YD wrote:
PeteS wrote:

Tom Del Rosso wrote:
"John Larkin" wrote in
message
It's like with Saabs: great cars to drive, between fires.
That's just a Saab story.

I've heard of tantalums exploding if reversed, but otherwise there
must be some parameter that's being pushed. Voltage not derated
enough?


Tants have a problem with surge currents more than voltage, which is
why they are not usually suitable for the input to a supply, and are
ok at the output provided it's a fairly stable load and can be soft
started. The critical rating for me is the rms and peak current v.
temp.

Cheers

PeteS

Guess I could look this up, but what makes tants more/less desirable
for which reasons?

- YD.

Tants have a very high capacitance density which is always desirable in
constrained space. Note that MLCC devices can also have high density,
although not as high as tants. MLCCs come with their own pros and cons,
of course.

They have low (but not very low) esr which can be a desirable trait,
particularly with LDOs and some switch mode converters (which was the
issue at the start of this thread). That comes down to (simplistically)
providing an output zero to maintain the feedback phase margin. Some
manufacturers have started to get the clue and provide converters of
this type that are stable with ceramics (esr in the 10s of milliohms or
lower). I've done converters that are stable with ceramics only but it
takes a little more work (it's necessary to add extra pole/zero pairs,
basically which means more components). Typical LDOs (such as the ones
from National) require an output esr in the range of 0.5 - 10 ohms for
stability incidentally.

The wet tants don't have particularly good esr characteristics across
extended temperature so sometimes it's a good idea to put a ceramic in
series with a small resistor in parallel with the tant to maintain the
output esr for loop stability reasons. Note that the increase in esr
increases the surge susceptibility.

Tants are very susceptible to high surge currents (di/dt rate seems to
be as much an issue as absolute surge) and can fail spectacularly
(pyrotechnically, in fact).

That's my view of them - how it affects where I use them depends on what
I need to do, of course. Personally, I only use them if I have no other
choice.

Cheers

PeteS