[SI-LIST] Re: Referencing multiple voltages in a stackup
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Fri, 22 Oct 2004 12:03:45 -0400
Oops, as Fred Balistreri points out, that should have been a 150 ps
edge, not a 150 ns edge down in paragraph 4.
Scott McMorrow wrote:
>Dudes,
>There is a need to expand beyond the view of the "plane" and refocus
>your thinking to power delivery vs. signal transmission. Everyone wants
>to conflate everything into one term called return current.
>Unfortuntately, this conflation also happens at the text book level,
>further complicating correct understanding. In fact, two totally
>seperate phenomena occur.
>
>First, for Kirchhoff's Current Law to be satisfied, there has to be an
>AC and a DC path from Vcc to Ground, and it must extend back to the
>regulator. This path, however does not need too, nor rarely does,
>follow the path of the signal. If there was no wire attached to the
>output of a driver, this current would still exist. This is the Power
>Delivery Mode current.
>
>Second, if we attach a wire to the driver, then current is diverted to
>the signal line. This current division is form of mode converstion into
>Signal Mode. In this process, most of the driver current is driven
>down(up) the line. It is in this area that really interesting things
>happen, depending upon whether a signal is ground referenced, vcc driver
>referenced or vcc other referenced. For example, if the driver is
>switching to a high state (positive logic) and the signal wire is
>referenced to ground, then very little has to happen at the boundary
>between the chip and the package. Since vcc current at the driver is
>already referenced to ground at the output transistors, and since there
>is usually a ground connection fairly close to the driver bump, there is
>no disruption in the path, and thus a smooth transition in the mode
>converstion process. (i.e. a significant percentage of the driver
>energy is transferred to the signal wire.) If the wire were infinitely
>long, then current would continue to travel out away from the package to
>infinity. This energy is lost to the system and must be replenished by
>the power supply regulator. Where does this current come from? It
>comes from between Vcc and ground. It comes from the power grid of the
>system. This would be the Power Delivery Mode.
>
>The power grid of the system is comprised of the regulator (VRM), power
>delivery traces or planes, decoupling capacitors, package balls, vias
>and bumps, and finally any on-die decoupling. Fortunately, or
>unfortunately depending on your point of view, the package, it's bumps,
>vias, balls, pads and pcb breakouts, has a fairly low cutoff frequency
>for power delivery purposes. Somewhere between 100 and 500 MHz not a
>hell of a lot of power supply energy is being passed through the package
>power grid. Why? Because there's just too much darn inductance. So, if
>you have a driver that is switching in 150 ns (2.33 GHz) you better hope
>that the transition from die to signal wire is extremely good, and that
>most of the energy has been transferred to the wire. Otherwise,
>whatever is left rattling around in the package and die must be
>decoupled locally with on-die capacitance that can absorb the impact of
>high frequency transients. If this is not done properly then noise at
>the die can grow to unacceptable levels. There is nothing that can be
>done at the PCB level which can resolve this issue, again, because of
>the power/ground pin inductance in the way.
>
>The goal of good silicon and package design is to convert as much of the
>switching energy into a signal mode. This relegates Kirchhoff to
>recharging the "batteries" for the next switching cycle, which is
>usually a lower frequency event than the switching transient. This
>lower frequency event occurs whether or not the signal path ever returns
>to the driver. For example, with an infinite line, energy converted to
>the signal mode travels out and away from the package infinitely, lost
>to the universe. On the other side, at the receiver, energy received
>from an infinitely long signal mode, needs to be converted back to Power
>Mode by the package and the receiver. If the receiver is terminated,
>and the package is designed well, then a high percentage of the energy
>is recovered. Whatever energy is not recovered by the receiver (and
>therefore terminated to Vcc and ground and decoupled by on die
>capacitors) is reflected back out to the universe and is lost. There is
>reciprocity between package design for drivers and receivers, much like
>there is reciprocity for transmitting and receiving antennas in the RF
>world.
