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Switching power supply question

T

Tim Williams

Jan 1, 1970
0
Yes but with a motor the type of load is normally well defined.(a
rotating mass of some sort) and is designed for a specific application
in mind.

Until you put an undefined load on that motor, which is kind of the point
of using it in the first place!

Tim
 
N

NoEmail

Jan 1, 1970
0
Joerg said:
Jim Thompson wrote:
First post must have been picked up by a wind gust (it's ugly out
here right now).
We discrete designers would not mind an extra pin one bit. We are
used to external parts from PWM chips. What's the big deal?

I've been waiting for someone to do some research on this topic. It
has been solved for a long time.

National Semiconductor has an article on LDO ESR:

"Capacitors are key to voltage regulator design"

http://www.national.com/nationaledge/jul02/article2.html

The LP2985 specs the minimum ESR at 5 milliohms. This is compatible
with ceramic output caps:

http://lis.epfl.ch/contest/flying07/docs/resources/LP2985.pdf

The copyright on the datasheet is May, 2003. About 5 years ago.

Analog Devices has solved the ESR problem completely:

"Solving Stability Problems of Low Dropout Regulators"

http://www.analog.com/en/content/0,2886,766_818_11812,00.html

The ADP3331 has no minimum ESR spec:

http://www.analog.com/UploadedFiles/Data_Sheets/ADP3331.pdf

The copyright on the datasheet is also 2003.

For reference, here's a bunch of articles from various sources
on LDO ESR. The url's are not shown since they degrade with time. But
google finds them quickly.

AN1148 PDF 205,195 Compensating Low-Dropout Regulators
AN682 PDF 879,026 New Generation of Low Dropout Regulators
AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO
SLVA068 PDF 68,595 LDO Fundamental Theory
SLVA072 PDF 291,538 LDO Technical Review
SLVA079 PDF 202,342 LDO Terms and Definitions
SLVA115 PDF 85,754 Regulator ESR Stability
SLYT151 PDF 468,245 Compensation Transient Response
SLYT187 PDF 254,973 Understanding the Stable ESR Range
SLYT194 PDF 246,355 LDO Linear Stability Analysis
SR003AN PDF 73,945 Compensation for Linear Regulators
SR004AN PDF 41,598 Linear Regulator Output Structures
 
J

John Larkin

Jan 1, 1970
0
But it's a Dollar a pop. Yikes.

C-load, r-r i/o, 16 MHz, 40 mA drive, works from 3 to 30 volts, all in
SOT-23.

But we live in different worlds. Our VME modules sell for $2400 to
$12K each. A $200 Hittite mmic is noticed; a $1 opamp or a $15
microprocessor isn't.

I could drive a low-current capacitive load from a regular opamp and a
few extra passives, but placing the passives costs 10-15 cents each,
and takes up space. So why bother?


John
 
J

Jim Thompson

Jan 1, 1970
0
[snip]
Analog Devices has solved the ESR problem completely:

"Solving Stability Problems of Low Dropout Regulators"

http://www.analog.com/en/content/0,2886,766_818_11812,00.html
[snip]

That's hilarious!

"James E. Salomon" should be "James E. Solomon".

I ought to know, since I was the young buck engineer, fresh out of
college, working for Solomon... I'm the one who actually did all the
math ;-)

Likewise on any of his noise analysis papers.

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
John said:
C-load, r-r i/o, 16 MHz, 40 mA drive, works from 3 to 30 volts, all in
SOT-23.

But we live in different worlds. Our VME modules sell for $2400 to
$12K each. A $200 Hittite mmic is noticed; a $1 opamp or a $15
microprocessor isn't.

I remember agonizing over a BOM item on a defense industry design, I
thought it was outrageously expensive and pondered how to roll a
discrete design. The engineering manager came along and asked why I had
my head buried between my hands. When I told him he burst into laughter.

Being a taxpayer I could not laugh though.

I could drive a low-current capacitive load from a regular opamp and a
few extra passives, but placing the passives costs 10-15 cents each,
and takes up space. So why bother?

Those must be Bay Area placement costs. But even from Europe I have
heard numbers of 3-4 Euro-Cents from several people. My whole design
philosophy has to change every time a client insists on domestic production.
 
