LTspice provide a built in exponential waveform generator. This can be very helpful to generate arbitrary exponential pulses to simulate special systems such as surge protector or ESD modeling. In this lesson, we will understand all the parameters that are involved to setup such a waveform. There are some undocumented commands that can useful when a burst of signal need to be created.

Equations to generate the exponential are given below, note variables are matched to above dialog box:

For t < Rise Delay:

Vinital+(Vpulse-Vinital)*(1-exp(-(time_Rise_Delay)/Rise_Tau))

For t < Fall Delay:

Vinital+(Vpulse-Vinital)*(1-exp(-(time*Rise_Delay)/Rise_Tau))-(Vpulse-Vinital)*(1-exp(-(time-Fall_Delay)/Fall_Tau))

An illustrated version is shown in following plot corrospoding : 

EXP(Vinital Vpulse Rise_Delay Rise_Tau Fall_Delay Fall_Tau) => EXP(0 1 1ms 1ms 10ms 2ms):

A hidden or undocumented feature of this command is to have multiple pulses in a burst fashion as it can help simulate the burst of exponential pulses in a specified period of time.

EXP(Vinital Vpulse Rise_Delay Rise_Tau Fall_Delay Fall_Tau Tpulse Npulse Tburst)

This is illustrated below:

EXP(0 1 1ms 1ms 10ms 2ms 30ms 5 200ms)

In conclusion, the goal of this post is to show how to exponential can be created using the LTspice source EXP command. With some undocumented features, exponential bursts of different period and cycles can be created. This could be useful to model surges and ESD events.