HD26C31 Datasheet, PDF (11/15 Pages) Hitachi Semiconductor – Quadruple Differential Line Drivers With 3 State Outputs

Output Characteristics ("L" Level)
0.5
Ta = 25°C
0.4
V_CC = 4.5 V
0.3
V_CC = 5.0 V
0.2
V_CC = 5.5 V
0.1
HD26C31
0
20 40 60 80 100
Output Current I_OL (mA)
Figure 2 I_OL vs. V_OL Characteristics
When termination resistance R_T is connected between the two transmission lines, as shown in figure 3 the
current path situation is that current I_OH on the side outputting a high level (in this case, the Y output) flows
to the side outputting a low level (in this case, the Z output) via R_T, with the result that the low level rise is
large.
If termination resistance R_T is dropped to GND on both transmit lines, as shown in figure 4 the current path
situation is that the current that flows into the side outputting a low level (in this case, the Z output) is only
the input bias current from the receiver. As this input bias current is small compared with the signal
current, it has almost no effect on the differential input signal at the receiver end.
Figure 5 shows the output voltage characteristic when termination resistance R_T is varied.
Also, when used in a party line system, etc., the low level rises further due to the receiver input bias
current, so that it is probably advisable to drop the termination resistance to GND.
However, the fact that it is possible to make the value of R_T equal to the characteristic impedance of the
transmission line offers the advantage of being able to hold the power dissipation on the side outputting a
high level to a lower level than in the above case.
Consequently, the appropriate use must be decided according to the actual operating conditions
(transmission line characteristics, transmission distance, whether a party line is used, etc.).
Figure 6 shows the output characteristics when termination resistance R_T is varied.
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