J

Joerg

Jan 1, 1970
0
NoEmail said:
I've been waiting for someone to do some research on this topic. It
has been solved for a long time.

National Semiconductor has an article on LDO ESR:

"Capacitors are key to voltage regulator design"

http://www.national.com/nationaledge/jul02/article2.html

The LP2985 specs the minimum ESR at 5 milliohms. This is compatible
with ceramic output caps:

http://lis.epfl.ch/contest/flying07/docs/resources/LP2985.pdf

Then one sunny day the cap mfg "improves" a cap series, resulting in
even lower ESR, phssst ... BANG!

Besides, the ESR "stable range" is usually a figure and not guaranteed
data. That alone is a turn-off point for me. In medical electronics
where I work a lot you can't rely on typical data.

I also had some non-datasheet-mentioned pathologies. For example on a
LM29xx series chip from National many moons ago. Someone absolutely
wanted to have it in there despite my advice against LDOs. Turned out it
did not "like" too high a source impedance. Made it oscillate, heat up
and then fry. Lengthy calls, a team at National on the other side. You
could hear large sheets being handled. Suddenly a faint "oh drat" could
be heard over the phone and then I knew we were in deep doodoo.

The copyright on the datasheet is May, 2003. About 5 years ago.

Analog Devices has solved the ESR problem completely:

"Solving Stability Problems of Low Dropout Regulators"

http://www.analog.com/en/content/0,2886,766_818_11812,00.html

The ADP3331 has no minimum ESR spec:

http://www.analog.com/UploadedFiles/Data_Sheets/ADP3331.pdf

The copyright on the datasheet is also 2003.

Take a look at the far right where the price is listed. The ADP3331 has
a realistic quantity street price around a Dollar, way above budget for
a lot of designs. Especially if you can do it without an LDO.

For reference, here's a bunch of articles from various sources
on LDO ESR. The url's are not shown since they degrade with time. But
google finds them quickly.

AN1148 PDF 205,195 Compensating Low-Dropout Regulators
AN682 PDF 879,026 New Generation of Low Dropout Regulators
AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO
SLVA068 PDF 68,595 LDO Fundamental Theory
SLVA072 PDF 291,538 LDO Technical Review
SLVA079 PDF 202,342 LDO Terms and Definitions
SLVA115 PDF 85,754 Regulator ESR Stability
SLYT151 PDF 468,245 Compensation Transient Response
SLYT187 PDF 254,973 Understanding the Stable ESR Range
SLYT194 PDF 246,355 LDO Linear Stability Analysis
SR003AN PDF 73,945 Compensation for Linear Regulators
SR004AN PDF 41,598 Linear Regulator Output Structures


Thanks for the links. I don't generally use LDOs but there may come a
situation where it has to be one.
 
N

NoName

Jan 1, 1970
0
[...]
Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!

That should be caught in incoming inspection.
Besides, the ESR "stable range" is usually a figure and not
guaranteed data. That alone is a turn-off point for me. In medical
electronics where I work a lot you can't rely on typical data.

It is never a good idea to use typical values in a design. From the
datasheet:

"The LP2985 was designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer: for capacitance
values in the 2.2 uF to 4.7 uF range, ceramics are the least
expensive and also have the lowest ESR values (which makes them best
for eliminating high- frequency noise). The ESR of a typical 2.2 uF
ceramic capacitor is in the range of 10 mohm to 20 mohm, which
easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

I'm sure you could require a guaranteed performance spec in a
purchase contract if you were willing to pay for it.

Anyway, this was just an example to show it is possible to improve
the ESR performance without requiring an extra pin with external
filter components.
I also had some non-datasheet-mentioned pathologies. For example
on a LM29xx series chip from National many moons ago. Someone
absolutely wanted to have it in there despite my advice against
LDOs. Turned out it did not "like" too high a source impedance.
Made it oscillate, heat up and then fry. Lengthy calls, a team at
National on the other side. You could hear large sheets being
handled. Suddenly a faint "oh drat" could be heard over the phone
and then I knew we were in deep doodoo.

This is an implementation problem in the design. Obviously it is
something you check before committing a part to production.

