LT3464

6

3464i

The LT3464 uses a constant off-time control scheme in

conjunction with Burst Mode

efficiency over a wide range of output current. Operation

can best be understood by studying the above diagram.

When the FB pin voltage is lower than the 1.25V reference,

the hysteretic comparator enables the power section,

causing the chip to start switching, thus charging the

output capacitor. When the output voltage increases

enough to overcome the hysteresis, the feedback com-

parator shuts off the power section leaving only low

power circuitry running until the output voltage falls

again. This cycle repeats, keeping the output voltage

within a small window. The switching action is as follows:

The switch turns on, and current through it starts to ramp

OPERATIO

up until the point where the current limit is reached, at

which point the switch turns off for a fixed amount of time.

While the switch is off the inductor is delivering current to

the load. When the off time expires, the switch turns on

again until the current limit is reached, and the cycle

repeats.

This chip includes an internal power Schottky diode and a

PNP transistor for output disconnect. The PNP transistor

disconnects the load from the input during shutdown. The

PNP control circuitry is designed to keep the PNP out of

saturation across a wide range of current, to keep quies-

cent current to a minimum and to provide current limiting

to protect the chip during short-circuit conditions.

Burst Mode is a registered a trademark of Linear Technolgy Corporation.

BLOCK DIAGRA

S

R

Q

Q

DELAY

–

–

+

–

+

12mV

OUT

OUT

1.25V

CTRL

SW

GND

OUT

ANTI

SAT

4

8

1

2

7

6

5

3

SHDN

FB

CAP

3464 BD

0.1

Ω