LT3791
20
3791fb
For more information www.linear.com/LT3791
SHORTLED Pin
The LT3791 provides an open-drain status pin,
SHORTLED, which
pulls low when the FB pin is below
400mV. The only time the FB pin will be below 400mV
is during start-up or if the LEDs are shorted. During
start-up the LT3791 ignores the voltage on the FB pin
until the soft-start capacitor reaches 1.75V. To prevent
false tripping after startup, a large enough soft-start
capacitor must be used to allow the output to get up to
approximately 40% to 50% of the final value.
OPENLED Pin
The LT3791 provides an open-drain status pin, OPENLED,
which pulls low when the FB pin is above 1.15V and the
voltage across V
(ISP-ISN)
is less than 10mV. If the open
LED clamp voltage is programmed correctly using the FB
pin, then the FB pin should never exceed 1.1V when the
LEDs are connected. Therefore, the only way for the FB
pin to exceed 1.15V is for an open LED event to occur.
Soft-Start, Fault Function
Soft-start reduces the input power sources’ surge currents
by gradually increasing the controller’s current limit (pro
-
portional to
an internally buffered clamped equivalent of
V
C
). The soft-start interval is set by the soft-start capacitor
selection according to the following equation
t
SS
=
14µA
• C
SS
Make sure C
SS
is large enough when there is loading
during start-up.
The SS pin is also used as a fault timer. Once an open
LED or a shorted LED fault is detected, a 1.4µA pull-
down current
source is activated. With a 500k pull-up
resistor to V
REF
on the SS pin, the LT3791 will latch off
until the EN/UVLO pin is toggled. Without any resistor
to V
REF
the SS pin enters a hiccup mode operation. The
1.4µA pulls SS down until 0.2V is reached, at which
point
the 14µ
A pull-up current source turns on. If the
fault condition
hasn’t been removed when SS reaches
1.75V, then the 1.4µA pull-down current source turns on
again initiating a new cycle. This will continue until the
fault is removed.
Loop Compensation
The LT3791 uses an internal transconductance error
amplifier whose V
C
output compensates the control loop.
The external inductor, output capacitor and the comp-
ensation resistor
and
capacitor determine the loop
stability.
The
inductor and output capacitor are chosen based on
performance, size and cost. The compensation resis
-
tor and capacitor at V
C
are set to optimize control loop
response and
stability. For typical LED applications, a
10nF compensation
capacitor at V
C
is adequate, and a
series resistor should always be used to increase the
slew rate on the V
C
pin to maintain tighter regulation of
LED current during fast transients on the input supply of
the converter.
applicaTions inForMaTion
