LT1946

4

sn1946 1946fs

PI FU CTIO S

pensation network to this pin, or use the internal compen-

FB (Pin 2): Feedback Pin. Reference voltage is 1.250V.

Connect resistive divider tap here. Minimize trace area at

SHDN (Pin 3): Shutdown Pin. Tie to 2.4V or more to enable

device. Ground to shut down. Do not float this pin.

GND (Pin 4): Ground. Tie directly to local ground plane.

SW (Pin 5): Switch Pin. This is the collector of the internal

NPN power switch. Minimize the metal trace area con-

nected to this pin to minimize EMI.

COMP (Pin 7): Internal Compensation Pin. Provides an

for internal compensation. Tie to GND if not used.

SS (Pin 8): Soft-Start Pin. Place a soft-start capacitor

here. Upon start-up, 4

µA of current charges the capacitor

to 1.5V. Use a larger capacitor for slower start-up. Leave

floating if not in use.

BLOCK DIAGRA

Σ

–

+

A2

FB

SHDN

SHUTDOWN

0.5V

DRIVER

Q1

0.01

Ω

120k

90pF

SW

GND

1946 BD

COMPARATOR

5

4

µA

1

SS

8

COMP

4

S

RQ

RAMP

GENERATOR

1.2MHz

OSCILLATOR

÷3

–

+

–

+

2

3

1.250V

REFERENCE

6

7

R1 (EXTERNAL)

FB

R2 (EXTERNAL)

A3

A1

Figure 2. Block Diagram