7

LTC3490

3490fa

APPLICATIO S I FOR ATIO

The LTC3490 requires only four external components to

operate: an inductor, an output capacitor, a switch and a

pull-down resistor. The inductor is nominally set at 3.3

µH

and the capacitor at 4.7

µF. Optional components include

an input capacitor and dimming resistors.

COMPONENT SELECTION

Inductor Selection

The high frequency operation of the LTC3490 allows the

use of small surface mount inductors. The minimum

inductance value is proportional to the operating fre-

quency and is limited by the following constraints:

L

f

H

and

L

VV

V

f Ripple V

H

IN MIN

OUT MAX

IN MIN

OUT MAX

≥

≥

()

3

()

(

)

()

()

•–

••

where:

f = Operating Frequency (Hz)

Ripple = Inductor Current Ripple (A)

The inductor current ripple is typically set to 20% to 40%

of the inductor current.

The peak inductor current is given by:

II

VI

R

R

I

VR

I

VV

V

Lf V

LPK

OUT

OUT

OUT

P

N

IN

IN

N

IN

IN

OUT

IN

OUT

=

+

()

+

()

•–

•

–•

–

•• •

2

where:

For high efficiency, choose an inductor with a high fre-

quency core material, such as ferrite, to reduce core

losses. The inductor should have low ESR (equivalent

able to handle the peak inductor current at full load without

saturating. In single cell applications, the inductor ESR

must be below 25m

Ω to keep the efficiency up and

maintain output current regulation. Dual cell applications

can tolerate significantly higher ESR (up to 75m

Ω) with

minimal efficiency degradation. Molded chokes or chip

inductors usually do not have enough core to support the

peak inductor currents in the 1A to 2A region. If radiated

noise is an issue, use a toroid, pot core or shielded bobbin

inductor to minimize radiated noise. See Table 1 for a list

of suggested inductors. Look closely at the manufacturers

data sheets; they specify saturation current differently.

Table 1. Inductor Information

INDUCTOR PART NUMBER

ESR (m

Ω) SATURATION CURRENT (A)

TOKO A918CY-3R3M

47

1.97

TYCO DN4835-3R3M

58

2.15

TDK SLF7045T-3R3M2R5

20

2.5

Output Capacitor Selection

The output capacitor value and equivalent series resis-

tance (ESR) are the primary factors in the output ripple.

The output ripple is not a direct concern for LED drive as

the LED will operate at the average current value. However

the peak pulsed forward current rating of the LED must not

be exceeded to avoid damaging the LED.