>
>So, we can summarize that there are two modes of current in a device:
>Power Delivery Mode and Signal Mode. We can further state that the
>package and drivers should be designed to facilitate conversion of
>energy from one mode to as much of the Signal Mode as possible. The
>function of the driver and the package is to convert as much energy from
>the Power Mode to the Signal Mode, much like an RF amplifier, feedline
>and antenna's job is to convert as much energy from the Power Mode into
>a propagating wave. An efficient conversion leaves little high
>frequency energy rattling around. The function of the receiver and
>package is to convert as much energy from the Signal Mode back to the
>Power Mode.
>
>So, what about an unterminated receiver. Oops ... conversion of energy
>is near zero. So what happens? Most all of the energy is reflected back
>out to the universe. And for a poorly matched driver? A significant
>amount of energy reflects back to the driver and needs to be absorbed by
>the silicon Power Mode control devices (i.e. on-die capacitors),
>increasing the amount of noice at the driver. And what if you combine
>an unterminated receiver with a poorly matched driver? Well, the energy
>just rattles around and around and around. In the SI world we look at
>these waveforms every day. They have overshoot, undershoot, ringing and
>have all sorts of nasty effects. So we slap on resistors to absorb and
>disipate the energy and keep it from slamming the die. But, when it
>slams the die there we go again causing energy to be converted from
>Signal Mode back to Power Mode .... yuck! Who does all the decoupling
>work? Why the silicon, of course ... until the energy drops below the
>cutoff frequency of the package and is capable of being decoupled by the
>PCB planes and decoupling capacitors. There it propgates out and away
>from the package in a circular wavefront.
>
>So, you ask the question, "why is there so much high freqency noise
>(>500 MHz) on my power and ground planes?" Well, my answer to this is
>"Because you did not ground reference all your high edge rate signals."
>Remember that we've said the purpose of the driver and the package is to
>convert energy from the Power Mode to the Signal Mode and vice versa.
>This allows the energy to be carried by a uniform transmission line.
>But, if the transmission line is not uniform then several things can
>occur. First, at an impedance discontinuity energy can be reflected
>back. We're all used to this at an unterminated receiver where all the
>energy is reflected back to the driver. This causes the driver to be
>responsible for returning the energy back to the Power Mode, and thus
>forcing the driver and it's package to do double duty. Second, there
>can be a mode discontinuity where a change in propagation mode is
>forced. Vias are the common example. At a via transition, Signal Mode
>(stripline or microstrip mode signals) is converted to the Via Mode. At
>the transition, Via Mode is converted near-simultaneously to another
>Signal Mode on the new trace layer, and to multiple Parallel Plate Modes
>(the dominate mode between power and ground planes.) Parallel Plate Mode
>energy is a predominant amount of the high frequency noise that you see
>on your planes. This energy will propagate outward in a circular
>wavefront until it is absorbed or reflected back. Mostly it reflects
>(although Istvan's concept of capacitors terminating to the
>characterisitc impedance of the plane solves the reflection , and
>therefore resonance, issues.) Unfortunately, at switching frequencies,
>the mounted inductance of a capacitor is such that it is a very poor
>terminator at best. Which is why it is a very bad idea to switch from a
>ground referenced trace layer to a Vcc referenced trace layer. Some of
>the energy has to be converted into parallel plate mode and has to
>travel through decoupling capacitors whose mounted inductance are such
>that they do not do a very good job. Let me repeat, if you transition a
>trace from a ground referenced layer to a Vcc referenced layer through a
>via, you will force noise into your power planes. But if you transition
>from ground referenced layers to ground referenced layers, the noise
>will be short circuited by ground to ground via connections all around
>the PCB.
>
>To solve this problem is simple. Don't transition to Vcc referenced
>trace layers. In order of noise priority it is wise to do the following.
>
>1) Place a ground layer underneath all packages.
>2) Keep trace routing on one ground referenced layer with two vias maximum.
