The TDA7480 is a class D audio amplifier assembled in Power DIP (PDIP20) package, specially designed for high efficiency applications, capable of 10W output power at a load of 8w/4w and a total harmonic distorsion of 10%. Requires a split-supply (max. ±20V).
The main selling point of this amplifier is the very low dissipated power compared to normal class AB amplifiers. Only a small "on board" copper area heatsink is required for normal operation. The IC has built-in stand-by and mute feature, overvoltage protection, short circuit protection and thermal overload protection.
The output of the amplifier is a high frequency square wave (~100Khz), rail to rail, with variable duty cycle. To obtain the audio signal, the output must be low pass filtered. The preamplifier provides the voltage gain of the overall amplifier. The second stage is the class D power stage, with a gain 1.5. The class D amplifier stage is done with a multivibrator. With no signal it generates a 50% duty cycle square wave, with signal applied, it changes the duty cycle. The switching frequency is set by the voltage on pin 9.
The most important filter capacitor is C5 between the pins 16 and 17. The value of the parasitic inductance between this capacitance and the IC pins is related to the amplitude of the spikes on the power supply pins at every commutations of the output. For any commutation, there is an abrupt variation of the current in the parasitic inductances in series to the supply. This abrupt variation increases as the output current increases and can be typically of a few amperes on 10ns. With this slew rate of the current, an inductance of a few nH (i.e. the lead inductance of the pin) generates voltage spikes in order of volts. These spikes can cause distortion / offset increase due to the non linear coupling on the internal elementary devices in the signal circuit, overvoltage on the IC, strong noise on the logic signals inside the chip causing incertain logic levels, dangerous for the IC. To avoid these spikes, it is mandatory to put a quality bypass capacitor at a distance lower than 5mm from the pins.
The capacitor C8 on pin 9 must be a low inductance type and should be placed close to the IC. The voltage on this pin sets the switching frequency. The purpose of C8 is to filter the high frequency noise from entering the IC as it could generate distortion/offset.
C4 on pin 8 sets the bandwidth of the class D amplifier. It is important that the reference ground of this capacitor is as near as possible to the IC signal ground.
C3 on pin 11 filters the high frequency noise that can enter in the input and that can cause intermodulation aliasing noise at the output. No signal at frequency greater than half of the switching frequency can enter in the IC without generating aliasing noise.
The electrolitic capacitors must have a good ESR, ESL at switching frequency (around 100Khz) and a sufficient maximum operating voltage.
The low pass filter placed after the switching stage is dimensioned to eliminate the high frequency PWM waves and to feed the audio signal to the loudspeaker. The losses in the filter capacitor C14 due to the ESR will be neglegible if multilayer film capacitors are used.Multilayer Mylar, Polypropilen or Policarbonate film capacitors are recomended, avoid using ceramic capacitors.
The losses in the inductor L1 at low frequencies are mainly due to the coil winding series resistance. At higher frequencies, where the Skin Effect becomes of importance, a multiwire winding could be effective to obtain the maximum in terms of efficiency. However, for most of the applications the use of a single wire winding with adequate cross section is enough. The inductor used for the filter must sustain a DC current greater than the specified current limitation value without saturation and the coil material must show very low hysteresis losses.
