MIC5270 negative LDO misbehaving...

[Phil Hobbs]

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.

Hanging a 300-ohm load on it makes the sawtooth speed up. That\'s
expected, because once the pass transistor kicks it out of regulation,
the capacitor has to bleed down until it happens again, and that happens
much faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it. A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit. I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual
capacitance was reasonably well known. Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine. Apparently this lulled me into a
false sense of security.

Razza frazza $#@*!!!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Fred Bloggs]

On Wednesday, October 7, 2020 at 3:42:12 PM UTC-4, Phil Hobbs wrote:

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.

Hanging a 300-ohm load on it makes the sawtooth speed up. That\'s
expected, because once the pass transistor kicks it out of regulation,
the capacitor has to bleed down until it happens again, and that happens
much faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it. A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit. I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual
capacitance was reasonably well known. Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine. Apparently this lulled me into a
false sense of security.

Must be too much ESR:
\"High-ESR capacitors may cause instability. Capacitors
with an ESR of 3Ω or greater at 100kHz may cause a high
frequency oscillation.\"
Low ESR out to 100KHz means a high frequency low loss type.

If the sawtooth is stable it\'s unlikely to be a coupling issue.

Razza frazza $#@*!!!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Wed, 7 Oct 2020 15:42:01 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.

We\'ve seen that, a linear reg making sawtooths. I think we used some
other Micrel reg that was bad news.

Maybe the current limiter circuit is less well-designed than the main
loop?

Hanging a 300-ohm load on it makes the sawtooth speed up. That\'s
expected, because once the pass transistor kicks it out of regulation,
the capacitor has to bleed down until it happens again, and that happens
much faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it. A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit. I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual
capacitance was reasonably well known. Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine. Apparently this lulled me into a
false sense of security.

Razza frazza $#@*!!!

Cheers

Phil Hobbs

Did you try the Williams stabilization technique, namely using a huge
aluminum lytic?

Here\'s my latest idea about supplies for my DDG rev C.

https://www.dropbox.com/s/gu9fsljyju558w4/P505C_Regs_1.jpg?raw=1

Designing this is like solving a 7-sided Rubics cube in the dark. I\'ll
have 17 regulator chips, four of them duals, and a couple of
references.

The big Zynq is rude and crude and draws nasty current bursts as code
executes. The +1 volt switcher was oscillating at 80 KHz and made the
Arms intermittent. The trigger FPGA is super sensitive to supply
noise. Both have sequencing rules. There are thermal issues. The
pick-and-place machine has run out of feeder slots, so I can\'t just
add a lot of parts.

A couple of places, we are using a 56uF polymer cap in series with a 1
ohm resistor, as a damper. That\'s crazy but works.

Switchers driving switchers have fun negative-resistance issues.

My LTM8078 positive to dual negative switcher idea has occasional
hangup issues too. Not in Spice of course. Hence the 79M05 and the
question mark.

Not complaining, of course.

 

[Tim Williams]

Try a cap across \"R2\" (in the datasheet circuit)? Or RC, or...

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

\"Phil Hobbs\" <pcdhSpamMeSenseless@electrooptical.net> wrote in message
news:0cb9954e-68a9-b76f-5cb9-066776ee665f@electrooptical.net...

So I have this little power supply board, which is sort of a collection of
our previous solutions to weird power supply needs, mainly needing very
quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15, quiet
+-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making -12V
produces an irregular sawtooth about a volt p-p which nothing seems to
fix.

Hanging a 300-ohm load on it makes the sawtooth speed up. That\'s
expected, because once the pass transistor kicks it out of regulation, the
capacitor has to bleed down until it happens again, and that happens much
faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it. A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit. I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual capacitance
was reasonably well known. Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine. Apparently this lulled me into a false
sense of security.

Razza frazza $#@*!!!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Fred Bloggs]

On Wednesday, October 7, 2020 at 4:40:53 PM UTC-4, John Larkin wrote:

On Wed, 7 Oct 2020 15:42:01 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.
We\'ve seen that, a linear reg making sawtooths. I think we used some
other Micrel reg that was bad news.

Maybe the current limiter circuit is less well-designed than the main
loop?