[...]
Take a look at the far right where the price is listed. The
ADP3331 has a realistic quantity street price around a Dollar, way
above budget for a lot of designs. Especially if you can do it
without an LDO.

I checked the parts in Digikey before posting. Certainly the price
is high - that's what you pay for in a monopoly. However, the part
is not affected by ESR, and it does not require an extra pin with
external filter components. So a clever design might get around the
patent and give the same results.
Thanks for the links. I don't generally use LDOs but there may
come a situation where it has to be one.

Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?
 
M

MooseFET

Jan 1, 1970
0
That should be caught in incoming inspection.

Rough ascii art mechanical drawing:
[should be]<-------1E6 miles ------------->[is]

I have seen many "improved" parts get past incoming inspection.
 
J

John Larkin

Jan 1, 1970
0
[...]
Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!

That should be caught in incoming inspection.


Are you suggesting that every lot of ceramic caps be tested, at
receiving, for ESR? And by extension, every received part (like 16-bit
octal DACs, FPGA's, laser diodes, kep nuts) be tested for all their
specs at receiving inspection?

John
 
J

Joerg

Jan 1, 1970
0
NoName said:
[...]
Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!

That should be caught in incoming inspection.

On ESR? Manufacturers routinely spec a guaranteed maximum, no minimums.
They will refuse to take back and give credit if you return a shipment
because ESR is too low. And rightfully so. I am not sure 1206 caps would
be suitable as tiles for the lobby or pavers for the patio but you'd
have to do something with all the "rejects" ;-)

It is never a good idea to use typical values in a design. From the
datasheet:

"The LP2985 was designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer: for capacitance
values in the 2.2 uF to 4.7 uF range, ceramics are the least
expensive and also have the lowest ESR values (which makes them best
for eliminating high- frequency noise). The ESR of a typical 2.2 uF
ceramic capacitor is in the range of 10 mohm to 20 mohm, which
easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

Excellent example. Quote from that very datasheet: "The output capacitor
must maintain its ESR within the stable region over the full operating
temperature range of the application to assure stability." Or how about
"...utilizing circuitry which allows the regulator to be stable across
the entire range of output current with an output capacitor whose ESR is
as low as 5 mohm." a page earlier? So what exactly does "as low as"
mean? Is it guaranteed? I doubt that. And how do you maintain that
value? Put a 5mohm resistor in series?

<goose bumps developing ...>

Then there is the typical graph on top of top of page 15. Where are
those limits guaranteed? Why are they not in the min-max tables? I'll
never use a part like that.

I'm sure you could require a guaranteed performance spec in a
purchase contract if you were willing to pay for it.

IME companies are often not even willing to simulate the thing when it
did go wrong. Maybe because then a dirty little secret could pop out.

Anyway, this was just an example to show it is possible to improve
the ESR performance without requiring an extra pin with external
filter components.

Sorry, I do not see that. Same old, same old, there is a minimum ESR
mentioned and IMHO that's not a good thing.

This is an implementation problem in the design. Obviously it is
something you check before committing a part to production.

Absolutely not. This was a stability problem of a chip that was not
mentioned in the datasheet. IOW not supposed to happen but did happen.
Needless to say the guys where rather apologetic on the phone because
they knew it wasn't supposed to happen.

[...]
Take a look at the far right where the price is listed. The
ADP3331 has a realistic quantity street price around a Dollar, way
above budget for a lot of designs. Especially if you can do it
without an LDO.

I checked the parts in Digikey before posting. Certainly the price
is high - that's what you pay for in a monopoly. However, the part
is not affected by ESR, and it does not require an extra pin with
external filter components. So a clever design might get around the
patent and give the same results.
Thanks for the links. I don't generally use LDOs but there may
come a situation where it has to be one.

Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?


Then I'd get shot ;-)

But seriously, all you have to do is run thorough simulations and pay
special attention to loop stability. In a discrete design it is not a
problem to add a 4700pF cap somewhere, on a chip that is quite
impossible. But I rarely design discrete LDOs these days because when
you are this close to the input voltage you might as well go straight to
a SEPIC or forward converter, which is what I usually do.
 
