5

LT1676

SHDN (Pin 1): When pulled below the shutdown mode

threshold, nominally 0.30V, this pin turns off the regula-

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

logically “ANDed” with the internal UVLO, set at nominally

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

capacitor to ground, typically 100pF or so is recom-

mended when the pin is left “open.”

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

capacitor to ground may be required right at the pin.

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

compensation capacitor free of large ground currents.

output switch. It also supplies power to the internal control

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.

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

SWOFF

1676 TD02

BOOST

SWON

SWDR

0

High dV/dt Mode

Low dV/dt Mode