[SI-LIST] Re: Decoupling for PLL
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: steve weir <weirsp@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 26 Aug 2004 17:20:55 -0700
steve weir wrote:
> Ray, this is a decoupling filter. We don't need to match the filter
> to the PS source which we can treat as essentially zero Z. I agree
> that if we wanted to matche the source we would have ourselves one big
> chunk of iron.
My point was, that if we don't match the impedances we will have
"heavy metal" to deal with. Therefore we either need a series R which
works OK for low current loads, or some sort of compensation ckt such as
3a. With those approaches we can use reasonable size inductances. I
agree, we don't "need" to match the filter if we can apply circuit
modifications to manage the peaking. A simple Order N=2 Butterworth
filter with a rolloff frequency of 1kHz, a source impedance of 10
milliOhms and a load impedance of 150 ohms works out to an L of 1330 nH
and a C of 13,300 uF, probably not something you'd want to implement in
any practical design, but if you did it would have a nice Butterworth
shape with no peaking and wouldn't require any series R or compensation
network.
>
> We need to match to the needs of the load. One way to go about that
> is to start with the minimum part count circuit, the simple series
> R-L-C, and scale for the load impedance. Most PLL's aren't very power
> hungry. If that doesn't work then circuit 3A is a nice way around the
> problem. The only limitation of 3A, and it is unlikely a problem in
> these circumstances is the stray inductance in the series R-C.
In order to manage the peaking the series resistor is effective as
we've both stated as long as the I*R drop is tolerable. I usually use an
iterative approach to determine the value of the series R using spice
simulations. Do you know of a way to calculate the optimum value for the
series R ?
The figure 3a compensator provides a load to the filter equal to the
characteristic impedance of the LC filter. Choose the compensator
capacitor to be about 4 times the filter C (not critical, bigger is OK)
and then select the compensator R so that R=SQRT(L/C) where R and C are
the R and C in your filter. I'd suspect the parasitic L in the
compensator components wouldn't be detrimental to the low frequencies
that the power filter is usually designed to roll off.
-Ray
>
> Regards,
>
> Steve.
.The ASCII schematics below illustrate the topologies under discussion
(best viewed with a fixed-width font)
Original "peaky" power filter:
L
___
+--------------UUU---+----------------+
| | |
' ' .-.
--- Low or zero Z --- | | PLL Load
- --- | |(few 100 ohms)
| | C '-'
| | |
=== === |
GND GND ===
GND
"De-peaked" circuit using series R
Few ohms L
___ ___
+-----|___|----UUU---+----------------+
| | |
' ' .-.
--- --- | | PLL Load
- Low or zero Z --- | |(few 100 ohms)
| | C '-'
| | |
=== === |
GND GND ===
GND
"De-peaked" using shunt compensator network:
L
___
+--------------UUU---+--------+-------+
| | | |
' ' | .-.
--- Low or zero Z --- --- | | PLL Load
- --- ---4*C | |(few 100 ohms)
| | C | '-'
| | . |
=== === | |
GND GND | ===
' GND
.-.
| |
| | SQRT(L/C)
'-'
|
===
GND
.
.
.
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