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Abiding by the USB 100mA, 500mA rule

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Winfield Hill
Guest

Tue May 05, 2020 6:45 pm   



Ricky C wrote...
Quote:

I kept searching your design for the pull down
and couldn't find it.


Added this morning, didn't see your post earlier.


--
Thanks,
- Win

Ricky C
Guest

Tue May 05, 2020 8:45 pm   



On Tuesday, May 5, 2020 at 12:45:33 PM UTC-4, Winfield Hill wrote:
Quote:
Ricky C wrote...

I kept searching your design for the pull down
and couldn't find it.

Added this morning, didn't see your post earlier.


Good that this was caught. I can see where this is a detail that could be missed. It was only after I realized there was no power pin on the device that it occurred to me there might be a problem.

I'm concerned about a similar issue with my design. Presently the entire design is powered off the supercap. Turning on the drive to the pass transistor to charge the supercap depends on providing a ground on a comparator O/C output as the supercap charges up from zero volts. I'm thinking of using a couple of Schottky diodes to wire OR the 5V rail and the supercap output to power this circuit. But is it needed? Don't know... I don't think a manufacturer will spec a chip at Vdd = 0V unless the chip is designed for that specifically.

It just occurred to me that I can get more capacity from the supercap if I charge it to the full 5V and use a regulator to drive the indicators. An LDO with less dropout than 1V will extend the useful voltage range as the cap discharges. If I have to wire OR the LDO output with the 5V rail it would be very useful if the LDO did not back feed power to the supercap.

Know of any very low dropout regulators for 10 mA that don't back feed?

Maybe the back feed is not important. Only the comparator needs power when the battery is zero voltage. I can wire OR power just for the comparators and power the indicators from an LDO. It is still important to have a low dropout voltage or it won't usefully extend the hold up time of the supercap. How low is a reasonable LDO drop out voltage for a 4 mA load?

In fact, I guess the comparator controlling the constant current charging circuit is not needed if charging the supercap to full 5V so no need for powering anything until the supercap is up to voltage. The only concern for the LDO is as low a dropout voltage as possible.

Thinking out loud here.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

Tom Gardner
Guest

Tue May 05, 2020 10:45 pm   



On 28/04/20 10:47, Winfield Hill wrote:
Quote:
Yes, and spread the drawing across six pages, in which
you have to search every label, to find out what's
connected to what.


That is indeed unsatisfactory, especially taken to extremes
where every net is names and there are very few wires. Relying
on a PDF viewer's "search" function is, um, suboptimal.

But there is a long-established alternative, which I'm
sure you must have seen and used.

For each "leaf" schematic, have all wires that goto other
leaf schematics terminated in a named symbol that looks
like a PCB edge connector. Then have an "interconnection"
schematic containing a block for each leaf schematic. Each
block has "pin" names matching those on the leaf schematic,
and the pins are connected by wires.

That way you can always trace from one pin to all others
merely by having your finger follow the line on the paper!

Why do you find that unacceptable?


Quote:
I like having it all available,
even if it gets a little crowded. Haha, I've noticed
many of my designs keep expanding until the page is
filled up. Once portions get moved to a 2nd or even
a 3rd page, the whole thing starts to get unwieldy.


I certainly like your explanations and constraints being
right next to the relevant part of the schematic.

But your schematics don't guide the reader by drawing their
eyes to the commonly used "design pattern". That's a real
disadvantage, just as a non-prettyprinted computer program
is a pain.

Unless you are counting pretty programs like
https://www.ioccc.org/2019/dogon/prog.c
or
https://www.ioccc.org/2012/tromp/tromp.c

Ricky C
Guest

Wed May 06, 2020 2:45 am   



On Tuesday, May 5, 2020 at 5:44:20 PM UTC-4, Tom Gardner wrote:
Quote:
On 28/04/20 10:47, Winfield Hill wrote:
Yes, and spread the drawing across six pages, in which
you have to search every label, to find out what's
connected to what.

That is indeed unsatisfactory, especially taken to extremes
where every net is names and there are very few wires. Relying
on a PDF viewer's "search" function is, um, suboptimal.

But there is a long-established alternative, which I'm
sure you must have seen and used.

For each "leaf" schematic, have all wires that goto other
leaf schematics terminated in a named symbol that looks
like a PCB edge connector. Then have an "interconnection"
schematic containing a block for each leaf schematic. Each
block has "pin" names matching those on the leaf schematic,
and the pins are connected by wires.

That way you can always trace from one pin to all others
merely by having your finger follow the line on the paper!

Why do you find that unacceptable?


Things similar to that are what I've seen "legislated" by various companies.. I think designers tend to want to take short cuts and make exceptions to rules, so they need to be enforced. As Win likes, for the designer who is familiar with the design it's better to have more on a page, while not so much for everyone else if it is too cramped and requires references without connecting lines.

One company I worked for dealt with the clutter on a page by using numbers in circles with an arrow pointing to the other matching circle with the same number and pointing back atcha.

I find often (especially digital designs with buses) the clutter is overwhelming and the drawing advantages are just lost entirely. At some point it might as well be some form of tabled net list to provide easy searching. The drawing is a bunch of small lists (IC, pin number, net name) scattered around the drawing anyway.

Someone I knew many years ago took over a design that was done that way and didn't like it. I think if it were done properly it might be better than a schematic for certain designs where the pictographs are not of much value..


Quote:
I like having it all available,
even if it gets a little crowded. Haha, I've noticed
many of my designs keep expanding until the page is
filled up. Once portions get moved to a 2nd or even
a 3rd page, the whole thing starts to get unwieldy.

I certainly like your explanations and constraints being
right next to the relevant part of the schematic.

But your schematics don't guide the reader by drawing their
eyes to the commonly used "design pattern". That's a real
disadvantage, just as a non-prettyprinted computer program
is a pain.


Mostly the problems we perceive is the lack of adequate white space to divide functional sections and more clearly show interconnects crossing that white space. However adding the white space makes the drawing larger or makes the details smaller if shrunk down to the same total size. With many here in the age group where visual acuity is not optimal, none of us want smaller details.

--

Rick C.

--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209

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