LTC3783 LINER [Linear Technology], LTC3783 Datasheet - Page 9

• Manufacturer Part Number: LTC3783
• Description: Full-Featured LED Driver with 1.5A Switch Current
• Manufacturer: LINER [Linear Technology]

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
100mV
R
⎛
⎜
⎝
SENSE
R2
R1
is higher than 1V, the LED current is clamped
V
CTRL
+ 1
SENSE
V
REF
⎞
⎟ • 1.01V
⎠
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 configuration, 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 configuration, 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
R2
V
R1
D
LED
V
V
BUCK
V
LED
• 1.01V + V
IN
LT3517
LED
– V
• t
FB
• t
IN
ΔI
SW
SW
Q2
(μs) • V
(μs) • V
Figure 4
R1
R2
3517 F04
BE(Q1)
ΔI
+
–
V
SW
OUT
IN
(
IN
is the switching period.
– V
R
SENSE
LED
LED
ARRAY
)
LT3517
IN
is the input
3517ff
9
(5)
(6)
(7)