LT3508
6
3508fd
PIN FUNCTIONS
BOOST1, BOOST2: The BOOST pins are used to provide
drive voltages, higher than the input voltage, to the internal
NPN power switches. Tie through a diode to a 2.8V or
higher supply, such as V
OUT
or V
IN
.
Exposed Pad: The exposed pad metal of the package pro-
vides both electrical contact to ground and good thermal
contact to the printed circuit board. The exposed pad must
be soldered to the circuit board for proper operation.
FB1, FB2: The LT3508 regulates each feedback pin to
0.800V. Connect the feedback resistor divider taps to
these pins.
GND: Tie the GND pins directly to the Exposed Pad and
ground plane.
PG1, PG2: The power good pins are the open-collector
outputs of an internal comparator. PG remains low until
the FB pin is within 10% of the fi nal regulation voltage.
As well as indicating output regulation, the PG pins can
be used to sequence the two switching regulators. These
pins can be left unconnected. The PG outputs are valid
when V
IN1
is greater than 3.7V and SHDN is high. The PG
comparators are disabled in shutdown.
RT/SYNC: The RT/SYNC pin is used to set the internal
oscillator frequency. Tie a 33.2k resistor from RT/SYNC
to GND for a 1MHz switching frequency. To synchronize
the part to an external frequency, drive the RT/SYNC pin
with a logic-level signal with positive and negative pulse
widths of at least 80ns.
SHDN: The shutdown pin is used to put the LT3508 in
shutdown mode. Pull the pin below 0.3V to shut down the
LT3508. The 2.63V threshold can function as an accurate
undervoltage lockout (UVLO), preventing the regulator
from operating until the input voltage has reached the
programmed level. Do not drive SHDN more than 6V
above V
IN1
.
SW1, SW2: The SW pins are the outputs of the internal
power switches. Connect these pins to the inductors, catch
diodes and boost capacitors.
TRACK/SS1, TRACK/SS2: The TRACK/SS pins are used
to soft-start the two channels, to allow one channel to
track the other output, or to allow both channels to track
another output. For tracking, tie a resistor divider to this
pin from the tracked output. For soft-start, tie a capacitor
to this pin. An internal 1.2µA soft-start current charges
the capacitor to create a voltage ramp at the pin. In the
TSSOP package, if these pins are unused, tie a 47pF or
greater capacitor from each pin to GND.
V
C1
, V
C2
: The V
C
pins are the outputs of the internal error
amps. The voltages on these pins control the peak switch
currents. These pins are normally used to compensate the
control loops, but can also be used to override the loops.
Pull these pins to ground with an open drain to shut down
each switching regulator separately.
V
IN1
: The V
IN1
pin supplies current to the LT3508 internal
circuitry and to the internal power switch connected to
SW1 and must be locally bypassed. V
IN1
must be greater
than 3.7V for channel 1 or channel 2 to operate. If V
IN1
is greater than 3.7V, channel 2 can operate with V
IN2
as
low as 3V.
V
IN2
: The V
IN2
pin supplies current to the internal power
switch connected to SW2 and must be locally bypassed.
Connect this pin directly to V
IN1
unless power for chan-
nel 2 is coming from a different source. V
IN2
must be
greater than 3V and V
IN1
must be greater than 3.7V for
channel 2 to operate.
