LT1533CS datasheet(13/20 Pages) LINER

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LT1533

APPLICATIONS INFORMATION

Output ripple current at maximum input (DC = 34.7%) is:

∆I

kHz

OUT =

−

()

12 1 2 34 7

800

•. %

•

The maximum inductor current is:

ImA

LMAX =+

150

196

Primary inductance should be greater than:

PRI ==

5 800

309

•

The magnetizing ripple current is approximately:

∆I

kHz

MAG =

12 0 5

3 6 309

225

.•

•

Peak switch current is:

ISW(PEAK) = 3.6 • 196mA + 225mA = 930mA

which is less than the 1A maximum switch current.

Note that you can discern your magnetizing ripple by

looking at the reflected inductance ripple and subtracting

the switch current ripple.

∆IMAG = N • ∆IL – ∆ISW

With knowledge of turns ratio and primary inductance

along with volt/sec requirements (to prevent saturation)

the transformer can be designed.

Transformers are available from Coiltronics for some

standard applications. Figure 5 lists them. Variations are

available from Coiltronics at 561-241-7876. Also, see

Linear Technology’s Application Notes AN19, AN44 and

AN70 for further information about magnetics.

Capacitors

Correct choice of input and output capacitors can be very

important to low noise switcher performance. Push-pull

topologies and other low noise topologies will in general

have continuous currents which reduce the requirements

Figure 5. Transformers for Typical Applications

for capacitance. However, noise depends more on the ESR

of the capacitors.

Input capacitors must also withstand surges that occur

during the switching of some types of loads. Some solid

tantalum capacitors can fail under these surge conditions.

Design Note 95 offers more information but the following

is a brief summary of capacitor types and attributes.

Aluminum Electrolytic: Low cost and higher voltage but in

general don’t use with this part because of high ESR and

poor high frequency performance.

Specialty Polymer Aluminum: Panasonic has come out

with their series CD capacitors. While they are only avail-

able for voltages below 16V, they have very low ESR and

good surge capability.

Solid Tantalum: Small size and low impedance. Typically

available for voltages below 50V. Possible problem with

surge currents (AVX TPS line addresses this issue).

OS-CON: Lower impedance than aluminum but only avail-

able for 25V or less. Form factor may be a problem.

Sometimes their very low ESR can cause loop stability

problems.

PRIMARY A

PRIMARY B

SECTION A

SECTION B

= TIED TOGETHER

*=HIGH TURNS RATIO VERSION OF CTX-02-13716-X1.

ACCOMMODATES LOW SUPPLY VOLTAGES OR

HIGH DROPOUT REGULATORS

1533 F05

PRIMARY A

PRIMARY B

SECTION A

TIE OUTPUT

COMMON TO

THIS POINT

SECTION B

NOMINAL

INPUT

VOLTAGE

OUTPUT

POWER

1.5W

3.0W

1.5W

3.0W

1.5W

10W

CONNECTION

DIAGRAM

COILTRONICS

PART

NUMBER

CTX02-13716-X1

CTX02-13665-X1

CTX02-13713-X1

CTX02-13664-X1

CTX02-13834-X3*

CTX02-13949-X1

NOMINAL

OUTPUT VOLTAGE

AFTER LINEAR

REGULATOR

12V

±15V

12V

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