5

LT1308A/LT1308B

nect a series RC from this pin to ground. Typical values

are 47k

FB (Pin 2): Feedback Pin. Reference voltage is 1.22V.

Connect resistive divider tap here. Minimize trace area at

SHDN (Pin 3): Shutdown. Ground this pin to turn off

switcher. To enable, tie to 1V or more. SHDN does not

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

plane. Ground plane should enclose all components

associated with the LT1308. PCB copper connected to

Pin 4 also functions as a heat sink. Maximize this area to

keep chip heating to a minimum.

SW (Pin 5): Switch Pin. Connect inductor/diode here.

Minimize trace area at this pin to keep EMI down.

right at the pin, connected directly to ground.

LBI (Pin 7): Low-Battery Detector Input. 200mV refer-

ence. Voltage on LBI must stay between –100mV and 1V.

Low-battery detector does not function with SHDN pin

grounded. If not used, float LBI pin.

LBO (Pin 8): Low-Battery Detector Output. Open collec-

tor, can sink 50

µA. A 1MΩ pull-up is recommended. LBO

is high impedance when SHDN is grounded.

–

+

–

+

–

+

–

+

+

+

Σ

COMPARATOR

RAMP

GENERATOR

R

BIAS

Q2

×10

Q1

FB

FB

ENABLE

*HYSTERESIS IN LT1308A ONLY

200mV

A = 3

FF

A2

A1

Q4

*

ERROR

AMPLIFIER

A4

0.03

Ω

DRIVER

SW

GND

1308 BD

Q3

Q

S

600kHz

OSCILLATOR

5

LBO

LBI

SHDN

SHUTDOWN

3

7

1

4

R6

40k

R5

40k

R1

(EXTERNAL)

R3

30k

R4

140k

2

6

8

R2

(EXTERNAL)

Figure 2. LT1308A/LT1308B Block Diagram