9

LT1308A/LT1308B

1308abfa

Waveforms for a LT1308B 5V to 12V boost converter

using a 10

μF ceramic output capacitor are pictured in

Figures 4 and 5. In Figure 4, the converter is operating in

continuous mode, delivering a load current of approxi-

mately 500mA. The top trace is the output. The voltage

increases as inductor current is dumped into the output

capacitor during the switch off time, and the voltage

decreases when the switch is on. Ripple voltage is in this

case due to capacitance, as the ceramic capacitor has little

ESR. The middle trace is the switch voltage. This voltage

The lower trace is the switch current. At the beginning of

the switch cycle, the current is 1.2A. At the end of the

switch on time, the current has increased to 2A, at which

point the switch turns off and the inductor current flows

into the output capacitor through the diode. Figure 5

depicts converter waveforms at a light load. Here the

converter operates in discontinuous mode. The inductor

current reaches zero during the switch off time, resulting

in some ringing at the switch node. The ring frequency is

set by switch capacitance, diode capacitance and induc-

tance. This ringing has little energy, and its sinusoidal

shape suggests it is free from harmonics. Minimizing the

copper area at the switch node will prevent this from

causing interference problems.

LAYOUT HINTS

The LT1308A/LT1308B switch current at high speed,

mandating careful attention to layout for proper perfor-

mance.

You will not get advertised performance with

careless layout. Figure 6 shows recommended component

placement for an SO-8 package boost (step-up) converter.

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

of the switching loops. Input capacitor C1

must be placed

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

inability to regulate or oscillation.

The negative terminal of output capacitor C2 should tie

close to the ground pin(s) of the LT1308A/LT1308B. Doing

this reduces dI/dt in the ground copper which keeps high

frequency spikes to a minimum. The DC/DC converter

ground should tie to the PC board ground plane at one

place only, to avoid introducing dI/dt in the ground plane.

1

2

8

7

3

4

6

5

L1

C2

D1

LBO

LBI

LT1308A

LT1308B

GND

SHUTDOWN

R1

R2

MULTIPLE

VIAs

GROUND PLANE

1308 F04

+

C1

+

Figure 6. Recommended Component Placement for SO-8

Package Boost Converter. Note Direct High Current Paths

Pin 2 (FB). Use Multiple Vias to Tie Pin 4 Copper to Ground

Plane. Use Vias at One Location Only to Avoid Introducing

Switching Currents into the Ground Plane

Figure 7 shows recommended component placement for

a boost converter using the TSSOP package. Placement is

similar to the SO-8 package layout.

Figure 5. Converter Waveforms in Discontinuous Mode

20mV/DIV

10V/DIV

500mA/DIV

500ns/DIV

Figure 4. 5V to 12V Boost Converter Waveforms in

Continuous Mode. 10

μF Ceramic Capacitor Used at Output

100mV/DIV

10V/DIV

1A/DIV

500ns/DIV