AN557 datasheet(38/52 Pages) STMICROELECTRONICS

38 / 52 page

AN557 APPLICATION NOTE

38/52

CATCH DIODE

Because of quickly rise and fall time of the current

(about 40-50ns) the use Schottky diode is recco-

manded. Ultra-fast diodes with 30-50ns of trr (re-

verse

recovery

time)

are

not

considered

sufficiently fast for this family of converters, since

they would give too elevated peaks of current at

the turn on of the internal power transistor, so high

thatcould affect the reliability of the complete sys-

tem, as well as drastically reduce the efficiency.

The oscilloscope photographs show the Output

Voltage and Output Current waveforms obtained

with diode having different trr value.

Figure 48. Schottky Diode.

Figure 49. Ultra Fast Diode (trr < 100ns).

In the test circuits used for this family of convert-

ers, Schottky diodes from 60V (breakdown re-

verse voltage) are suggested since the device can

support a max. input power voltage of 55V (for

specific applications, Schottky diodes with a re-

verse breakdown voltage higher or equal to the

maximum supply voltage should be used), with

current rating and packaging to satisfy all the con-

ditions of duty cycle, and therefore also of power

dissipation.

COIL

Concerning the coil, a molypermalloy toroidal

cores has been suggested, so that it would be

easy for everybody to obtain samples, wrap them

with a right number of turns in order to evaluate

and correlate the measurements and performance

of the devices.

In addition since the devices are dynamically test-

ed 100% in production, with a "jig" of testing which

uses the same coil suggested in the applications,

in the case of contests for example on a guaran-

teed parameter like the efficiency, should be easi-

er to solve the objections; in this case should be

remembered that changing the magnetic material,

the dimension, the wire and the number of the

winding, also change the losses in the coil reduc-

ing the total efficiency of the application.

This can be easily verify using for example toroi-

dal cores in iron powered rather that those sug-

gested in molypermalloy.

Moreover, it is important to dimension properly the

coil in order to avoid its saturation, a good choice

is to dimension that its saturation; current is not

equal to the maximum nominal current capable to

deliver to the load, but rather higher by about 20%

then the maximum guaranted current of the de-

vice, in short circuit condition.

Only in this way it is possible to guarantee that the

coil never saturate in all the possible working con-

ditions, i.e.: in presence of a load transient, in short

circuit in output and in the case of elevated tem-

perature of the magnetic part.

At last, it should be remembered that the suggest-

ed inductors values, are referred to the inductors

values that the coil must have at the maximum out-

put current of the application. Oscilloscope Photo-

graphs showing the device output voltage and

current waveforms obtained with different induc-

tor.

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