7

LTC1504A

Ideally, the LTC1504A requires a low impedance bypass

right at the chip and a larger reservoir capacitor that can be

located somewhat farther away. This requirement usually

can be met with a ceramic capacitor right next to the

LTC1504A and an electrolytic capacitor (usually 10

µF to

100

µF,dependingonexpectedloadcurrent)locatedsome-

where nearby. In certain cases, the bulk capacitance

requirement can be met by the output bypass of the input

supply. Applications running at very high load currents or

at input supply voltages greater than 6V may require the

local ceramic capacitor to be 1

µF or greater. In some

cases, both the low impedance and bulk capacitance

requirements can be met by a single capacitor, mounted

very close to the LTC1504A. Low ESR organic semicon-

ductor (OS-CON) electrolytic capacitors or surge tested

surface mount tantalum capacitors can have low enough

impedance to keep the LTC1504A happy in some circuits.

Often the RMS current capacity of the input bypass capaci-

tors is more important to capacitor selection than value.

Buck converters like the LTC1504A are hard on input

capacitors, since the current flow alternates between the

full load current and near zero during every clock cycle. In

current flow in the input capacitor is half of the total load

current plus half the ripple current in the inductor—

perhaps 300mA in a typical 500mA load current applica-

tion. This current flows through the ESR of the input

bypass capacitor, heating it up and shortening its life,

sometimes dramatically. Many ordinary electrolytic ca-

pacitors that look OK at first glance are not rated to

withstand such currents—check the RMS current rating

before you specify a device! If the RMS current rating isn’t

specified, it should not be used as an input bypass capaci-

tor. Again, low ESR electrolytic and surge tested tantalums

usually do well in LTC1504A applications and have high

RMS current ratings. The local ceramic bypass capacitor

usually has negligible ESR, allowing it to withstand large

RMS currents without trouble. Table 1 shows typical

surface mount capacitors that make acceptable input

bypass capacitors in LTC1504A applications.

Inductor

The LTC1504A requires an external inductor to be con-

nected from the switching node SW to the output node

Table 1. Representative Surface Mount Input Bypass Capacitors

PART

VALUE

ESR

MAX RMS

TYPE

HEIGHT

AVX

TPSC226M016R0375

22

µF 0.38Ω

0.54A

Tantalum

2.6mm

TPSD476M016R0150

47

µF 0.15Ω

0.86A

Tantalum

2.9mm

TPSE107M016R0125

100

µF 0.13Ω

1.15A

Tantalum

4.1mm

1206YC105M

1

µF

Low

>1A

X7R Ceramic 1.5mm

1210YG106Z

10

µF

Low

>1A

Y5V Ceramic 1.7mm

Sanyo

16SN33M

33

µF 0.15Ω

1.24A

OS-CON

7mm

16SN68M

68

µF

0.1

Ω

1.65A

OS-CON

7mm

16CV100GX

100

µF 0.44Ω 0.23A*

Electrolytic

6mm

16CV220GX

220

µF 0.34Ω 0.28A*

Electrolytic

7.7mm

Sprague

593D476X0016D2W

47

µF 0.17Ω

0.93A

Tantalum

2.8mm

593D107X0016E2W

100

µ 0.15Ω

1.05A

Tantalum

4mm

*Note: Use multiple devices in parallel or limit output current to prevent capacitor overload.

where the load is connected. Inductor requirements are

fairly straightforward; it must be rated to handle continu-

ous DC current equal to the maximum load current plus

half the ripple current and its value should be chosen

based on the desired ripple current and/or the output

current transient requirements. Large value inductors

lower ripple current and decrease the required output

capacitance, but limit the speed that the LTC1504A can

change the output current, limiting output transient re-

sponse. Small value inductors result in higher ripple

currents and increase the demands on the output capaci-

tor, but allow faster output current slew rates and are often

smaller and cheaper for the same DC current rating. A

typical inductor used in an LTC1504A application might

have a maximum current rating between 500mA and 1A

and an inductance between 33

µH and 220µH.

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 mate-

rials are small and don’t radiate much energy, but gener-

ally cost more than powdered iron rod 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 LTC1504A requires to operate. Table 2

shows some typical surface mount inductors that work

well in LTC1504A applications.