Datasheet

LT3478/LT3478-1
7
34781f
PI FU CTIO S
UUU
SW (Pins 1, 2): Switch Pin. Collector of the internal NPN
power switch. Both pins are fused together inside the IC.
Connect the inductor and diode here and minimize the
metal trace area connected to this pin to minimize EMI.
V
IN
(Pin 3): Input Supply. Must be locally bypassed with
a capacitor to ground.
V
S
(Pin 4): Inductor Supply. Must be locally bypassed
with a capacitor to ground. Can be shorted to V
IN
if only
one supply is available (see L (Pin 5) function).
L (Pin 5): Inductor Pin. An internal resistor between V
S
and L pins monitors inductor current to protect against
inrush current. Exceeding 6A immediately turns off the
internal NPN power switch and discharges the soft-start
pin. Input current monitoring can be disabled by connect-
ing the inductor power supply directly to the L pin and
leaving the V
S
pin open (requires local bypass capacitor
to GND on L pin; not V
S
pin).
V
OUT
(Pin 6): Output voltage of the converter. Connect a
capacitor from this pin to ground. Internal circuitry moni-
tors V
OUT
for protection against open LED faults.
LED (Pin 7): Connect the LED string from this pin to
ground. An internal (LT3478-1)/external (LT3478) resistor
between the V
OUT
and LED pins senses LED current for
accurate control.
OVPSET (Pin 8): Programs V
OUT
overvoltage protection
level (OVP) to protect against open LED faults. OVP =
(OVPSET • 41)V. OVPSET range is 0.3V to 1V for an OVP
range of typically 12.3V to 41V.
V
C
(Pin 9): Output of the transconductance error amplifi er
and compensation pin for the converter regulation loop.
V
REF
(Pin 10): Bandgap Voltage Reference. This pin can
supply up to 100µA. Can be used to program CTRL1,
CTRL2, OVPSET pin voltages using resistor dividers to
ground.
⎯
S
⎯
H
⎯
D
⎯
N (Pin 11): The
⎯
S
⎯
H
⎯
D
⎯
N pin has an accurate 1.4V
threshold and can be used to program an undervoltage
lockout (UVLO) threshold for system input supply using a
resistor divider from supply to ground. A 10µA pin current
hysteresis allows programming of undervoltage lockout
(UVLO) hysteresis.
⎯
S
⎯
H
⎯
D
⎯
N above 1.4V turns the part on
and removes a 10µA sink current from the pin.
⎯
S
⎯
H
⎯
D
⎯
N = 0V
reduces V
IN
current < 3µA.
⎯
S
⎯
H
⎯
D
⎯
N can be directly connected
to V
IN
. If left open circuit the part will be turned off.
CTRL1 (Pin 12): CTRL1 pin voltage is used to program
maximum LED current (CTRL2 = V
REF
). CTRL1 voltage
can be set by a resistor divider from V
REF
or an external
voltage source. Maximum LED current is given by:
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
()
(,.)•
.
LT Max LED Current
Min CTRL
R
SE
3478
105
01
=
NNSE
Amps
(linear for 0.1V < CTRL1< 0.95V ; CTRL2 = V
REF
) For maxi-
mum LED current, short CTRL1 and CTRL2 pins to V
REF
.
CTRL2 (Pin 13): The CTRL2 pin is available for program-
ming a decrease in LED current versus temperature
(setting temperature breakpoint and slope). This feature
allows the output LED(s) to be programmed for maximum
allowable current without damage at higher temperatures.
This maximizes LED usage and increases reliability. A
CTRL2 voltage with negative temperature coeffi cient is
created using an external resistor divider from V
REF
with
temperature dependant resistance. If not used, CTRL2
should be tied to V
REF
.
PWM (Pin 14): Input pin for PWM dimming control. Above
1V allows converter switching and below 1V disables
switching with V
C
pin level maintained. With an external
MOSFET placed in series with the ground side of the LED
string, a PWM signal driving the PWM pin and MOSFET
gate provides accurate dimming control. The PWM signal
can be driven from 0V to 15V. If unused, the pin should
be connected to V
REF
.
R
T
(Pin 15): A resistor to ground programs switching
frequency between 200kHz and 2.25MHz.
SS (Pin 16): Soft-Start Pin. Placing a capacitor here pro-
grams soft-start timing to limit inductor inrush current
during start-up due to the converter. When inductor current