That chip has a real high theta-JA. Could be an oscillatory partial thermal shutdown effect. Datasheet is mostly static so no telling.

Hanging a 300-ohm load on it makes the sawtooth speed up. That\'s
expected, because once the pass transistor kicks it out of regulation,
the capacitor has to bleed down until it happens again, and that happens
much faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it. A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit. I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual
capacitance was reasonably well known. Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine. Apparently this lulled me into a
false sense of security.

Razza frazza $#@*!!!

Cheers

Phil Hobbs
Did you try the Williams stabilization technique, namely using a huge
aluminum lytic?

Here\'s my latest idea about supplies for my DDG rev C.

https://www.dropbox.com/s/gu9fsljyju558w4/P505C_Regs_1.jpg?raw=1

Designing this is like solving a 7-sided Rubics cube in the dark. I\'ll
have 17 regulator chips, four of them duals, and a couple of
references.

The big Zynq is rude and crude and draws nasty current bursts as code
executes. The +1 volt switcher was oscillating at 80 KHz and made the
Arms intermittent. The trigger FPGA is super sensitive to supply
noise. Both have sequencing rules. There are thermal issues. The
pick-and-place machine has run out of feeder slots, so I can\'t just
add a lot of parts.

A couple of places, we are using a 56uF polymer cap in series with a 1
ohm resistor, as a damper. That\'s crazy but works.

Switchers driving switchers have fun negative-resistance issues.

My LTM8078 positive to dual negative switcher idea has occasional
hangup issues too. Not in Spice of course. Hence the 79M05 and the
question mark.

Not complaining, of course.

 

[Piotr Wyderski]

Phil Hobbs wrote:

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit.

Yeah, right, the unlimited capacitive load capability. I have a follower
based on the LM8262 supposed to be a rail splitter. 0.1% thermally
coupled resistor array at the input with 0.05% matching, 100uF ceramic
at the output and the resulting voltage is off by 30mV, 4x the max from
the datasheet. Consistently wrong on two prototype units.

Will need to disassemble it part by part to find the core issue, but I
don\'t see any oscillations.

Best regards, Piotr

 

[Phil Hobbs]

On 2020-10-07 16:16, Fred Bloggs wrote:

On Wednesday, October 7, 2020 at 3:42:12 PM UTC-4, Phil Hobbs wrote:
So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.

Hanging a 300-ohm load on it makes the sawtooth speed up. That\'s
expected, because once the pass transistor kicks it out of regulation,
the capacitor has to bleed down until it happens again, and that happens
much faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it. A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit. I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual
capacitance was reasonably well known. Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine. Apparently this lulled me into a
false sense of security.

Must be too much ESR:
\"High-ESR capacitors may cause instability. Capacitors
with an ESR of 3Ω or greater at 100kHz may cause a high
frequency oscillation.\"
Low ESR out to 100KHz means a high frequency low loss type.

Thanks.

Loop instability usually leads to oscillation at some frequency near the
unity gain cross, with a fairly well-defined period and a distorted
sinusoidal waveform.

This one is a sawtooth whose frequency depends very strongly on the
load, and whose waveform is very far from periodic. That\'s why I was
looking for some interaction with the (much faster) switching junk on
the LDO\'s input.

> If the sawtooth is stable it\'s unlikely to be a coupling issue.

The maximum frequency sawtooth I\'ve seen is around 20 +- 20 kHz. (It
really is very irregular.)

Tomorrow I\'ll disconnect it from the switchers and run it off a lab supply.

Cheers

Phil Hobbs

(Who would like to think that the other technical things he has to worry
about are more important, except that they aren\'t.)

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-10-07 16:57, Tim Williams wrote:

Try a cap across \"R2\" (in the datasheet circuit)?  Or RC, or...

Tim

I did all the usual loop compensation things--no joy.

Thanks

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-10-07 18:07, Piotr Wyderski wrote:

Phil Hobbs wrote:

The part is specified for ceramic output caps of at least 1 uF, and
the datasheet claims that the value can be increased without limit.

