Use LTspice to Model Decap and Bondwire Inductance

Use ltspice to model decap and bondwire inductance: realistic ESL/ESR and bondwire parasitics in LTspice for accurate HF simulation modeling

You might think capacitors and bondwires are boring, just passive cans and thin wires. But in high-speed and power supply design, their hidden quirks can trip you up like a stealthy spiderweb. Ignoring Equivalent Series Inductance (ESL) or bondwire inductance is like brushing past a sliver, you don’t notice the pain until it stings.

In LTspice, embracing these little gremlins transforms simple sims into powerful predictors of real-world behavior. You’re no longer designing in theory, you’re war-testing your circuit before it ever hits a PCB.

Capturing the Soul of Decaps: ESL, ESR, and the Magic Resonance

When you drop an ideal capacitor into a schematic, it behaves like a saint: delivering smooth decoupling everywhere. But real decaps sniff at perfection. They come with ESL, ESR, and sometimes dielectric absorption, all packed into a skinny package. Analog Devices’ classic tutorial reminds us that low-impedance ground connections and minimal loop area are non-negotiable for effective decoupling. Skimping here is like leaving your shield at home, ask yourself if you really want high-frequency noise crashing your power rails.

How to Embed ESL and ESR in LTspice

Here’s your cheat-sheet for injecting realism:

1. Insert a capacitor symbol in LTspice.
2. Double-click it and fill in “Series Inductance”, this is your ESL.
3. Optionally, add Series Resistance, that’s your ESR.

Many brands today include SPICE models with built-in ESL/ESR for the win. That saves you setup time and mirrors real-life behavior better than your guesswork.

Bondwire Inductance: Tiny Wire, Huge Consequence

Those thin wires connecting an IC die to its packaging look harmless, until fast edges shake out overshoots and ringing that can baffle even seasoned designers. Bondwire inductance lurks under the radar in high-frequency switching or digital I/O.

You can model it in LTspice by:

• Inserting a discrete inductor in series with the bondwire path. A solid ballpark value is around 1 nH/mm of wire, chain that up with your bondwire length estimate.

• For shared paths or complex structures, you can use coupled inductors with a K directive to include mutual inductance effects.

• Deep dive? LTspice supports behavioral, current-dependent inductors, though that gets into advanced territory fast.

Stitching Together ESL and Bondwire for Realistic Power Path Simulation

Here’s a practical workflow gem that engineers use, but rarely write out in cozy, friendly detail:

1. Place your decap, and carve in realistic ESL and ESR values.
2. Insert bondwire inductance right at the power pin, don’t skip it.
3. Peek at PCB trace inductance, sometimes a tiny inductor there is worth modeling.
4. Apply a fast pulsed load, like a MOSFET stepping hard from zero to load current. That edge wakes up the parasitics.
5. Run .tran to see transient overshoot and ringing, then .ac to examine resonances and impedance peaks.

You’ll see voltage spikes, resonance peaking, and other quirks you just cannot get with ideal parts. It’s like hearing the haunted violin under a quiet piano, not expected, but oh-so-real.

Advanced Twists: Saturation, Hysteresis, and the CHAN Model

Yes, bondwires don’t saturate, but if you love pushing your simulations into high realism, modeling saturation for inductors (especially in power rails or beads) shows you what resilience your design can actually handle. All-About-Circuits talks about nonlinear models, hysteresis, and the CHAN model in LTspice to simulate realistic inductor behavior.

And Hackaday walks you through capturing flux-based imperfections, parasitic capacitances, core losses, the whole nine yards, especially handy if you’re creating your own SPICE subcircuits.

A Simulated Experiment: Swapping Between Ideal and Realistic Parts

Picture this:

• Setup A: Ideal 10 μF decap, zero ESL/ESR, no bondwire. Run a 1 A, 10 ns pulse. You get smooth, boring voltage.
• Setup B: Same decap, but 1 nH ESL plus 20 mΩ ESR, plus a 2 nH bondwire. Now run that pulse. You see ~100–200 mV overshoot, ringing that rings like a chime. Fast edges reveal your scrappy truths.

That’s how you wake your sim up, make it suffer a bit, and it tells you secrets it would hide otherwise.

Layout Matters: Beyond SPICE

You can simulate until your eyes dry, but those lead lengths, trace loops, and ground plane breaks on your PCB wreak havoc in real life. Analog Devices’ tutorial again reminds us: tie decaps to power pins via short traces and ground planes or your ESL climbs, and you’re back in noise city.

Developer Wisdom: Best Practices Checklist

Keep this cheat list taped to your monitor:

• Use manufacturer models whenever possible, they embed ESL/ESR for you.
• Approximate bondwire inductance: 1 nH/mm, tweak from package datasheet or PCB layout.
• Don’t skip load-edge testing. The devil always hides in fast transitions.
• Use .step to sweep ESL/bondwire values and see which parameter kills or preserves your signal.
• Confirm with physical measurements, sometimes measurement laughs at simulation.

Key Takings

• Decap ESL/ESR and bondwire inductance bite you in fast transitions if you ignore them.
• LTspice lets you model those parasitics, use the built-in fields or discrete inductors/resistors.
• A couple of nH around decoupling network or bondwire can wreck power rail stability, spikes/overshoot included.
• Saturation models like CHAN and behavioral sources bring elite realism for hardcore design.
• PCB layout = your parasitic’s playground, minimize via lengths and loops for best decoupling.

Additional Resources:

• Decoupling Capacitors: Mastering Power Integrity in Electronic Design: Engineering tutorial explaining practical capacitor placement, ESR/ESL modeling, and strategies to reduce power-supply noise effectively.