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L5994 - L5994A

Transformer Catch Diode (L5994A only)

The diode which steers the current generated by the secondary winding of the +5.1V transformer should be a

fast-recovery one, with a breakdown voltage greater than:

with a certain safety margin. The diode has to withstand a pulsed current whose peak value is approximately:

while its RMS value is given by:

Transformer Filter Capacitors

The most stringent requirement on the input filter capacitor (connected between V13IN and ground) is its RMS

ripple current rating, which should be at least:

The working voltage should be higher than the voltage generated when the regulator is lightly loaded. Also for

this part the use of high quality electrolytic or OS-CON capacitors is advised.

Layout and Grounding

The electrical design is only the first step in the development of a switching converter. Since currents ranging

from mA to some A, both DC and switched, live together on the same circuitboard, the PCB layout is vital for a

correct operation of the circuit but is not an easy task.

A proper layout process generally includes careful component placing, proper grounding, correct traces routing,

and appropriate trace widths. Fortunately, since low voltages are involved in this kind of applications, isolation

requirements are of no concern.

Referring to literature for a detailed analysis of this matter, only few important points will be here reminded.

1) All current returns (signal ground, power ground, etc.) should be mutually isolated and should be con-

nected only at a single ground point. Ground planes may be extremely useful both to arrange properly

current returns and to minimize radiation (see next 2 points), even though they cannot solve every prob-

lem

2) Noise coupling between adjacent circuitry can be reduced minimizing the area of the loop where current

flows. This is particularly important where there are high pulsed currents, that is the circuit including the

input filter capacitor, the power switch, the synchronous rectifier and the output capacitor. The next pri-

ority should be given to the gate drive circuits.

3) Magnetic field radiation (and stray inductance) can be reduced by keeping all traces which carry

switched currents as short as possible.

4) The Kelvin-connected traces of current sense should be kept short and close together.

5) For high current paths, the traces could be doubled on the other side of the PCB whenever possible: this

will reduce both the resistance and the inductance of the wiring.

6) In general, traces carrying signal currents should run far from traces carrying pulsed currents or with

quickly swinging voltages. From this viewpoint, particular care should be taken of the high impedance

paths (feedback input, current sense traces...). It could be a good idea to route signal traces on one PCB

V

RR

V

IN LIN

5.1

–

() η V

IN M AX

5.1

–

()

⋅

+

=

I

13PK

I

OUT12

–

V

IN MIN

V

IN M IN

5.1

–

---------------------------------

⋅

I

13R M S

I

OUT12

V

IN M IN

V

IN M IN

5.1

–

---------------------------------

⋅

=

I

13AC

I

OUT12

5.1

V

IN M IN

5.1

–

---------------------------------

⋅

=