D said:
On Fri, 27 Apr 2007 00:10:02 GMT, Joerg
D from BC wrote:
On Thu, 26 Apr 2007 13:29:50 -0700, Joerg
D from BC wrote:
[snip]
I can guess at why you're not finding one....
1) This is near the drop out voltage of typical regulators
Drop-out doesn't matter. You just feed in enough volts. It's easy to
buy 2V linear supplies, even 0.5V duals. But no 3.3V.
[snip]
Ohh come on.... it does matter a bit..
Let's say the drop out voltage is 3Volts..
Regulator dissipation is 2A*3V=6 watts (or more)
A monolithic regulator might have some yucky drift too at 6Watts..
That's nearly = to the load dissipation at 6.6Watts.
I'm saying that a large dropout voltage results in a less efficient
linear regulator.
Sure but we live in Caleefohniah where the power can be anywhere between
110V and 130V, or sometimes 0V. So you need dropout just to accommodate
that. Dissipation doesn't matter right now and I am not a friend of
monolithic regulators for anything above a few hundred mA. The LM317 is
pretty much as far as I will go in that domain. I like the uA723 a lot,
plus some nice fat pass transistors.
The latest linear regulator I've played with is the LP2951.
Datasheet says a 3.3V version is available.
Of course pass transistors will be needed for 2A.
For example:
Digikey LP2950ACZ-3.3-ND is in stock.
The National LDO series is where I got my first big black eye. I was
already prejudiced against LDO but a client really liked it so they said
we should use it. The client is king, so I had to. Fired up the prototype,
the output voltage came up sluggishly and a "high amperage smell"
developed. Hung a scope onto the rail and it was singing the blues,
oscillating like crazy. Turned out it didn't like it when the source
impedance got too high but we had no choice. Naturally, that was not
mentioned in the data sheet. App Engineering told us more or less "You are
right, it cannot handle hi-Z, sorry about that". Great. I forgot the exact
part number. IIRC its wasn't LP but something like LM2931. I forgot
because I vowed never to design one in again. And I never did, threw away
the datasheet.
Not that I want to diss one particular vendor here. I've come across more
than one hardcore LDO problem. For example a TPS71550 that "didn't like
it" when the supply came up too fast. Phsssst ... BANG. Was that mentioned
anywhere? Oh no. TI wanted scope plots and what not. I had sent them the
rather simple schematic but they were not willing to throw it onto SPICE.
Would have been easy but not for the customer since they do not release
the innards anymore. So I voted that one off the island as well ;-)