AN557 Datasheet, PDF (38/52 Pages) STMicroelectronics – EASY APPLICATION DESIGN WITH THE L4970A

AN557 APPLICATION NOTE
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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