LT1676 Просмотр технического описания (PDF) - Linear Technology

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LT1676

Wide Input Range, High Efficiency, Step-Down Switching Regulator

Linear Technology 

LT1676

PIN FUNCTIONS

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

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

tor and reduces VIN 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 VIN. This action is

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

6.7V, such that minimum VIN 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 VSW node, so a small

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

mended when the pin is left “open.”

VCC (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 VIN. If the output capacitor is located more

than an inch from the VCC pin, a separate 0.1µF bypass

capacitor to ground may be required right at the pin.

VSW (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 VC

compensation capacitor free of large ground currents.

VIN (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 VCC 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.

VC (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.

TIMING DIAGRAMS

High dV/dt Mode

VIN

VSW

0

SWDR

SWON

BOOST

SWOFF

1676 TD01

VIN

VSW

0

SWDR

SWON

BOOST

SWOFF

Low dV/dt Mode

1676 TD02

5

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