8

LTC1504

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 LTC1504 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 LTC1504 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 materials are

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

than powdered iron rod core inductors with similar electrical

characteristics. The choice of which style inductor to use

oftendependsmoreonthepricevssizerequirementsandany

radiated field/EMI requirements than on what the LTC1504

requires to operate. Table 2 shows some typical surface

mount inductors that work well in LTC1504 applications.

Table 2. Representative Surface Mount Inductors

CORE

CORE

PART

VALUE

MAX DC

TYPE

MATERIAL

HEIGHT

CoilCraft

DT3316-473

47

µH

1A

Shielded

Ferrite

5.1mm

DT3316-104

100

µH

0.8A

Shielded

Ferrite

5.1mm

DO1608-473

47

µH

0.5A

Open

Ferrite

3.2mm

DO3316-224

220

µH

0.8A

Open

Ferrite

5.5mm

Coiltronics

CTX50-1

50

µH

0.65A

Toroid

KoolM

4.2mm

CTX100-2

100

µH

0.63A

Toroid

KoolM

µ

6mm

CTX50-1P

50

µH

0.66A

Toroid

Type 52

4.2mm

CTX100-2P

100

µH

0.55A

Toroid

Type 52

6mm

Sumida

CDRH62-470

47

µH

0.54A

Shielded

Ferrite

3mm

CDRH73-101

100

µH

0.50A

Shielded

Ferrite

3.4mm

CD43-470

47

µH

0.54A

Open

Ferrite

3.2mm

CD54-101

100

µH

0.52A

Open

Ferrite

4.5mm

Output Capacitor

The output capacitor affects the performance of the

LTC1504 in a couple of ways: it provides the first line of

defense during a transient load step and it has a large effect

on the compensation required to keep the LTC1504 feed-

back loop stable. Transient load response of an LTC1504

circuit is controlled almost entirely by the output capacitor

and the inductor. In steady load operation, the average

current in the inductor will match the load current. When

the load current changes suddenly, the inductor is sud-

denly carrying the wrong current and requires a finite

amount of time to correct itself—at least several switch

cycles with typical LTC1504 inductor values. Even if the

LTC1504 had psychic abilities and could instantly assume

the correct duty cycle, the rate of change of current in the

inductor is still related to its value and will not change

instantaneously.

Until the inductor current adjusts to match the load cur-

rent, the output capacitor has to make up the difference.

Applications that require exceptional transient response

(2% or better for instantaneous full-load steps) will re-

quire relatively large value, low ESR output capacitors.

Applications with more moderate transient load require-

ments can often get away with traditional standard ESR

electrolytic capacitors at the output and can use larger

valued inductors to minimize the required output capaci-

tor value. Note that the RMS current in the output capacitor

is slightly more than half of the inductor ripple current—

much smaller than the RMS current in the input bypass

capacitor. Output capacitor lifetime is usually not a factor

in typical LTC1504 applications.

Large value ceramic capacitors used as output bypass

capacitors provide excellent ESR characteristics but can

cause loop compensation difficulties. See the Loop Com-

pensation section.

Loop Compensation

Loop compensation is strongly affected by the output

capacitor. From a loop stability point of view, the output

inductor and capacitor form a series RLC resonant circuit,

with the L set by the inductor value, the C by the value of

the output capacitor and the R dominated by the output

capacitor’s ESR. The amplitude response and phase shift

due to these components is compensated by a network of

Rs and Cs at the COMP pin to (hopefully) close the

feedback loop in a stable manner. Qualitatively, the L and

Kool M

µ is a registered trademark of Magnetics, Inc..