AVR-Chat

Hi,

I have to agree witrh those talking about power problems, it sounds to me also like a typical "brown out" problem....

If it was happening to me, I would be using a 'scope and a good DVM - Together! To analyse the problem as best I could.....

If I understood you correctly, you made changes to the power in some manner (why as any normal PC supply delivers a good stable voltage!)

BUT, a PC power supply is generally a switched mode power supply and putting a linear regulator after it is HIGHLY NOT recommended!!!!

Also a linear regulator needs some "headroom", after a PC power supply THERE IS NO HEADROOM left here !!!! I think this is your problem!! 

The linear regulator is dropping anything up to 2.5 volts.......depending upon its type and the actual load of course.....

I am a great believer in "Belt & Braces (suspenders for you yanks!!)", but here its WRONG WRONG WRONG.....

And doing it without checking/using a DVM/Scope is also WRONG WRONG WRONG.....

I hope this helps you further.....

Greetings from

Andy Mathison

[Non-text portions of this message have been removed]

Wow Andy!  So... fiery!

The changes were made because we weren't sure what a PC power supply outputs at power-up when the load on the 5V is only in the tens-hundreds milli-amps.  Using a scope to determine that is irrelevant, because another PSU might do something else.  Still, I did check it.

You would be right about the headroom if I wasn't using an LP3966 "3A Fast Ultra Low Dropout Linear Regulators" that drops only 80mV at 300mA.
Plenty headroom :-)

What's wrong with a linear regulator after a switched mode regulator, please?  I'm really interested in that?

What makes you think I didn't test it with DVM/Scope?  I did, saw a curve, and couldn't tell if it's acceptable or not.
If you were doing it with a DVM/scope, how would you define what should work or not?


I'm no yank, but I don't get either "Belt & Braces" nor the "suspenders" expression... what's it supposed to mean?

Thanks,

Cat

> To: AVR-Chat@yahoogroups.com
> From: andrewdavid.mathison@freenet.de
> Date: Fri, 12 Mar 2010 18:59:43 +0100
> Subject: [AVR-Chat] Re: Code stuck waiting for SPI transmission but  only after power-up
> 
> Hi,
> 
> I have to agree witrh those talking about power problems, it sounds to me also like a typical "brown out" problem....
> 
> If it was happening to me, I would be using a 'scope and a good DVM - Together! To analyse the problem as best I could.....
> 
> If I understood you correctly, you made changes to the power in some manner (why as any normal PC supply delivers a good stable voltage!)
> 
> BUT, a PC power supply is generally a switched mode power supply and putting a linear regulator after it is HIGHLY NOT recommended!!!!
> 
> Also a linear regulator needs some "headroom", after a PC power supply THERE IS NO HEADROOM left here !!!! I think this is your problem!! 
> 
> The linear regulator is dropping anything up to 2.5 volts.......depending upon its type and the actual load of course.....
> 
> I am a great believer in "Belt & Braces (suspenders for you yanks!!)", but here its WRONG WRONG WRONG.....
> 
> And doing it without checking/using a DVM/Scope is also WRONG WRONG WRONG.....
> 
> I hope this helps you further.....
> 
> Greetings from
> 
> Andy Mathison
> 
> [Non-text portions of this message have been removed]
> 
> 
> 
> ------------------------------------
> 
> Yahoo! Groups Links
> 
> 
> 
 		 	   		  

[Non-text portions of this message have been removed]

At 05:26 AM 13/03/2010, you wrote:
>You would be right about the headroom if I wasn't using an LP3966 
>"3A Fast Ultra Low Dropout Linear Regulators" that drops only 80mV at 300mA.

Ha ha...but did you pay attention to the input and output
capacitors recommended?

I didn't the 1st time I used a LDO.....

I must have missed it but what is the input voltage to the LDO?

