LT3517
APPLICATIONS INFORMATION
than 1V, the LED current is:
VCTRL
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:
ILED
=
(2)
10 •RSENSE
R1
R2
When V
to be:
is higher than 1V, the LED current is clamped
CTRL
VOUT
=
•1.01V + VBE(Q2)
(5)
100mV
RSENSE
ILED
=
(3)
+
R1
R
SENSE
V
OUT
LED
ARRAY
The LED current programming feature possibly increases
total dimming range by a factor of ten.
–
Q2
LT3517
FB
The CTRL pin should not be left open (tie to V
if not
REF
R2
used). The CTRL pin can also be used in conjunction with
a PTC thermistor to provide overtemperature protection
for the LED load.
3517 F04
Figure 4
Inductor Selection
2V
V
REF
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:
45.3k
49.9k
CTRL
5k
PTC
3517 F02
Figure 2
DBUCK • tSW(µs)• V – V
(
)
LED
IN
L µH =
( )
(6)
ΔI
Setting Output Voltage
VLED
VIN
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:
DBUCK
=
V
is the voltage across the LED string, V is the input
IN
LED
R1
R2
⎛
⎞
⎠
voltage to the converter, and t is the switching period.
VOUT
=
+1 •1.01V
(4)
SW
⎜
⎝
⎟
In the boost configuration, the inductor can be estimated
using the formula:
V
OUT
DBOOST • tSW(µs)• VIN
L µH =
( )
(7)
R1
LT3517
ΔI
FB
VLED – VIN
R2
DBOOST
=
3517 F03
VLED
Figure 3
3517fa
9
Linear [ Linear ]
