LT1777 Linear Technology, LT1777 Datasheet - Page 6

• Manufacturer Part Number: LT1777
• Description: Low Noise Step-Down Switching Regulator
• Manufacturer: Linear Technology

PIN
LT1777
GND (Pins 1, 8, 9, 16): These corner package pins are
mechanically connected to the die paddle and thus aid in
conducting away internally generated heat. As these are
electrically connected to the die substrate, they must be
held at ground potential. A direct connection to the local
ground plane is recommended.
NC (Pins 2, 11, 15): Package Pins 2, 11 and 15 are
unconnected.
SHDN (Pin 3): When pulled below the shutdown mode
threshold, nominally 0.5V, this pin turns off the regulator
and reduces V
peres (shutdown mode).
When this pin is held above the shutdown mode threshold,
but below the lockout threshold, the part will be opera-
tional with the exception that output switching action will
be inhibited (lockout mode). A user-adjustable undervolt-
age lockout can be implemented by driving this pin from
an external resistor divider to V
“ANDed” with the internal UVLO, nominally set at 6.7V,
such that minimum V
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 V
ground, typically 100pF or so is recommended when the
pin is left open.
V
from 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
inch from the V
to ground may be required right at the pin.
V
external sense inductor to limit power path dI/dt. The
sense inductor is placed between the V
the cathode of the freewheeling (power) diode, and the V
pin is connected to the diode. As the voltage across the
inductor reaches 2V
6
CC
D
U
(Pin 5): This pin is used in conjunction with a small
: (Pin 4): Pin to power the internal control circuitry
FUNCTIONS
IN
. If the output capacitor is located more than an
U
IN
CC
input current to a few tens of microam-
pin, a separate 0.1 F bypass capacitor
U
IN
SW
BE
can be increased above 6.7V, but
, drive to the output transistor is
node, so a small capacitor to
IN
. This action is logically
SW
output node and
D
removed or supplied accordingly to limit dI/dt (see Appli-
cations Information).
V
and has large currents flowing through it. Keep the traces
to the switching components as short as possible to
minimize electromagnetic radiation and voltage spikes.
SGND (Pin 7): This is the device signal ground pin. The
internal reference and feedback amplifier are referred to it.
Keep the ground path connection to the FB divider and the
V
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 which meets the
input ripple current requirements is needed here (see
Applications Information).
SYNC (Pin 12): Pin to synchronize internal oscillator to
external frequency reference. It is directly logic compat-
ible 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 13): 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. Coupling
from high speed noise to this pin can cause irregular
operation. (See Switch Node Considerations section.)
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 R/C combination) between this node and ground.
Coupling from high speed noise to this pin can cause
irregular operation. (See Switch Node Considerations
section.)
SW
C
IN
C
compensation capacitor free of large ground currents.
(Pin 14): This is the control voltage pin which is the
(Pin 10): This is the high voltage supply pin for the
(Pin 6): This is the emitter node of the output switch
CC
pin is left