>3) If you have to transition through more than two vias, transition from
>ground referenced to ground referenced layer.
>4) Surround high speed signal transitions (such as clocks) with ground
>vias to contain the energy in the transition.
>5) Place ground stitch vias in all unused board space to form energy
>containment boundaries.
>
>Now it's time to get another cup of coffee.
>
>
>best regards,
>
>scott
>
>
>
>
>Gourari, Alexandre wrote:
>
>
>
>>Good point, Jim, and I agree with what Geoff said. The only point I'd like
>>to clarify, probably for myself mostly ;-))
>>Considering single CMOS totem pole driver with decoupling cap large enough
>>to store energy necessary to supply switching current you may want to look
>>at the receiver side now. The load worth considering for switching consists
>>of 2 caps, input-Vss and input-Vcc. Depending on signal transition direction
>>receiver's input current charges one and discharges the other. That effect
>>creates return currents in the both planes at either transition. Obviously
>>the difference in cap values and load side termination affects magnitude of
>>these return currents, but IMHO return current shall be considered for both
>>planes.
>>
>>Best regards,
>>Alex Gourari
>>
>>
>>-----Original Message-----
>>From: Geoff Stokes [mailto:gstokes@xxxxxxxxx]
>>Sent: Friday, October 22, 2004 7:55 AM
>>To: si-list@xxxxxxxxxxxxx)
>>Subject: [SI-LIST] Re: Referencing multiple voltages in a stackup
>>
>>
>>Hi Jim
>>
>>This link was posted here a few days ago.
>>http://www.ewh.ieee.org/r5/denver/rockymountainemc/archive/2004/Sep/Maxwell.
>>pdf
>>I recommend you download it in case it disappears at some point.
>>
>>Page 57 says that for high frequency, ground is a concept that does not
>>exist. There is a fair amount of discussion to back that view. The high
>>frequency components of a signal return through various paths, including Vdd
>>plane decoupling caps and 0V plane. So basically you are right if you add
>>the fact that the (distributed) load current might be dumped either in the
>>0V or Vdd conductors/planes, whereas, as you point out in many cases it must
>>return to the FET drain or source. Actually the power planes are AC
>>"ground" planes. They carry currents and develop associated local voltage
>>drops and give rise to system imperfections in terms of pulse shapes and
>>noise immunity at critical devices. But the important point is that in
>>order for the return current coupling, radiation and susceptibility all to
>>be kept to a minimum, the immediate current-carrying parts of power and 0V
>>planes must be well-stitched together, using vias and decoupling caps.
>>
>>Cheers
>>Geoff
>>
>>
>>
>>
>>
>>>-----Original Message-----
>>>From: Peterson, James F (FL51) [mailto:james.f.peterson@xxxxxxxxxxxxx]
>>>Sent: 22 October 2004 12:23
>>>To: si-list@xxxxxxxxxxxxx
>>>Subject: [SI-LIST] Re: Referencing multiple voltages in a stackup
>>>
>>>
>>>With regards to Chris's comments below:
>>>(paraphrasing : return currents reluctantly using the Vcc
>>>plane until they
>>>can get to a ground plane.) with a typical driver topology
>>>(totem pole), and
>>>the driver switched to a logic "1" state, don't return
>>>currents want to use
>>>this Vcc plane. Don't they need to return to the pullup FET
>>>(Kirchhoff's
>>>current law)? If this is true, it seems the return currents
>>>would want to
>>>hop on the Vccio plane to get back to that FET.....I've
>>>always been curious
>>>about this and have heard different arguments.
>>>thanks,
>>>Jim Peterson
>>>Honeywell
>>>
>>>-----Original Message-----
>>>From: si-list-bounce@xxxxxxxxxxxxx
>>>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
>>>Sent: Thursday, October 21, 2004 6:01 PM
>>>To: chris.mcgrath@xxxxxxxx; si-list@xxxxxxxxxxxxx
>>>Subject: [SI-LIST] Re: Referencing multiple voltages in a stackup
>>>
>>>
>>>Chris,
>>>
>>>You need to be very careful with that proposal. In a perfect world,
>>>signals return only against the reference ( typically ground
>>>) for the I/O,
>>>and only operate on one side of that reference plane. It is
>>>not much worse
>>>to operate on both sides of the plane. The antipad for the
>>>signal via
>>>typically provides the return signal path from one side of
>>>the plane to the
>>>other.
