Why no depletion mode LDOs?

[Joerg]

Occasionally I need an LDO and I hate those things because of inherent
instabilities. So I usually do not use commercial ones. My own LDO
designs sometimes contain depletion mode FETs when I don't have the
space for a charge pump or radio silence is required. Downside is there
seem not to be many folks doing that, not much of a variety of FETs to
pick from and the bigger depletion mode devices cost over a buck. Ouch.

Why are there no commercial LDOs with depletion mode devices? Or are
there? At least I am not the only one with such "wicked thoughts",
there's even some in Arizona:

http://repository.asu.edu/attachments/56281/content/Lepkowski_asu_0010E_10141.pdf

Of course there is the issue of channel saturation but for smaller stuff
they work.

 

[Joerg]

I seem to recall that there's something about the construction of the
usual power FET that makes it only work in enhancement mode -- so a
depletion mode power FET would be considerably harder to make. This is
probably why they're expensive and rare.

Well, if you look at the SOA of this dude here that's mighty impressive:

http://ixdev.ixys.com/DataSheet/DS100260(IXTH-T16N20D2.pdf

But ... five bucks :-(

The idea of an LDO with a charge pump so that you can go common-source is
interesting -- are there any commercial devices that do this?

No idea. Dang, should have patented it

With tubes life was quite a bit easier when it came to regulators since
they were pretty much all depletion mode devices. But the filament was a
pain because sometimes the tube couldn't take much voltage between it
and the cathode.

 

[John S]

I seem to recall that there's something about the construction of the
usual power FET that makes it only work in enhancement mode -- so a
depletion mode power FET would be considerably harder to make. This is
probably why they're expensive and rare.

Supertex makes a number of depletion mode FETs available through Mouser
for under a dollar in single quantities.

 

[Joerg]

Supertex makes a number of depletion mode FETs available through Mouser
for under a dollar in single quantities.

However, big ones like their DN2625 are also around a buck.

 

[Joerg]

you can get LDOs with a seperate input pin for a higher voltage bias,
so
most of it is there

A lone pin doesn't qualify as "most of it" though

 

[John S]

However, big ones like their DN2625 are also around a buck.

Oh. My mistake. I did not see where you stated you needed "big" ones.

 

[Joerg]

Oh. My mistake. I did not see where you stated you needed "big" ones.

I don't need them this big but reasonable, the usual few hundred mA. No
problem, I just wondered why none of the big semi mfgs do that.

 

[Joerg]

LTC's LTC3035 has an internal charge pump they have others with an
inductive booster.

http://www.linear.com/product/LTC3035

I'd bet LTC was the first company to figure this out. Unfortunately that
little LDO costs over a buck :-(

Mine's gonna cost less

 

[Joerg]

Most systems have some higher voltage around. So an LDO could have a
source follower topology with an extra input for the gate drive. I've
built my own LDOs that way.

Usually I have only one voltage coming in and have to make anything else
from that. Not a big deal to create a voltage above the input rail but
it'll always cost a few diodes and caps, meaning valuable space.

 

[Joerg]

even with one voltage coming in it isn't unusual that you need a few
closely spaced low voltages for say a an FPGA or cpu core and IO
so you can have a switcher for the highest and LDOs for the rest
and use the board input for the gate drive

Most of the time I do not use FPGA or any programmable has just one
supply. But there are situations where there simply is not enough space
for a switcher.

 

[Michael Wieser]

Occasionally I need an LDO and I hate those things because of inherent
instabilities. So I usually do not use commercial ones. My own LDO
designs sometimes contain depletion mode FETs when I don't have the
space for a charge pump or radio silence is required. Downside is there
seem not to be many folks doing that, not much of a variety of FETs to
pick from and the bigger depletion mode devices cost over a buck. Ouch.

Why are there no commercial LDOs with depletion mode devices? Or are
there? At least I am not the only one with such "wicked thoughts",
there's even some in Arizona:

http://repository.asu.edu/attachments/56281/content/Lepkowski_asu_0010E_10141.pdf

Of course there is the issue of channel saturation but for smaller stuff
they work.

http://www.austriamicrosystems.com/Products/Power-Management/LDOs

hth

-
Michael Wieser
--

 

[Joerg]

http://www.austriamicrosystems.com/Products/Power-Management/LDOs

hth

Which one? The ones I checked are of the usual PMOS architecture, like
this one:

http://www.austriamicrosystems.com/...92/version/4/file/AS1351_Data_Sheet_v1-21.pdf

 

[Tim Williams]

Sounds like HMOS -- IIRC, the 8088 was among them, essentially regular NMOS
circuitry, except the drain load resistor (which is a diffusion anyway and
subject to nonlinear MOS effects, if used as such) was constructed a bit
differently, and doped / implanted enough to make it behave as a saturated
(that is, current saturation region) NMOS transistor, a crude CCS.

Tim

 

[josephkk]

I don't need them this big but reasonable, the usual few hundred mA. No
problem, I just wondered why none of the big semi mfgs do that.

They are also big bureaucracies, think it through.

?-)

 

[Joerg]

They are also big bureaucracies, think it through.

Yeah, bureaucracy usually stifles everything in its path.

But, what can I say, one of them did come through: Infineon. In Germany
there is one of those eternal jokes going around, that Siemens (from
which they were spun off) is in reality a huge bank with an attached
electric division. Long story short, their customer service used to be
the pits and now their web site is thoroughly screwed up. Even the
support form does not work. Then I usually write to their investor
relations folks or whatever, because their clientele won't put up with
captchas annd such nonsense. Got an answer and ... SPICE models! They
have a whole line of depletion mode FETs, the BSP1xx series. It's like
going to a Levy's jeans rack, you just pick you size. In this case Rdson
and channel saturation current.

 

[josephkk]

Yeah, bureaucracy usually stifles everything in its path.

But, what can I say, one of them did come through: Infineon. In Germany
there is one of those eternal jokes going around, that Siemens (from
which they were spun off) is in reality a huge bank with an attached
electric division. Long story short, their customer service used to be
the pits and now their web site is thoroughly screwed up. Even the
support form does not work. Then I usually write to their investor
relations folks or whatever, because their clientele won't put up with
captchas annd such nonsense. Got an answer and ... SPICE models! They
have a whole line of depletion mode FETs, the BSP1xx series. It's like
going to a Levy's jeans rack, you just pick you size. In this case Rdson
and channel saturation current.

Freaky.

?-)