How are abs max ratings set?...

P

Phil Hobbs

Guest
In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in
another thread. Swapping out the mylar cap for a polypropylene reduced
the soakage tail on the ramp, but didn\'t eliminate it.

--
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


--
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
 
On Sun, 29 Jan 2023 16:43:36 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in
another thread. Swapping out the mylar cap for a polypropylene reduced
the soakage tail on the ramp, but didn\'t eliminate it.

--
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
Click to expand...

The upper ESD diode in the 74VHC74A is essentially a zener to ground,
not a pn diode to Vcc. One could test one to see where it actually
zeners.

Mylar caps make terrible ramps.

I like to test parts past abs max to see what happens. It\'s rare to
find a part that won\'t tolerate 2x abs max voltage, and 5:1 isn\'t
unusual.
 
John Larkin wrote:
On Sun, 29 Jan 2023 16:43:36 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:


In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in
another thread. Swapping out the mylar cap for a polypropylene reduced
the soakage tail on the ramp, but didn\'t eliminate it.


The upper ESD diode in the 74VHC74A is essentially a zener to ground,
not a pn diode to Vcc. One could test one to see where it actually
zeners.

Mylar caps make terrible ramps.
Click to expand...

Mylar isn\'t too bad at this speed (1-100 Hz). The original version of
this circuit had about a 200 us reset pulse, controlling the gate of a
2N7002 directly. The capacitor was a 330 nF, 200V mylar, and the
discharge arrangement was that 2N7002 with a 20-ohm resistor in series.
The ramp reset pulse was about 200 us, i.e. about 15 time constants.
There was about a 2-ms tail following the reset. The polyprop ones are
probably 3x tighter, but not orders of magnitude.

I have some polystyrenes in my drawer, but they\'re only available up to
the early nanofarads. (I have a few NOS ones up to 10 nF.)

I made a little RC differentiator gizmo with a polystyrene cap and an
OPA141, and verified that the rest of the ramp was linear to better than
12 bits, which was about the limit of the measurement. Making the reset
pulse 10 ms wide fixed it completely.

I like to test parts past abs max to see what happens. It\'s rare to
find a part that won\'t tolerate 2x abs max voltage, and 5:1 isn\'t
unusual.
Click to expand...

Seems like the chip makers decide how tight they can make the spec
without hurting sales.

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
 
On Sun, 29 Jan 2023 19:32:32 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
On Sun, 29 Jan 2023 16:43:36 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:


In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in
another thread. Swapping out the mylar cap for a polypropylene reduced
the soakage tail on the ramp, but didn\'t eliminate it.


The upper ESD diode in the 74VHC74A is essentially a zener to ground,
not a pn diode to Vcc. One could test one to see where it actually
zeners.

Mylar caps make terrible ramps.

Mylar isn\'t too bad at this speed (1-100 Hz). The original version of
this circuit had about a 200 us reset pulse, controlling the gate of a
2N7002 directly. The capacitor was a 330 nF, 200V mylar, and the
discharge arrangement was that 2N7002 with a 20-ohm resistor in series.
The ramp reset pulse was about 200 us, i.e. about 15 time constants.
There was about a 2-ms tail following the reset. The polyprop ones are
probably 3x tighter, but not orders of magnitude.

I have some polystyrenes in my drawer, but they\'re only available up to
the early nanofarads. (I have a few NOS ones up to 10 nF.)

I made a little RC differentiator gizmo with a polystyrene cap and an
OPA141, and verified that the rest of the ramp was linear to better than
12 bits, which was about the limit of the measurement. Making the reset
pulse 10 ms wide fixed it completely.

I like to test parts past abs max to see what happens. It\'s rare to
find a part that won\'t tolerate 2x abs max voltage, and 5:1 isn\'t
unusual.

Seems like the chip makers decide how tight they can make the spec
without hurting sales.

Cheers

Phil Hobbs
Click to expand...

Some engineers are reluctant to run parts at abs max, so guardband
down. I figure the manufacturer has already guardbanded.

One interesting case is reverse biasing polarized capacitors, which is
rarely specified.

Another is RF parts, where the abs max voltage is often the max
suggested supply max, and it\'s assumed but not stated that the actual
drain voltage will swing to 2x the supply.
 
John Larkin wrote:
On Sun, 29 Jan 2023 19:32:32 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
On Sun, 29 Jan 2023 16:43:36 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:


In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in
another thread. Swapping out the mylar cap for a polypropylene reduced
the soakage tail on the ramp, but didn\'t eliminate it.


The upper ESD diode in the 74VHC74A is essentially a zener to ground,
not a pn diode to Vcc. One could test one to see where it actually
zeners.

