LT1533CS datasheet(14/20 Pages) LINER

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LT1533

APPLICATIONS INFORMATION

Ceramic: Generally used for high frequency and high

voltage bypass. If all ceramic capacitors are used, they can

have such a low ESR as to cause loop stability problems.

Often they can resonate with their ESL before ESR be-

comes effective.

Input Capacitor

The requirements for the input capacitor are less stringent

for this part. Input current ripple is lower because of the

push-pull action and low noise features of the part. How-

ever, the input capacitor should have low ESR at high

frequencies since this will be an important factor concern-

ing how much conducted noise is created. Values of input

capacitor will typically be in the 1

µF to 22µF range with

ESR under 0.3

Ω.

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 condi-

tion. Several manufacturers have developed a line of solid

tantalum capacitors specially tested for surge capability

(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 derating capaci-

tor 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. Typical required

ESR will be in the 0.05

Ω to 0.3Ω range.

The specific value for capacitance will depend on topol-

ogy. A typical output capacitor is an AVX type TPS, 22

µF

and 25V with a guaranteed ESR less than 0.2

Ω. To further

reduce ESR, multiple output capacitors can be used in

parallel. The value in microfarads is not particularly impor-

tant. A small 22

µF tantalum capacitor will have high ESR

and higher output voltage ripple. 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

Switching Diodes

In general, switching diodes should be Schottky diodes

such as 1N5818 or MBR130 (1A/30V). Low output current

applications may use 1N4148 switching diodes.

Unregulated Applications

The LT1533 can be used to create a low noise “DC

transformer” unregulated power supply. DC transformers

are open-loop switching regulators where the output

voltage is controlled by the turns ratio of the transformer.

A DC transformer provides a low cost isolated supply.

For such applications, the DUTY pin of the LT1533 should

be grounded. This will force the outputs into a 50% on,

50% off mode. Note that because of slew control there will

be some variance from 50%. Figure 6 shows a 5V to

±12V

DC transformer.

One concern with this type of application is having both

switch outputs transition at the same time. This can cause

both primary side windings to have positive EMF added to

the winding, causing the current to run away. Since this

part controls slew rate this won’t happen. It is possible to

see slightly increased total current draw when both drivers

are on, but this will be controlled and observable. Since the

outputs share a common sense resistor, the outputs will

turn off when the total current in both exceeds the limit set

by the VC pin.

The FB pin should be DC biased between 0.7V and 1.2V to

prevent frequency shifting from occurring. This also en-

sures that the VC pin is set to its upper clamp, providing

peak output current.

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