lt3518euf-trpbf Linear Technology Corporation, lt3518euf-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt3518euf-trpbf
• Description: Full-featured Led Driver With 2.3a Switch Current
• Manufacturer: Linear Technology Corporation

LT3518
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
suffi cient. Table 2 shows some recommended capacitor
vendors.
Table 2. Ceramic Capacitor Manufacturers
VENDOR
Taiyo Yuden
AVX
Murata
TDK
Loop Compensation
The LT3518 uses an internal transconductance error
amplifi er whose V
The external inductor, output capacitor, and the compen-
sation 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 V
stability. For typical LED applications, a 10nF compensation
capacitor at V
required. A compensation resistor may be used to increase
the slew rate on the V
of LED current during fast transients on V
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 suffi ciently low leakage current. Table 3 has some
recommended component vendors.
APPLICATIONS INFORMATION
12
C
is adequate, and a series resistor is not
C
C
output compensates the control loop.
PHONE
[phone]
[phone]
[phone]
[phone]
are selected to optimize control loop
C
pin to maintain tighter regulation
WEB
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.tdk.com
IN
or CTRL.
Table 3. Schottky Diodes
PART NUMBER
On Semiconductor
MBRS260T3
Diodes Inc.
DFLS140L
Zetex
ZLLS2000TA
International Rectifi er
10MQ060N
Board Layout
The high speed operation of the LT3518 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
signifi cantly reduces the start-up current spike and
output voltage overshoot. A typical value for the soft-start
capacitor is 0.1μF .
V
IN
R
60
40
40
60
supply to the LT3518 should
(V)
IN
terminal of the
I
AVE
2.2
1.5
2
1
(A)
3518fb