Let's use LTspice - Basics - Semiconductor business -Macnica

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Design Analog Power SupplyAnalog Devices

Introduction

Let's use LTspice - In the introductory section, we gave an overview of LTspice and introduced its simple functions.
In this article, I will explain how to start LTspice and the basic operation method while designing a simple circuit.

Let's use LTspice series list is here

Circuit design

I would like to start designing circuits with LTspice right away.

This time, we will make a step-down power supply circuit using the linear regulator LT3045 from Analog Devices.
Design by referring to the following circuit on the top page of the data sheet.

Article header library 125445 pic01 1 — Reference circuit

Start from a new schematic

1. Click File > New Schematic and a blank layout screen will appear.

2. Start circuit design!

Article header library 125445 pic02 1 — Create a new screen

Placing ICs and components

1. Click component and enter "LT3045". If so, click the OK button.

Article header library 125445 pic03 1 — GUI (Component)

Article header library 125445 pic04 1 — Select Component Symbol (LT3045)

2. Select Resistor, Capacitor and Ground from the toolbar and place them as shown below.
(These can be rotated with Ctrl+R)

Article header library 125445 pic05 1 — Place components in circuit

3. Click component again and enter "voltage". Place a Voltage Source.

Article header library 125445 pic06 1 — Select Component Symbol (voltage)

4. Select Wire from the toolbar to wire the circuit.

5. Select Label from the toolbar and name the input and output nodes "IN" and "OUT".

There are two advantages of attaching labels.

Unlike the node name inside SPICE, it is easy for people to understand by looking at the waveform name (on the simulation result screen).
Since nodes with the same name have the same potential, they are connected without wiring in complex circuit configurations.

Article header library 125445 pic07 1 — Add wires and labels

Part parameter selection

1. Right click on R, C and voltage source (V1) and change the values respectively.
(It can be edited by moving the mouse over the part you want to change and right-clicking.)

Article header library 125445 pic08 1 — Select applied voltage

2. Select the part you want to use from the library of parts, if it exists. You can open the parts library by selecting "Select Capacitor" or "Select Resistor" on the parameter selection screen.

Article header library 125445 pic09 2 — Part parameter selection and library

※Notes

Unit labels on LTspice are the same as those in the International System of Units, but please note the following three points.

For 10^6, use MEG(meg) instead of M(m).
For 1 farad, just enter 1 instead of 1F.
μ representing 10^-6 is entered as "u" on LTspice.

Run the simulation

Now, the design of the circuit diagram is completed. Now we are ready to run the simulation.

The simulation is executed according to the following procedure.

1. Click the Run button on the toolbar to display "Edit Simulation Command".

Article header library 125445 pic10 1 — GUI (Run)

2. Set the "Stop time" here (5msec here) and click OK to start the simulation.

If you want to know how to set up transient analysis,Transient analysis (.tran)Please refer to the article.

Article header library 125445 pic11 2 — Edit Simulation Command (Stop time)

3. Click the point you want to observe in the waveform viewer!

We were able to confirm that the design values of an input voltage of 5 V, an output voltage of approximately 3 V, and an output current of approximately 500 mA were obtained.

Article header library 125445 pic12 1 — Run the simulation

bonus

Following the steps above, I created the typical application circuit on the top page of the datasheet.
However, LTspice has a nice feature to call a JIG circuit (sample test).

Please pay attention to the screen where the LT3045 is selected from Component in step 1 of the "IC and component placement" section.
Above the input is a button that says "Open this macromodel's test fixture".

* As of April 2022, LTspice has been upgraded and changed to "Open this macromodel's example circuit".

Article header library 125445 pic13 1 — Choice of test circuit

Click here to output a pre-prepared test circuit as shown below.
(Please note that some ICs are not prepared.)

With the test circuit, the simulation can be run by simply clicking the Run button.
It is possible to change the conditions just by changing the parameters of the peripheral parts, so please take advantage of it!

Article header library 125445 pic14 1 — Simulation in test circuit

At the end

How was your circuit design with LTspice?
Since the circuit can be designed with simple operations as described above, it is possible to easily check the characteristics.

In addition, since it is easy to change the parameters of peripheral constants, it is the most suitable tool for operation verification before board fabrication.

LTspice is not only easy to use, but also surpasses other tools in terms of simulation model accuracy.
Please try once.

If you want to quickly master how to use it, please select and take the LTspice seminar from the Analog Circuit Technology Seminar.