5

LT1373

frequency compensation, current limiting and soft start. It

is the output of the error amplifier and the input of the

current comparator. Loop frequency compensation can be

to ground.

FB (Pin 2): The feedback pin is used for positive output

voltage sensing and oscillator frequency shifting. It is the

inverting input to the error amplifier. The noninverting

input of this amplifier is internally tied to a 1.245V

reference. Load on the FB pin should not exceed 100

µA

when the NFB pin is used. See Applications Information.

NFB (Pin 3): The negative feedback pin is used for negative

output voltage sensing. It is connected to the inverting

input of the negative feedback amplifier through a 400k

source resistor.

S/S (Pin 4): Shutdown and Synchronization Pin. The S/S

pin is logic level compatible. Shutdown is active low and

the shutdown threshold is typically 1.3V. For normal

floating. To synchronize switching, drive the S/S pin be-

tween 300kHz and 340kHz.

drops below 2.5V. Undervoltage lockout stops switching

GND S (Pin 6): The ground sense pin is a “clean” ground.

The internal reference, error amplifier and negative feed-

back amplifier are referred to the ground sense pin. Con-

nect it to ground. Keep the ground path connection to the

free of large ground currents.

GND (Pin 7): The ground pin is the emitter connection of

the power switch and has large currents flowing through it.

It should be connected directly to a good quality ground

plane.

switch and has large currents flowing through it. Keep the

traces to the switching components as short as possible to

minimize radiation and voltage spikes.

–

+

NEGATIVE

FEEDBACK

AMP

NFB

S/S

FB

400k

200k

0.08

Ω

GND

LT1373 • BD

GND SENSE

1.245V

REF

5:1 FREQUENCY

SHIFT

250kHz

OSC

SYNC

SHUTDOWN

DELAY AND RESET

LOW DROPOUT

2.3V REG

ANTI-SAT

LOGIC

DRIVER

SW

SWITCH

–

+

COMP

ERROR

AMP

CURRENT

AMP

PI FU CTIO S

BLOCK DIAGRA