8

LTC1504A

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

TP3-470

47

µH

0.55A

Toroid

Ferrite

2.2mm

TP3-470

47

µH

0.72A

Toroid

Ferrite

3mm

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

LTC1504A 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 LTC1504A

feedback loop stable. Transient load response of an

LTC1504A circuit is controlled almost entirely by the

output capacitor and the inductor. In steady load opera-

tion, the average current in the inductor will match the load

current. When the load current changes suddenly, the

inductor is suddenly carrying the wrong current and

requires a finite amount of time to correct itself—at least

several switch cycles with typical LTC1504A inductor

values. Even if the LTC1504A 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 cannot 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 LTC1504A 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

C of the output stage form a 2nd order roll-off with 180

°

of phase shift; the R due to ESR forms a single zero at a

somewhat higher frequency that reduces the roll-off to

first order and reduces the phase shift to 90

°.

If the output capacitor has a relatively high ESR, the zero

comes in well before the initial phase shift gets all the way

to 180

° and the loop only requires a single small capacitor

from COMP to GND to remain stable (Figure 4a). If, on the

other hand, the output capacitor is a low ESR type to

maximize transient response, the ESR zero can increase in

frequency by a decade or more and the output stage phase

shift can get awfully close to 180

° before it turns around

and comes back to 90

°. Large value ceramic, OS-CON

electrolytic and low impedance tantalum capacitors fall

into this category. These loops require an additional zero

to be inserted at the COMP pin; a series RC in parallel with

a smaller C to ground will usually ensure stability. Figure 4b

shows a typical compensation network which will opti-

mize transient response with most output capacitors.

Adjustable output parts can add a feedforward capacitor

across the feedback resistor divider to further improve

Kool M

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