5

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

sn1767 1767fas

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 included

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 Perfor-

mance Characteristics section). An impedance of less

than 5k

Ω (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 0.22

Ω FET structure.

This pin powers the internal circuitry and internal regula-

tor. 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

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.

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

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

section in Applications Information 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 under-

voltage lockout (UVLO), preventing the regulator from

operating until the input voltage has reached a predeter-

mined level. Float or pull high to put the regulator in the

operating mode.

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.