My customer asks:
I'm trying to find out what the absolute maximum output power from the LM4673 Audio Power Amplifier if I use a 3.6Volt supply (I'm hoping for less than 1 Watt). The datasheet gives a level of 870mWatts at 10% distortion but I need to know what happens if we seriously overdrive it, i.e. 30% distortion.
I'm trying to protect the speaker from damage. For the application the amount of distortion is secondary to volume so they want to run the PA into saturation. The problem is that the last couple of dBs of output power don't depend on amplitude but on the amount of saturation of the output, e.g. a sine wave at 3.6Volts is 800mW but a square at 3.6 Volts is 1.6 Watts. The speaker we use is rated for 1Watt maximum so what I'm hoping is that the absolute power we could ever get out of the part is less than that.
Measuring one device under the customer's conditions, the LM4673 max's out at 1.16W. I've attached a scope shot of the output waveform, post measurment filter. The signal on the scope is a normalized monitor output of the Audio Precsion, so the vertical scale is not accurate. However, the frequency and waveform shape are, so you can see its a 1kHz squarewave output. There is no active power limiter in the device, the modulator headroom and output stage RDSon are what's contributing most to the power limiting. This was measured on one device at room temp, so I can't guarantee that the LM4673 will limit to that power level under all possible operating conditions.
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