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LTC3601

3601f

APPLICATIONS INFORMATION

exceeded. This collapse will result in an abrupt increase

in inductor ripple current, so it is important to ensure the

core will not saturate.

Different core materials and shapes will change the

size/current and price/current relationship of an induc-

tor. 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 core inductors

with similar characteristics. The choice of which style

inductor to use mainly depends on the price versus size

requirements and any radiated ﬁeld/EMI requirements.

New designs for surface mount inductors are available

from Toko, Vishay, NEC/Tokin, Cooper, Coilcraft, TDK and

Würth Electronik. Table 1 gives a sampling of available

surface mount inductors.

Table 1. Inductor Selection Table

INDUCTANCE

(μH)

DCR

(mΩ)

MAX

CURRENT

(A)

DIMENSIONS

(mm)

HEIGHT

(mm)

Würth Electronik WE-PD2 Typ MS Series

0.56

0.82

1.2

1.7

2.2

9.5

6.5

5.4

4.8

3.6

5.2 × 5.8

Vishay IHLP-2020BZ-01 Series

0.47

0.68

2.2

8.8

12.4

50.1

11.5

4.2

5.2 × 5.5

Toko DE3518C Series

0.56

1.2

1.7

3.3

2.4

2.1

3.5 × 3.7

1.8

Sumida CDRH2D18/HP Series

0.56

0.82

1.1

3.7

2.9

2.5

3.2 × 3.2

Cooper SD18 Series

0.47

0.82

1.2

1.5

2.2

20.1

24.7

29.4

34.5

39.8

3.58

3.24

2.97

2.73

2.55

5.5 × 5.5

1.8

Coilcraft LPS4018 Series

0.56

2.2

4.8

2.8

2.7

4 × 41.7

TDK VLS252012 Series

0.47

1.5

2.2

126

155

3.3

2.4

1.8

2.5 × 21.2

CIN and COUT Selection

The input capacitance, CIN,isneededtoﬁlterthetrapezoidal

wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring a low ESR

input capacitor sized for the maximum RMS current is

recommended. The maximum RMS current is given by:

RMS

OUT MAX

OUT

()

–

where IOUT(MAX) equals the maximum average output

current. This formula has a maximum at VIN = 2VOUT,

where IRMS = IOUT/2. This simple worst-case condition

is commonly used for design because even signiﬁcant

deviations do not offer much relief. Note that ripple cur-

rent ratings from capacitor manufacturers are often based

on only 2000 hours of life which makes it advisable to

further de-rate the capacitor or choose a capacitor rated

at a higher temperature than required.

Several capacitors may be paralleled to meet the require-

ments of the design. For low input voltage applications

sufﬁcient bulk input capacitance is needed to minimize

transient effects during output load changes. Even though

the LTC3601 design includes an overvoltage protection

circuit, care must always be taken to ensure input volt-

age transients do not pose an overvoltage hazard to the

part.

The selection of COUT is primarily determined by the effec-

tive series resistance (ESR) that is required to minimize

voltage ripple and load step transients. The output ripple,

ΔVOUT, is determined by:

VOUT < IL

ESR + 1

8• f • COUT

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

RMS current handling requirements. Dry tantalum, special

polymer, aluminum electrolytic, and ceramic capacitors are

all available in surface mount packages. Special polymer

capacitors offer low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

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