Is it killing FM reception or only AM? If FM, you may be able to do
better filtering rather easily (such as ferrites on leads). If AM,
then it is likely that considerable shielding may be necessary OR you
may be able to slow down the pulse edges enough to attenuate some of
the higher harmonics. What oscillator frequency are they using?

On Thu, Jul 2, 2015 at 1:10 PM, Bob Blick <bobblick@ftml.net> wrote:
> Question for the resident MC34063A expert (you know who you are!)
>
> I am trying to clarify my understanding of this switching regulator
> chip, so please correct me if I'm wrong.
>
> 1. Oscillator's frequency is set by the timing capacitor only.
> 2. Oscillator's duty cycle is not adjustable but is slightly dependent
> on frequency.
> 3. Current limit pin's threshold is 300 millivolts below V+.
> 4. Current limiting causes oscillator's ON pulse to shorten.
> 5. Voltage feedback inhibits output pulses.
>
> When in normal buck operation and light loading this results in each
> cycle's ON pulse to be its full natural width (no current limiting), but
> the voltage feedback inhibits most of the pulses. On an oscilloscope
> this looks like short pulses at low repetition rate. Loading the circuit
> adds more pulses until the pulses are in bursts. It's continuous mode
> for the inductor during those bursts and I can see the inductor current
> rising through each burst. Further loading and the bursts have enough
> pulses in them to cause the inductor current to rise and current
> limiting kicks in near the end of each burst. Further loading finally
> gives no gap between pulses and there's full continuous mode and we've
> reached the limit. Further loading and the output voltage drops.
>
> The reason I'm asking this question is because the various things I plug
> in the 12V outlet in my car seem to mostly have these regulators and it
> really ruins radio reception. Looking at this chip makes me think
> there's not much hope of fixing the problem without going to a different
> type of switching regulator. No amount of added filtering or upgrading
> of parts is going to fix this bursty design.
>
> Does that seem about right?
>
> Thanks for any comments or sources for alternative ready-made adapters
> that are better.
>
> Bob
>
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