Civil Engineering Reference
In-Depth Information
excitation to this RC structure. The insensitivity of the effective capacitance
to
also enables flexibility in the choice of
a reasonable value of
that.
is used is the largest capacitance that the cell is expected to drive.
When the gate drives a capacitive load under a step input‚ if the falling
output waveform is assumed to be of the form
then the value of
can be obtained from the 50% and 90% time points
(denoted as
and
respectively) as
The reason for choosing
and
rather than any other threshold is a result
of empirical observations.
The values of and for a gate driving a purely capacitive load may
easily be found using any of the techniques described in Section 4.3‚ such as
the use of
equations.
Finding
and
This procedure involves a set of iterations‚ whereby the average load current
driven by the Thevenin model for the driver through an O'Brien-Savarino
model‚ as shown in Figure 4.9(a)‚ is equated to that through a capacitor
shown in Figure 4.9(b). Equivalently‚ this may be thought of as equating the
total charge delivered to both circuits over a given time period‚
The value
of
is typically set to
Since the value of the effective capacitance is unknown‚ this procedure in-
volves iterations to compute the values of
and
Specifically‚ the
iterations involve the following steps:
Step 1
The value is computed by solving the following equation‚ illus-
trated for a falling output transition:
