LT3755-1 LINER [Linear Technology], LT3755-1 Datasheet - Page 20

• Manufacturer Part Number: LT3755-1
• Description: 60V 4-Switch Synchronous
• Manufacturer: LINER [Linear Technology]

LT3791
APPLICATIONS INFORMATION
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
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
VC). The soft-start interval is set by the soft-start capacitor
selection according to the following equation
Make sure C
during start-up.
20
t
SS
=
14µA
1.2V
SS
•C
is large enough when there is loading
(ISP-ISN)
SS
is less than 10mV. If the open
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 100k pull-up resistor
to V
normally. With a 500k pull-up resistor to V
pin, the LT3791 will latch off until the EN/UVLO pin is
toggled. Without any resistor to V
a hiccup mode operation. The 1.4µA pulls SS down until
0.2V is reached, at which point the 14µA 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 cur-
rent 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 ampli-
fier whose VC output compensates the control loop. The
external inductor, output capacitor and the compensation
resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at VC are set to optimize control loop re-
sponse and stability. For typical LED applications, a 10nF
compensation capacitor at VC is adequate, and a series
resistor should always be used to increase the slew rate
on the VC pin to maintain tighter regulation of LED current
during fast transients on the input supply of the converter.
REF
on the SS pin, the LT3791 will continue to switch
REF
the SS pin enters
REF
on the SS
3791f