Anciliary caps for voltage regs

[Cursitor Doom]

Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

 

[Phil Hobbs]

On 2020-04-18 16:37, Cursitor Doom wrote:

Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

78XX regulators are very forgiving as long as you use at least a
microfarad or so right next to the input. The output is an NPN emitter,
so is pretty low-Z at all frequencies of interest. You can make one
oscillate fiercely if you use 10 nF on the input and feed it with long
floppy wires.

79XX chips are much more like LDOs because the output is the collector
of an NPN. With those, use a regular aluminum or tantalum cap on the
output.

You size the output cap to get the transient load regulation you need.

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

 

[Cursitor Doom]

On Sat, 18 Apr 2020 17:12:41 -0400, Phil Hobbs wrote:

78XX regulators are very forgiving as long as you use at least a
microfarad or so right next to the input. The output is an NPN emitter,
so is pretty low-Z at all frequencies of interest. You can make one
oscillate fiercely if you use 10 nF on the input and feed it with long
floppy wires.

Thanks for the tip, Phil. I'll keep it in mind for next time I need to
make a regulator oscillate fiercely.

But seriously, you usually see these devices with a tantalum on one side
and an electolytic on the other. I tried looking up to see why they often
specify tants, but found some article which stated their higher ESR than
other types of the same size was advantageous in this application,
although that seems heavily counter-intuitive to me, I have to say!

 

On Saturday, April 18, 2020 at 4:37:53 PM UTC-4, Cursitor Doom wrote:

Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

Isn't 0.33/.1=3.3?

The answer to your question is in figures 34 and 35. The larger input capacitor attenuates the ripple applied to the input thereby improving output transient response. It's usually a 10:1 ratio. They just don't want to scare people away with a 1uF.

 

[Pimpom]

On 4/19/2020 4:22 PM, Cursitor Doom wrote:

On Sat, 18 Apr 2020 17:12:41 -0400, Phil Hobbs wrote:

78XX regulators are very forgiving as long as you use at least a
microfarad or so right next to the input. The output is an NPN emitter,
so is pretty low-Z at all frequencies of interest. You can make one
oscillate fiercely if you use 10 nF on the input and feed it with long
floppy wires.

Thanks for the tip, Phil. I'll keep it in mind for next time I need to
make a regulator oscillate fiercely.

But seriously, you usually see these devices with a tantalum on one side
and an electolytic on the other. I tried looking up to see why they often
specify tants, but found some article which stated their higher ESR than
other types of the same size was advantageous in this application,
although that seems heavily counter-intuitive to me, I have to say!

You already got a good answer from Phil Hobbs. Here's my 2 cents:
My own rule of thumb is to place a 78xx regulator close to the
main filter - within a few inches of copper track at most - in
which case there's no need for any other input capacitor. When
that's not possible, I place an electrolytic cap of 10uF or
higher close to the regulator.

Most manufacturers say that the output cap is not needed for
stability but improves transient response. A 0.1uF ceramic cap
works fine unless more is needed due to other considerations.

When I have to place the regulator on a heatsink away from the
PCB, I solder one ceramic cap each from input and output to
ground directly onto the leads.

 

[Robert Baer]

Cursitor Doom wrote:

Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

There is no such thing as "first principles"; everyone designs their
3-terminal regulator differently, so some require very little bypassig,
and others require a lot.

 

[Phil Hobbs]

On 2020-04-20 21:57, Robert Baer wrote:

Cursitor Doom wrote:
Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

  There is no such thing as "first principles"; everyone designs their
3-terminal regulator differently, so some require very little bypassig,
and others require a lot.

I don't think CD was talking about conservation of momentum or classical
equipartition of energy. I think most of us probably start from rules
of thumb to choose a capacitor type, search our in-house parts
catalogues and/or Digikey, and then check the datasheets before choosing
the exact part. Capacitor datasheets are horrible.

The parameters that matter are the capacitance and ESR. There's also
ESL, but linear regulators are slow, so that rarely matters for loop
stability. (It can matter a lot if there's some ugly switching supply
upstream.) Time/temperature ratings also matter for wet electros.

