lt3517 Linear Technology Corporation, lt3517 Datasheet - Page 12

• Manufacturer Part Number: lt3517
• Description: Full-featured Led Driver With 1.5a Switch Current
• Manufacturer: Linear Technology Corporation

LT3517
APPLICATIONS INFORMATION
Switching Frequency
There are two methods to set the switching frequency of
LT3517. Both methods require a resistor connected at R
pin. Do not leave the R
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 R
See Table 4 below or see the Oscillator Frequency vs R
graph in the Typical Performance Characteristics for resis-
tor 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 R
to generate a switching frequency 20% lower than the
external clock when external clock is absent.
12
Switching Frequency (kHz)
1000
1500
2000
2500
250
500
T
pin open. Also, do not load this pin
T
T
R
T
pin and be able
90.9
39.2
16.9
9.53
6.04
4.02
( kΩ )
T
pin and GND.
T
T
In general, a lower switching frequency should be used
where either very high or very low switching duty cycle
operation is required, or high effi ciency is desired. Selection
of a higher switching frequency will allow use of smaller
value external components and yield a smaller solution
size and profi le.
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.
3517f