LT3519 LINER [Linear Technology], LT3519 Datasheet - Page 10

• Manufacturer Part Number: LT3519
• Description: LED Driver with Integrated Schottky Diode
• Manufacturer: LINER [Linear Technology]

LT3519
APPLICATIONS INFORMATION
Programming the Turn-On and Turn-Off Thresholds
with the SHDN/UVLO Pin
The falling SHDN/UVLO value can be accurately set by
the resistor divider. A small 2μA pull-down current is ac-
tive when SHDN/UVLO is below the 1.22V threshold. The
purpose of this current is to allow the user to program
the rising hysteresis. The following equations should be
used to determine the values of the resistors:
Inductor Selection
The inductor used with the LT3519 should have a saturation
current rating of 1A or greater. For buck mode LED driv-
ers, 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:
V
voltage to the converter, and f
In the boost confi guration, the inductor can be estimated
using the formula:
10
LED
L µH
D
L µH
D
V
V
BUCK
BOOST
IN FALLING
IN RISING
( )
( )
is the voltage across the LED string, V
(
(
Figure 3. SHDN/UVLO Threshold Programming
=
=
=
=
D
f
V
f
OSC
)
V
OSC
(
BUCK
LED
=
IN
)
V
D
=
LED
2
BOOST
(MHz) • 0.15A
V
LT3519
µA R
R
(MHz) • 0.15A
LED
• V
1
– V
R
(
+
•
2
R
IN
IN
1 1 + V
SHDN/UVLO
• V
2
)
– V
• .
IN
1 22
OSC
IN FALLING
LED
(
is the switching frequency.
V
)
µH • A • MHz
µH • A • MHz
V
IN
R1
R2
3519 F03
)
V
V
IN
is the input
Table 1. Recommended Inductor Vendors
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. For best
reliability, this capacitor should have low ESR and ESL and
have an adequate ripple current rating. A 2.2μF ceramic
type capacitor is usually suffi cient for LT3519.
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
PHONE
[phone]
[phone]
[phone]
[phone]
IN
pin of the LT3519. A
WEB
www.sumida.com
www.toko.com
www.cooperet.com
www.vishay.com
3519f