5

LT1308

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 ground and

700mV. 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 10

µA. A 1MΩ pullup is recommended. LBO

is high impedance when SHDN is grounded.

–

+

–

+

–

+

–

+

+

+

Σ

COMPARATOR

RAMP

GENERATOR

R

BIAS

Q2

×10

Q1

FB

FB

ENABLE

200mV

A = 3

FF

A2

A1

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)

LAYOUT HINTS

The LT1308 switches current at high speed, mandating

careful attention to layout for proper performance.

You will

not get advertised performance with careless layouts.

Figure 2 shows recommended component placement.

Follow this closely in your PC layout. Note the direct path

close (< 5mm) to the IC package. As little as 10mm of wire

inability to regulate or oscillation. A 10

µF ceramic bypass

capacitor is the only input capacitance required

provided

the battery has a low inductance path to the circuit. The

battery itself provides the bulk capacitance the device

requires for proper operation. If the battery is located some

Figure 2. Recommended Component Placement. Traces

Pin is Kept Low. Lead Length to Battery Should Be Kept Short.

Ground Plane Should Be Placed Under All Components

1

2

3

4

8

7

6

5

L

D

LT1308

GND

MULTIPLE

VIAs

GROUND PLANE

1308 F02