LT1676 Linear Technology, LT1676 Datasheet - Page 5

• Manufacturer Part Number: LT1676
• Description: Wide Input Range/ High Efficiency/ Step-Down Switching Regulator
• Manufacturer: Linear Technology

PIN
SHDN (Pin 1): When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regula-
tor and reduces V
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
logically “ANDed” with the internal UVLO, set at nominally
6.7V, such that minimum V
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
capacitor to ground, typically 100pF or so is recom-
mended when the pin is left “open.”
V
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
than an inch from the V
capacitor to ground may be required right at the pin.
V
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
TIMING DIAGRAMS
CC
SW
W U
U
(Pin 2): This pin is used to power the internal control
(Pin 3): This is the emitter node of the output switch
BOOST
SWOFF
SWDR
SWON
V
SW
FUNCTIONS
V
IN
0
U
IN
. If the output capacitor is located more
IN
input current to a few tens of micro-
U
High dV/dt Mode
CC
pin, a separate 0.1 F bypass
W
IN
can be increased above
SW
node, so a small
IN
. This action is
1676 TD01
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
compensation capacitor free of large ground currents.
V
output switch. It also supplies power to the internal control
circuitry during start-up conditions or if the V
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
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.
IN
C
(Pin 8): This is the control voltage pin which is the
(Pin 5): This is the high voltage supply pin for the
SWOFF
BOOST
SWDR
SWON
V
SW
V
IN
0
Low dV/dt Mode
LT1676
CC
1676 TD02
pin is left
5
C