Mylar caps make terrible ramps.

Mylar isn\'t too bad at this speed (1-100 Hz). The original version of
this circuit had about a 200 us reset pulse, controlling the gate of a
2N7002 directly. The capacitor was a 330 nF, 200V mylar, and the
discharge arrangement was that 2N7002 with a 20-ohm resistor in series.
The ramp reset pulse was about 200 us, i.e. about 15 time constants.
There was about a 2-ms tail following the reset. The polyprop ones are
probably 3x tighter, but not orders of magnitude.

I have some polystyrenes in my drawer, but they\'re only available up to
the early nanofarads. (I have a few NOS ones up to 10 nF.)

I made a little RC differentiator gizmo with a polystyrene cap and an
OPA141, and verified that the rest of the ramp was linear to better than
12 bits, which was about the limit of the measurement. Making the reset
pulse 10 ms wide fixed it completely.

I like to test parts past abs max to see what happens. It\'s rare to
find a part that won\'t tolerate 2x abs max voltage, and 5:1 isn\'t
unusual.

Seems like the chip makers decide how tight they can make the spec
without hurting sales.


Some engineers are reluctant to run parts at abs max, so guardband
down. I figure the manufacturer has already guardbanded.

One interesting case is reverse biasing polarized capacitors, which is
rarely specified.

Another is RF parts, where the abs max voltage is often the max
suggested supply max, and it\'s assumed but not stated that the actual
drain voltage will swing to 2x the supply.
Yup.
Click to expand...

It\'s inconvenient that there\'s so little indication of how the ratings
are arrived at.

Wet electros appear fine when you exceed their voltage ratings by a
smallish amount like 10%, but their lifetime drops very very rapidly
with overvoltage.

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
 
On Mon, 30 Jan 2023 04:20:21 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
On Sun, 29 Jan 2023 19:32:32 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
On Sun, 29 Jan 2023 16:43:36 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:


In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in
another thread. Swapping out the mylar cap for a polypropylene reduced
the soakage tail on the ramp, but didn\'t eliminate it.


The upper ESD diode in the 74VHC74A is essentially a zener to ground,
not a pn diode to Vcc. One could test one to see where it actually
zeners.

Mylar caps make terrible ramps.

Mylar isn\'t too bad at this speed (1-100 Hz). The original version of
this circuit had about a 200 us reset pulse, controlling the gate of a
2N7002 directly. The capacitor was a 330 nF, 200V mylar, and the
discharge arrangement was that 2N7002 with a 20-ohm resistor in series.
The ramp reset pulse was about 200 us, i.e. about 15 time constants.
There was about a 2-ms tail following the reset. The polyprop ones are
probably 3x tighter, but not orders of magnitude.

I have some polystyrenes in my drawer, but they\'re only available up to
the early nanofarads. (I have a few NOS ones up to 10 nF.)

I made a little RC differentiator gizmo with a polystyrene cap and an
OPA141, and verified that the rest of the ramp was linear to better than
12 bits, which was about the limit of the measurement. Making the reset
pulse 10 ms wide fixed it completely.

I like to test parts past abs max to see what happens. It\'s rare to
find a part that won\'t tolerate 2x abs max voltage, and 5:1 isn\'t
unusual.

Seems like the chip makers decide how tight they can make the spec
without hurting sales.


Some engineers are reluctant to run parts at abs max, so guardband
down. I figure the manufacturer has already guardbanded.

One interesting case is reverse biasing polarized capacitors, which is
rarely specified.

Another is RF parts, where the abs max voltage is often the max
suggested supply max, and it\'s assumed but not stated that the actual
drain voltage will swing to 2x the supply.
Yup.

It\'s inconvenient that there\'s so little indication of how the ratings
are arrived at.
Click to expand...

Very inconvenient. I wonder how guardband standards emerge. It\'s kind
of a regional cultural thing, I expect. I like to measure parts to see
how conservative the max ratings are. I can recall only one opamp, a
Harris HFA1130, that was rated 11 volts abns max and would die about
there. 11 was an unusual number.

Wet electros appear fine when you exceed their voltage ratings by a
smallish amount like 10%, but their lifetime drops very very rapidly
with overvoltage.
Click to expand...

Polymers are interesting. I tested a bunch for a couple of months to
make sure I could use them reverse biased.

One thing that the semi people can\'t be trusted about is mosfet max
power and current.

FDD86367 is a dpak spec\'d at 227 watts and 100 amps CW.

IR established industry standards for insane power claims. One of
their TO247 fets claims 1000 amps and 1000 watts.


Sometimes the way to get extreme performance is to push a part or two.
That\'s a calculated risk.
 
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