IBIS VCC Sweep for Model Generation

The Vcc Sweep

By Timothy Coyle

Understanding Vcc Sweep Range and Relativity

The IBIS Specification has some requirements on how the IV data should be generated that often confuses people so we are going to talk about them here.

The IV Curve Sweep Range

The IBIS Specification recommends a voltage sweep of –Vcc to 2*Vcc when generating any of the IV data curves. The idea behind this is to capture a wide enough data range to cover a transmission line system simulation. So if you have a signal going from a driver to a receiver and there are reflections that keep bouncing back and forth until the signal settles out than your IV table data should have sufficient data points for the simulator to account for the interactions between the driver and receiver with the reflected waveforms. The –Vcc to 2*Vcc sweep recommendation comes from the fact that a maximum negative reflection from a shorted transmission line would be –Vcc and the maximum positive reflection from an open transmission line would be 2*Vcc.

However keep in mind that this is only a recommendation and the Vcc sweep range can be any user specified range. For some IO buffer designs it may not make sense to take the Vcc sweep all the way down to –Vcc or all the way out to 2*Vcc. This provision also allows for the use of lab measured data where a full –Vcc to 2*Vcc sweep may not be feasible while feasible in simulation.

The Vcc Relative Curve

The Power Clamp IV curve and the Pullup IV curve are Vcc relative. When doing the minimum and maximum process corner sweeps, the Vcc has to be made relative to the supply voltage for that process corner and the sweep voltage has to track the variation in Vcc when doing a ground relative sweep. For example, assume a device has typical supply voltage of 3.3 V, a minimum supply voltage of 3.0 V, and maximum supply voltage of 3.6 V. For the typical case, the voltage sweep is 3.3 V to 6.6 V. For the minimum case, the sweep voltage is 3.0 V to 6.3 V. The difference between the typical and minimum supply voltage is -0.3 V, so the sweep voltage for the minimum process corner has to be adjusted by –0.3 V. The same can be done for the maximum supply voltage, except the difference is +0.3 V. The sweep voltage range for each process corner is 3.3 V, which keeps each data point the same ‘distance’ from Vcc.

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