AVR-Chat

> On Wed, Nov 19, 2008 at 4:30 AM, mago Umandam <magzky02@yahoo.com> wrote:
>> Yes, Thanks for response. switch regulator seems to be a good solution.
>> i googled and came to the switch regulator LM5008. I have no 1st hand
>> experience in switch regulator. This 8 pin SMT  IC is capable handling
>> 70V drop at 300mA? Datasheet says efficiency is about 82% at 80Vin
>> 300mA. Will order the part and test and see what happens:)
>
> That's not the only solution..
>
> You could implement a switching pre-regulator, to a volt or so above
> the linear reg's dropout, and use the linear to de-noise the SMPS
> output.
> You can select the SMPS operating frequency to fall at the point of
> your linear reg's maximum point.
>
> In a lot of bench supplies, they use a relay to switch the line
> transformer between two or more output voltages as you change the
> voltage setting.
>
> Same idea, keep the dissipation reasonable, but use linear regulation
> for easy low noise output.
>

Be aware that NO regulator, linear or switcher, that I know of, will
regulate down to zero volts. Almost all are limited by the reference
voltage as the minimum output value.

Dave's suggestion of a switcher followed by a linear is a good one if you
want to minimize noise. It  certainly adds complexity and keeping the the
output of the pre-regulator within the "right" range above the output
regulator is a challenge. Whether or not it is worth the effort is up to
you to decide.

Controlling a regulator with a micro is not simple. There are some schemes
that treat the regulator like an op-amp and the feedback input like the
inverting input of the op-amp. You can inject current through a resistor
to that node to shift the output voltage and that current can come from a
DAC. Or, if you design things correctly, the "resistor" can be THE DAC.

Jim Wagner
Oregon Research Electronics