Datasheet

LT3518
10
3518fe
APPLICATIONS INFORMATION
Figure 4
For a buck or a buck-boost confi guration, the output voltage
is typically level-shifted to a signal with respect to GND as
illustrated in the Figure 4. The output can be expressed as:
V
OUT
=
R1
R2
•1.01V + V
BE(Q1)
(5)
LT3518
FB
Q1
R1
+
–
LED
ARRAY
V
OUT
R2
3518 F04
R
SENSE
Inductor Selection
The inductor used with the LT3518 should have a satura-
tion current rating of 2A or greater. For buck mode LED
drivers, the inductor value should be chosen to give a
ripple current “ΔI” of ~30% to 40% of the LED current.
In the buck mode, the inductor value can be estimated
using the formula:
LµH
()
=
D
BUCK
•t
SW
(µs) • V
IN
–V
LED
()
ΔI
D
BUCK
=
V
LED
V
IN
(6)
V
LED
is the voltage across the LED string, V
IN
is the input
voltage to the converter, and t
SW
is the switching period.
In the boost confi guration, the inductor can be estimated
using the formula:
LµH
()
=
D
BOOST
•t
SW
(µs) • V
IN
ΔI
D
BOOST
=
V
LED
–V
IN
V
LED
(7)
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
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 LT3518. 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 due to
the current returned though the Schottky diode when the
switch is off. For best reliability, this capacitor should have
low ESR and ESL and have an adequate ripple current
rating. The RMS input current is:
I
IN(RMS)
=I
LED
•(1–D)•D
(8)
where D is the switch duty cycle. A 2.2µF ceramic type
capacitor is usually suffi cient.
Output Capacitor Selection
The selection of output capacitor depends on the load
and converter confi guration, i.e., step-up or step-down.
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. 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 capacitor is usually
suffi cient. Very high performance PWM dimming appli-
cations may require a larger capacitor value to support
the LED voltage during PWM transitions.