LT1776CS8 datasheet(5/20 Pages) LINER

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LT1776

TYPICAL PERFORMANCE CHARACTERISTICS

VC Pin Switching Threshold,

Boost Threshold, Clamp Voltage

vs Temperature

TEMPERATURE (

°C)

–50

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

LT1776 G10

–25

100

125

CLAMP

VOLTAGE

BOOST

THRESHOLD

SWITCHING

THRESHOLD

TEMPERATURE (

°C)

–50

750

700

650

600

550

500

450

400

LT1776 G12

–25

100

125

Error Amplifier Transconductance

vs Temperature

FB PIN VOLTAGE (V)

1.0

100

–50

–100

–150

1.4

1776 G11

1.1

1.2

1.3

1.5

°C

–55

°C

125

°C

Feedback Amplifier Output

Current vs FB Pin Voltage

PIN FUNCTIONS

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

threshold, nominally 0.30V, this pin turns off the regulator

and reduces VIN input current to a few tens of microam-

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 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 one 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 which meets the

input ripple current requirements is needed here. (See

Applications 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

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