F

Fred Bloggs

Jan 1, 1970
0
Joerg said:
NoName said:
[...]
National Semiconductor has an article on LDO ESR:
"Capacitors are key to voltage regulator design"


The LP2985 specs the minimum ESR at 5 milliohms. This is
compatible with ceramic output caps:


Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!


That should be caught in incoming inspection.

On ESR? Manufacturers routinely spec a guaranteed maximum, no minimums.
They will refuse to take back and give credit if you return a shipment
because ESR is too low. And rightfully so. I am not sure 1206 caps would
be suitable as tiles for the lobby or pavers for the patio but you'd
have to do something with all the "rejects" ;-)

It is never a good idea to use typical values in a design. From the
datasheet:

"The LP2985 was designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer: for capacitance
values in the 2.2 uF to 4.7 uF range, ceramics are the least
expensive and also have the lowest ESR values (which makes them best
for eliminating high- frequency noise). The ESR of a typical 2.2 uF
ceramic capacitor is in the range of 10 mohm to 20 mohm, which
easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

Excellent example. Quote from that very datasheet: "The output capacitor
must maintain its ESR within the stable region over the full operating
temperature range of the application to assure stability." Or how about
"...utilizing circuitry which allows the regulator to be stable across
the entire range of output current with an output capacitor whose ESR is
as low as 5 mohm." a page earlier? So what exactly does "as low as"
mean? Is it guaranteed? I doubt that. And how do you maintain that
value? Put a 5mohm resistor in series?

<goose bumps developing ...>

Then there is the typical graph on top of top of page 15. Where are
those limits guaranteed? Why are they not in the min-max tables? I'll
never use a part like that.

I'm sure you could require a guaranteed performance spec in a
purchase contract if you were willing to pay for it.

IME companies are often not even willing to simulate the thing when it
did go wrong. Maybe because then a dirty little secret could pop out.

Anyway, this was just an example to show it is possible to improve
the ESR performance without requiring an extra pin with external
filter components.

Sorry, I do not see that. Same old, same old, there is a minimum ESR
mentioned and IMHO that's not a good thing.

This is an implementation problem in the design. Obviously it is
something you check before committing a part to production.

Absolutely not. This was a stability problem of a chip that was not
mentioned in the datasheet. IOW not supposed to happen but did happen.
Needless to say the guys where rather apologetic on the phone because
they knew it wasn't supposed to happen.

[...]
Take a look at the far right where the price is listed. The
ADP3331 has a realistic quantity street price around a Dollar, way
above budget for a lot of designs. Especially if you can do it
without an LDO.


I checked the parts in Digikey before posting. Certainly the price
is high - that's what you pay for in a monopoly. However, the part
is not affected by ESR, and it does not require an extra pin with
external filter components. So a clever design might get around the
patent and give the same results.
For reference, here's a bunch of articles from various sources on
LDO ESR. The url's are not shown since they degrade with time.
But google finds them quickly.
AN1148 PDF 205,195 Compensating Low-Dropout Regulators
AN682 PDF 879,026 New Generation of Low Dropout Regulators
AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO
SLVA068 PDF 68,595 LDO Fundamental Theory
SLVA072 PDF 291,538 LDO Technical Review
SLVA079 PDF 202,342 LDO Terms and Definitions
SLVA115 PDF 85,754 Regulator ESR Stability
SLYT151 PDF 468,245 Compensation Transient Response
SLYT187 PDF 254,973 Understanding the Stable ESR Range
SLYT194 PDF 246,355 LDO Linear Stability Analysis
SR003AN PDF 73,945 Compensation for Linear Regulators
SR004AN PDF 41,598 Linear Regulator Output Structures
Thanks for the links. I don't generally use LDOs but there may
come a situation where it has to be one.


Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?



Then I'd get shot ;-)

But seriously, all you have to do is run thorough simulations and pay
special attention to loop stability. In a discrete design it is not a
problem to add a 4700pF cap somewhere, on a chip that is quite
impossible. But I rarely design discrete LDOs these days because when
you are this close to the input voltage you might as well go straight to
a SEPIC or forward converter, which is what I usually do.

