LT1767 Просмотр технического описания (PDF) - Linear Technology

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LT1767

Monolithic 1.5A, 1.25MHz Step-Down Switching Regulators

Linear Technology 

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

PIN FUNCTIONS

FB: The feedback pin is used to set output voltage using

an external voltage divider that generates 1.2V at the pin

with the desired output voltage. The fixed voltage 1.8V,

2.5V, 3.3V and 5V versions have the divider network in-

cluded internally and the FB pin is connected directly to

the output. If required, the current limit can be reduced

during start up or short-circuit when the FB pin is below

0.5V (see the Current Limit Foldback graph in the Typical

Performance Characteristics section). An impedance of

less than 5kΩ (adjustable part only) at the FB pin is needed

for this feature to operate.

BOOST: The BOOST pin is used to provide a drive voltage,

higher than the input voltage, to the internal bipolar NPN

power switch. Without this added voltage, the typical

switch voltage loss would be about 1.5V. The additional

boost voltage allows the switch to saturate and voltage

loss approximates that of a 0.22Ω FET structure.

VIN: This is the collector of the on-chip power NPN switch.

This pin powers the internal circuitry and internal regu-

lator. At NPN switch on and off, high dI/dt edges occur

on this pin. Keep the external bypass capacitor and catch

diode close to this pin. All trace inductance in this path

will create a voltage spike at switch off, adding to the VCE

voltage across the internal NPN.

GND: The GND pin acts as the reference for the regulated

output, so load regulation will suffer if the “ground” end of

the load is not at the same voltage as the GND pin of the

IC. This condition will occur when load current or other

currents flow through metal paths between the GND pin

and the load ground point. Keep the ground path short

between the GND pin and the load and use a ground plane

when possible. Keep the path between the input bypass

and the GND pin short. The GND pin of the MS8 package

is directly attached to the internal tab. This pin should

be attached to a large copper area to improve thermal

resistance. The exposed pad of the MS8E package is also

connected to GND. This should be soldered to a large

copper area to improve its thermal resistance.

VSW: The switch pin is the emitter of the on-chip power

NPN switch. This pin is driven up to the input pin voltage

during switch on time. Inductor current drives the switch

pin negative during switch off time. Negative voltage must

be clamped with an external catch diode with a VBR < 0.8V.

SYNC: The sync pin is used to synchronize the internal

oscillator to an external signal. It is directly logic compat-

ible and can be driven with any signal between 20% and

80% duty cycle. The synchronizing range is equal to initial

operating frequency, up to 2MHz. See Synchronization in

Applications Information section for details. When not in

use, this pin should be grounded.

SHDN: The shutdown pin is used to turn off the regulator and

to reduce input drain current to a few microamperes. The

1.33V threshold can function as an accurate undervoltage

lockout (UVLO), preventing the regulator from operating

until the input voltage has reached a predetermined level.

Float or pull high to put the regulator in the operating mode.

VC: The VC pin is the output of the error amplifier and the

input of the peak switch current comparator. It is normally

used for frequency compensation, but can do double duty

as a current clamp or control loop override. This pin sits

at about 0.35V for very light loads and 0.9V at maximum

load. It can be driven to ground to shut off the output.

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For more information www.linear.com/LT1767

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