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'[EE] FET switch voltage drop'
2007\03\28@220341 by Zik Saleeba

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I was having a conversation with the chief design engineer at my place
of work yesterday. He was saying he'd have to install an extra 3.3V
voltage regulator in our design because I'd just discovered that we
needed a second switched 3.3V supply. I suggested we used a high-side
FET switch instead  - a suggestion he dismissed by saying the voltage
drop across the FET was around 0.6V which would put the supply well
out of spec.

This man is a chief hardware designer of 40 years experience. I'm just
a computer scientist who does electronics for a hobby. Am I crazy or
is he completely wrong on this one? I'd be expecting a voltage drop in
the millivolts at the low currents we're talking about - not 0.6V. Can
someone set me straight here?

Cheers,
Zik

2007\03\28@222239 by David VanHorn

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On 3/28/07, Zik Saleeba <spam_OUTzikTakeThisOuTspamzikzak.net> wrote:
>
> I was having a conversation with the chief design engineer at my place
> of work yesterday. He was saying he'd have to install an extra 3.3V
> voltage regulator in our design because I'd just discovered that we
> needed a second switched 3.3V supply. I suggested we used a high-side
> FET switch instead  - a suggestion he dismissed by saying the voltage
> drop across the FET was around 0.6V which would put the supply well
> out of spec.


Probably rather high, for a low RDSon fet at moderate current, that is
completely turned on.  He was thinking bipolar.

2007\03\28@230805 by Herbert Graf

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face
On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote:
> I was having a conversation with the chief design engineer at my place
> of work yesterday. He was saying he'd have to install an extra 3.3V
> voltage regulator in our design because I'd just discovered that we
> needed a second switched 3.3V supply. I suggested we used a high-side
> FET switch instead  - a suggestion he dismissed by saying the voltage
> drop across the FET was around 0.6V which would put the supply well
> out of spec.
>
> This man is a chief hardware designer of 40 years experience. I'm just
> a computer scientist who does electronics for a hobby. Am I crazy or
> is he completely wrong on this one? I'd be expecting a voltage drop in
> the millivolts at the low currents we're talking about - not 0.6V. Can
> someone set me straight here?

Assuming you are driving the gate high enough, the FET should appear as
a low value resistor, so any voltage drop will be directly related to
the current going through it. Assuming a very low current the voltage
drop will be very small.

The engineer was probably thinking BJTs. I wouldn't bother debating, get
a spice program and show him what the situation would really be. TTYL

2007\03\28@231925 by Zik Saleeba

face picon face
Thanks guys. I can still claim not to have lost my mind then :)

Yes, I think he was thinking bipolar initially and then was probably
too embarrassed to admit his mistake.

Cheers,
Zik

On 3/29/07, Herbert Graf <.....mailinglist3KILLspamspam@spam@farcite.net> wrote:
{Quote hidden}

> -

2007\03\29@014414 by Jesse Lackey

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face
Pop quiz: which of the 7 deadly sins is your chief hooo-ha guilty of?

Please don't have him work for me.  Any good engineer knows he/she will
make mistakes.  How the unavoidable errors are handled and risk managed
are as important as creative design abilities.

A cheap PFET will do it easily, assuming we aren't talking amps here.
Alternatively 3.3V regulators with enable pins are common and also
pretty cheap.  I've done it both ways.

J

Zik Saleeba wrote:
> Yes, I think he was thinking bipolar initially and then was probably
> too embarrassed to admit his mistake.
>
>
> On 3/29/07, Herbert Graf <mailinglist3spamKILLspamfarcite.net> wrote:
>> On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote:
>>>
>>> This man is a chief hardware designer of 40 years experience. I'm just
>>> a computer scientist who does electronics for a hobby. Am I crazy or

2007\03\29@022047 by Zik Saleeba

face picon face
Which of the deadly sins is he guilty of? You know I got as far as
considering deadly sin #1 - lust - and then I felt unaccountably ill.

Workplace safety announcement: considering your workmates engaging in
lust is not good for productivity. And it does your head in.

Cheers,
Zik

On 3/29/07, Jesse Lackey <.....jsl-mlKILLspamspam.....celestialaudio.com> wrote:
{Quote hidden}

> -

2007\03\29@030712 by Apptech

face
flavicon
face
> Yes, I think he was thinking bipolar initially and then was probably
> too embarrassed to admit his mistake.

It's not true for bipolar either.
Vbe is about 0.6V but at low currents Vce saturation can be under 0.1V with
ease.

In fact, Vce can be under 10 mV if it must be make it so'd.
This may require Ic to be >> Ic, but sometimes this is acceptable :-) ).
eg I have used a bipolar in preference to a FET to switch an input voltage
from a lead acid battery to a divider to an ADC input with saturation
voltage of around 10 mV and base drive much larger than the small divider
current. In that case the voltage drop matters and the styill small base
current is irrelevant.


       Russell.

2007\03\29@032905 by wouter van ooijen

face picon face
> I'd
> be expecting a voltage drop in the millivolts at the low
> currents we're talking about - not 0.6V. Can someone set me
> straight here?

