Datasheet FDA901 (STMicroelectronics) - 8

FDA901 General introduction 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. FDA901 includes: digital I2C and I2S interfaces, internal 24 bits DAC conversion, digital signal processing for 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- channel application (front/rear - left/right). By means of I2C register settings it is possible to choose a 2-channel 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);
DB3856
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Rev 1 page 8/17
Document Outline 1 Block diagram and pins description 1.1 Block diagram 1.2 Pins description 2 Electrical specifications 2.1 Absolute maximum ratings 2.2 Thermal data 3 General information 3.1 New feedback topology 3.2 LC filter design 3.3 Load possibilities 4 Package information 4.1 LQFP64 (10x10x1.4 mm exp. pad up) package information 4.2 LQFP64 (10x10x1.4 mm exp. pad up) marking information Revision history