Model CMOS Buffer gate behaviorally
Logic
The CMOS Buffer block represents a CMOS Buffer logic gate behaviorally:
The block output logic level is HIGH
if
the logic level of the gate input is 1.
The block output logic level is LOW
otherwise.
The block determines the logic levels of the gate inputs as follows:
If the gate voltage is greater than the threshold voltage, the block interprets the input as logic 1.
Otherwise, the block interprets the input as logic 0.
The threshold voltage is the voltage value at midpoint between the High level input voltage parameter value and the Low level input voltage parameter value.
Note: To improve simulation speed, the block does not model all the internal individual MOSFET devices that make up the gate. See Basic Assumptions and Limitations for details. |
The block models the gate as follows:
The gate inputs have infinite resistance and finite or zero capacitance.
The gate output offers a selection of two models: Linear
and Quadratic
.
For more information, see Selecting the Output Model for Logic Blocks. Use the Output
current-voltage relationship parameter to specify the output
model.
You can specify propagation delay for both output
models. For Linear
output, the block sets
the value of the gate output capacitor such that the resistor-capacitor
time constant equals the Propagation delay parameter
value. For Quadratic
output, the gate input
demand is lagged to approximate the Propagation delay parameter
value.
The block output voltage depends on the output model selected:
For Linear
model, output
high is the High level output voltage parameter
value, and output low is the Low level output voltage parameter
value.
For Quadratic
model, the
output voltage for High and Low states is a function of the output
current, as explained in Quadratic Model Output and Parameters. For zero load
current, output high is Vcc (the Supply voltage parameter
value), and output low is zero volts.
The block does not model the internal individual MOSFET devices
that make up the gate (except for the final MOSFET pair if you select
the Quadratic
option for the Output
current-voltage relationship parameter). This limitation
has the following implications:
The block does not accurately model the gate's response to input noise and inputs that are around the logic threshold voltage.
The block does not accurately model dynamic response.
Circuits that involve a feedback path around a set of logic gates may require a nonzero propagation delay to be set on one or more gates.
Voltage value below which the block interprets the input voltage
as logic LOW
. The default value is 2
V.
Voltage value above which the block interprets the input voltage
as logic HIGH
. The default value is 3
V.
Fixed capacitance that approximates the input capacitance for
a MOSFET gate. The MOSFET capacitance depends on the applied voltage.
When you drive this block with another gate, the Average
input capacitance produces a rise time similar to that
of the MOSFET. You can usually find this capacitance value on a manufacturer
datasheet. The default value is 5
pF. Setting this
value to zero may result in faster simulation times.
Select the output model, Linear
or Quadratic
.
The default value is Linear
.
Voltage value at the output when the output logic level is LOW
.
The default value is 0
V. This parameter is available
when you select the Linear
option for the Output
current-voltage relationship parameter.
Voltage value at the output when the output logic level is HIGH
.
The default value is 5
V. This parameter is available
when you select the Linear
option for the Output
current-voltage relationship parameter.
Value of the series output resistor that is used to model the
drop in output voltage resulting from the output current. The default
value is 25
Ω. You can derive this value
from a datasheet by dividing the high-level output voltage by the
maximum low-level output current. This parameter is available when
you select the Linear
option for the Output
current-voltage relationship parameter.
Supply voltage value applied to the gate in your circuit. The
default value is 5
V. This parameter is available
when you select the Quadratic
option for
the Output current-voltage relationship parameter.
The gate supply voltage for which mask data output resistances
and currents are defined. The default value is 5
V.
This parameter is available when you select the Quadratic
option
for the Output current-voltage relationship parameter.
A row vector [ R_OH1 R_OH2 ]
of two resistance values. The first value R_OH1 is
the gradient of the output voltage-current relationship when the gate
is logic HIGH and there is no output current. The second value R_OH2 is
the gradient of the output voltage-current relationship when the gate
is logic HIGH and the output current is I_OH. The
default value is [ 25 250 ]
Ω. This parameter
is available when you select the Quadratic
option
for the Output current-voltage relationship parameter.
The resulting current when the gate is in the logic HIGH state,
but the load forces the output voltage to zero. The default value
is 63
mA. This parameter is available when you
select the Quadratic
option for the Output
current-voltage relationship parameter.
A row vector [ R_OL1 R_OL2 ]
of two resistance values. The first value R_OL1 is
the gradient of the output voltage-current relationship when the gate
is logic LOW and there is no output current. The second value R_OL2 is
the gradient of the output voltage-current relationship when the gate
is logic LOW and the output current is I_OL. The
default value is [ 30 800 ]
Ω. This parameter
is available when you select the Quadratic
option
for the Output current-voltage relationship parameter.
The resulting current when the gate is in the logic LOW state,
but the load forces the output voltage to the supply voltage Vcc.
The default value is -45
mA. This parameter is
available when you select the Quadratic
option
for the Output current-voltage relationship parameter.
Time it takes for the output to swing from LOW
to HIGH
or HIGH
to LOW
after
the input logic levels change. The default value is 25
ns.
The gradient of the voltage-current relationship for the protection
diodes when forward biased. The default value is 5
Ω.
This parameter is available when you select the Quadratic
option
for the Output current-voltage relationship parameter.
The voltage above which the protection diode is turned on. The
default value is 0.6
V. This parameter is available
when you select the Quadratic
option for
the Output current-voltage relationship parameter.
Specify whether the initial output state of the block is High
or Low
.
This parameter is used for both linear and quadratic output states,
provided that the Propagation delay parameter
is greater than zero and the Solver Configuration block
does not have the Start simulation from steady state option
selected. The default value is Low
.
The block has the following ports:
A
Electrical input port
J
Electrical output port