When using two or more LM4651/52 chipsets in a system an output signal is heard when there is no input signal. What is the cause and how can it be eliminated?
When using two LM4651/52 class D chipsets the modulation frequency or switching frequency (SWF) of each chipset is independently set by the ROSC resistor connected between the OSC pin (pin 16) and GND. Due to resistor variation, fab process variation and the operating temperature the SWF of each chipset will vary slightly. This difference between the SWF of each chip results in a “beat frequency” that occurs in the audio band. For example, if the SWF is set to 100kHz for each chip and there is just a 0.1% difference then it is possible a 100Hz beat frequency will be heard when there is no input signal. The most likely cause of the beat frequency is due to the output filter inductors coupling between channels. Coupling between PCB traces can also be a cause or very poor supply bypassing on the power lines. Possible solutions are listed below: A. The simplest and lowest cost solution is to set the SWF far enough apart to eliminate any possible beat frequency from developing in the audio
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