[SI-LIST] Re: Decoupling for PLL

Steve-

Thanks for the details of your synthesis procedure. Makes perfect sense 
technically, but I now see where our "disconnect" is.

In the majority of the designs that I've worked on real estate is at a 
dear premium. The system designers grouse about having to put in a 
couple 0603 parts (L and C) [and even louder if they are 0805] to 
implement a power filter. When we advise them to include an extra R to 
tame the peaking invariably we get to listen to 15 minutes of their PCB 
real estate woes. Usually the L's that we had to work with were 
relatively low inductance ferrite beads or ocassionaly actual inductors. 
In any case 0805 was about the largest physical size that was allowed 
for the filter components(sometimes a 3215 bead if we were real lucky). 
B case tantalum caps and inductors in the mH range were pretty much out 
of the question. Hence we didn't have the luxury synthesizing a proper 
filter as you've suggested. Also, when we did apply the series resistor 
fix to filters made of relatively small  LC parts, the ohmic value 
usually turned out to be < 5 ohms (more like 1-2) in order to keep the 
DC levels within narrow ranges. So in the end we usually ended up having 
to devise suboptimal filters in order to comply with board area 
constraints. Although they may have been suboptimal, they did indeed 
provide the requisite filtering at the necessary frequencies without 
peaking or excessive DC drop.

Thanks again!

-Ray


steve weir wrote:

> Ray, being an old stick in the mud, the synthesis procedure I use is 
> pretty trivial for the series R-L-C.
>
> Since Butterworth is our friend, the ratio of the characteristic Z of 
> sqrt( L/C ) will be 1.4X our target Z, which will equal R.  So the 
> procedure is:
>
> 1) Set R from the acceptable IR drop.  Let's take your example of 100 
> ohms.
> 2) Determine L and C from Zchar and Wn.  For Butterworth Zchar = .707 
> * Ztarget
> a. C = 1/( Zchar * Wn ) = 1/( 71 * 6280 ) = 2.26uF
> b. L = Zchar / Wn. = 71 / 6280 = 11.3mH
>
> A drum inductor at that high of an inductance already has about 40 
> ohms resistance, so we just plop a 62 ohm resistor in series and we 
> are done.  We have a filter that is well behaved from DC up for any 
> load from open circuit down to the saturation current of the 
> inductor.  Where I/R drop is an issue we scale the impedance, but the 
> inductor physical size is essentially fixed by the voltage swing and 
> the cut-off frequency.  For example if we drop down by 10:1 in 
> inductance, the winding resistance similarly falls in the same package 
> and the saturation current rises as the square root.  The same 
> inductor size and cost will apply whether we synthesize a 100 ohm, 10 
> ohm or 1 ohm circuit.  All that is going to change is the size of C, 
> and the size of the series R that we add.
>
> If we drop this to a 1 ohm circuit then L drops to 113uH, and C 
> increases to 226uF ( B box tantalum at low voltage ).  For a bit more 
> reasonable cut-off frequency of 10KHz we get to drop the energy 
> storage, and the 1 ohm circuit becomes 11.3uH and 22.6uF, both very 
> tolerable values.
>
> If we go to the shunt load form then we can keep moving L down and C 
> up, but that 1KHz cut-off frequency is going to keep the LC product 
> fixed, and we will need to invert Zchar to 1.4 Ztarget, meaning that 
> the ratio of L/C will be 4X that of the comparable R-L-C.  So for any 
> given characteristic impedance, 3A results in a higher L than the 
> R-L-C.  That was a blessing to voltage mode converters, because the 
> problem was getting L down.
>
> Regards,
>
>
> Steve.

.
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: