LT3518 LINER [Linear Technology], LT3518 Datasheet - Page 11

• Manufacturer Part Number: LT3518
• Description: Full-Featured LED Driver with 2.3A Switch Current
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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:
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:
V
voltage to the converter, and t
In the boost confi guration, the inductor can be estimated
using the formula:
LED
L µH
D
L µH
D
V
OUT
BUCK
BOOST
( )
( )
is the voltage across the LED string, V
=
=
=
=
R2
R1
=
D
D
V
V
BUCK
BOOST
V
LED
LT3518
IN
• 1.01V + V
LED
V
FB
LED
– V
• t
• t
SW
IN
SW
I
R1
R2
3518 F04
(µs) • V
Figure 4
BE(Q1)
(µs) • V
+
–
I
V
OUT
SW
(
IN
IN
is the switching period.
– V
R
SENSE
LED
LED
ARRAY
)
IN
is the input
(5)
(6)
(7)
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
VENDOR
Sumida
Toko
Cooper
Vishay
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
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:
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.
I
IN(RMS)
=I
LED
• (1– D) • D
PHONE
[phone]
[phone]
[phone]
[phone]
IN
pin of the LT3518. A
WEB
www.sumida.com
www.toko.com
www.cooperet.com
www.vishay.com
LT3518
11
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(8)