5
LT1676
PIN FUNCTIONS
UUU
SHDN (Pin 1):
When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regula-
tor and reduces V
IN
input current to a few tens of micro-
amperes (shutdown mode).
When this pin is held above the shutdown mode thresh-
old, but below the lockout threshold, the part will be
operational with the exception that output switching
action will be inhibited (lockout mode). A user-adjustable
undervoltage lockout can be implemented by driving this
pin from an external resistor divider to V
IN
. This action is
logically “ANDed” with the internal UVLO, set at nominally
6.7V, such that minimum V
IN
can be increased above
6.7V, but not decreased (see Applications Information).
If unused, this pin should be left open. However, the high
impedance nature of this pin renders it susceptible to
coupling from the high speed V
SW
node, so a small
capacitor to ground, typically 100pF or so is recom-
mended when the pin is left “open.”
V
CC
(Pin 2): This pin is used to power the internal control
circuitry off of the switching supply output. Proper use of
this pin enhances overall power supply efficiency. During
start-up conditions, internal control circuitry is powered
directly from V
IN
. If the output capacitor is located more
than an inch from the V
CC
pin, a separate 0.1µF bypass
capacitor to ground may be required right at the pin.
V
SW
(Pin 3): This is the emitter node of the output switch
and has large currents flowing through it. This node
moves at a high dV/dt rate, especially when in “boost”
mode. Keep the traces to the switching components as
short as possible to minimize electromagnetic radiation
and voltage spikes.
GND (Pin 4): This is the device ground pin. The internal
reference and feedback amplifier are referred to it. Keep
the ground path connection to the FB divider and the V
C
compensation capacitor free of large ground currents.
V
IN
(Pin 5): This is the high voltage supply pin for the
output switch. It also supplies power to the internal control
circuitry during start-up conditions or if the V
CC
pin is left
open. A high quality bypass capacitor that meets the input
ripple current requirements is needed here. (See Applica-
tions Information.)
SYNC (Pin 6): Pin used to synchronize internal oscillator
to the external frequency reference. It is directly logic
compatible and can be driven with any signal between
10% and 90% duty cycle. The sync function is internally
disabled if the FB pin voltage is low enough to cause
oscillator slowdown. If unused, this pin should be grounded.
FB (Pin 7): This is the inverting input to the feedback
amplifier. The noninverting input of this amplifier is inter-
nally tied to the 1.24V reference. This pin also slows down
the frequency of the internal oscillator when its voltage is
abnormally low, e.g., 2/3 of normal or less. This feature
helps maintain proper short-circuit protection.
V
C
(Pin 8): This is the control voltage pin which is the
output of the feedback amplifier and the input of the
current comparator. Frequency compensation of the over-
all loop is effected by placing a capacitor, (or in most cases
a series RC combination) between this node and ground.
SWOFF
1676 TD01
BOOST
SWON
SWDR
0
V
IN
V
SW
SWOFF
1676 TD02
BOOST
SWON
SWDR
0
V
IN
V
SW
High dV/dt Mode Low dV/dt Mode
TIMING DIAGRAMS
WUW
