LT3518 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Fabricante

LT3518

Full-Featured LED Driver with 2.3A Switch Current

Linear Technology 

LT3518

APPLICATIONS INFORMATION

For a buck or a buck-boost configuration, the output voltage

is typically level-shifted to a signal with respect to GND as

illustrated in the Figure 4. The output can be expressed as:

VOUT

=

R1

R2

•

1.01V

+

VBE(Q1)

(5)

+

R1

VOUT

LT3518 Q1

–

FB

R2

3518 F04

RSENSE

LED

ARRAY

Figure 4

Inductor Selection

The inductor used with the LT3518 should have a satura-

tion current rating of 2A or greater. For buck mode LED

drivers, the inductor value should be chosen to give a

ripple current “ΔI” of ~30% to 40% of the LED current.

In the buck mode, the inductor value can be estimated

using the formula:

L (μH)

=

DBUCK

•

tSW (μs) •

ΔI

(VIN

–

VLED )

(6)

DBUCK

=

VLED

VIN

VLED is the voltage across the LED string, VIN is the input

voltage to the converter, and tSW is the switching period.

In the boost configuration, the inductor can be estimated

using the formula:

L (μH) = DBOOST • tSW (μs) • VIN

ΔI

(7)

DBOOST

=

VLED – VIN

VLED

Table 1 provides some recommended inductor vendors.

Table 1. Inductor Manufacturers

VENDOR

PHONE

Sumida

(408) 321-9660

Toko

(408) 432-8281

Cooper

(561) 998-4100

Vishay

(402) 563-6866

WEB

www.sumida.com

www.toko.com

www.cooperet.com

www.vishay.com

Input Capacitor Selection

For proper operation, it is necessary to place a bypass

capacitor to GND close to the VIN pin of the LT3518. A

1μF or greater capacitor with low ESR should be used. A

ceramic capacitor is usually the best choice.

In the buck mode configuration, the capacitor at the input

to the power converter has large pulsed currents due to

the current returned though the Schottky diode when the

switch is off. For best reliability, this capacitor should have

low ESR and ESL and have an adequate ripple current

rating. The RMS input current is:

IIN(RMS) = ILED • (1– D) • D

(8)

where D is the switch duty cycle. A 2.2μF ceramic type

capacitor is usually sufficient.

Output Capacitor Selection

The selection of output capacitor depends on the load

and converter configuration, i.e., step-up or step-down.

For LED applications, the equivalent resistance of the LED

is typically low, and the output filter capacitor should be

sized to attenuate the current ripple.

To achieve the same LED ripple current, the required filter

capacitor value is larger in the boost and buck-boost mode

applications than that in the buck mode applications. For

LED buck mode applications, a 1μF ceramic capacitor

is usually sufficient. For the LED boost and buck-boost

mode applications, a 2.2μF ceramic capacitor is usually

sufficient. Very high performance PWM dimming appli-

cations may require a larger capacitor value to support

the LED voltage during PWM transitions.

3518fe

10

Share Link: