Constant current driver interfering with audio amp

naigon

Hi everyone,

I'm having some trouble with a project I'm working on. I'm using an STCS2 to drive a high power LED and a TDA7052A to run sound. The trouble I'm having is that when I pulse the LED driver to change the brightness it is causing audible fuzz in the audio amp (at the rate of the pulse I'm assuming). The issue doesn't happen if I use two independent power sources for the audio amp and LED driver, but when they are both on one power source it does have the issue. I've attached two images of the circuit I'm using; they are basically just cut and pastes from the data sheets for the respective components.

I guess that the fast switching of the drain in the STCS2 is causing some type of power irregularities (spikes or drops) but don't know how to debug further. Any help would be greatly appreciated. I'm pretty sure the code is correct as its canned stuff (pulse is pwm from the prop education, wave player is david sloan's 4 from the objex). Plus, it works fine when I'm pulsing a small led instead of the luxeon.

Here are the links to the data sheets for clicking:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00172746.pdf
http://www.nxp.com/documents/data_sheet/TDA7052A_AT.pdf

Thanks!
Scott

Leon

Use two supplies. A similar problem often arises when using a motor.

naigon

Hey Leon,

Thanks for the reply. I don't think two supplies is really a solution, because the final version needs to completely fit into a tube 1.3" inner diameter with maybe 9" of length, so 4 batteries is a bit too large. Is there a circuit that could be used to isolate the power? The LED while powerful doesn't cause issues alone; if I run it with a simple resistor audio still plays well.

Thanks again everyone!

Phil Pilgrim (PhiPi)

Try putting a choke in series with the power going to the amp.

-Phil

Beau Schwabe

Isolate your audio circuit with a diode-capacitor filter.

naigon

Hey Beau,

Thanks for the suggestion, after adding a diode-capacitor circuit to both the Led positive and the audio amp positive the noise is extremely reduced, still noticeable with your ear near it but from 5 feet back you can't hear it. I'm assuming that varying diode/capacitor combinations will filter even better. Right now I have the standard 1N4001 diode and a 1uF cap. Are there better choices for this application?

Phil,
I'm going to get a choke this week and test that as well for the audio amp, as the filter seems to make the biggest difference when applied to the LED positive as opposed to the audio amp.

Cheers,
Scott

Beau Schwabe

naigon,

I'm surprised the 1uF is doing much at all, you need something like 100uF or 1000uF depending on the load requirements on your Audio amp.

naigon

Yes you are right; I read it wrong it was actually 100uF. Also I found a 1000uF in my old parts drawer, and used that with 4x100uf in parallel, and now the noise is completely gone; though I found it to be more effective when placed in front of the LED instead of the audio amp.

Marking this as resolved, thank you, thank you, thank you!

bradharper

I have the same issue, involving the exact same chips:

http://forums.parallax.com/showthread.php?129187-Filtering-PWM-BAM-Noise

I'll definitely experiment with the solution mentioned here.

@naigon
Did you end up filtering both the LED+ and Audio+? What current are you pushing the Luxeon with, and what cap value(s) did you settle on?

naigon

bradharper wrote: »

@naigon
Did you end up filtering both the LED+ and Audio+? What current are you pushing the Luxeon with, and what cap value(s) did you settle on?

I found that filtering the LED side was the only thing required, specifically the positive line to the LED, not necessarily the LED driver. I also noticed slight noise when doing other tasks on the chip so you may want to filter your 3.3v regulator too; I didn't bother since I turn off the audio by disconnecting with an NPN at this point.

Currently I'm running 1A and with 2000uF noise is a little noticeable, 3000uF its gone. I suspect that up to 2A would require 4000uF to 5000uF, but haven't tested. Of course I need to check to make sure they make a cap that big in a small enough SMD package for the final version, otherwise this LED driver will need to be replaced with another.