AVR-Chat

Chuck Hackett wrote:
> My railroad signal application will require powering the boards over
> a long distance (> 1,000 feet) from a head-end power supply.  To
> overcome problems with the voltage drop along the cable run I am
> looking at using a switching regulator to cut the possible 24-36vdc
> power bus (falling to maybe 10-12 v at the far end) down to the 5v
> required on the board.     
> 
> I am looking into switching regulators because they are more
> efficient at dropping higher voltages down to 5v levels.  I expect a
> current draw of 100ma on up to a possible max of 700ma.  
> 
> One example would be the LT-1076-5
>
(http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,P198
3)
> 
> My concern is the noise generated by the regulator's switching
> activity. 
> 
> Assuming I use a circuit suggested by the manufacturer, keep traces
> short, and keep the regulator circuit away from input lines am I
> likely to run into problems with noise causing the AVR to operate
> erratically?   
> 
> My board does not involve sensitive A/D inputs.  The only A/D input
> is a low-impedance, slow changing signal that goes through a low pass
> filter before it gets to the AVR.  
> 
> I had thought of powering an intermediate internal power bus from the
> switcher and then using a linear regulator down to 5v but I'd rather
> avoid the extra real-estate and cost.  
> 

We have done a lot of AVR stuff powered with National Semi's "simple
switchers" with no problems, they usually just work like a 7805 but a
lot less heat!!. They have a design tool called Webench on their website
to tell you the right components to use around the switcher for trouble
free operation.

E.g. LM2592HVT-5.0 will give you a nice efficient 5V @ 2A from up to 60V
input. It's a TO-220/5 package so easy to make prototypes with.


-- 
Tim Mitchell

On 17 Sep 2008 at 9:12, Tim Mitchell wrote:

> Chuck Hackett wrote:
> > My railroad signal application will require powering the boards
> over
> > a long distance (> 1,000 feet) from a head-end power supply.  To
> > overcome problems with the voltage drop along the cable run I am
> > looking at using a switching regulator to cut the possible
> 24-36vdc
> > power bus (falling to maybe 10-12 v at the far end) down to the
> 5v
> > required on the board.     
> > 
> > I am looking into switching regulators because they are more
> > efficient at dropping higher voltages down to 5v levels.  I expect
> a
> > current draw of 100ma on up to a possible max of 700ma.  
> > 
> > One example would be the LT-1076-5
> >
> (http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,
> P198
> 3)
> > 
> > My concern is the noise generated by the regulator's switching
> > activity. 
> > 
> > Assuming I use a circuit suggested by the manufacturer, keep
> traces
> > short, and keep the regulator circuit away from input lines am I
> > likely to run into problems with noise causing the AVR to
> operate
> > erratically?   
> > 
> > My board does not involve sensitive A/D inputs.  The only A/D
> input
> > is a low-impedance, slow changing signal that goes through a low
> pass
> > filter before it gets to the AVR.  
> > 
> > I had thought of powering an intermediate internal power bus from
> the
> > switcher and then using a linear regulator down to 5v but I'd
> rather
> > avoid the extra real-estate and cost.  
> > 
> 
> We have done a lot of AVR stuff powered with National Semi's
> "simple
> switchers" with no problems, they usually just work like a 7805 but
> a
> lot less heat!!. They have a design tool called Webench on their
> website
> to tell you the right components to use around the switcher for
> trouble
> free operation.
> 
> E.g. LM2592HVT-5.0 will give you a nice efficient 5V @ 2A from up to
> 60V
> input. It's a TO-220/5 package so easy to make prototypes with.
> 
> 
> -- 
> Tim Mitchell
> 
> 
> ------------------------------------
> 

	I have used this design: 
	
	Ultra Simple 5V Switchmode Regulator
	http://sound.westhost.com/appnotes/an006.htm

	A nice cheap switching regulator.

	There is an even better version here:

	Simple Switching Regulator
	http://www.techlib.com/electronics/regulators.html

	Mark Jordan

On Wed, Sep 17, 2008 at 8:50 PM, Enki <enkitec@gmail.com> wrote:
>        I have used this design:
>
>        Ultra Simple 5V Switchmode Regulator
>        http://sound.westhost.com/appnotes/an006.htm
>
>        A nice cheap switching regulator.
>
>        There is an even better version here:
>
>        Simple Switching Regulator
>        http://www.techlib.com/electronics/regulators.html

I will not recommend this kind of circuitrs unless for cost
reasons. They tend to be quite noisy.

I did use similar circuits for very wide input AC/DC sensors
since there are no PWM ICs which works from 12-240V DC and
20-240V AC.

Xiaofan

> I will not recommend this kind of circuitrs unless for cost
> reasons. They tend to be quite noisy.

Well, Nationals simple switchers aren't too wonderful, but unless you
lay out the board with no consideration for the circulating currents,
they certainly won't interfere with the AVRs operation.

Noise is something that you minimize in switchers by paying attention
to the components you are using, and to the layout itself.

Unless you have something fairly sensitive in analog going on, an SMPS
is a good choice.
Keeping the voltage on the bus high is a good way to minimize i^2R losses.

Be sure that you are taking into account the interaction between the
supplies though, (this would apply equally to linear supplies).
When unit X draws current, it will affect the current available to the
other units near it on the bus.

On 17 Sep 2008 at 21:29, Xiaofan Chen wrote:

> On Wed, Sep 17, 2008 at 8:50 PM, Enki <enkitec@gmail.com> wrote:
> >        I have used this design:
> >
> >        Ultra Simple 5V Switchmode Regulator
> >        http://sound.westhost.com/appnotes/an006.htm
> >
> >        A nice cheap switching regulator.
> >
> >        There is an even better version here:
> >
> >        Simple Switching Regulator
> >        http://www.techlib.com/electronics/regulators.html
> 
> I will not recommend this kind of circuitrs unless for cost
> reasons. They tend to be quite noisy.
>

	The first one, yes.
	The second circuit is a lot better.
	No difference compared to one MC34063 based.

	Mark Jordan

Don't go through all the trouble of trying to design a low noise switcher.
You can get one from these folks.  I think these supplies are about $11-$12
and they are pretty good.

http://www.astrodyne.com/catalog.asp?pagetype=search&prtno=asd105&srchtype=w
eb&code=usa

I have dealt with Astrodyne for years, and they make good, inexpensive
stuff.

Steve

-----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Enki
Sent: Wednesday, September 17, 2008 10:06 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] RE: Powering an AVR with a switching regulator

On 17 Sep 2008 at 21:29, Xiaofan Chen wrote:

> On Wed, Sep 17, 2008 at 8:50 PM, Enki <enkitec@gmail.com> wrote:
> >        I have used this design:
> >
> >        Ultra Simple 5V Switchmode Regulator
> >        http://sound.westhost.com/appnotes/an006.htm
> >
> >        A nice cheap switching regulator.
> >
> >        There is an even better version here:
> >
> >        Simple Switching Regulator
> >        http://www.techlib.com/electronics/regulators.html
> 
> I will not recommend this kind of circuitrs unless for cost
> reasons. They tend to be quite noisy.
>

	The first one, yes.
	The second circuit is a lot better.
	No difference compared to one MC34063 based.

	Mark Jordan