LT3517

11

3517ff

Table 3. Schottky Diodes

PART NUMBER

Diodes Inc.

DFLS160

60

1

Zetex

ZLLS1000TA

40

1

International Rectifier

10MQ060N

60

1.5

Board Layout

The high speed operation of the LT3517 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

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.

Switching Frequency

There are two methods to set the switching frequency of

LT3517. Both methods require a resistor connected at RT

pin. Do not leave the RT pin open. Also, do not load this pin

with a capacitor. A resistor must always be connected for

proper operation. One way to set the frequency is simply

connecting an external resistor between the RT pin and

GND. See Table 4 below or see the Oscillator Frequency vs

resistor values and corresponding switching frequencies.

Switching Frequency (kHz)

250

90.9

500

39.2

1000

16.9

1500

9.53

2000

6.04

2500

4.02

The other way is to make the LT3517 synchronize with

an external clock via SYNC pin. For proper operation, a

resistor should be connected at the RT pin and be able

to generate a switching frequency 20% lower than the

external clock when external clock is absent.

In general, a lower switching frequency should be used

where either very high or very low switching duty cycle

operation is required, or high efficiency is desired. Selection

of a higher switching frequency will allow use of smaller

value external components and yield a smaller solution

size and profile.

Thermal Considerations

The LT3517 is rated to a maximum input voltage of 30V

for continuous operation, and 40V for nonrepetitive one

second transients. Careful attention must be paid to the

internal power dissipation of the LT3517 at higher input

voltages to ensure that the maximum junction temperature

is not exceeded. This junction limit is especially important

when operating at high ambient temperatures. The Exposed

Pad on the bottom of the package must be soldered to a

ground plane. This ground should then be connected to

an internal copper ground plane with thermal vias placed

directly under the package to spread out the heat dissipated

by the LT3517.

APPLICATIONS INFORMATION