Datasheet

LT3519/LT3519-1/LT3519-2
10
3519fa
APPLICATIONS INFORMATION
For open LED protection of a boost type LED driver, set
the resistor from the output to the FB pin such that the
expected V
FB
during normal operation will not exceed
1.1V. For a buck mode or buck-boost mode LED driver,
the output voltage is typically level-shifted to a signal with
respect to GND as illustrated in Figure 2. The open LED
voltage level can be expressed as:
V
OUT
= V
BE(Q1)
+
R1
R2
• 1.22V
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 2.2μA pull-down current is
active 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:
V
IN(FALLING)
=
R1+R2
R2
• 1.22V
V
IN(RISING)
= 2.2µA •R1+ V
IN(FALLING)
R1
R
SENSE(EXT)
LED
ARRAY
V
OUT
R2
100k
–
+
3519 F02
FB
Q1
LT3519/
LT3519-1/
LT3519-2
Figure 2. Open LED Protection FB Resistor Connector
for Buck Mode or Buck-Boost Mode LED Driver
R1
V
IN
R2
3519 F03
LT3519/
LT3519-1/
LT3519-2
SHDN/UVLO
Figure 3. SHDN/UVLO Threshold Programming