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LTC3122

3122fa

For more information www.linear.com/LTC3122

applicaTions inForMaTion

Output and Input Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used to minimize the output voltage ripple. Multilayer

ceramic capacitors are an excellent choice as they have

extremely low ESR and are available in small footprints.

X5R and X7R dielectric materials are preferred for their

ability to maintain capacitance over wide voltage and tem-

perature ranges. Y5V types should not be used. Although

ceramic capacitors are recommended, low ESR tantalum

capacitors may be used as well.

When selecting output capacitors, the magnitude of the

peak inductor current, together with the ripple voltage

specification, determine the choice of the capacitor. Both

the ESR (equivalent series resistance) of the capacitor and

the charge stored in the capacitor each cycle contribute

to the output voltage ripple.

The ripple due to the charge is approximately:

VRIPPLE(CHARGE) ≈

IP • VIN

COUT • VOUT • ƒ

where IP is the peak inductor current.

The ESR of COUT is usually the most dominant factor for

ripple in most power converters. The ripple due to the

capacitor ESR is:

VRIPPLE(ESR) =ILOAD • RESR •

VOUT

VIN

where RESR = capacitor equivalent series resistance.

Theinputfiltercapacitorreducespeakcurrentsdrawnfrom

the input source and reduces input switching noise. A low

ESR bypass capacitor with a value of at least 4.7µF should

be located as close to the VIN pin as possible.

Low ESR and high capacitance are critical to maintain low

output voltage ripple. Capacitors can be used in parallel

for even larger capacitance values and lower effective

ESR. Ceramic capacitors are often utilized in switching

converter applications due to their small size, low ESR and

low leakage currents. However, many ceramic capacitors

experience significant loss in capacitance from their rated

value with increased DC bias voltage. It is not uncommon

for a small surface mount capacitor to lose more than 50%

of its rated capacitance when operated near its rated volt-

age. As a result it is sometimes necessary to use a larger

capacitor value or a capacitor with a larger value and case

size, such as 1812 rather than 1206, in order to actually

realize the intended capacitance at the full operating volt-

age. Be sure to consult the vendor’s curve of capacitance

vs DC bias voltage. Table 2 shows a sampling of capacitors

suited for LTC3122 applications.

Table 2. Representative Output Capacitors

MANUFACTURER,

PART NUMBER

VALUE

(µF)

VOLTAGE

(V)

SIZE L × W × H (mm)

TYPE, ESR (mΩ)

AVX,

12103D226MAT2A

3.2 × 2.5 × 2.79,

X5R Ceramic

Kemet,

C2220X226K3RACTU

5.7 × 5.0 × 2.4,

X7R Ceramic

Kemet,

A700D226M016ATE030

7.3 × 4.3 × 2.8,

Alum. Polymer, 30mΩ

Murata,

GRM32ER71E226KE15L

3.2 × 2.5 × 2.5,

X7R Ceramic

Nichicon,

PLV1E121MDL1

8 × 8 × 12,

Alum. Polymer, 25mΩ

Panasonic,

ECJ-4YB1E226M

3.2 × 2.5 × 2.5,

X5R Ceramic

Sanyo,

25TQC22MV

7.3 × 4.3 × 3.1,

POSCAP, 50mΩ

Sanyo,

16TQC100M

100

7.3 × 4.3 × 1.9,

POSCAP, 45mΩ

Sanyo,

25SVPF47M

6.6 × 6.6 × 5.9,

OS-CON, 30mΩ

Taiyo Yuden,

TMK325BJ226MM-T

3.2 × 2.5 × 2.5,

X5R Ceramic

TDK,

CKG57NX5R1E476M

6.5 × 5.5 × 5.5,

X5R Ceramic

Cap-XX

GS230F

1.2Farads

4.5

39 × 17 × 3.8

28mΩ

Cooper

A1030-2R5155

1.5Farads

2.5

Ø = 10, L = 30

60mΩ

Maxwell

BCAP0050-P270

50Farads

2.5

Ø = 18, L = 40

20mΩ

For applications requiring a very low profile and very large

capacitance, the GS, GS2 and GW series from Cap-XX

and PowerStor Aerogel Capacitors from Cooper all offer

very high capacitance and low ESR in various low profile

packages.

Amethodforimprovingtheconverter’stransientresponse

usesasmallfeed-forwardseriesnetworkofacapacitorand

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