LTC3783 LINER [Linear Technology], LTC3783 Datasheet - Page 11

• Manufacturer Part Number: LTC3783
• Description: Full-Featured LED Driver with 1.5A Switch Current
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Table 3. Schottky Diodes
PART NUMBER
Diodes Inc.
DFLS160
Zetex
ZLLS1000TA
International Rectifier
10MQ060N
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
bypass capacitor on the V
be placed as close as possible to the V
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
IN
V
R
60
40
60
supply to the LT3517 should
(V)
IN
terminal of the
I
AVE
1.5
1
1
(A)
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
R
resistor values and corresponding switching frequencies.
Table 4. Switching Frequency vs R
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.
T
graph in the Typical Performance Characteristics for
Switching Frequency (kHz)
1000
1500
2000
2500
250
500
T
R
T
90.9
39.2
16.9
9.53
6.04
4.02
( kΩ )
LT3517
11
3517ff