Nonsense, see http://focus.ti.com/lit/an/slva072/slva072.pdf , explains
simply what is going on in sections 8 and 9 and why you end up with a
range of ESR. You can use a compensating series resistor...
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg a écrit :
NoName said:
[...]
National Semiconductor has an article on LDO ESR:
"Capacitors are key to voltage regulator design"

The LP2985 specs the minimum ESR at 5 milliohms. This is
compatible with ceramic output caps:

Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!

That should be caught in incoming inspection.

On ESR? Manufacturers routinely spec a guaranteed maximum, no minimums.
They will refuse to take back and give credit if you return a shipment
because ESR is too low. And rightfully so. I am not sure 1206 caps would
be suitable as tiles for the lobby or pavers for the patio but you'd
have to do something with all the "rejects" ;-)

It is never a good idea to use typical values in a design. From the
datasheet:

"The LP2985 was designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer: for capacitance
values in the 2.2 uF to 4.7 uF range, ceramics are the least
expensive and also have the lowest ESR values (which makes them best
for eliminating high- frequency noise). The ESR of a typical 2.2 uF
ceramic capacitor is in the range of 10 mohm to 20 mohm, which
easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

Excellent example. Quote from that very datasheet: "The output capacitor
must maintain its ESR within the stable region over the full operating
temperature range of the application to assure stability." Or how about
"...utilizing circuitry which allows the regulator to be stable across
the entire range of output current with an output capacitor whose ESR is
as low as 5 mohm." a page earlier? So what exactly does "as low as"
mean? Is it guaranteed? I doubt that. And how do you maintain that
value? Put a 5mohm resistor in series?

<goose bumps developing ...>

Then there is the typical graph on top of top of page 15. Where are
those limits guaranteed? Why are they not in the min-max tables? I'll
never use a part like that.

I'm sure you could require a guaranteed performance spec in a
purchase contract if you were willing to pay for it.

IME companies are often not even willing to simulate the thing when it
did go wrong. Maybe because then a dirty little secret could pop out.

Anyway, this was just an example to show it is possible to improve
the ESR performance without requiring an extra pin with external
filter components.

Sorry, I do not see that. Same old, same old, there is a minimum ESR
mentioned and IMHO that's not a good thing.

This is an implementation problem in the design. Obviously it is
something you check before committing a part to production.

Absolutely not. This was a stability problem of a chip that was not
mentioned in the datasheet. IOW not supposed to happen but did happen.
Needless to say the guys where rather apologetic on the phone because
they knew it wasn't supposed to happen.

[...]
Take a look at the far right where the price is listed. The
ADP3331 has a realistic quantity street price around a Dollar, way
above budget for a lot of designs. Especially if you can do it
without an LDO.

I checked the parts in Digikey before posting. Certainly the price
is high - that's what you pay for in a monopoly. However, the part
is not affected by ESR, and it does not require an extra pin with
external filter components. So a clever design might get around the
patent and give the same results.
For reference, here's a bunch of articles from various sources on
LDO ESR. The url's are not shown since they degrade with time.
But google finds them quickly.
AN1148 PDF 205,195 Compensating Low-Dropout Regulators
AN682 PDF 879,026 New Generation of Low Dropout Regulators
AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO
SLVA068 PDF 68,595 LDO Fundamental Theory
SLVA072 PDF 291,538 LDO Technical Review
SLVA079 PDF 202,342 LDO Terms and Definitions
SLVA115 PDF 85,754 Regulator ESR Stability
SLYT151 PDF 468,245 Compensation Transient Response
SLYT187 PDF 254,973 Understanding the Stable ESR Range
SLYT194 PDF 246,355 LDO Linear Stability Analysis
SR003AN PDF 73,945 Compensation for Linear Regulators
SR004AN PDF 41,598 Linear Regulator Output Structures
Thanks for the links. I don't generally use LDOs but there may
come a situation where it has to be one.

Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?


Then I'd get shot ;-)

But seriously, all you have to do is run thorough simulations and pay
special attention to loop stability. In a discrete design it is not a
problem to add a 4700pF cap somewhere, on a chip that is quite
impossible. But I rarely design discrete LDOs these days because when
you are this close to the input voltage you might as well go straight to
a SEPIC or forward converter, which is what I usually do.

But it's so easy to add esr without adding series resistance...
 
J

Joerg

Jan 1, 1970
0
Fred said:
NoName said:
[...]

National Semiconductor has an article on LDO ESR:


"Capacitors are key to voltage regulator design"


http://www.national.com/nationaledge/jul02/article2.html


The LP2985 specs the minimum ESR at 5 milliohms. This is
compatible with ceramic output caps:


http://lis.epfl.ch/contest/flying07/docs/resources/LP2985.pdf


Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!


That should be caught in incoming inspection.

On ESR? Manufacturers routinely spec a guaranteed maximum, no
minimums. They will refuse to take back and give credit if you return
a shipment because ESR is too low. And rightfully so. I am not sure
1206 caps would be suitable as tiles for the lobby or pavers for the
patio but you'd have to do something with all the "rejects" ;-)

Besides, the ESR "stable range" is usually a figure and not
guaranteed data. That alone is a turn-off point for me. In medical
electronics where I work a lot you can't rely on typical data.


It is never a good idea to use typical values in a design. From the
datasheet:

"The LP2985 was designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer: for capacitance
values in the 2.2 uF to 4.7 uF range, ceramics are the least
expensive and also have the lowest ESR values (which makes them best
for eliminating high- frequency noise). The ESR of a typical 2.2 uF
ceramic capacitor is in the range of 10 mohm to 20 mohm, which
easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

Excellent example. Quote from that very datasheet: "The output
capacitor must maintain its ESR within the stable region over the full
operating temperature range of the application to assure stability."
Or how about "...utilizing circuitry which allows the regulator to be
stable across the entire range of output current with an output
capacitor whose ESR is as low as 5 mohm." a page earlier? So what
exactly does "as low as" mean? Is it guaranteed? I doubt that. And how
do you maintain that value? Put a 5mohm resistor in series?

<goose bumps developing ...>

Then there is the typical graph on top of top of page 15. Where are
those limits guaranteed? Why are they not in the min-max tables? I'll
never use a part like that.

I'm sure you could require a guaranteed performance spec in a
purchase contract if you were willing to pay for it.

IME companies are often not even willing to simulate the thing when it
did go wrong. Maybe because then a dirty little secret could pop out.

Anyway, this was just an example to show it is possible to improve
the ESR performance without requiring an extra pin with external
filter components.

Sorry, I do not see that. Same old, same old, there is a minimum ESR
mentioned and IMHO that's not a good thing.

I also had some non-datasheet-mentioned pathologies. For example
on a LM29xx series chip from National many moons ago. Someone
absolutely wanted to have it in there despite my advice against
LDOs. Turned out it did not "like" too high a source impedance.


Made it oscillate, heat up and then fry. Lengthy calls, a team at
National on the other side. You could hear large sheets being
handled. Suddenly a faint "oh drat" could be heard over the phone
and then I knew we were in deep doodoo.


This is an implementation problem in the design. Obviously it is
something you check before committing a part to production.

Absolutely not. This was a stability problem of a chip that was not
mentioned in the datasheet. IOW not supposed to happen but did happen.
Needless to say the guys where rather apologetic on the phone because
they knew it wasn't supposed to happen.

[...]

Take a look at the far right where the price is listed. The
ADP3331 has a realistic quantity street price around a Dollar, way
above budget for a lot of designs. Especially if you can do it
without an LDO.


I checked the parts in Digikey before posting. Certainly the price
is high - that's what you pay for in a monopoly. However, the part
is not affected by ESR, and it does not require an extra pin with
external filter components. So a clever design might get around the
patent and give the same results.

For reference, here's a bunch of articles from various sources on
LDO ESR. The url's are not shown since they degrade with time.


But google finds them quickly.


AN1148 PDF 205,195 Compensating Low-Dropout Regulators
AN682 PDF 879,026 New Generation of Low Dropout Regulators
AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO
SLVA068 PDF 68,595 LDO Fundamental Theory
SLVA072 PDF 291,538 LDO Technical Review
SLVA079 PDF 202,342 LDO Terms and Definitions
SLVA115 PDF 85,754 Regulator ESR Stability
SLYT151 PDF 468,245 Compensation Transient Response
SLYT187 PDF 254,973 Understanding the Stable ESR Range
SLYT194 PDF 246,355 LDO Linear Stability Analysis
SR003AN PDF 73,945 Compensation for Linear Regulators
SR004AN PDF 41,598 Linear Regulator Output Structures


Thanks for the links. I don't generally use LDOs but there may
come a situation where it has to be one.


Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?



Then I'd get shot ;-)

But seriously, all you have to do is run thorough simulations and pay
special attention to loop stability. In a discrete design it is not a
problem to add a 4700pF cap somewhere, on a chip that is quite
impossible. But I rarely design discrete LDOs these days because when
you are this close to the input voltage you might as well go straight
to a SEPIC or forward converter, which is what I usually do.

Nonsense, see http://focus.ti.com/lit/an/slva072/slva072.pdf , explains
simply what is going on in sections 8 and 9 and why you end up with a
range of ESR. You can use a compensating series resistor...

Better read the one below ;-)

http://www.analog.com/UploadedFiles/Technical_Articles/3752210Publication_V-Ref.pdf
 
J

Joerg

Jan 1, 1970
0
Fred said:
Joerg a écrit :
NoName said:
[...]

National Semiconductor has an article on LDO ESR:

"Capacitors are key to voltage regulator design"

http://www.national.com/nationaledge/jul02/article2.html

The LP2985 specs the minimum ESR at 5 milliohms. This is
compatible with ceramic output caps:

http://lis.epfl.ch/contest/flying07/docs/resources/LP2985.pdf

Then one sunny day the cap mfg "improves" a cap series, resulting
in even lower ESR, phssst. BANG!

That should be caught in incoming inspection.

On ESR? Manufacturers routinely spec a guaranteed maximum, no
minimums. They will refuse to take back and give credit if you return
a shipment because ESR is too low. And rightfully so. I am not sure
1206 caps would be suitable as tiles for the lobby or pavers for the
patio but you'd have to do something with all the "rejects" ;-)

Besides, the ESR "stable range" is usually a figure and not
guaranteed data. That alone is a turn-off point for me. In medical
electronics where I work a lot you can't rely on typical data.

It is never a good idea to use typical values in a design. From the
datasheet:

"The LP2985 was designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer: for capacitance
values in the 2.2 uF to 4.7 uF range, ceramics are the least
expensive and also have the lowest ESR values (which makes them best
for eliminating high- frequency noise). The ESR of a typical 2.2 uF
ceramic capacitor is in the range of 10 mohm to 20 mohm, which
easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

Excellent example. Quote from that very datasheet: "The output
capacitor must maintain its ESR within the stable region over the full
operating temperature range of the application to assure stability."
Or how about "...utilizing circuitry which allows the regulator to be
stable across the entire range of output current with an output
capacitor whose ESR is as low as 5 mohm." a page earlier? So what
exactly does "as low as" mean? Is it guaranteed? I doubt that. And how
do you maintain that value? Put a 5mohm resistor in series?

<goose bumps developing ...>

Then there is the typical graph on top of top of page 15. Where are
those limits guaranteed? Why are they not in the min-max tables? I'll
never use a part like that.

I'm sure you could require a guaranteed performance spec in a
purchase contract if you were willing to pay for it.

IME companies are often not even willing to simulate the thing when it
did go wrong. Maybe because then a dirty little secret could pop out.

Anyway, this was just an example to show it is possible to improve
the ESR performance without requiring an extra pin with external
filter components.

Sorry, I do not see that. Same old, same old, there is a minimum ESR
mentioned and IMHO that's not a good thing.

I also had some non-datasheet-mentioned pathologies. For example
on a LM29xx series chip from National many moons ago. Someone
absolutely wanted to have it in there despite my advice against
LDOs. Turned out it did not "like" too high a source impedance.

Made it oscillate, heat up and then fry. Lengthy calls, a team at
National on the other side. You could hear large sheets being
handled. Suddenly a faint "oh drat" could be heard over the phone
and then I knew we were in deep doodoo.

This is an implementation problem in the design. Obviously it is
something you check before committing a part to production.

Absolutely not. This was a stability problem of a chip that was not
mentioned in the datasheet. IOW not supposed to happen but did happen.
Needless to say the guys where rather apologetic on the phone because
they knew it wasn't supposed to happen.

[...]

