TAS54x4C Hardware Design Guidelines
1.3.2
AM Mitigation
A potential issue in a radio is that a switching power supply or a switching amplifier may interfere with the
AM radio reception. The interference is typically a tone that is found at a harmonic of the PWM frequency.
The AM mitigation system provides a method to change the PWM switching frequency using I
2
C
commands. The controller that provides the I
2
C commands would require the AM frequency information so
that the correct PWM frequency could be selected. No noise, clicks, or pops occur during the frequency
change so the frequency can be changed while audio is present.
lists the recommended switching
frequencies and shows that the harmonics fall outside the associated AM frequency band which avoids
tones in the AM band.
Table 2. Recommended Switching Frequencies for AM Mode Operation
US
EUROPEAN
AM FREQUENCY (kHz)
SWITCHING FREQUENCY
AM FREQUENCY (kHz)
SWITCHING FREQUENCY
(kHz)
(kHz)
522 to 540
417
540 to 917
500
540 to 914
500
917 to 1125
417
914 to 1122
417
1125 to 1375
500
1122 to 1373
500
1375 to 1547
417
1373 to 1548
417
1547 to 1700
357
1548 to 1701
357
1.3.3
Spread Spectrum Oscillator
Situations occur where the AM-tuned frequency is not available in the system. In these situations potential
interference can be overcome by creating an oscillator that varies slightly over time which is typically
called spread spectrum or dither. The oscillator in the TAS54x4C devices can be dithered by changing the
current in the REXT pin. The current in the REXT pin can be varied by up to ±2.5% of the nominal value
without sacrificing functionality. This variation moves the oscillator frequency enough to avoid the AM
receiver from locking on the frequency and producing a tone. The next design characteristic for dither is to
determine the dither frequency or how fast the REXT pin current changes. Any change in the PWM
frequency is demodulated as an output signal. Because the TAS54x4C devices have feedback this
change is reduced by the amount of feedback in the amplifier. The TAS54x4C devices have the greatest
amount of open loop gain at DC and will therefore have the greatest amount of feedback at DC.
Obviously, DC cannot work, so using a frequency close to DC, but not so low the AM interference is not
eliminated. Testing proves that 15 to 20 Hz is a good frequency. The optimum dither oscillator shape is a
triangle wave. The dithered oscillator tracks the voltage change of this low frequency oscillator. The linear
ramps with the abrupt changes of a triangle wave will provide the best spread spectrum oscillator.
The EMI measurements can be improved with spread spectrum oscillators. The results vary depending on
the type of measurement. Peak measurements typically do not show any improvements in the
measurements, but quasi-peak measurements may improve the EMI by 6 to 10 dB.
1.4
Output
Class D amplifiers require special consideration when working with the output stages. These amplifiers
require a proper demodulation filter in order to recover the audio signal with a flat frequency response. In
the TAS54x4C devices the output stages are BTL so the demodulation filter must be a fully balanced filter.
Another issue regarding the recovery of a quality audio signal is overshoot on the PWM signal. Overshoot
can be reduced using snubbers.
7
SLOA196 – June 2014
TAS54x4C Design Guide
Copyright © 2014, Texas Instruments Incorporated
