SPICE Controller incorporating LTspice technology
Many analog signals are analogies of situations in real life. An altimeter readout shows altitude in terms a pilot understands
Are your passive tolerances what you need?
Many analog circuits involve small-signal amplification, which makes them very prone to noise. It’s very important to get an analog circuit working optimally, because this yields maximum noise immunity.
Something similar to this audio power amplifier circuit drives the big, boxy monitor speaker above my work-from-home desk. But how can the designer be sure the audio won’t sound like it’s being played through a fuzz box?
A power amplifier drives a 4Ω speaker, but how will I know the sound won’t be distorted?
This is where simulation can help. So which simulator? LTspice™ is free to download from Analog Devices, and that’s encouraging, because, as their name suggests, they are a leading supplier of analog and related parts. The “LT” stands for Linear Technology, who were acquired by Analog Devices in 2017.
With LTspice™ you can sweep parameters to find the range of input amplitude that yields an unclipped output signal and select passive component tolerances to balance performance and cost.
Low-budget, ten percent tolerance passives may not be the best choice when they cause a large potential difference in output amplitude
Isn’t SPICE ancient? Why do we still need it?
The first version of SPICE was developed at Berkeley and released in 1973. I remember. I was already working in my first job then. It turned out to be a fundamental development that led to many more specialized versions over many years. For example, HSPICE includes transmission-line modeling for high-speed PCB design.
SPICE, unlike, say, IBIS-based tools, simulates at a very basic level. If the behavior of digital input-output signals is all you need to consider, then an IBIS, or IBIS-AMI (for SerDes) simulator usually offers a better solution.
If you have to model detailed electrical internal device behavior, however, and especially for analog circuits, SPICE is often the default choice.
In all things in life, we use models. The detail we need in those models depends on whether that detail affects the outcome.
We can, for example, look at a capacitor as simply its capacitance. This may or may not be good enough for our purposes. It certainly isn’t good enough for high-frequency design. Capacitors are physical components. There are no perfect capacitor models – just models that take increasing numbers of effects into account, like ESL (equivalent series inductance) and ESR (equivalent series resistance).
The beauty of SPICE (“Simulation Program with Integrated Circuit Emphasis”) is that it models detailed effects in active components too, to model nonlinear behavior. It’s as useful in integrated circuit design as it is in analog circuit verification, because when you get right down to it, all devices are analog on the inside.
But if we had to simulate integrated circuits with all their internal detail in SPICE to do PCB design, it would take so long, we would never produce a board. We need SPICE for analog simulation and luckily, the bespoke analog circuits we need to simulate on PCBs are usually much more manageable.
With its long history, SPICE still carries echoes of the past. Its input is still sometimes called a “SPICE deck”, originally meaning a deck of punched cards. There are naming conventions that can surprise the modern eye. You can’t use “IC” as the start of a reference designator, because “I” stands for current. In the old days, you had to type netlists in by hand, so these conventions saved time.
All this means you have to be careful when you write a SPICE netlist, or these issues will catch you out. Driving SPICE from eCADSTAR Schematic Editor has the big advantage that you can set all this up, either in your eCADSTAR parts library or in the schematic itself, and the data that you simulate is connected directly to your design data.
eCADSTAR recognizes that the format for expressing values like resistance, capacitance, and inductance, may be different in SPICE to what you use in your schematic, and your reference designators may need changing. That’s why you can set these differently, even at the library part level, for SPICE simulation.
Robust technology needs clean power—simulation really helps
Companies like Analog Devices supply an extensive set of SPICE models. One of the most important settings to get right is the mapping from eCADSTAR pins to SpiceOrder – the position of each of the terminals in a spice model. eCADSTAR refers to SpiceOrder as SPICE Pin. For two-pin components, that resolves automatically, though it still matters for viewing results in SPICE analysis. A larger component, like the DC/DC converter below, requires a more careful setup.
Pin numbers on the device may be different from SpiceOrder values
There must be the same number of pins in the part definition as there are in the SPICE model.
You can find the SpiceOrder for a supplied model in its .asy file.
The signal label is associated with the SpiceOrder here—for example “RUN” is associated with SpiceOrder “2”
In eCADSTAR Library Editor, SpiceOrder becomes SPICE Pin and you can associate it with the signal label in the same way as in the SPICE .asy file
To see how this regulator performs on power-up, I added two extra components to the schematic: a 3.3V DC voltage source, seen on the left, and a test load on the right, taking the place of the 1.8V memory that will be on the finished PCB.
A 1.43Ω resistor takes the place of the 1.8V memory, yielding a continuous current of 1.26A
I set this up as a transient analysis before starting an LTspice™ simulation.
We can model DC power-up with a transient analysis
The 1.8V supply stabilizes at just over 1.4ms
But what would happen if there were significant ripples on the 3.3V input voltage? This time, ignoring the time it takes for the input voltage to reach 3.3V, we make it a 1kHz, 300mV peak amplitude sine wave with a 3.3V offset.
The output voltage (bottom trace) shows only minimal disturbance from the 300mV amplitude ripple on the input voltage (top trace)
We can continue with all kinds of simulations. For example, we can model fluctuating power demand caused by bus switching to see if the 1.8V remains stable. SPICE is not a digital simulator so the way to do it is to add more dynamic elements to the circuit to mimic the changing load.
Why not just simulate directly in LTSPICE?
The big benefits of using the SPICE Controller are repeatability, data management, and consistency. Your setup remains with your design data when you archive and it’s there when you introduce engineering changes.
The circuits you simulate are the same ones you use in the manufactured PCB.
Jane Berrie•Signal Integrity Expert, Zuken Tech Center, BristolJane Berrie has been involved in EDA for PCB signal integrity since the 1980s. Her articles have appeared in many publications worldwide - too many times to mention. Jane is also a past session chair for 3D IC design at the annual Design Automation Conference. Jane’s also an innovator with a unique perspective, who constantly works on new solutions in the fast-evolving world of electronic design. In her spare time, Jane has organized themed charity events - including two in aid of lifeboats and red squirrel survival. Jane is also a regular disco-goer.