LT3724
PIN FUNCTIONS
V
CC
(Pin 12):
The V
CC
pin is the internal bias supply
decoupling node. Use a low ESR 1µF ceramic capacitor
to decouple this node to PGND. Most internal IC func-
tions are powered from this bias supply. An external
diode connected from V
CC
to the BOOST pin charges the
bootstrapped capacitor during the off-time of the main
power switch. Back driving the V
CC
pin from an external
DC voltage source, such as the V
OUT
output of the buck
regulator supply, increases overall efficiency and reduces
power dissipation in the IC. In shutdown mode this pin
sinks 20µA until the pin voltage is discharged to 0V.
NC (Pin 13):
No Connection.
SW (Pin 14):
In step-down applications the SW pin is
connected to the cathode of an external clamping Schottky
diode, the source of the power MOSFET and the induc-
tor. The SW node voltage swing is from V
IN
during the
on-time of the power MOSFET, to a Schottky voltage drop
below ground during the off-time of the power MOSFET.
In start-up and in operating modes where there is insuf-
ficient inductor current to freewheel the Schottky diode, an
internal switch is turned on to pull the SW pin to ground
so that the BOOST pin capacitor can be charged. Give
careful consideration in choosing the Schottky diode to
limit the negative voltage swing on the SW pin.
TG (Pin 15):
The TG pin is the bootstrapped gate drive
for the top N-Channel MOSFET. Since very fast high cur-
rents are driven from this pin, connect it to the gate of
the power MOSFET with a short and wide, typically 0.02”
width, PCB trace to minimize inductance.
BOOST (Pin 16):
The BOOST pin is the supply for the
bootstrapped gate drive and is externally connected to a
low ESR ceramic boost capacitor referenced to SW pin.
The recommended value of the BOOST capacitor, C
BOOST
,
is 50 times greater than the total input capacitance of the
topside MOSFET. In most applications 0.1µF is adequate.
The maximum voltage that this pin sees is V
IN
+ V
CC
,
ground referred.
Exposed Pad (SGND) (Pin 17):
The exposed leadframe is
internally connected to the SGND pin. Solder the exposed
pad to the PCB ground for electrical contact and optimal
thermal performance.
3724fd
7
LINER [ LINEAR TECHNOLOGY ]
