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LT1534/LT1534-1

APPLICATIONS INFORMATION

Fast Voltage Slew Edges

A very fast voltage slew under certain operating conditions

may produce ringing on the COL voltage waveform. While

there is small harmonic energy in this, it can be eliminated

by placing an RC network of 10

Ω in series with 1000pF

from the COL pin to ground.

Switching Diodes

In general, switching diodes should be Schottky diodes

such as 1N5817-19 or MBR320-330.

Choosing the Inductor

For a boost converter, inductor selection involves trade-

offs of size, maximum output power, transient response

and filtering characteristics. Higher inductor values pro-

vide more output power and lower input ripple. However,

they are physically larger and can impede transient re-

sponse. Low inductor values have high magnetizing cur-

rent, which can reduce maximum power and increase

input current ripple.

The following procedure can be used to handle these

trade-offs:

1. Assume that the average inductor current for a boost

converter is equal to load current times VOUT/VIN and

decide whether the inductor must withstand continu-

ous overload conditions. If average inductor current at

maximum load current is 0.5A, for instance, a 0.5A

inductor may not survive a continuous 1.5A overload

condition. Also be aware that boost converters are not

short-circuit protected, and under output short condi-

tions, only the available current of the input supply

limits inductor current.

Input Capacitor

The ESR of this capacitor acts with high frequency current

components to produce much of the conducted noise of

the switcher. Values of 1

µF to 47µF are typical with ESR

less than 0.3

Ω. Place the capacitor close to the IC and

inductor.

The input capacitor can see a high surge current when a

battery of high capacitance source is connected “live.”

Some solid tantalum capacitors can fail under this con-

dition. Several manufacturers have developed a line of

solid tantalum capacitors specially tested for surge capa-

bility (e.g., AVX TPS series). However, even these units

may fail if the input voltage approaches the maximum

voltage rating of the capacitor. AVX recommends derat-

ing capacitor voltage by 2:1 for high surge applications.

Output Filter Capacitor

Output capacitors are usually chosen on the basis of ESR

since this will determine output ripple. However, low ESR

is also needed for low output noise and this will typically

be the tougher requirement. Typically required ESR will be

less than 0.2

Ω . Typical capacitance values are in the 47µF

to 500

µF range. Again keep connection length as short as

possible. Table 1 shows some typical surface mount

capacitors.

Table 1

SIZE

CAPACITOR

ESR (MAX

Ω)

E CASE

AVX TPS, Sprague 593D

0.1 to 0.3

AVX TAJ

0.7 to 0.9

D CASE

AVX TPS, Sprague 593D

0.1 to 0.3

AVX TAJ

0.9 to 2.0

Panasonic CD

0.05 to 0.18

C CASE

AVX TPS

0.2 (Typ)

AVX TAJ

1.8 to 3.0

B CASE

AVX TAJ

2.5 to 10

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