> i'm doing a project that has a SMPS to step-up a couple AA to 5V.
> The SMPS is a TI TL499A. Has a linear and switch regulator inside 
> the
> package.

> And i still get noise of the SMPS on my audio output.
> The audio noise from the ISD is like a smooth crackling (frying).
> The audio noise from the PWM is of a very high frequency.
> The audio noise is proportional to the switching freq of the SMPS. 
> The lower
> the batteris, the noisier the audio get.
>
> When i power the circuit through the Linear regulator inside the 
> chip, all
> the switching noise disappears. Its very clean.
>
> Does someone have an idea of what i can do/should improve on my 
> design ?

What output filtering do you have from SMPS?
A series inductor (tens of uH+, higher the better within reason) with 
capacitor before and after should help.

Ground connection from SMPS should meet ISD and system ground at a 
single point. There should be NO ground loops - all ground paths 
should star from the one point as much as possible.

The linear regulator in the SMPS chip may tell you something about 
their expectations :-)



        RM

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