Yeah, right, the unlimited capacitive load capability. I have a follower
based on the LM8262 supposed to be a rail splitter. 0.1% thermally
coupled resistor array at the input with 0.05% matching, 100uF ceramic
at the output and the resulting voltage is off by 30mV, 4x the max from
the datasheet. Consistently wrong on two prototype units.

Will need to disassemble it part by part to find the core issue, but I
don\'t see any oscillations.

    Best regards, Piot

My sympathies. BTW amps used with giant output caps can still be
oscillating internally--try putting sense resistors in the supply leads,
and see.

Re: capacitive loads in general.

There are op amps, e.g. my long-lost fave LM6361, which have their
compensation cap close to the output, so that a capacitive load
basically just overcompensates the amp rather than adding an additional
feedback pole.

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-10-07 17:20, Fred Bloggs wrote:

On Wednesday, October 7, 2020 at 4:40:53 PM UTC-4, John Larkin wrote:
On Wed, 7 Oct 2020 15:42:01 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.
We\'ve seen that, a linear reg making sawtooths. I think we used some
other Micrel reg that was bad news.

Maybe the current limiter circuit is less well-designed than the main
loop?

That chip has a real high theta-JA. Could be an oscillatory partial thermal shutdown effect. Datasheet is mostly static so no telling.

Thanks. The sawtooth happens even at very light loads.

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Sylvia Else]

On 08-Oct-20 6:42 am, Phil Hobbs wrote:

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps.  Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.

Hanging a 300-ohm load on it makes the sawtooth speed up.  That\'s
expected, because once the pass transistor kicks it out of regulation,
the capacitor has to bleed down until it happens again, and that happens
much faster with a load.

1 nF and 10 nF caps from output to FB don\'t help.

I thought it might be the switching spikes on the input confusing it.  A
330-ohm high current bead (BLMAG331SN1) and a 1-uF bypass on the input
knocked the spikes down pretty well but didn\'t help the oscillation.

The part is specified for ceramic output caps of at least 1 uF, and the
datasheet claims that the value can be increased without limit.  I tried
4.7 uF and 1 uF, both with good C(V) curves so that the actual
capacitance was reasonably well known.  Didn\'t help.

I tried changing chips, but that made no difference.

Micrel brags about how stable and quiet the chip is, so presumably it
works in some regimes--in fact we use it in another gizmo to make -4
from -5 to -6, and it works fine.  Apparently this lulled me into a
false sense of security.

Razza frazza $#@*!!!

Cheers

Phil Hobbs

Perhaps their claim that \"The output does not require ESR to maintain
stability\" is overly optimistic, and they do require some.

Sylvia.

 

[John Larkin]

On Wed, 7 Oct 2020 19:39:32 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-10-07 18:07, Piotr Wyderski wrote:
Phil Hobbs wrote:

The part is specified for ceramic output caps of at least 1 uF, and
the datasheet claims that the value can be increased without limit.

Yeah, right, the unlimited capacitive load capability. I have a follower
based on the LM8262 supposed to be a rail splitter. 0.1% thermally
coupled resistor array at the input with 0.05% matching, 100uF ceramic
at the output and the resulting voltage is off by 30mV, 4x the max from
the datasheet. Consistently wrong on two prototype units.

Will need to disassemble it part by part to find the core issue, but I
don\'t see any oscillations.

    Best regards, Piot

My sympathies. BTW amps used with giant output caps can still be
oscillating internally--try putting sense resistors in the supply leads,
and see.

Re: capacitive loads in general.

There are op amps, e.g. my long-lost fave LM6361, which have their
compensation cap close to the output, so that a capacitive load
basically just overcompensates the amp rather than adding an additional
feedback pole.

Cheers

Phil Hobbs

Many RRO amps have one end of the comp cap on the output pin. Or two
caps, c-b on PNP and NPN common-emitter output transistors. So adding
an external cap to ground just makes the single pole slower. Nice for
making precision power or reference rails.

LM8261 is specifically c-loadable. OPA197 is happy with, say, 5 uF or
more. AD8565 likes big caps.

 

[server]

On Wed, 7 Oct 2020 20:04:47 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-10-07 17:20, Fred Bloggs wrote:
On Wednesday, October 7, 2020 at 4:40:53 PM UTC-4, John Larkin wrote:
On Wed, 7 Oct 2020 15:42:01 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

So I have this little power supply board, which is sort of a collection
of our previous solutions to weird power supply needs, mainly needing
very quiet supplies.

