FDA903U datasheet(22/80 Pages) STMICROELECTRONICS

22 / 80 page

General information

FDA903U

22/80

DS12498 Rev 2

General information

5.1

LC filter design

The audio performance of a Class D amplifier is heavily influenced by the characteristics of

the output LC filter. The choice of its components is quite critical because a lot of constraints

have to be fulfilled at the same time: size, cost, filter for EMI suppression, efficiency. In

particular, both the inductor and the capacitor exhibit a non linear behavior: the value of the

inductance is a function of the instantaneous current in it and similarly the value of the

capacitor is a function of the voltage across it.

In the classical approach, where the feedback loop is closed right at the output of the power

stage, the LC filter is placed outside the loop and these nonlinearities cause the Total

Harmonic Distortion (THD) to increase. The only way to avoid this phenomenon would be to

use components which are highly linear, but this means they are also bigger and/or more

expensive.

Furthermore, when the LC filter is outside the loop, its frequency response heavily depends

on the impedance of the loudspeaker; this is one of the most critical aspects of Class-D

amplifiers. In standard class D this can be mitigated, but not solved, by means of additional

damping networks, increasing cost, space and power dissipation. FDA903U, instead,

provides a very flat frequency response over audio-band which cannot be achieved by

standard class D without feedback after LC filter.

Since the demodulator group is now in the feedback path, some constraints regarding the

inductor and capacitor choice are still present but of course less stringent than in the case of

a typical switching application.

5.2

Load possibilities

FDA903U supports several load possibilities, driving 2 Ω, 4 Ω and higher ohmic loads.

Possible channel configurations are:



1 x 4 ohm (or higher) (up to 18 V)



1 x 2 ohm (up to 16 V)

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