>>>
>>>Substantively worse is the common practice of offset
>>>striplines in a Vcc
>>>Sig Sig Gnd sandwich. Most of the return current has to work
>>>its way to
>>>nearby decoupling capacitors on the surface and back down to
>>>the opposing
>>>power layer. This works OK for non-demanding signals, but makes for
>>>resonant cavities that can be vexing.
>>>
>>>Then we get to what I understand as your proposal. Be
>>>afraid, be very
>>>afraid. For that proposal to fly, you need to thoroughly
>>>evaluate the
>>>effective inductance between whatever chunk of metal you are
>>>attempting to
>>>use as an image plane and the rest of the return signal path,
>>>and be able
>>>to show that the L*di/dt is not going to first create an EMC
>>>nightmare and
>>>secondly will not create signaling problems. You could
>>>readily create a
>>>situation where no amount of decoupling will make the board work.
>>>
>>>Steve
>>>At 02:09 PM 10/21/2004 -0700, Chris McGrath wrote:
>>>
>>>
>>>
>>>
>>>
>>>>I'm putting together a stackup with roughly 20 layers that requires
>>>>distribution of five different voltages and I'm wondering what effect
>>>>running 3.3V logic (133MHz) adjacent to a lower voltage
>>>>
>>>>
>>>>
>>>>
>>>reference plane
>>>
>>>
>>>
>>>
>>>>(such as 1.5V) would have on the lower voltage reference plane. =20
>>>>
>>>>I understand the concept of the power plane operating as a reference
>>>>plane, but with 4 mil cores throughout the board, I am worried about
>>>>coupling switching noise from 3.3V signals into lower
>>>>
>>>>
>>>>
>>>>
>>>voltage reference
>>>
>>>
>>>
>>>
>>>>planes adjacent to the 3.3V signals. =20
>>>>
>>>>As I see it, the conservative approach would be to only route 3.3V
>>>>signals next to the 3.3V plane or next to a ground plane. However,
>>>>given the rise times involved (~ 1ns), I tend to believe
>>>>
>>>>
>>>>
>>>>
>>>that sufficient
>>>
>>>
>>>
>>>
>>>>decoupling and stitching together of ground planes in the area would
>>>>suppress any noise that could potentially couple into the
>>>>
>>>>
>>>>
>>>>
>>>lower voltage
>>>
>>>
>>>
>>>
>>>>planes. My understanding is that for the higher speed
>>>>
>>>>
>>>>
>>>>
>>>signals (over 1
>>>
>>>
>>>
>>>
>>>>GHz), it is not wise to route adjacent to any reference other than
>>>>ground and the voltage reference for the GHZ signals. My question
>>>>mainly revolves around signals running at less than 1 GHz.
>>>>
>>>>I am interested in hearing any opinions you may have on the
>>>>
>>>>
>>>>
>>>>
>>>topic. We
>>>
>>>
>>>
>>>
>>>>often talk about stackups but I could not find anything in
>>>>
>>>>
>>>>
>>>>
>>>the archives
>>>
>>>
>>>
>>>
>>>>that addressed the issue of stackups with a number of different
>>>>voltages.
>>>>
>>>>Thanks,
>>>>Chris
>>>>
>>>>
>>>>------------------------------------------
>>>>Chris McGrath
>>>>Sr. Hardware Engineer
>>>>ADIC
>>>>ph: (607) 241-4858
>>>>eM: chris.mcgrath@xxxxxxxx
>>>>------------------------------------------
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>
>
>
--
Scott McMorrow
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