So for this amplifier IC - PAM8403 - which is a 3W + 3W class d, what is the current requirement or in other terms, what will be the load current. I know since this is an audio amplifier, load current is not always fixed. But as a generalization, what will be the value.
I was planning to use a 3.7V 2600mAh cell to power it, through a 5V regulator. I doubt that it is 0.6A per channel. For other ratio of voltage and power, the result is not the same.

  1. Just want to know how long that cell can power this amp.

In datasheets, different values for power based on different values of voltages are listed. That is understandable but for the same voltage and load, they also put forth that if THD + N is 1%, power will be different and if THD + N is 10%, power output is little more than the former, even though the voltage supplied and the connected load is same.

  1. Is there a particular way to select/switch an appropriate Total harmonic Distortion + Noise figure, while designing?


EDIT - The speaker driver is rated at 4 ohm.

  • \$\begingroup\$ Load current is dependent on load/speaker impedance. \$\endgroup\$ – Andy aka Jan 27 '15 at 14:58
  • \$\begingroup\$ Speaker impedance is 4 Ohm. \$\endgroup\$ – curiousCoder Jan 27 '15 at 14:59

The efficiency of the chip means that if your speakers are producing 3 watts (6 watt in total), the power taken by the chip from 5 volts is going to be maybe 7 watts. That's an efficiency of about 86%.

The data sheet says "Efficiency up to 90%" so expect somewhere around 85% I'd say but it is not clear cut how accurate this figure is.

With 7 watts from a 5 volt supply, the dc current is 1.4 amps and if you have used a boost regulator to give you 5 volts from (say) 3.7 volts, the current from the battery will be at least 2.1 amps given the efficiency of the booster that I assumed to be 90%.

Regards THD read it this way.... the device can provide more power to the speakers but the distortion will be greater because the chip is getting close to its overload condition. Selecting the THD figures isn't really appropriate - if the distortion appears to high to the ear then back-off the signal fed to the device so it doesn't distort.

  • \$\begingroup\$ So basically the entire system can run for little more than an hour. So how come many portable speakers run for more than 5 or 6 hours when at full volume, even though they have they same or almost same specs? \$\endgroup\$ – curiousCoder Jan 27 '15 at 15:19
  • 3
    \$\begingroup\$ Basically because a music signal is not really a full-blown continuous sinewave taking 3w per channel. As a rule of thumb the peak to RMS ratio is about 6:1. It's also called crest-factor en.wikipedia.org/wiki/Crest_factor \$\endgroup\$ – Andy aka Jan 27 '15 at 15:24
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    \$\begingroup\$ In simple terms, for 3 watts sine into 4 ohms you need an RMS voltage of 3.5 volts and the peak will be about 4.9 volts (hence the 5v supply to the chip and H bridge driver). The long-term RMS for music will be (say) 6 times lower at 0.8167 volts and this produces a power of about 167mW. This of course impacts on battery life greatly. \$\endgroup\$ – Andy aka Jan 27 '15 at 15:28

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