[FAQ] TINA/Spice: Differences between a Transient and an Average Model

There are two types of Power models commonly found in the TI Power product folders. Each has a specific usage.

Average Model

The Average model is a simplified model of the control loop of the device. It is used for AC simulation to provide a Bode Plot (AC Transfer Characteristic) for use in determining the stability of the Power supply designed with the TI product. The Bode Plot can help determine the locations of the poles and zeros of the design and will provide a Crossover Frequency and a Phase Margin at that frequency. The Average model symbol usually will have fewer pins than the IC as many of the functions or features do not affect the loop characteristics. Average models typically simulate very quickly. Here is an example of an average model and the Bode Plot provided:

As shown, there are a couple of parameters that are passed to the Average model and fewer pins than the actual IC. Also, a very large inductor and capacitor are used in the injection circuit. These are required for an accurate Bode Plot to be created. The compensation network, load capacitance, inductor value, and load are the same values from the design.

Transient Model

The transient model is a model created for use in time domain simulations. The model will contain the switching behavior of the device and will contain all of the features of the device, including faults, interrupts, and time dependent properties. Typically, the transient model will come in two versions, Startup and Steady State. The Startup model will contain all of the features in the part as well as showing the actual start up of the device. The Steady State model will normally contain initial conditions to put the device as close to steady state as possible. There is usually a parameter associated with the model that will allow either Startup or Steady State to be chosen. Transient simulations typically take longer to simulate as they capture the switching nature of the device. This requires many time steps and the time step may have to be very small to give accurate results.

Here is an example of a transient model:

The model has all of the IC pins and can be used for any time based simulation. You can add current loads, test enables, thresholds, observe the ripple, check the frequency of operation, etc. with this model. It also contains a MODE varaible that can be used to change from a Startup model (MODE=0) based on the SS capacitor value or a Steady State model (MODE=1) to get to steady state opeartion quicker (in simulation time).

Both Average and Transient models can be very helpful when designing your circuit. If you need help designing the circuit before simulating, be sure to use the WEBENCH® Power Designer.