Power supply considerations for MCU project using motors and audio amplifier

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Kian

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Hi all,

I am working on a microcontroller project that is powered by a single 3.7V 2000mAh lipo battery. The microcontroller is powered by a LDO which provides 3.3V output.

I have 2 DC vibration motors interfaced to my MCU. The motors are rated for 3.7V (https://www.digikey.sg/en/products/detail/ineed-motor/IND-YZN20-5848/15796061) but can operate over 2-5V. They driven using a NMOS using the following reference design (https://www.precisionmicrodrives.com/discrete-driver-circuits-for-vibration-motors).

At the same time, I am using an mp3 audio module to play sound through a 8 ohms 1.5W speaker. The audio module has an operating voltage between 2.4V to 5.2V. The datasheet shows a typical supply voltage of 3.7V also.

My question is, would it make sense to connect the motor supply and audio module supply directly to the battery? However, as the battery gets drained during usage, the voltage will start to drop. I presume this will cause the motors will become weaker and the sound from the audio module could become softer. If I want to avoid this problem, would it make sense to use a boost converter to maintain a 3.7V or even 5V supply for connecting to the motor and audio module? Or is there another solution?

Thanks in advance!
 
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If everything ran from 5V, the maximum current at that would be under 1A.

I'd probably use a 5V boost converter. The motors would need to be driven with N Channel MOSFETs, to keep the gate voltage relative to the MCU 0V.

Note that with a nominally 3.6V or 3.7V lithium cell, the battery voltage will vary from 4.2V at full charge down to ~3.5V when dead flat.
The regulators must be able to cope with that range!

So the boost needs to be above the maximum battery voltage & the LDO needs to work down to the minimum voltage.

If one of the output voltages was within or too near the battery range, you would need a buck-boost that could both increase and decrease the voltage to the output, which is more expensive.

If you are using voltage regulator modules, get one that's rated 2 - 3A
Most sellers of cheap modules are non technical and use whatever figures seem to look best, often a short-term rating rather than continuous for output current. I've had so-called "1A" modules fail at around 300mA continuous load.
 
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It would most likely be perfectly fine to feed the motors and audio directly from the battery, I'd certainly try it before I'd bother adding a boost converter, which is also going to considerably reduce battery life time.

I'm presuming the vibration motors are tiny mobile phone types?, and that you're aware that you're not likely to get 1.5W from the speaker?, regardless of boost converter or not. Assuming zero losses (which is impossible) you can get a maximum of 0.35W to an 8 ohm speaker from a 5V supply, and about 1.5W if the amplifier is bridged (as it may be). In reality these figures will be considerably less, due to the usual losses. At 3.7V (and 100% efficiency) the single ended value drops to 0.2W.
 
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When the battery is fully charged at 4.2V, the MP3 module's bridged amplifier produces about 1.2W into 8 ohms with 10% awful sounding clipping distortion into an 8 ohms speaker or about 0.75W at low distortion. When the amplifier's supply voltage drops then its output level when clipping distortion begins also drops. When you hear clipping distortion then you must turn down the volume, the amplifier does not do it.
 
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An audio amplifier amplifies the input by a given multiple. As the battery voltage drops, the output signal gets clipped because the voltage cannot accommodate the gain of the amplifier. The output volume doesn't just drop proportionally (the amplifier gain isn't proportional to battery level (in most applications).
 
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