Audio amplifier to drive a PCB coil  - Page 1
Forum section: electronics / projects

author: DigitalDesigner
author_rank: Contributor
timestamp: May 04, 2025, 05:19:56 am
content: Hi,I need to drive a coil on a PCB for a magnetics application (not audio) and I would like to use an audio amplifier like the PAM8302A (datasheet attached here). The reason for choosing a class D is efficiency. The frequency range is up to 18KHz.The supply available on the board is only 3V3, the coil DC resistance is 16R and the impedance is 122uH. The maximum current the coil needs to be driven (signal at its peak) is 190mA.A couple of questions if someone could please help:1. Can I use the PAM8302A with that coil from a 3V3 supply? What are the parameters that I need to check and/or be aware of for this applications?2. Can that IC drive that coil at 190mA from a 3V3 rail and 16R? In the datasheet it mentions driving 4R and 8R so I assume it can supply the current, but I have no idea if the inductance causes some other issues? Or perhaps the higher resistance/impedance causes some other issues?3. If that is not suitable, are there other ICs which are?Thank you

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2025, 09:44:00 am
content: Quote from: DigitalDesigner on May 04, 2025, 05:19:56 amSpeakers are inductive so it should work fine.

author: jwet
author_rank: Frequent Contributor
timestamp: May 05, 2025, 04:10:10 pm
content: The amp is certainly capable of doing something like this- it can drive the load I think- but you're kind of running out of headroom for your current.Can you provide 5v to the amp? This will likely help but might not be enough still.Your load will look like 16 ohms in series with 122 uH- the complex impedance will be 16 + j13.8 ohms at 18 Khz. The complex and real components are nearly equal. This means that current and voltage will be out of phase at about 45 degrees. Go through the numbers or throw it at a simulator to see that what you'll get is what you expect.

author: zapta
author_rank: Super Contributor
timestamp: May 05, 2025, 04:17:46 pm
content: In the datasheet, the typical load inductance is 4.7uH, and there is no specification of max inductance. Yours is 122uH. I don't know what it means but worth trying.https://www.diodes.com/datasheet/download/PAM8302A.pdf

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 05, 2025, 05:57:13 pm
content: At a glance it looks like it should work. The datasheet makes it look very simple, just a simple PWM modulator with no internal feedback. You might be able to infer its linear range for your load by examining the THD plots.The large inductance is going to limit the achievable peak output current at high frequencies (122uH at 18KHz is 13.8ohms of reactance).

author: macboy
author_rank: Super Contributor
timestamp: May 06, 2025, 12:13:13 pm
content: 190 mA RMS requires about 3.04 V RMS, but that is a sine wave with a voltage swing of +/- 4.3 V. You can't get there with a 3.3V supply unless that amp does some fancy tricks like using a charge pump to produce a higher voltage rail.Look at the datasheet spec for power delivered into 8 ohms at 3.3 V supply. Calculate the voltage being delivered to the 8 ohm load at that power. Assume that the voltage will be the same for the 16 ohm load then calculate the current for 16 ohms. I assume it will fall far short of 190 mA.

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 06, 2025, 01:01:21 pm
content: Quote from: macboy on May 06, 2025, 12:13:13 pmOP said they need 190mA peak, not RMS.Looking closer at the datasheet, at Vdd=3.6V and RL=8ohm, the THD starts to shoot up at 0.7W out. That's equivalent to 3.347V peak, about 93% of the supply rail. So at Vdd=3.3V you might get 3.07V peak, which would be enough to drive 191.8mA peak into a 16 ohm load. So very marginal, but possible.

author: DigitalDesigner
author_rank: Contributor
timestamp: May 11, 2025, 01:54:04 am
content: Thank you for all the replies.From your feedback it looked like it could work so I decided to rapid prototype one and it does work. Like mtwieg said it is marginal but I had other parameters I could relax so it now works.Thank you everybody!