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Ldo regulators

L

Ldo

Jan 1, 1970
0
I'm interested in interfacing to a board that sports a ldo regulator that
fix the supply to 3.3V. The regulator is REG102NA-3.3 Actually i don't need
such a regulator beacuse the only supply i have is already 3.3V. I was
wondering if i can feed the regulator with 3.3V (Vin = Vout in this case, no
dropout) and have the board working anyway.
 
L

Lostgallifreyan

Jan 1, 1970
0
I'm interested in interfacing to a board that sports a ldo regulator
that fix the supply to 3.3V. The regulator is REG102NA-3.3 Actually i
don't need such a regulator beacuse the only supply i have is already
3.3V. I was wondering if i can feed the regulator with 3.3V (Vin =
Vout in this case, no dropout) and have the board working anyway.


Something about this post makes me want to find a blanket to bite.

Ok, if you have a regulated 3.3V supply, and a regulator on a board that
would do the same thing, why not just bypass the onboard regulator?
Whatever you do, you're not going to get a zero volt drop from a voltage
regulator, unless it's a power converter. Even so, there's no point in this
case. Just check your 3.3V supply carefully, and if it's good, bypass the
onboard regulator.
 
J

Joerg

Jan 1, 1970
0
Lostgallifreyan said:
Something about this post makes me want to find a blanket to bite.

A piece of wood may be better. Less fuzz between the teeth after letting
go of it :)

Ok, if you have a regulated 3.3V supply, and a regulator on a board that
would do the same thing, why not just bypass the onboard regulator?
Whatever you do, you're not going to get a zero volt drop from a voltage
regulator, unless it's a power converter. Even so, there's no point in this
case. Just check your 3.3V supply carefully, and if it's good, bypass the
onboard regulator.

Plus with all luck it might begin to oscillate. Many LDOs will easily
start to sing. Some have an undocumented failure mode where they choke
up upon seeing or thinking to see a rising source impedance. Then the
current rises and rises. Bzzzt - pop - POOF.
 
L

Lostgallifreyan

Jan 1, 1970
0
A piece of wood may be better. Less fuzz between the teeth after
letting go of it :)

I must practise this. If my crowns will let me. >:)
Plus with all luck it might begin to oscillate. Many LDOs will easily
start to sing. Some have an undocumented failure mode where they choke
up upon seeing or thinking to see a rising source impedance. Then the
current rises and rises. Bzzzt - pop - POOF.

Nice thought. I tend to avoid LDO regulators, preferring to reduce the drop
at the rectifier, but I might have to use one some time. Any advice on
which types, or makes, are generally well behaved?
 
J

Joerg

Jan 1, 1970
0
Lostgallifreyan said:
I must practise this. If my crowns will let me. >:)




Nice thought. I tend to avoid LDO regulators, preferring to reduce the drop
at the rectifier, but I might have to use one some time. Any advice on
which types, or makes, are generally well behaved?


Just got a black eye with the TPS71550. It seems to become unstable with
too much input impedance, resulting in self destruction (they don't have
an overtemp limit) and occasionally taking everything connected into the
grave with it. TI hinted that there might be an issue with source
impedance but insisted on scope plots before they'd investigate further.
It is hard to take scope plots if it blows after a few hundred cycles
and you don't know when. So, I guess we'll ditch that regulator now. I
had sent them my rather simple schematic and they could have thrown that
on the simulator. Nope, not going to happen, they said :-(

Thing is, I can't simulate because they won't release the innards of it.
Ok, I had a 7uA to 2mA load change. Muffled it by easing it on over a
millisecond. They said that might still make it choke up if there was a
high input impedance. My take is that this stuff needs to be mentioned
in datasheets. Either that or good enough SPICE models.

Then there was a National part, LM29xx or something like that with the
same issue. At least they vowed to change the data sheet and mention
that problem.

Then there were a whole lot of others. Forgot the P/Ns and various mfgs
because these weren't my designs. I did the re-design and flung all the
LDOs out the front door immediately.

Anyhow, I won't use LDOs unless there is absolutely no other way.
Usually there is.
 
L

Lostgallifreyan

Jan 1, 1970
0
Just got a black eye with the TPS71550. It seems to become unstable with
too much input impedance, resulting in self destruction (they don't have
an overtemp limit) and occasionally taking everything connected into the
grave with it. TI hinted that there might be an issue with source
impedance but insisted on scope plots before they'd investigate further.
It is hard to take scope plots if it blows after a few hundred cycles
and you don't know when. So, I guess we'll ditch that regulator now. I
had sent them my rather simple schematic and they could have thrown that
on the simulator. Nope, not going to happen, they said :-(

If they won't accept your proof at that basic level, they won't accept
anything more elaborate. You made the right call, don't waste your time.
Thing is, I can't simulate because they won't release the innards of it.
Ok, I had a 7uA to 2mA load change. Muffled it by easing it on over a
millisecond. They said that might still make it choke up if there was a
high input impedance. My take is that this stuff needs to be mentioned
in datasheets. Either that or good enough SPICE models.

