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I'm a hobbyst trying to design a simple circuit for amplifying the PWM audio output of an ATTINY85. I'm using an LM4890 (1W amp) and a 1W microspeaker. The problem comes when I supply 3V with a small coin cell, CR1616, which has a small capacity of 60mAh. The circuit works but the battery is drained fast. I checked the datasheet for the LM4890 but I'm still not sure how to calculate the resistor values for using less gain or reducing power consumption. I'm considering to add a regulator for the battery so the sound frequencies can be more stable. Any suggestions are highly appreciated. Below is the circuit:

enter image description here

LM4890 datasheet: http://www.ti.com/lit/ds/symlink/lm4890.pdf

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  • \$\begingroup\$ The CR1616 is a tiny cell, so only if you use this circuit for short periods of time and power down everything when not in use (not producing sound) can the battery last a reasonable time. I see that the SHUTDOWN pin of the LM4890 is connected to 3 V so it is always on. How do you intend to use this circuit? Does it always need to produce a sound when it is on? If not then use the microcontroller to shutdown the amplifier when not used. \$\endgroup\$ – Bimpelrekkie Mar 14 at 11:26
  • \$\begingroup\$ When the circuit is on it will always be outputting sound, constantly, so unfortunately the shutdown has no use for this purpose. \$\endgroup\$ – Chu Mar 14 at 11:28
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    \$\begingroup\$ It won't. Coin cells are designed for loads in the microamp range, and waste most of their energy in their own internal resistance when drawing such a heavy load. There's no free lunch here -- you need to reset your expectations. Either use a much beefier battery (perhaps a couple of AAA cells?) or reevaluate your power budget, and produce a lot less audio power. \$\endgroup\$ – Dave Tweed Mar 14 at 11:39
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    \$\begingroup\$ I'm getting only about 2 minutes of operation with a new battery Then obviously the battery is much, much too small for your application. It is not even the 5 mA quiescent current but simply the power that needs to go into the speaker. It is far too much for a CR1616. Even other small Lithium cells will not cut it. You really need to go to AAA size or larger batteries for the amount of power your application needs. \$\endgroup\$ – Bimpelrekkie Mar 14 at 11:47
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    \$\begingroup\$ You may want to consider a class D amplifier. The LM4890 is a linear amp, so it is, at best, a class B device which, in turn, means that it is, at best, 50% efficient -- and that's only at full volume with a nice clean sine wave input. Lower volumes or higher crest factor inputs will be less efficient. A class D amp would be bulkier, but would use less (possibly far less) energy for the energy sent to the speaker. But conservation of energy is still a thing. \$\endgroup\$ – TimWescott Mar 14 at 15:24

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