Take a look at the far right where the price is listed. The
ADP3331 has a realistic quantity street price around a Dollar, way
above budget for a lot of designs. Especially if you can do it
without an LDO.

I checked the parts in Digikey before posting. Certainly the price
is high - that's what you pay for in a monopoly. However, the part
is not affected by ESR, and it does not require an extra pin with
external filter components. So a clever design might get around the
patent and give the same results.

For reference, here's a bunch of articles from various sources on
LDO ESR. The url's are not shown since they degrade with time.

But google finds them quickly.

AN1148 PDF 205,195 Compensating Low-Dropout Regulators
AN682 PDF 879,026 New Generation of Low Dropout Regulators
AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO
SLVA068 PDF 68,595 LDO Fundamental Theory
SLVA072 PDF 291,538 LDO Technical Review
SLVA079 PDF 202,342 LDO Terms and Definitions
SLVA115 PDF 85,754 Regulator ESR Stability
SLYT151 PDF 468,245 Compensation Transient Response
SLYT187 PDF 254,973 Understanding the Stable ESR Range
SLYT194 PDF 246,355 LDO Linear Stability Analysis
SR003AN PDF 73,945 Compensation for Linear Regulators
SR004AN PDF 41,598 Linear Regulator Output Structures

Thanks for the links. I don't generally use LDOs but there may
come a situation where it has to be one.

Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?


Then I'd get shot ;-)

But seriously, all you have to do is run thorough simulations and pay
special attention to loop stability. In a discrete design it is not a
problem to add a 4700pF cap somewhere, on a chip that is quite
impossible. But I rarely design discrete LDOs these days because when
you are this close to the input voltage you might as well go straight
to a SEPIC or forward converter, which is what I usually do.

But it's so easy to add esr without adding series resistance...

Then someone hangs a heavily bypassed cicuit onto it ... WHADDABANG!
 
J

Joerg

Jan 1, 1970
0
NoName wrote:

[...]
Your complaints about LDO's are well known. You spoke of rolling
your own when needed. Any chance you could post a schematic so
we could see how these problems are solved?


Ok, one hint, don't want to leave you hanging here: The statement in
most papers that an emitter- or source-follower cannot be pulled into
saturation and thus cannot become an LDO is not necessarily true ;-)

There are ways to generate voltages above Vin level ...
 
J

Joel Koltner

Jan 1, 1970
0
Joerg said:
There are ways to generate voltages above Vin level ...

You mean even without any switching circuitry (which obviously makes it easy
to generate higher voltage) and only DC input (AC is the same as
switching...), right?
 
J

Joerg

Jan 1, 1970
0
Joel said:
You mean even without any switching circuitry (which obviously makes it easy
to generate higher voltage) and only DC input (AC is the same as
switching...), right?

You do need a wee oscillator :)

Sometimes you can hitch a ride on an oscillator that is in the system
anyway, as long as that comes on line well before the regulator needs to
go into really low dropout. Or use a free Schmitt somewhere. For example
a CMOS variety that already starts at 2-3V.
 
J

Joel Koltner

Jan 1, 1970
0
Joerg said:
You do need a wee oscillator :)

And here I thought you were going really exotic on us. :)

Thanks for the information!
 
J

Joerg

Jan 1, 1970
0
Joel said:
And here I thought you were going really exotic on us. :)

Thanks for the information!

I remember a really exotic version from the tube days. Someone needed a
negative grid voltage at next to nothing in current. He put a couple of
9V batteries in there and they pretty much lasted their usual shelf life
(possibly they even survived him). You just had to peek into the
amplifier every other year, making sure they didn't leak.
 
T

Tim Williams

Jan 1, 1970
0
Joerg said:
I remember a really exotic version from the tube days. Someone needed a
negative grid voltage at next to nothing in current. He put a couple of
9V batteries in there and they pretty much lasted their usual shelf life
(possibly they even survived him). You just had to peek into the
amplifier every other year, making sure they didn't leak.

A C battery? Pffbt, that's *ancient*...

Now how about this:
- Run the phono amp heaters from the output tubes' cathode current (class
A)
- Run the phono/tape head amp heaters from an RF oscillator (or a number of
other possible uses)

Tim
 
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