LT3513

13

3513fa

INDUCTOR SELECTION AND MAXIMUM OUTPUT

CURRENT

A good first choice for the inductor value is:

L

VV

OUT

F

=

+

1 8

.

is in μH. The inductor’s RMS current rating must be greater

than the maximum load current and its saturation current

should be at least 30% higher. For highest efficiency, the

series resistance (DCR) should be less than 0.1

Ω. Table 1

lists several vendors and types that are suitable.

Table 1. Inductor Vendors

VENDOR

URL

PART SERIES

TYPE

Coilcraft

www.coilcraft.com

MSS7341

Shielded

Murata

www.murata.com

LQH55D

Open

TDK

www.component.tdk.com

SLF7045

SLF10145

Shielded

Shielded

Toko

www.toko.com

DC62CB

D63CB

D75C

D75F

Shielded

Shielded

Shielded

Open

Sumida

www.sumida.com

CR54

CDRH74

CDRH6D38

CR75

Open

Shielded

Shielded

Open

The optimum inductor for a given application may differ

from the one indicated by this simple design guide. A larger

value inductor provides a higher maximum load current,

and reduces the output voltage ripple. If your load is lower

than the maximum load current, then you can relax the

value of the inductor and operate with higher ripple cur-

rent. This allows you to use a physically smaller inductor

or one with a lower DCR resulting in higher efficiency. Be

aware that the maximum load current depends on input

voltage. A graph in the Typical Performance Character-

istics section of this data sheet shows the maximum

load current as a function of input voltage and inductor

result in discontinuous mode operation, which further

reduces maximum load current. For details of maximum

output current and discontinuous mode operation, see

Linear Technology’s Application Note 44. Finally, for duty

inductance is required to avoid subharmonic oscillations,

see Application Note 19.

The current in the inductor is a triangle wave with an average

value equal to the load current. The peak switch current

is equal to the output current plus half the peak-to-peak

inductor ripple current. The LT3513 limits its switch cur-

rent in order to protect itself and the system from overload

faults. Therefore, the maximum output current that the

LT3513 will deliver depends on the switch current limit, the

inductor value, and the input and output voltages. When

the switch is off, the potential across the inductor is the

output voltage plus the catch diode drop. This gives the

peak-to-peak ripple current in the inductor:

ΔI

DC V

V

Lf

L

OUT

F

+

()

1–

•

where f is the switching frequency of the LT3513 and L

is the value of the inductor. The peak inductor and switch

current is:

II

I

I

SW PK

LPK

OUT

L

+

Δ

2

To maintain output regulation, this peak current must be

1.5A at DC = 0.75 as shown in the Typical Performance

Characteristics section. The maximum output current is

a function of the chosen inductor value:

II

I

ADC

I

OUT MAX

LIM

LL

()

–.

•

– .

•

–

==

()

ΔΔ

2

25

1 0 57

2

Choosing an inductor value so that the ripple current is

small will allow a maximum output current near the switch

current limit. One approach to choosing the inductor is to

start with the simple rule given above, look at the available

inductors and choose one to meet cost or space goals.

Then use these equations to check that the LT3513 will

be able to deliver the required output current. Note again

that these equations assume that the inductor current is

is less than

OPERATION