I've never used a SPICE model in my life. I'm not at all a high-powered
tech though, I guess I don't need to. Even so, it's just a model. Plenty of
people who ARE high=powered techs and better will say similar things. As
with laser diodes, it's far better if the company will push the envelope on
four samples from different batches and post that data in an addition to
the data sheet. It's easier for them, and it means caveat emptor for the
buyer, who at least has something real, not a model or a guess, to go on.
Then there was a National part, LM29xx or something like that with the
same issue. At least they vowed to change the data sheet and mention
that problem.

I like their parts, generally. That news is good. I buy my LM317's from
them, in (very) modest bulk. :) I'd try their LDO's so long as they tell it
like it is.
Then there were a whole lot of others. Forgot the P/Ns and various mfgs
because these weren't my designs. I did the re-design and flung all the
LDOs out the front door immediately.

Anyhow, I won't use LDOs unless there is absolutely no other way.
Usually there is.

Yes. I think if the overhead voltage is pared down to the point where there
is not, it's a bad design anyway.
 
L

Ldo

Jan 1, 1970
0
Sorry, i was in a hurry and i wrote the post before going to bed. And sorry,
i also took the only blanket left (it was pretty cold and i couldn't get to
sleep).

There are 2 boards involved.
- Board A provides a regulated 3.3V
- Board B gets its power from board A
- Board B has the LDO regulator (3.3V) and i guess it was designed to be
used with supplies greater than 3.3V.

I cannot cut the piece of board containg the regulator because it's not me
who made the boards. Maybe i could try to bite the regulator and see if it
breaks.

Now i would like to know whether applying 3.3V at Vin i get 3.3V at Vout
anyway (regulated or not) If i can't, i cannot use the board B, unless i
find 5V somewhere.
 
J

Joerg

Jan 1, 1970
0
Lostgallifreyan said:
If they won't accept your proof at that basic level, they won't accept
anything more elaborate. You made the right call, don't waste your time.

That was pretty much verbatim my thought process after the last phone
call with them. TI is a really good company but I was a bit taken aback
by their unwillingness to throw it onto the simulator. It would have
taken a guy there maybe 30 minutes in total and they might have
discovered something serious with their own product.

BTW I had a similar issue on Monday, totally non-technical. Well, maybe
not totally. The property tax bill looked iffy. So I called. After a bit
of run-around I got the auditor's office on the line and they had me fax
it in. They called back, profoundly apologizing. The final number was ok
(they said...) but the line item display was pretty hosed. Nobody had
noticed that the computer was doing this since who knows when...
I've never used a SPICE model in my life. I'm not at all a high-powered
tech though, I guess I don't need to. Even so, it's just a model. Plenty of
people who ARE high=powered techs and better will say similar things. As
with laser diodes, it's far better if the company will push the envelope on
four samples from different batches and post that data in an addition to
the data sheet. It's easier for them, and it means caveat emptor for the
buyer, who at least has something real, not a model or a guess, to go on.

If the models are good SPICE is ok. Once I discovered a reliability
issue that was impossible to measure with the usual scope technology
available in the early 90's.
I like their parts, generally. That news is good. I buy my LM317's from
them, in (very) modest bulk. :) I'd try their LDO's so long as they tell it
like it is.




Yes. I think if the overhead voltage is pared down to the point where there
is not, it's a bad design anyway.


Depends. When you have to use three AA cells and the uC needs at least
3.6V you are between a rock and a hard spot. But I have wised up and use
switchers now, mostly sans PWM chip for cost and other reasons. Wasn't
really possible in the TPS case though. That will require some nifty
discrete linear design with 0201 parts and stuff. Guess I'll need better
reading glasses now.
 
L

Lostgallifreyan

Jan 1, 1970
0
Sorry, i was in a hurry and i wrote the post before going to bed. And
sorry, i also took the only blanket left (it was pretty cold and i
couldn't get to sleep).

No worries, Joerg's suggestion of wood should do nicely.
There are 2 boards involved.
- Board A provides a regulated 3.3V
- Board B gets its power from board A
- Board B has the LDO regulator (3.3V) and i guess it was designed to
be used with supplies greater than 3.3V.

I cannot cut the piece of board containg the regulator because it's
not me who made the boards. Maybe i could try to bite the regulator
and see if it breaks.

