LTC3121 datasheet(13/26 Pages) LINER

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LTC3121

3121fa

For more information www.linear.com/LTC3121

applicaTions inForMaTion

The inductor should have low DCR (series resistance of

the windings) to reduce the I2R power losses, and must be

able to support the peak inductor current without saturat-

ing. Molded chokes and most chip inductors usually do

not have enough core area to support the peak inductor

currents of 2A to 3A seen on the LTC3121. To minimize

radiated noise, use a shielded inductor.

See Table 1 for suggested components and suppliers

Table 1. Recommended Inductors

PART NUMBER

VALUE

(µH)

DCR

(mΩ)

ISAT

(A)

SIZE (mm)

W × L × H

Coilcraft XAL4020-222ME

Coilcraft XAL4030-332ME

Coilcraft XAL4030-472ME

Coilcraft XAL5050-682ME

Coilcraft XAL6060-223ME

Coilcraft MSS1260T-333ML

2.2

3.3

4.7

6.8

5.6

5.5

4.5

6.0

5.6

4.34

4.3 × 4.3 × 2.1

4.3 × 4.3 × 3.1

5.3 × 5.3 × 5.1

6.3 × 6.3 × 6.1

12.3 × 12.3 × 6.2

Coiltronics DR73-2R2-R

Coiltronics DR74-4R7-R

Coiltronics DR125-330-R

Coiltronics DR127-470-R

2.2

4.7

5.52

4.37

3.84

5.28

7.6 × 7.6 × 3.55

7.6 × 7.6 × 4.35

12.5 × 12.5 × 6

12.5 × 12.5 × 8

Sumida CDR7D28MNNP-2R2NC

Sumida CDR7D28MNNP-6R8NC

2.2

6.8

4.9

3.5

7.6 × 7.6 × 3

Taiyo-Yuden NR5040T3R3N

3.3

3.8

5 × 5 × 4

TDK LTF5022T-2R2N3R2-LC

TDK SPM6530T-3R3M

TDK VLP8040T-4R7M

2.2

3.3

4.7

3.2

6.8

4.4

5 × 5.2 × 2.2

7.1 × 6.5 × 3

8 × 7.7 × 4

Würth WE-PD7447789002

Würth WE-PD7447789003

Würth WE-PD7447779006

Würth WE-HCI7443251000

Würth WE-PD744770122

Würth WE-PD744770133

Würth WE-PD7447709470

2.2

3.3

4.7

6.8

4.8

4.2

3.3

8.5

3.6

4.5

7.3 × 7.3 × 3.2

7.3 × 7.3 × 4.5

10 × 10 × 5

12 × 12 × 8

12 × 12 × 10

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 LTC3121 applications.

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