Datasheet

LT3519/LT3519-1/LT3519-2
11
3519fa
APPLICATIONS INFORMATION
ESL and have an adequate ripple current rating. A 2.2μF
ceramic type capacitor is usually suffi cient for LT3519
(400kHz version). A capacitor of proportionately less
value for LT3519-1/LT3519-2 (higher frequency version)
can be used.
Output Capacitor Selection
The selection of output capacitor depends on the load
and converter confi guration, i.e., step-up or step-down
and the operating frequency. For LED applications, the
equivalent resistance of the LED is typically low, and the
output fi lter capacitor should be sized to attenuate the
current ripple.
To achieve the same LED ripple current, the required fi lter
capacitor value is larger in the boost and buck-boost mode
applications than that in the buck mode applications. Lower
operating frequencies will require proportionately higher
capacitor values. For LED buck mode applications, a 1μF
ceramic capacitor is usually suffi cient. For the LED boost
and buck-boost mode applications, a 2.2μF ceramic ca-
pacitor is usually suffi cient. Very high performance PWM
dimming applications may require a larger capacitor value
to support the LED voltage during PWM transitions.
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. Recommended Ceramic Capacitor Vendors
VENDOR PHONE WEB
TDK [phone] www.tdk.com
Kemet [phone] www.kemet.com
Murata [phone] www.murata.com
Taiyo Yuden [phone] www.t-yuden.com
Inductor Selection
The inductor used with the LT3519/LT3519-1/LT3519-2
should have a saturation current rating of 1A or greater.
For buck mode LED drivers, the inductor value should be
chosen to give a ripple current 150mA or more. In the
buck mode, the inductor value can be estimated using
the formula:
LµH
()
=
D
BUCK
•V
IN
–V
LED
()
f
OSC
(MHz) • 0.15A
µH • A •MHz
V
⎛
⎝
⎜
⎞
⎠
⎟
D
BUCK
=
V
LED
V
IN
V
LED
is the voltage across the LED string, V
IN
is the input
voltage to the converter, and f
OSC
is the switching frequency.
In the boost confi guration, the inductor can be estimated
using the formula:
LµH
()
=
D
BOOST
•V
IN
f
OSC
(MHz) • 0.15A
µH • A •MHz
V
⎛
⎝
⎜
⎞
⎠
⎟
D
BOOST
=
V
LED
–V
IN
()
V
LED
Table 1. Recommended Inductor Vendors
VENDOR PHONE WEB
Sumida [phone] www.sumida.com
Toko [phone] www.toko.com
Cooper [phone] www.cooperet.com
Vishay [phone] www.vishay.com
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
IN
pin of the LT3519/
LT3519-1/LT3519-2. A 1μF or greater capacitor with low
ESR should be used. A ceramic capacitor is usually the
best choice.
In the buck mode confi guration, the capacitor at the input
to the power converter has large pulsed currents. For
best reliability, this capacitor should have low ESR and