lt3517 Linear Technology Corporation, lt3517 Datasheet - Page 9

• Manufacturer Part Number: lt3517
• Description: Full-featured Led Driver With 1.5a Switch Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Setting Output Voltage
For a boost application, the output voltage can be set by
selecting the values of R1 and R2 (see Figure 3) according
to the following equation:
than 1V, the LED current is:
When V
to be:
The LED current programming feature possibly increases
total dimming range by a factor of ten.
The CTRL pin should not be left open (tie to V
used). The CTRL pin can also be used in conjunction with
a PTC thermistor to provide overtemperature protection
for the LED load.
I
V
LED
I
LED
OUT
=
CTRL
=
=
10 •
R
100
SENSE
R2
R1
is higher than 1V, the LED current is clamped
V
CTRL
R
mV
V
+ 1 • 1.01V
SENSE
REF
LT3517
45.3k
5k
PTC
Figure 2
Figure 3
FB
2V
V
OUT
R1
R2
3517 F03
3517 F02
49.9k
CTRL
REF
if not
(4)
(2)
(3)
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 LT3517 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:
L µH
D
LED
( )
BOOST
L µH
D
V
OUT
( )
BUCK
is the voltage across the LED string, V
=
=
=
D
=
=
BOOST
R
R
V
D
LED
2
1
V
BUCK
V
LED
V
• .
LT3517
IN
LED
1 01
– V
• t
FB
• t
SW
ΔI
IN
V V
SW
Q2
+
(µs) • V
(µs) • V
Figure 4
R1
R2
3517 F04
BE Q
ΔI
(
+
–
V
SW
2
OUT
IN
(
)
IN
is the switching period.
– V
R
SENSE
LED
LED
ARRAY
)
LT3517
IN
is the input
9
3517f
(5)
(6)
(7)