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Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
References: <email@example.com> <3E2B540B.4720FB1E@ieee.org>
NNTP-Posting-Date: Mon, 20 Jan 2003 06:00:39 GMT
Organization: AT&T Broadband
Date: Mon, 20 Jan 2003 06:00:40 GMT
nospam, analog wrote:
>> The cheapest would be to add a series diode between the regulator
>> output and the 10uF capacitor.
> The 10u capacitor is required for regulator stability.
True, it *is* the suggested value. But the output impedance curves in
the Harris data sheet indicate that the device is stable with an output
capacitor as little as 0.01uF, although this leads to pronounced
impedance peaking at 200kHz. Reading between the lines of the
impedance curves, it appears the smallest damping network that would
provide a good margin of stability would be something like a 22nF
capacitor connected in series with a 22 ohm resistor.
> I would be surprised if the regulator didn't honk with a diode in
> series especially on light loading.
The aforementioned damping network would take of that without killing
the pot during worse case switching - if it were even needed at all.
> Probably honk with the diode rectifying the ac imposed on the output
> which will keep the diode reverse biased and so the capacitor
> isolated for most of the time.
Actually, with one or two milliamps of bleeder current on the output,
the ac resistance of the isolation diode might provide just the right
small signal load impedance to ensure stability.
Still, this solution has other problems (the long settling time) that
would probably make one of the other choices more attractive and
render this discussion more or less moot.
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