Datasheet

LT3796/LT3796-1
16
3796fa
For more information www.linear.com/3796
Open LED Detection
The LT3796/LT3796-1 provide an open-collector status pin,
VMODE, that pulls low when the FB1 pin is above 1.19V and
V
(ISP-ISN)
is less than 25mV. If the open LED clamp volt-
age is programmed correctly using the resistor divider,
then the FB1 pin should never exceed 1.15V when LEDs
are connected, therefore, the only way for the FB1 pin to
be within 60mV of the 1.25V regulation voltage is for an
open LED event to have occurred.
LED Over Current Protection Feature
The ISP and ISN pins have a short-circuit protection feature
independent of the LED current sense feature. This feature
prevents the development of excessive switching currents
and protects the power components. The short-circuit
protection threshold (375mV, typ) is designed to be 50%
higher than the default LED current sense threshold. Once
the LED over current is detected, the GATE pin is driven
to GND to stop switching, and TG pin is pulled high to
disconnect the LED array from the power path.
A typical LED short-circuit protection scheme for boost
or buck-boost mode converter is shown in Figure 4. The
Schottky diode D2 should be put close to the drain of
M2 on the board. It protects the LED
+
node from swing-
ing well below ground when being shorted to ground
through a long cable. Usually, the internal protection loop
takes about 1µs to respond. Including PNP helper Q1 is
recommended to limit the transient short-circuit current.
With the PNP helper, the short-circuit current can be
limited to 2A, whereas the short-circuit current can reach
to 20A without the PNP helper as shown in Figure 5 and
Figure 6 respectively. Refer to boost LED driver with output
short-circuit protection and LED current monitor for the
test schematic. Note that the impedance of the short-circuit
cable affects the peak current.
APPLICATIONS INFORMATION
Figure 4. The Simplified LED Short-Circuit Protection
Schematic for Boost/Buck-Boost Mode LED Driver
Figure 5. Short-circuit Current without PNP Helper
LT3796/
LT3796-1
37961 F04
Q1
R
SNS
R
LED
LED
+
GND (BOOST) OR
V
IN
(BUCK-BOOST MODE)
LED
STRING
D2
ISP
GATE
SENSE
V
IN
V
IN
C1
M1
M2
ISN
TG
C2
L1
D1
I
M2
10A/DIV
FAULT
10V/DIV
37961 F05
1µs/DIV
LED
+
50V/DIV
Figure 3b. Feedback Resistor Connection for Buck Mode or
Buck-Boost Mode LED Driver Using External PNP
LT3796/
LT3796-1
37961 F03b
R10
Q1
LED
ARRAY
V
OUT
FB1
R9
R
SENSE
–
+