Datasheet

LT3796/LT3796-1
22
3796fa
For more information www.linear.com/3796
of approximately 20mV magnitude. The following equations
are useful to estimate the inductor value (T
SW
= 1/f
OSC
):
L
BUCK
=
T
SW
• R
SENSE
• V
LED
(V
IN
– V
LED
)
V
IN
• 0.02V
L
BUCK, BOOST
=
T
SW
• R
SENSE
• V
LED
• V
IN
(V
LED
+V
IN
)• 0.02V
L
BOOST
=
T
SW
• R
SENSE
• V
IN
(V
LED
– V
IN
)
V
LED
• 0.02V
Table 6 provides some recommended inductor vendors.
Table 6. Inductor Manufacturers
VENDOR WEB
Sumida www.sumida.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Vishay www.vishay.com
Coilcraft www.coilcraft.com
Loop Compensation
The LT3796/LT3796-1 use an internal transconductance
error amplifier whose V
C
output compensates the con-
trol 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 V
C
are selected to optimize control
loop response and stability. For typical LED applications,
a 22nF 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 to
the converter.
Soft-Start Capacitor Selection
For many applications, it is important to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltage overshoot. The soft-start interval is set by the
soft-start capacitor selection according to the equation:
T
SS
= C
SS
•
2V
28
µ
A
A typical value for the soft-start capacitor is 0.1µF. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch. Soft-start also operates
as fault protection, which forces the converter into hiccup
or latchoff mode. Detailed information is provided in the
Fault Protection: Hiccup Mode and Latchoff Mode section.
Fault Protection: Hiccup Mode and Latchoff Mode
If an LED overcurrent condition, INTV
CC
undervoltage, or
thermal limit happens, an open-drain pull-down on FAULT
asserts. The TG pin is pulled high to disconnect the LED
array from the power path, and the GATE pin is driven low.
If the soft-start pin is charging and still below 1.7V, then
it will continue to do so with a 28µA source. Once above
1.7V, the pull-up source is disabled and a 2.8µA pull-down
is activated. While the SS pin is discharging, the GATE is
forced low. When SS pin is discharged below 0.2V, a new
cycle is initiated. This is referred as hiccup mode operation.
If the fault still exists when SS crosses below 0.2V, then
a full SS charge/discharge cycle has to complete before
switching is enabled and the FAULT flag is deasserted.
If a resistor is placed between V
REF
pin and SS pin to hold
SS pin higher than 0.2V during a fault, then the LT3796/
LT3796-1 will enter latchoff mode with GATE pin low, TG
pin high and FAULT pin low. To exit latchoff mode, the EN/
UVLO pin must be toggled low to high.
APPLICATIONS INFORMATION