LT3760
10
3760fc
operaTion
to define the turn on/off voltages for V
IN
. SHDN/UVLO pin
current switches from 2.4µA to 0µA when SHDN/UVLO
pin voltage exceeds 1.476V.
The LT3760 constant switching frequency is programmable
from 100kHz up to 1MHz using a single resistor at the RT
pin to ground. A SYNC pin is also provided to allow an
external clock to define the converter switching frequency.
The GATE output provides a ±0.8A peak gate drive for an
external N-channel power MOSFET to generate a boosted
output voltage V
OUT
using a single inductor, Schottky
diode and output capacitor. With LED strings connected
from V
OUT
to every LED pin, the lowest voltage on each
LED pin is monitored and compared to an internal 1V
reference. V
OUT
is regulated to ensure the lowest LED pin
voltage of any connected LED string is maintained at 1V.
If any of the LED strings are open, the LT3760 will ignore
the open LED pin. If all of the LED strings are open V
OUT
charges up until a user programmable OVP (overvoltage
protection) level is reached. This programmable OVP level
allows the user to protect against LED damage when the
LED strings
are opened and then reconnected.
Since
the LT3760 boost controller uses a current mode
topology, the V
C
pin voltage determines the peak current
in the inductor of the converter and hence the duty cycle
of the GATE switching waveform. The basic loop uses a
pulse from an internal oscillator to set an RS flip-flop and
turn on the external power MOSFET. Current increases
in the MOSFET and inductor until the V
C
commanded
peak switch current is exceeded and the MOSFET is then
turned off. Inductor current is sensed during the GATE on
period by a sense resistor RS in the source of the external
N-channel power MOSFET. As with all current mode
converters, slope compensation is added to the control
path to ensure stability for duty cycles above 50%. Any
over current fault condition in the MOSFET turns off the
MOSFET and triggers soft start internally. In this fault mode
the LT3760 only allows MOSFET turn-on approximately
every 2ms. This hiccup mode significantly reduces the
power rating required for the MOSFET.
LED current programming and dimming can be achieved
using the I
SET
, CTRL and PWM pins. A single resistor at
the I
SET
pin programs LED current. Analog dimming of
LED brightness is achieved using the CTRL pin below 1V.
PWM dimming of LED brightness is achieved by control-
ling the duty cycle of the PWM pin.
For robust operation the LT3760 monitors system
conditions and performs soft start for startup after any of
the following faults: V
IN
, SHDN or INTV
CC
voltages too low
or MOSFET current too high. The LT3760, when entering
these faults, discharges an internal soft start node and
prevents switching at the GATE pin. When exiting these
faults the LT3760 ramps up an internal soft start node to
control V
C
pin voltage rise and hence control MOSFET peak
switch current rise. In addition the soft start period gradually
ramps up switching frequency from approximately 33%
to 100% of full scale.
The LT3760 monitors each LED pin voltage. If the LED
string has an open fault (V(LED
X
)<0.5V) the FAULT flag
is pulled low.
For LED protection, the LT3760 CTRL pin allows an LED
current derating curve to be programmed versus the
ambient temperature of the LED strings. An NTC resistor
placed close to the LEDs decreases CTRL pin voltage and
hence decreases LED current as LED ambient temperature
increases.
The
LT3760 also allows it’s own junction temperature
to be monitored and regulated by derating LED currents
when a junction temperature programmed by the T
SET
pin is exceeded.
