3

General information

FDA901 is a fully digital single chip class D amplifier with high immunity to the demodulation filter effects. The

high integration level and the on-board signal processing allow excellent audio performance to be achieved.

Thanks to the digital input and to the feedback strategy in the power stage that make the amplifier immune from

the output filter components non-linearity, the number and size of the external components are minimized.

A number of features is included to reduce EMI and the fully digital approach provides a strong GSM immunity.

interpolation and noise shaping, innovative self-diagnostic functions and automatic detection of wrong load

connections or variation of the load, internal PLL for a clock generation.

3.1

New feedback topology

FDA901 adopts an innovative feedback topology, where the LC filter is included in the feedback loop making the

amplifier highly insensitive to the characteristics of such a demodulation circuit. This solution optimizes the system

performance in terms of THD and frequency response in any load conditions.

Regardless of the big phase shifting introduced by the output filter the device shows an adequate phase margin

for any load condition. The system stability has been tested taking into account:

•

PWM switching variation (from 300 to 440 kHz)

•

Silicon temperature variation (from -40 to 150 °C)

•

Load variation (both inductive and capacitive considered)

•

LC demodulator filter variation and tolerance

•

Voltage supply variation (from 6 to 25 V)

The system has been designed to guarantee a phase margin > 45 deg for any working condition.

The new feedback topology assures a strong control of voltage and current across the load making the diagnostic

load detection reliable. Moreover this topology allows to reach exceptionally good values of output damping factor.

3.2

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, EMI filtering and 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 increasing cost, volume and power dissipation. FDA901, 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.

3.3

Load possibilities

FDA901 supports several load possibilities and configurations. The default configuration is suitable for a 4-

solution with parallel outputs (left/right) or a 2.1 configuration for sub-woofer application.

Possible channel configurations:

•

4 x 4 ohm (up to 25 V);

•

4 x 2 ohm (up to 18 V);

FDA901

General introduction

DB3856 - Rev 1

page 8/17