LT3518 Просмотр технического описания (PDF) - Analog Devices

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LT3518

Full-Featured LED Driver with 2.3A Switch Current

Analog Devices 

LT3518

Applications Information

Use only ceramic capacitors with X7R, X5R or better dielec-

tric as they are best for temperature and DC bias stability

of the capacitor value. All ceramic capacitors exhibit loss

of capacitance value with increasing DC voltage bias, so it

may be necessary to choose a higher value capacitor to get

the required capacitance at the operation voltage. Always

check that the voltage rating of the capacitor is sufficient.

Table 2 shows some recommended capacitor vendors.

Table 2. Ceramic Capacitor Manufacturers

VENDOR

PHONE

WEB

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

AVX

(843) 448-9411

www.avxcorp.com

Murata

(770) 436-1300

www.murata.com

TDK

(847) 803-6100

www.tdk.com

Loop Compensation

The LT3518 uses an internal transconductance error

amplifier whose VC output compensates the control loop.

The external inductor, output capacitor, and the compen-

sation resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor at VC are selected to optimize control loop

stability. For typical LED applications, a 10nF compensation

capacitor at VC is adequate, and a series resistor is not

required. A compensation resistor may be used to increase

the slew rate on the VC pin to maintain tighter regulation

of LED current during fast transients on VIN or CTRL.

Diode Selection

The Schottky diode conducts current during the interval

when the switch is turned off. Select a diode rated for

the maximum SW voltage. If using the PWM feature for

dimming, it is important to consider diode leakage, which

increases with the temperature, from the output during the

PWM low interval. Therefore, choose the Schottky diode

with sufficiently low leakage current. Table 3 has some

recommended component vendors.

Table 3. Schottky Diodes

PART NUMBER

On Semiconductor

MBRS260T3

Diodes Inc.

DFLS140L

Zetex

ZLLS2000TA

International Rectifier

10MQ060N

VR (V)

60

40

40

60

IAVE (A)

2

1

2.2

1.5

Board Layout

The high speed operation of the LT3518 demands careful

attention to board layout and component placement. The

Exposed Pad of the package is the only GND terminal of

the IC and is also important for thermal management of

the IC. It is crucial to achieve a good electrical and thermal

contact between the Exposed Pad and the ground plane of

the board. To reduce electromagnetic interference (EMI),

it is important to minimize the area of the SW node. Use

a GND plane under SW and minimize the length of traces

in the high frequency switching path between SW and

GND through the diode and the capacitors. Since there is

a small DC input bias current to the ISN and ISP inputs,

resistance in series with these inputs should be minimized

and matched, otherwise there will be an offset. Finally,

the bypass capacitor on the VIN supply to the LT3518

should be placed as close as possible to the VIN terminal

of the device.

Soft-Start

For many applications, it is necessary to minimize the

inrush current at start-up. The built-in soft-start circuit

significantly reduces the start-up current spike and

output voltage overshoot. A typical value for the soft-start

capacitor is 0.1µF.

For more information www.linear.com/LT3518

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