LT3517

10

3517ff

Table 1 provides some recommended inductor vendors.

Table 1. Inductor Manufacturers

VENDOR

PHONE

WEB

Sumida

(408) 321-9660

www.sumida.com

Toko

(408) 432-8281

www.toko.com

Cooper

(561) 998-4100

www.cooperet.com

Vishay

(402) 563-6866

www.vishay.com

Input Capacitor Selection

For proper operation, it is necessary to place a bypass

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:

(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.

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 LT3517 uses an internal transconductance error ampli-

fier whose VC output compensates the control loop. The

external inductor, output capacitor, and the compensa-

tion 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

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.

APPLICATIONS INFORMATION