How about setting us straight first - what current are you talking
about?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu



2007\03\29@034241 by Zik Saleeba

face picon face
Always the detail man Wouter!

I checked the docs - looks like it's 57mA.

Cheers,
Zik

On 3/29/07, wouter van ooijen <wouterspamspam_OUTvoti.nl> wrote:
{Quote hidden}

> -

2007\03\29@042041 by wouter van ooijen

face picon face
> Always the detail man Wouter!

It never hurts to verify. In some contexts 2A would be considered a
small current...

> I checked the docs - looks like it's 57mA.

I recall you were switching 3.3V - so take care which FET you take
('default' FETs require ~ 8V to switch fully on) , but I think 1 Ohm
RDSon (57 mV drop) should be no problem.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu



2007\03\29@044034 by Alan B. Pearce

face picon face
>> Always the detail man Wouter!
>
>It never hurts to verify. In some contexts 2A would be considered
>a small current...
>
>> I checked the docs - looks like it's 57mA.
>
>I recall you were switching 3.3V - so take care which FET you take
>('default' FETs require ~ 8V to switch fully on) , but I think 1 Ohm
>RDSon (57 mV drop) should be no problem.


On the other hand selecting something from this table might be advantageous
...

http://www.zetex.com/3.0/3-3-2b.asp?rid=21 especially the three in the lower
block which have Vgs=1.8V figures.

2007\03\29@061924 by Forrest W Christian

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Zik Saleeba wrote:

>I checked the docs - looks like it's 57mA.
>  
>
2n7000@5v,50ma is 1.7Ohm Typical.

57ma through 1.7ohms produces a voltage drop of 0.0969V.

I'm sure you can find ones with lower Ron.

-forrest


2007\03\29@072115 by Tom Sefranek

face picon face
Zik Saleeba wrote:

>I was having a conversation with the chief design engineer at my place
>of work yesterday. He was saying he'd have to install an extra 3.3V
>voltage regulator in our design because I'd just discovered that we
>needed a second switched 3.3V supply. I suggested we used a high-side
>FET switch instead  - a suggestion he dismissed by saying the voltage
>drop across the FET was around 0.6V which would put the supply well
>out of spec.
>
>This man is a chief hardware designer of 40 years experience. I'm just
>a computer scientist who does electronics for a hobby. Am I crazy or
>is he completely wrong on this one? I'd be expecting a voltage drop in
>the millivolts at the low currents we're talking about - not 0.6V. Can
>someone set me straight here?
>
>Cheers,
>Zik
>  
>
When I think high side switch, I choose P-Channel devices.
(Pulling the gate to ground to turn it on.)
A random SOT-23 device with logic level and 1 amp.  NDS352AP
Do the math, the on resistance is less than an ohm, times the current,
(60 ma.).

Tom

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 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org


2007\03\29@074126 by Alan B. Pearce

face picon face
>2n7000@5v,50ma is 1.7Ohm Typical.

The zetex ones I posted the link to have 0.075 ohm at 1.8V drive, 0.055 ohm
at 2.5V drive and over 4A capable. Others at the top of the table give 0.04
ohm at 2.5V drive, and 0.02 ohm at 4.5V drive and 10A capable. All are 20V
BVdss.

>57ma through 1.7ohms produces a voltage drop of 0.0969V.

0.075 ohm gives 4.2mV
0.055 ohm gives 3.1mV
0.040 ohm gives 2.28mV
0.020 ohm gives 1.14mV

2007\03\29@091018 by Derward

picon face
Look at this one  http://www.st.com/stonline/books/ascii/docs/11028.htmk .
I have used this one(STP140NF55) and it has .008 ohms and will take 80 amps.
They are in a 220 package.
I buy them from Mouser at less than $2.00.

Derward Myrick


{Original Message removed}

2007\03\29@100241 by fred jones

picon face
I saw these claims of unbelievable current and wondered if a FET in a TO220
case could really handle 80amps.  I found this artical and found it useful
for understanding that question.  I think it is a good read if you have
time.

www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
FJ
=======================================================
From:  "Derward" <KILLspamwdmyrickKILLspamspamearthlink.net>

Look at this one  http://www.st.com/stonline/books/ascii/docs/11028.htmk .
I have used this one(STP140NF55) and it has .008 ohms and will take 80 amps.
They are in a 220 package.
I buy them from Mouser at less than $2.00.

Derward Myrick


{Original Message removed}

2007\03\29@100753 by Alan B. Pearce

face picon face
>will take 80 amps.

Seems a bit of overkill for 57mA ... ;)

2007\03\29@103414 by Derward

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I know it is overkill,  but it does not cost much and very low on
resistance.

I use them in a product and they work great.

