LT3754
8
3754fc
pin FuncTions
LED
x
(Pin 1-8,17-24): 16 LED Driver Outputs. Each output
contains an open collector constant current sink. LED
currents are programmable from 10mA to 50mA using a
single resistor at the I
SET
pin. Connect the cathode of each
LED string to an LED pin. Connect the anode of each LED
string to V
OUT
. Channels can be paralleled for greater LED
current or individually disabled (connect LED to V
OUT
).
SENSE (Pin 9): The Current Sense Input for the Control
Loop. Connect this pin to the sense resistor in the source
of the external power MOSFET.
GATE (Pin 10): Drives the gate of an N-channel MOSFET
from 0V to INTV
CC
.
INTV
CC
(Pin 11): A 7V LDO supply generated from V
IN
and
used to power the GATE driver and some control circuitry.
Must be bypassed with a 4.7µF capacitor to GND.
V
IN
(Pin 12): Input Supply Pin. Must be locally bypassed
with a 1µF capacitor to ground.
SHDN/UVLO (Pin 13): The SHDN/UVLO pin has an accurate
1.476V threshold and can be used to program an under volt-
age lockout (UVLO) threshold for system input supply using
a resistor divider from supply to ground. A 2.4µA pin cur-
rent
hysteresis allows programming of UVLO hysteresis.
SHDN/UVLO above 1.476V turns the part on and removes
a 2.4µA sink current from the pin. SHDN/UVLO < 0.7V
reduces V
IN
current < 20µA. If the shutdown function is
not required, it should be forced above 1.476V or con-
nected directly to V
IN
.
FAULT (Pin 14): Active low if any or all LED strings have
an open fault. If fault(s) removed, FAULT flag returns high.
Fault status is only updated during PWM high state and
latched during PWM low.
SYNC (Pin 15): Allows synchronization of boost converter
switching frequency to an external clock. RT resistor should
be programmed for f
OSC
20% below SYNC frequency. If
unused, connect to GND.
V
OUT
(Pin16): Boosted Output Voltage of the Converter.
Connect a capacitor from this pin to ground. Connect the
anode of each LED (string) to V
OUT
.
RT (Pin 25): A resistor to ground programs switching
frequency f
OSC
between 0.1MHz and 1MHz.
V
C
(Pin 26): Output of Both Transconductance Error
Amplifiers for the Converter Regulation Loop. The most
commonly used gm error amplifier (LED) regulates V
OUT
to ensure no LED pin falls below 1.1V. The other gm error
amplifier (OVP) is activated if all
LEDs fail open and a
regulated
maximum V
OUT
is required. Connect a resistor
and capacitor in series from the V
C
pin to ground.
PWM (Pin 27): Input Pin for PWM Dimming Control. Above
1.4V allows converter switching and below 0.7V disables
switching. The PWM signal can be driven from 0V to 6V.
If unused, connect to V
REF.
OVP
SET
(Pin 28): Programs maximum allowed V
OUT
regulation level if all LEDs are open circuit.
CTRL (Pin 29): CTRL pin voltage below 1V controls
maximum LED current. CTRL voltage can be set by a
resistor divider from V
IN
, V
REF
or an external voltage source.
LED current derating versus temperature is achievable
if the voltage programmed at the CTRL pin has a negative
temperature coefficient using an external resistor divider
from V
REF
pin with temperature dependent resistance.
T
SET
(Pin 30): Programs LT3754 junction temperature
breakpoint past which LED current will begin to derate.
V
REF
(Pin 31): 1.485V Reference Output Pin. This pin can
supply up to 150µA. Can be used to program CTRL, T
SET
and OVP
SET
pin voltages using resistor dividers to ground.
I
SET
(Pin 32): Resistor to Ground Programs LED pin cur-
rent. See Table 6 in the Applications Information Section.
Exposed Pad (Pin 33): GND. The ground for the IC and
the converter. The package has an exposed pad (Pin 33)
underneath the IC which is the best path for heat out of
the package. Pin 33 should be soldered to a continu-
ous copper ground plane under the device to reduce
die temperature and increase the power capability of
the LT3754.
