LT3517HFE-PBF(RevA) Ver la hoja de datos (PDF) - Linear Technology

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LT3517HFE-PBF
(Rev.:RevA)

Full-Featured LED Driver with 1.5A Switch Current

Linear Technology 

LT3517

APPLICATIONS INFORMATION

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 RT

graph in the Typical Performance Characteristics for resis-

tor values and corresponding switching frequencies.

Table 4. Switching Frequency vs RT

Switching Frequency (kHz)

250

500

1000

1500

2000

2500

RT ( kΩ )

90.9

39.2

16.9

9.53

6.04

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.

3517fa

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