LT3754
10
3754fc
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 LT3754 constant switching frequency is program-
mable 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 switch-
ing 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 1.1V reference. V
OUT
is regulated to ensure
the lowest LED pin voltage of any connected LED string
is maintained at 1.1V. If any of the LED strings are open,
the LT3754 regulation loop 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 LT3754 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 LT3754 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 LT3754 monitors system
conditions and performs soft-start for start-up after any of
the following faults: V
IN
, SHDN or INTV
CC
voltages too low or
MOSFET current too high. The LT3754, when entering these
faults, discharges an internal soft-start node and prevents
switching at the GATE pin. When exiting these faults the
LT3754 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 LT3754 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 LT3754 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 LT3754 also allows its own junction temperature to
be monitored and regulated by derating LED currents
when a junction temperature programmed by the T
SET
pin is exceeded.
