D282R2

2.2μH

2.04A

23mΩ

3mm

Sumida

CDC5D232R2

2.2μH

2.16A

30mΩ

2.5mm

Taiyo

Yuden

N06DB2R2M

2.2μH

3.2A

29mΩ

3.2mm

Taiyo

Yuden

N05DB2R2M

2.2μH

2.9A

32mΩ

2.8mm

Murata

LQN6C2R2M04

2.2μH

3.2A

24mΩ

5mm

Würth

744042001

1μH

2.6A

20mΩ

2mm

Catch Diode Selection

Although unnecessary in most applications, a small

improvement in efﬁciency can be obtained in a few ap-

plications by including the optional diode D1 shown in

Figure 1, which conducts when the synchronous switch

is off. When using Burst Mode operation or pulse skip

mode, the synchronous switch is turned off at a low current

and the remaining current will be carried by the optional

diode. It is important to adequately specify the diode peak

current and average power dissipation so as not to exceed

the diode ratings. The main problem with Schottky diodes

is that their parasitic capacitance reduces the efﬁciency,

usually negating the possible beneﬁts for LTC3446 circuits.

Another problem that a Schottky diode can introduce is

higher leakage current at high temperatures, which could

reduce the low current efﬁciency.

Remember to keep lead lengths short and observe proper

grounding to avoid ringing and increased dissipation when

using a catch diode.

Input Capacitor (CIN) Selection

In continuous mode, the input current of the converter is a

square wave with a duty cycle of approximately VOUTB/VIN.

To prevent large voltage transients, a low equivalent series

resistance (ESR) input capacitor sized for the maximum

RMS current must be used. The maximum RMS capacitor

current is given by:

IRMS IMAX

VOUTB(VIN VOUTB)

VIN

where the maximum average output current IMAX equals

the peak current minus half the peak-to-peak ripple cur-

rent, IMAX = IMAXP – ΔIL/2.

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

= IOUT/2. This simple worst case is commonly used to

design because even signiﬁcant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple cur-

rent ratings are often based on only 2000 hours lifetime.

This makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

the size or height requirements of the design. An additional

0.1μF to 1μF ceramic capacitor is also recommended on

VIN for high frequency decoupling, when not using an all

ceramic capacitor solution.

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