PICdude wrote:
> I'm sure I've used the LM2678 before -- it's an older chip, but worked  =

> well. (National's switchers work well with minimum of futzing).  But I  =

> tend towards the newer chips as they run at higher clock speeds and  =

> passives come down in size, bringing footprint and cost down.  Their  =

> webench tool gives you an estimated BOM cost so that's a good place to  =

> look for a net answer.

I had a play with WEBENCH, and the stupid thing kept crashing out =

(usually taking Firefox with it). The parametric search thing isn't much =

better...

At this point I couldn't give a flying f00 whether it's a synchronous or =

non-synchronous regulator, has an internal switch or wants an external =

FET, or whatever. All I want is a 5V / 5A switching regulator that can =

take 12V or so on the input (or more, more is good).

At this point I seem to have a few options:

   - 5V 5A external PSU. Cost ~=A325, comes with standard power plug.
     Problem scenario 1: someone plugs in a 12V PSU and blows the power
       sequencer / SMPSU chip and potentially the FPGA and PIC too (if
       the FET shorts out). Not good...
     Problem scenario 2: PSU plugged in has the wrong polarity. Same
       result as above. A nice big Schottky diode should stop this
       happening...
     Potential solution: custom power connector. I don't even want to
       think how much THAT would cost.

   - 12V 3A external PSU and LM2678. Cost ~=A325 for the PSU brick, =A312 i=
sh
     for the National SMPSU chip and support components (and an RPP
     Schottky).
     Problem scenario 1: Someone plugs in a 12V PSU with reversed
       polarity. Add a Schottky diode across the power plug to push the
       PSU into short-circuit protect mode. Should work as long as the
       PSU is smaller than ~50W (i.e. not a Nemic-Lambda EWS600 or
       something equally brutish).

In an ideal world I'd buy a box of Cisco ADP-20s and use them -- they're =

cheap on the surplus market, made by a fairly reputable PSU manufacturer =

(Delta), use a stock-standard Molex connector, and provide +12V, +5V and =

-12V. Problem is that I'm left up the creek if/when Cisco discontinue =

them (which would seem to be "soon").

What would be even better is a bunch of PSUs for external USB drive =

caddies -- but those seem to be extremely hard to find as spare parts.

Why is it always the "simple" things that cause the most trouble?

> Alternatively, isn't there some type of protection mechanism you could  =

> put in?  Or perhaps change to a custom power connector?

See above -- a Schottky diode will deal with reverse-polarity, a =

Zener+SCR crowbar will *theoretically* deal with overvoltage (but IME =

they're slow to react -- slow enough that the SMPSU chip will be quite =

dead by the time the SCR triggers).

To put it simply: I don't want to have units coming back from the field =

that have been damaged (needing a new crowbar SCR, Zener, etc counts as =

"damaged") just because some complete fool plugged the wrong PSU in. =

8-40V input covers just about all the PSUs I've got within easy reach. I =

think I might be able to Rube Goldberg my Farnell L30B into giving me =

60V, but that's pushing it. My two standard bench PSUs (Instek PSP2010s) =

are 20V/10A.

-- =

Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/

-- =

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