It uses an LMR23630 sync buck at 2.1 MHz to make +12 from +24, which is
good for a couple of amps. Its output feeds a 450-kHz AOZ1282CI async
buck making -16.

There are linears and cap multipliers to make +15, -12, quiet +-15,
quiet +-5, and one or two others.

It all works very nicely except that the Micrel MIC5270YM5 LDO making
-12V produces an irregular sawtooth about a volt p-p which nothing seems
to fix.
We\'ve seen that, a linear reg making sawtooths. I think we used some
other Micrel reg that was bad news.

Maybe the current limiter circuit is less well-designed than the main
loop?

That chip has a real high theta-JA. Could be an oscillatory partial thermal shutdown effect. Datasheet is mostly static so no telling.

Thanks. The sawtooth happens even at very light loads.

Cheers

Phil Hobbs

I wonder if one edge is an internal RF oscillation, and the other edge
is recovery.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Steve Wilson]

jlarkin@highlandsniptechnology.com wrote:

On Wed, 7 Oct 2020 20:04:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

Thanks. The sawtooth happens even at very light loads.

Cheers

Phil Hobbs

I wonder if one edge is an internal RF oscillation, and the other edge
is recovery.

Squegging?

Science teaches us to trust
-SW

 

[Piotr Wyderski]

John Larkin wrote:

> LM8261 is specifically c-loadable.

And hence I am puzzled with what I see. I admit I didn\'t have time to
debug this hopefully minor issue, but this little subcircuit should have
worked correctly from the very beginning. The part is genuine, and the
meter readings exceed the datasheet worst cases many times. Weird.

I will follow Phil\'s piece of advice.

Best regards, Piotr

 

[piglet]

On 07/10/2020 11:07 pm, Piotr Wyderski wrote:

Phil Hobbs wrote:

The part is specified for ceramic output caps of at least 1 uF, and
the datasheet claims that the value can be increased without limit.

Yeah, right, the unlimited capacitive load capability. I have a follower
based on the LM8262 supposed to be a rail splitter. 0.1% thermally
coupled resistor array at the input with 0.05% matching, 100uF ceramic
at the output and the resulting voltage is off by 30mV, 4x the max from
the datasheet. Consistently wrong on two prototype units.

Will need to disassemble it part by part to find the core issue, but I
don\'t see any oscillations.

    Best regards, Piotr

I am about to use that part so thanks for the alert. Have you eliminated
input bias current, datasheet worst case is 2700nA?

piglet

 

[Piotr Wyderski]

piglet wrote:

I am about to use that part so thanks for the alert. Have you eliminated
input bias current, datasheet worst case is 2700nA?

No, my dear piglet, not at all. Moreover, I have just identified this as
the root cause. Holy cow, didn\'t know they are still making parts like this.

The follower is otherwise very stable even with a 100uF capacitor, so I
dismiss the alert. The \"high impedance\" input was biasing this precious
precision divider.

Best regards, Piotr

 

[John Doe]

piglet <erichpwagner@hotmail.com> wrote:

I am about to use that part so thanks for the alert. Have you
eliminated input bias current, datasheet worst case is 2700nA?

That means the maximum current required at the input is 2700 nano
amps? Just curious.

 

[Piotr Wyderski]

John Doe wrote:

That means the maximum current required at the input is 2700 nano
amps? Just curious.

It means that this \"infinite impedance non-inverting opamp input\" is
injecting 2.7uA into the circuit connected to it.

Best regards, Piotr

 

[Piotr Wyderski]

piglet wrote:

> Have you eliminated input bias current, datasheet worst case is 2700nA?

A quick and dirty fix comprised of two 0603 1.2k/0.1% resistors put in
place of that thermally coupled 0605 array has resulted in a spot-on
VCC/2. Luckily, there are arrays in this range, so no new board spin is
needed. The opamp offset is negligible.

For the last few years I have been using mostly CMOS opamps and lost my
vigilance. I have just received a very valuable lesson and appreciate
you being a part of it.

Best regards, Piotr