HD74AC Datasheet, PDF (5/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

FACT Descriptions and Family Characteristics
1.7.2 Dynamic Output Drive
Traditionally, in order to predict what incident wave voltages would occur in a system, the designer was
required to do an output analysis using a Bergeron diagram. Not only is this a long and time-consuming
operation, but the designer needed to depend upon the accuracy and reliability of the manufacturer-supplied
‘typical’ output I/V curve. Additionally, there was no way to guarantee that any supplied device would
meet these ‘typical’ performance values across the operating voltage and temperature limits. Hitachi has
taken the necessary steps to guarantee incident wave switching on transmission lines with impedances as
low as 50 Ω.
Figure 2 shows a Bergerton diagram for switching both high-to-low and low-to-high. On the right side of
the graph (I _out > 0), are the V_OH and I_IH curves for FACT logic while on the left side (I_out < 0), are the curves
for V_OL and I_IL. Although we will only discuss here the low-to-high transition, the information presented
may be applied to a high-to-low transition.
7
V_IN/I_IN
6
Line 2
5
Slope = 50 Ω
4
HIGH-to-LOW
3
Line 1
2
V_OL/I_OL
Slope = 50 Ω
V_OH/I_OH
1
LOW-to-HIGH
0
–1
–2
–0.2
–0.1
0
0.1
0.2
Current (A)
Figure 2 Gate Driving 50 Ω Line Reflection Diagram
Begin analysis at the V_OL (quiescent) point. This is the intersection of the V_OL/I_OL curve for the output and
the V_IN/I_IN curve for the input. For CMOS inputs and outputs, this point will be approximately 100 mV.
Then draw a 50 Ω load line from this intersection to the V_OH/I_OH curve as shown by Line 1. This
intersection is the voltage that the incident wave will have. Here it occurs at approximately 3.95 V. Then
draw a line with a slope of –50 Ω from this first intersection point to the V_IN/I_IN curve as shown by Line 2.
This second intersection will be the first reflection back from the input gate. Continue this process of
drawing the load line from each intersection to the next. Lines terminating on the V_OH/I_OH curve should
have positive slopes while lines terminating on the V_IN/I_IN curve should have negative slopes.
Each intersection point predicts the voltage of each reflected wave on the transmission line. Intersection
points on the V_OH/I_OH curve will be waves travelling from the driver to the receiver while intersection points
on the V_IN/I_IN curve will be waves travelling from the receiver to the driver.
Figure 3 and 4 show the resultant waveforms. Each division on the time scale represents the propagation
delay of the transmission line.
4