Bizzare behaviour from SG/UC3525

[Pooh Bear]

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Misread. Should be N.Inv IN________|

The max CMV of the error amp is 5.2V, so it could
be possible to connect pin 2 to the 5.1V ref and
pin 2 at least 0.7V below it. This should put
the output of the error amp at 3.8Vdc.

That is indeed what I've done.

Graham

 

[Pooh Bear]

I'm beginning to wonder whether a more modern chip
(LT1105?) might be easier/cheaper..............

Me too ! The 3525 was chosen simply since almost *every* similar design
I've seen uses it.

LT are a bit pricey normally though. I'm wondering about On-Semi.

Incidentally, another company ( Italian FWIW ) makes their 'audio smps'
with one of IR's 'electronic ballast driver' chips. Basically any old
cheap way to make the necessary waveforms !

Graham

 

[Pooh Bear]

Off a 1989 Silicon General data sheet, 6th page,
labelled Shutdown Options. Just above the test
fixture circuit.... which also leaves pin 10 open
when running.

Cor blimey !

Can you still get a *genuine* SG data sheet for this part ?

Just to be sure. Pull COMP to 0v to do a shutdown
via the PWM stage and R-S flipflop.



Resistor needed in series with the pulldown Tr then.
Yup.



Thanks. Best guess atm would be up near the 5v1 Vref.

BTW: It's only a 60uA pulldown current. If I assume
47uF, and a required pulldown from 4V to 0.5V,
then that would mean that the Shutdown pin has to
be held high for 2.7 seconds before a full (safe)
soft re-start can happen.

That accords with my results.

Graham

 

[Frithiof Andreas Jensen]

How much ( value, type ) would you typically use ? 0.1uF box film polyester type
here. An MLC type presents no probs to me but modern film types are pretty

hot.

I seem to remember that I had *three* different values on there, probably:
100 nF, 10 nF and 500 pF and a funny layout too, there was an application
note on the proper placing of the capacitors so they were put in first, then
all the rest had to fit wherever it could.

I think there were shottky's on the outputs too so that they could not pull
the substrate negative.

That sounds suspiciously likely !

Those fast drivers are always a pain in the bum!