Derward


----- Original Message -----
From: "Alan B. Pearce" <RemoveMEA.B.PearceTakeThisOuTspamrl.ac.uk>
To: "Microcontroller discussion list - Public." <spamBeGonepiclistspamBeGonespammit.edu>
Sent: Thursday, March 29, 2007 9:07 AM
Subject: Re: [EE] FET switch voltage drop


> >will take 80 amps.
>
> Seems a bit of overkill for 57mA ... ;)
> --

2007\03\29@103656 by Derward

picon face
I agree with you, but I only use them at 2 amps.  I was just quoting their
specs.

Derward


----- Original Message -----
From: "fred jones" <TakeThisOuTboattowEraseMEspamspam_OUThotmail.com>
To: <RemoveMEpiclistspamTakeThisOuTmit.edu>
Sent: Thursday, March 29, 2007 9:02 AM
Subject: Re: [EE] FET switch voltage drop


{Quote hidden}

>>-

2007\03\29@105659 by Alan B. Pearce

face picon face
>I saw these claims of unbelievable current and wondered if a
>FET in a TO220 case could really handle 80amps.  I found this
>artical and found it useful for understanding that question.
>I think it is a good read if you have time.
...

Certainly a good read.

I have been using some chips where the current rating is specifically
defined as "package limited" and have versions of the chip in 2 packages,
one TO-39, and the other SMD0.5, with the latter having a significantly
higher current rating. The distributor assures me they have the same die
inside.

For many of these modern power switching FET chips, the current limitation
seems to be the package lead resistance.

2007\03\29@111901 by Dr Skip

picon face
David VanHorn wrote:
>  He was thinking bipolar.
>  
There are treatments available for that these days.... ;)

-Skip

2007\03\29@114750 by David VanHorn

picon face
On 3/29/07, Dr Skip <RemoveMEdrskipspam_OUTspamKILLspamgmail.com> wrote:
>
> David VanHorn wrote:
> >  He was thinking bipolar.
> >
> There are treatments available for that these days.... ;)


There's gotta be a way to work a lithium battery in there somehow.

2007\03\30@145314 by Rikard Bosnjakovic

picon face
On 3/29/07, fred jones <RemoveMEboattowTakeThisOuTspamspamhotmail.com> wrote:

> http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html

Argh. Why would anyone writing a technical article use the font Comic Sans?


--
- Rikard - http://bos.hack.org/cv/


'[EE] FET switch voltage drop'
2007\04\05@110704 by alan smith
picon face
Here is a part I've used in the past, pretty cheap too.  Depending on how much current he needs at 3.3V, a LDO could be as cheap but then add the caps in....
 
 MIC94062
 
 logic level drive, internal pup (I think..?) and is rated up to 2A


---------------------------------
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with theYahoo! Search movie showtime shortcut.

2007\04\05@134202 by Dwayne Reid

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At 12:53 PM 3/30/2007, Rikard Bosnjakovic wrote:
>On 3/29/07, fred jones <EraseMEboattowspamspamspamBeGonehotmail.com> wrote:
>
> > http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
>
>Argh. Why would anyone writing a technical article use the font Comic Sans?

I dunno what the problem is.  I find it easy to read, easy on the eyes.

In other words, I thought it looked just fine.

What are your objections to that specific font?

dwayne

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2007\04\05@140124 by Bob Barr

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On Thu, 05 Apr 2007 11:41:57 -0600, Dwayne Reid wrote:

>At 12:53 PM 3/30/2007, Rikard Bosnjakovic wrote:
>>On 3/29/07, fred jones wrote:
>>
>> > www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
>>
>>Argh. Why would anyone writing a technical article use the font Comic Sans?
>
>I dunno what the problem is.  I find it easy to read, easy on the eyes.
>
>In other words, I thought it looked just fine.
>
>What are your objections to that specific font?
>

I find it to be a very readable font as well, especially for reading
on the web.

Most of the objections to it that I've heard seem to be relate more to
the name of the font rather its appearance. Unfortunately, the use of
the word "comic" implies a notion of non-professionalism to many
people.


Regards, Bob

2007\04\05@142142 by Peter P.

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Bob Barr <bbarr <at> california.com> writes:

> I find it to be a very readable font as well, especially for reading
> on the web.

Imho if one reads a lot (like 10+ hours a day every day), one becomes very picky
about fonts, to the point of using the browser's style switch to kill any funny
fonts and substitute one's preferred one whenever possible. The Comic font is ok
but it has just a little too much 'decoration' for easy quick reading. Most
people who need to read a lot strongly prefer something like Arial Sans or
Schumacher Clean or Adobe Helvetica. All those little extra decorations add up
to the 'workload' and eventually imho one tires faster because of them. They are
ok for titles or inscriptions on diagrams or advrtising, but for content,
something simple and smooth is best.

just an opinion from someone who reads a little more than most on the www,
Peter P.


2007\04\05@142428 by Gerhard Fiedler

picon face
Bob Barr wrote:

>>> > http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html

>>> Argh. Why would anyone writing a technical article use the font Comic
>>> Sans?

>> What are your objections to that specific font?

> Most of the objections to it that I've heard seem to be relate more to
> the name of the font rather its appearance.

And probably to the fact that it's a standard Windows font :)

Gerhard

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