7

LT1308A/LT1308B

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

layouts. Figure 4 shows recommended component place-

ment for a 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

oscillation.

The negative terminal of output capacitor C2 should tie

close to Pin 4 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.

A SEPIC (Single-Ended Primary Inductance Converter)

schematic is shown in Figure 5. This converter topology

produces a regulated output over an input voltage range

that spans (i.e., can be higher or lower than) the output.

Recommended component placement for a SEPIC is

shown in Figure 6.

COMPONENT SELECTION

Inductors

Suitable inductors for use with the LT1308A/LT1308B

must fulfill two requirements. First, the inductor must be

able to handle current of 2A steady-state, as well as

support transient and start-up current over 3A without

inductance decreasing by more than 50% to 60%. Second,

the DCR of the inductor should have low DCR, under 0.05

Ω

so that copper loss is minimized. Acceptable inductance

values range between 2

µH and 20µH, with 4.7µH best for

most applications. Lower value inductors are physically

smaller than higher value inductors for the same current

capability.

Table 1 lists some inductors we have found to perform well

in LT1308A/LT1308B application circuits. This is not an

exclusive list.

Table 1

VENDOR

PART NO.

VALUE

PHONE NO.

Murata

LQH6C4R7

4.7

µH

770-436-1300

Sumida

CDRH734R7

4.7

µH

847-956-0666

Coiltronics

CTX5-1

5

µH

561-241-7876

Capacitors

Equivalent Series Resistance (ESR) is the main issue

regarding selection of capacitors, especially the output

capacitors.

The output capacitors specified for use with the LT1308A/

LT1308B circuits have low ESR and are specifically

designed for power supply applications. Output voltage

ripple of a boost converter is equal to ESR multiplied by

switch current. The performance of the AVX TPSD227M006

220

µF tantalum can be evaluated by referring to Figure 4.

When the load is 800mA, the peak switch current is

Ripple can be further reduced by paralleling ceramic units.

Table 2 lists some capacitors we have found to perform

well in the LT1308A/LT1308B application circuits. This is

not an exclusive list.

Table 2

VENDOR

SERIES

PART NO.

VALUE

PHONE NO.

AVX

TPS

TPSD227M006

220

µF, 6V 803-448-9411

AVX

TPS

TPSD107M010

100

µF, 10V 803-448-9411

Taiyo Yuden

X5R

LMK432BJ226

22

µF, 10V 408-573-4150

Taiyo Yuden

X5R

TMK432BJ106

10

µF, 25V 408-573-4150

Diodes

We have found Motorola MBRS130 and International

Rectifier 10BQ015 to perform well. For applications where

10BQ040.