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LT3517
APPLICATIONS INFORMATION
I LED =
than1V,theLEDcurrentis:
V CTRL
10 ? R SENSE
(2)
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:
When V CTRL is higher than 1V, the LED current is clamped
to be:
V OUT =
R1
R2
? 1.01V + V BE(Q1)
(5)
I LED =
100mV
R SENSE
(3)
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 REF if not
used). The CTRL pin can also be used in conjunction with
a PTC thermistor to provide overtemperature protection
for the LED load.
LT3517
FB
Q2
+
R1
V OUT
–
R2
3517 F04
Figure 4
R SENSE
LED
ARRAY
V REF
45.3k
5k
PTC
2V
49.9k
CTRL
3517 F02
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 OUT = ?
+ 1 ? ? 1.01V
? R1 ?
? R2
?
L ( ) =
D BUCK ? t SW (μs) ? ( V IN – V LED )
D BUCK =
Figure2
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:
(4)
μH (6)
ΔI
V LED
V IN
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 configuration, the inductor can be estimated
using the formula:
L ( μH ) =
D BOOST =
LT3517
FB
V OUT
R1
R2
D BOOST ? t SW (μs) ? V IN
ΔI
V LED – V IN
V LED
(7)
3517 F03
Figure 3
3517fg
For more information www.linear.com/LT3517
9
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