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LT1308ACFTR

High Current, Micropower Single Cell, 600kHz DC/DC Converters

Linear Technology 

LT1308A/LT1308B

APPLICATIONS INFORMATION

Waveforms for a LT1308B 5V to 12V boost converter using

a 10μF 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 approximately 500mA.

The top trace is the output. The voltage increases as induc-

tor 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 alternates between a

VCESAT and VOUT plus the diode drop. 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 discontinu-

ous 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 inductance. 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, man-

dating careful attention to layout for proper performance.

You will not get advertised performance with careless

layout. Figure 6 shows recommended component place-

ment 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 or PC

trace from CIN to VIN will cause problems such as 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.

GROUND PLANE

LBI LBO

C1

+

VIN

VOUT

100mV/DIV

VSW

10V/DIV

ISW

500mA/DIV

500ns/DIV

1308 F04

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

Continuous Mode. 10μF Ceramic Capacitor Used at Output

VOUT

20mV/DIV

VSW

10V/DIV

ISW

500mA/DIV

500ns/DIV

1308 F05

Figure 5. Converter Waveforms in Discontinuous Mode

R1

1

8

R2

2 LT1308A

7

L1

SHUTDOWN

3 LT1308B

6

4

5

MULTIPLE

VIAs

GND

+

C2

D1

VOUT

1308 F04

Figure 6. Recommended Component Placement for SO-8

Package Boost Converter. Note Direct High Current Paths

Using Wide PC Traces. Minimize Trace Area at Pin 1 (VC) and

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.

1308abfb

9

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