LT3988
6
3988f
pin FuncTions
BD: Internal boost diodes are connected between the BD
pin and the BOOST pins. Connect BD to a 3V or higher
supply, such as V
OUT
.
BOOST1, BOOST2: The BOOST pins are used to provide
drive voltages, higher than the input voltage, to the internal
NPN power switches.
DA1, DA2: Tie the DA pin to the anode of the external
Schottky catch diode. If the DA pin current exceeds 1.32A,
which could occur in an overload or short-circuit condi-
tion, switching is disabled until the DA pin current falls
below 1.32A.
EN/UVLO: This pin is used to shut down the LT3988. It can
be driven from a logic level, tied directly to the input, or
used as an undervoltage lockout by connecting a resistor
divider from V
IN1
.
FB1, FB2: The LT3988 regulates each feedback pin to 0.750V.
Connect the feedback resistor divider taps to these pins.
GND: The exposed pad metal of the package provides 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.
RT: The RT pin is used to set the internal oscillator fre-
quency. Tie a resistor from RT to GND for the desired
switching frequency.
SYNC: To synchronize the part to an external frequency,
drive the SYNC pin with a logic-level signal with positive
and negative pulse widths of at least 100ns. If the SYNC
function is not used, connect the SYNC pin to ground. If
using SYNC, minimize coupling to RT and FB2, and add
decoupling capacitors as needed up to 22pF.
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. Leave
these pins disconnected if unused.
V
IN1
: The V
IN1
pin supplies current to the LT3988 internal
circuitry and to the internal power switch connected to
SW1 and must be locally bypassed. V
IN1
must be greater
than 3.9V (typ) for channel 1 or channel 2 to operate.
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 channel
2 is coming from a different source. V
IN2
must be greater
than 2.6V (typ) and V
IN1
must be greater than 3.9V (typ)
for channel 2 to operate. If V
IN2
is driven above 60V, V
IN2
must be connected to V
IN1
.
Typical perForMance characTerisTics
V
IN1
Overvoltage Lockout
vs Temperature
TRACK/SS Pin Current
vs Temperature
TEMPERATURE (°C)
–50
IN1
64
63
62
61
0 50–25 25 100
3988 G18
15075 125
V
IN1
RISING
V
IN1
FALLING
TEMPERATURE (°C)
–50
TRACK/SS PIN CURRENT (µA)
1.38
1.34
1.26
1.30
1.22
0 50–25 25 100
3988 G19
15075 125