Regards

John Samperi

********************************************************
Ampertronics Pty. Ltd.
11 Brokenwood Place Baulkham Hills, NSW 2153 AUSTRALIA
Tel. (02) 9674-6495       Fax (02) 9674-8745
Website  http://www.ampertronics.com.au
*Electronic Design * Custom Products * Contract Assembly
********************************************************

> >You would be right about the headroom if I wasn't using an LP3966 
> >"3A Fast Ultra Low Dropout Linear Regulators" that drops only 80mV at 300mA.
> 
> Ha ha...but did you pay attention to the input and output
> capacitors recommended?
> I didn't the 1st time I used a LDO.....

Not as much as I should have, HAHA.  Seems OK tough, 100uF Tantalum 600mOhm both on input and output.
Doesn't meet input requirements (should be < 150mOhm); I might put an X5R 1uF in parallel... I don't think it oscillates or anything though.

> I must have missed it but what is the input voltage to the LDO?

5V (maybe 5.2V) from PC PSU.

Cat

> 
> Regards
> 
> John Samperi
> 
 		 	   		  

[Non-text portions of this message have been removed]

At 08:06 AM 13/03/2010, you wrote:
>5V (maybe 5.2V) from PC PSU.

So your target is less than 5V supply? Again I don't quite remember. (3.3V??)

Regards

John Samperi

********************************************************
Ampertronics Pty. Ltd.
11 Brokenwood Place Baulkham Hills, NSW 2153 AUSTRALIA
Tel. (02) 9674-6495       Fax (02) 9674-8745
Website  http://www.ampertronics.com.au
*Electronic Design * Custom Products * Contract Assembly
********************************************************

4.5V
It's complicated why :-)

> To: AVR-Chat@yahoogroups.com
> From: samperi@ampertronics.com.au
> Date: Sat, 13 Mar 2010 08:52:43 +1100
> Subject: RE: [AVR-Chat] Re: Code stuck waiting for SPI transmission but  only after power-up
> 
> At 08:06 AM 13/03/2010, you wrote:
> >5V (maybe 5.2V) from PC PSU.
> 
> So your target is less than 5V supply? Again I don't quite remember. (3.3V??)
> 
> Regards
> 
> John Samperi
> 
> ********************************************************
> Ampertronics Pty. Ltd.
> 11 Brokenwood Place Baulkham Hills, NSW 2153 AUSTRALIA
> Tel. (02) 9674-6495       Fax (02) 9674-8745
> Website  http://www.ampertronics.com.au
> *Electronic Design * Custom Products * Contract Assembly
> ********************************************************
> 
> 
> 
> ------------------------------------
> 
> Yahoo! Groups Links
> 
> 
> 
 		 	   		  

[Non-text portions of this message have been removed]

>
>
> > I didn't the 1st time I used a LDO.....
>
> Not as much as I should have, HAHA. Seems OK tough, 100uF Tantalum 600mOhm
> both on input and output.
> Doesn't meet input requirements (should be < 150mOhm); I might put an X5R
> 1uF in parallel... I don't think it oscillates or anything though.
>

See Linear Tech. AN88 "Ceramic Input Capacitors Can Cause Overvoltage
Transients".
http://cds.linear.com/docs/Application%20Note/an88.pdf

Tantalums on the input need to be surge rated.


-- 
http://www.wearablesmartsensors.com/
http://www.softwaresafety.net/
http://www.designer-iii.com/
http://www.unusualresearch.com/


[Non-text portions of this message have been removed]

At 09:53 AM 13/03/2010, you wrote:
>4.5V It's complicated why :-)

No, it makes sense. So you have about 0.5V-0.7V headroom
which should be OK. Have you measured that it is 4.5V? ie
is the R1/R2 divider calculated correctly for 4.5V and do
you have the small cap there also?

I would not set the BOD at say, 4.3V but somewhat lower
depending on the chip used.

Regards

John Samperi

********************************************************
Ampertronics Pty. Ltd.
11 Brokenwood Place Baulkham Hills, NSW 2153 AUSTRALIA
Tel. (02) 9674-6495       Fax (02) 9674-8745
Website  http://www.ampertronics.com.au
*Electronic Design * Custom Products * Contract Assembly
********************************************************