Re: Using a uC for DC-DC conversion?

From: Ken Smith (kensmith_at_green.rahul.net)
Date: 10/12/04 

Date: Tue, 12 Oct 2004 22:04:48 +0000 (UTC)

In article <L5Xad.2632$6q2.2342@newssvr14.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]
>What I mean are trade-offs with respect to PWM granularity, for example.
>Suppose the counter runs off 5MHz. In order to arrive at reasonable cost
>for the magnetics the PWM output needs to be, say, 300KHz or higher.
>That leaves only 4 bits of granularity. So it would idle like a Harley
>with worn spark plugs, something that may be ok.

Some of Linear's chips use "burst mode". I've seen switchers that used
"hit and miss" regulation where the transistor either turned on for a
fixed time or didn't. This is basically a granularity of 1 bit.

In both cases, the output capacitor must be larger to reduce the ripple.

> Then there is the
>feedback. Most uCs with an on-board ADC are out of price range so slope
>may have to do. On top of that there may be a limit of one slope
>conversion unless eternal muxing is done.

Don't some low cost micros have comparitors? You don't really need
anything like a good adc. You only really care right near the regulation
point. Chances are a "too high" / "too low" signal would be enough to
make a regulator that worked.

[...]
>making the code that runs the PWM safe and fast enough. After all, one
>minor hangup in this area could result in a plume of smoke.

Design carefully. If you leave the ground pin of a switcher chip
disconnected you get the same result.

> Next, there
>is the trend to ever lower VCC levels which renders the task of turning
>on a FET hard enough non-trivial.

Use a PIC they still run at 5V. If you have to make a level shifter, you
may as well use a switcher chip in place of it. 

-- 
--
kensmith@rahul.net   forging knowledge

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