Mhmm. I'd stick to blankets. And pitch pine.
Now i would like to know whether applying 3.3V at Vin i get 3.3V at
Vout anyway (regulated or not) If i can't, i cannot use the board B,
unless i find 5V somewhere.

There is a voltage drop in a linear regulator that applies no matter what
goes in, so you won't get your full 3.3V out. I haven't tried, but I think
you might be ok to short the input and output of the regulator on board B
if it's a standard three terminal type. Even if this made it try to draw
more power it can't destroy itself when there is no way that current can
actually go through it, as it's shorted. It would be simpler than removing
it, and easy to undo later if you get a different supply.
 
D

Dave

Jan 1, 1970
0
Ldo said:
I'm interested in interfacing to a board that sports a ldo regulator that
fix the supply to 3.3V. The regulator is REG102NA-3.3 Actually i don't
need
such a regulator beacuse the only supply i have is already 3.3V. I was
wondering if i can feed the regulator with 3.3V (Vin = Vout in this case,
no
dropout) and have the board working anyway.

According to the data sheet for the REG102NA-3.3, it states the minimum
input voltage for normal operation is Vin + 0.4V. You probably won't be
able to get this part to act correctly with a 3.3V input.

Can you unsolder the REG102NA-3.3 part and add a jumper to allow the
external regulated 3.3V into the board?

Dave
 
L

Ldo

Jan 1, 1970
0
Lostgallifreyan said:
No worries, Joerg's suggestion of wood should do nicely.


Mhmm. I'd stick to blankets. And pitch pine.


There is a voltage drop in a linear regulator that applies no matter what
goes in, so you won't get your full 3.3V out. I haven't tried, but I think
you might be ok to short the input and output of the regulator on board B
if it's a standard three terminal type. Even if this made it try to draw
more power it can't destroy itself when there is no way that current can
actually go through it, as it's shorted. It would be simpler than removing
it, and easy to undo later if you get a different supply.

I haven't bought the Board B yet, but i guess it should deploy the ldo in a
"small"
package, the Board itself is simple but not so cheap. I'm not very good in
soldering, let's say i'm not very patient in jobs that are not creative
(someone could say soldering is an art, i agree but that's not my case).
That said i'm afraid that if i try to do the "short" i could end up in
biting my hands this time!

I have to build a small adapter board, i hoped i could get away with just
connectors. Now i should use charge pump ICs to raise 3.3V to 5V and the
feed it to the ldo (it's sad i know). MAX619 might be it, but i'm afraid
that it doesn't provide enough current (50 mA). Perhaps ICs from the same
family have more power.
 
L

Ldo

Jan 1, 1970
0
Dave said:
According to the data sheet for the REG102NA-3.3, it states the minimum
input voltage for normal operation is Vin + 0.4V. You probably won't be
able to get this part to act correctly with a 3.3V input.

Can you unsolder the REG102NA-3.3 part and add a jumper to allow the
external regulated 3.3V into the board?

Dave

I know about +0.4V. But i was surprised to see a home made PIC programmer
working with 5V when it actually had a 7805 regulator inside. 7805 should
require 5V + 3V at least if i remember. I thought "if it works with 7805
maybe...."

Board B is not very cheap so i wouldn't put to the test my skills in
soldering and unsoldering!!
I'm investigating on charge pumps instead.
 
J

Joerg

Jan 1, 1970
0
Ldo said:
I haven't bought the Board B yet, but i guess it should deploy the ldo in a
"small"
package, the Board itself is simple but not so cheap. I'm not very good in
soldering, let's say i'm not very patient in jobs that are not creative
(someone could say soldering is an art, i agree but that's not my case).
That said i'm afraid that if i try to do the "short" i could end up in
biting my hands this time!

I have to build a small adapter board, i hoped i could get away with just
connectors. Now i should use charge pump ICs to raise 3.3V to 5V and the
feed it to the ldo (it's sad i know). MAX619 might be it, but i'm afraid
that it doesn't provide enough current (50 mA). Perhaps ICs from the same
family have more power.

That can be a bit much for many charge pumps. You could also use a
little boost converter chip from National or TI. You'll need an inductor
but it's small. If you want to take the easy route I believe TI makes
complete modules that can simply be dropped in.
 
L

Ldo

Jan 1, 1970
0
I have to build a small adapter board, i hoped i could get away with
just
That can be a bit much for many charge pumps. You could also use a
little boost converter chip from National or TI. You'll need an inductor
but it's small. If you want to take the easy route I believe TI makes
complete modules that can simply be dropped in.

I also found TPS60130 (300 mA). The power required by the chip on board B is
240 mW. So i guess it should do. But its package is too small for my
patience, i was hoping for a DIP.
 
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