Within each family, ESR generally decreases as the capacitance goes up.
For a given capacitance and voltage rating, wet aluminum electros have
the highest ESR, then tantalums. Multilayer ceramics have the lowest.
Film caps and alpos are between tants and MLCCs. Alpos are an
interesting case--you can choose quite a wide range of ESRs for the same
value and voltage rating, which is frequently quite useful for
stabilizing LDOs.

Most of the time I use ceramic output caps and put a zero-ohm jumper
between the regulator output and the cap, as I posted in the "off-label
uses" thread.

Regarding the horribleness of capacitor datasheets, high-valued MLCCs
are the worst. You absolutely have to look at the _characteristics_
sheet (Samsung) or the online tool (Murata, Kemet, Taiyo Yuden, several
others).

I have a blog post about this from a couple of years ago, most of which
is still relevant:
<https://electrooptical.net/News/high-value-ceramic-capacitors-they-stink-and-you-cant-get-them-anyway>

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

 

[Flyguy]

On Saturday, April 18, 2020 at 1:37:53 PM UTC-7, Cursitor Doom wrote:

Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

It pretty much depends upon the frequency response of the load and the amount of ripple on the incoming dc. If you have a full-wave rectified input the cap will have to supply the power between the half sine waves, which can be up to 8 msec (4 msec min) for a 60 Hz power source. Size the cap to provide this current, which is where the load instantaneous current demands come into play.

 

[Phil Hobbs]

On 2020-04-21 10:18, Phil Hobbs wrote:

On 2020-04-20 21:57, Robert Baer wrote:
Cursitor Doom wrote:
Gentlemen,

As a mere hobbyist, every time I need to use a 5V or 12V fixed voltage
regulator such as the L78 series in TO-220 package I have to look up the
data sheet to remind myself which size and type of cap goes across the
input and ditto the output. What I'd like to know is, do you hot-shot
designers also need to look these up or can you deduce the values and
types needed from first principles?
Here's an example (on page 5 of the datasheet linked to below) in which
they show a 33:1 ratio between input and output but don't specify the
dilectric of the caps in question for some reason (which seems like a
serious omission to me but WTF do I know?)


http://www.farnell.com/datasheets/2307057.pdf

   There is no such thing as "first principles"; everyone designs
their 3-terminal regulator differently, so some require very little
bypassig, and others require a lot.


I don't think CD was talking about conservation of momentum or classical
equipartition of energy.  I think most of us probably start from rules
of thumb to choose a capacitor type, search our in-house parts
catalogues and/or Digikey, and then check the datasheets before choosing
the exact part.  Capacitor datasheets are horrible.

The parameters that matter are the capacitance and ESR.  There's also
ESL, but linear regulators are slow, so that rarely matters for loop
stability.  (It can matter a lot if there's some ugly switching supply
upstream.)  Time/temperature ratings also matter for wet electros.

Within each family, ESR generally decreases as the capacitance goes up.
For a given capacitance and voltage rating, wet aluminum electros have
the highest ESR, then tantalums.  Multilayer ceramics have the lowest.
Film caps and alpos are between tants and MLCCs.  Alpos are an
interesting case--you can choose quite a wide range of ESRs for the same
value and voltage rating, which is frequently quite useful for
stabilizing LDOs.

Most of the time I use ceramic output caps and put a zero-ohm jumper
between the regulator output and the cap, as I posted in the "off-label
uses" thread.

Regarding the horribleness of capacitor datasheets, high-valued MLCCs
are the worst.  You absolutely have to look at the _characteristics_
sheet (Samsung) or the online tool (Murata, Kemet, Taiyo Yuden, several
others).

I have a blog post about this from a couple of years ago, most of which
is still relevant:
https://electrooptical.net/News/high-value-ceramic-capacitors-they-stink-and-you-cant-get-them-anyway

Oh, and of course MLCCs with high-k dielectrics can also be
piezoelectric, which is loads of fun.

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