Bob,
I am not at all an expert in the MC34063A, but as you said, it is widely us=
ed in regulators for modern low voltage device chargers. I faced a similar =
problem with ham radio receivers, and I solved it with a
simple modification.
With a scope, I evaluated the amplitude of the residual noise at full load.
Then I modified the feedback divider to raise the output voltage by a bit m=
ore than the peak voltage of the noise, and added a LDO regulator with prop=
er capacitor decoupling, and got rid of all the noise.
Lastly, I put the whole thing in an aluminum bow that I connected to the gr=
ound for EMI shielding, and
a large capacitor on the 12 V input. That solved it in my SDR receiver.

Just my $0.02,
Jean-Paul
AC9GH




> On Jul 2, 2015, at 1:10 PM, Bob Blick <bobblick@ftml.net> wrote:
>=20
> Question for the resident MC34063A expert (you know who you are!)
>=20
> I am trying to clarify my understanding of this switching regulator
> chip, so please correct me if I'm wrong.
>=20
> 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.
>=20
> 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.
>=20
> 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.
>=20
> Does that seem about right?
>=20
> Thanks for any comments or sources for alternative ready-made adapters
> that are better.
>=20
> Bob
>=20
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>=20
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