LTC3542

9

3542fa

design. An additional 0.1μF to 1μF ceramic capacitor is

also recommended on VIN for high frequency decoupling,

when not using an all ceramic capacitor solution.

Output Capacitor (COUT) Selection

The selection of COUT is driven by the required ESR to

minimize voltage ripple and load step transients. Typically,

once the ESR requirement is satisfied, the RMS current

rating generally far exceeds the IRIPPLE(P-P) requirement,

except for an all ceramic solution. The output ripple (

ΔVOUT)

is determined by:

ΔΔ

V

I

ESR

C

OUT

L

OUT

≈+

⎛

⎝⎜

⎞

⎠⎟

1

8•

•

ƒO

where fO is the switching frequency, COUT is the output

capacitance and

ΔIListheinductorripplecurrent.Forafixed

output voltage, the output ripple is highest at maximum

input voltage since

ΔIL increases with input voltage.

If tantalum capacitors are used, it is critical that the capaci-

tors are surge tested for use in switching power supplies.

An excellent choice is the AVX TPS series of surface mount

tantalums, available in case heights ranging from 2mm to

4mm. These are specially constructed and tested for low

ESR so they give the lowest ESR for a given volume. Other

capacitor types include Sanyo POSCAP, Kemet T510 and

T495 series, and Sprague 593D and 595D series. Consult

the manufacturer for other specific recommendations.

Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high

ripple current rating, high voltage rating and low ESR

are tempting for switching regulator use. However, the

ESR is so low that it can cause loop stability problems.

Since the LTC3542’s control loop does not depend on

the output capacitor’s ESR for stable operation, ceramic

capacitors can be used to achieve very low output ripple

and small circuit size. X5R or X7R ceramic capacitors are

recommended because these dielectrics have the best

temperature and voltage characteristics of all the ceramics

for a given value and size.

Great care must be taken when using only ceramic input

and output capacitors. When a ceramic capacitor is used

at the input and the power is being supplied through long

wires, such as from a wall adapter, a load step at the output

can induce ringing at the VIN pin. At best, this ringing can

couple to the output and be mistaken as loop instability.

At worst, the ringing at the input can be large enough to

damage the part. For more information, see Application

Note 88. The recommended capacitance value to use is

10μF for both input and output capacitors.

Table 1. Representative Surface Mount Inductors

MANUFACTURER

PART NUMBER

VALUE

(μH)

MAX DC

CURRENT

(A)

DCR

(

Ω)

SIZE (mm3)

Sumida

CDRH2D11-2RM

2.2

0.780

0.098

3.2

× 3.2 × 1.2

CDRH3D16

2.2

1.2

0.075

3.8

× 3.8 × 1.8

CMD4D11

2.2

0.95

0.116

4.4

× 5.8 × 1.2

CDH2D09B

3.3

0.85

0.15

2.8

× 3 × 1

CLS4D09

4.7

0.75

0.15

4.9

× 4.9 × 1

Murata

LQH32CN

2.2

0.79

0.097 2.5

× 3.2 × 1.55

LQH43CN

4.7

0.75

0.15

4.5

× 3.2 × 2.6

TDK

IVLC453232

2.2

0.85

0.18

4.8

× 3.4 × 3.4

VLF3010AT-

2R2M1R0

2.2

1.0

0.12

2.8

× 2.6 × 1

APPLICATIONS INFORMATION