HD26LS31 Datasheet, PDF (8/15 Pages) Hitachi Semiconductor – Quadruple Differential Line Drivers With 3 State Outputs

HD26LS31
HD26LS31 Line Driver Applications
The HD26LS31 is a line driver that meets the EIA RS-422A conditions, and has been designed to supply a
high current for differential signals to a bus line. Its features are listed below.
• Operates on a single 5 V power supply.
• High output impedance when power is off
• Three-state output
• On-chip current limiter circuit
• Sink current and source current both 40 mA
A block diagram is shown in figure 1. The enable function is common to all four drivers, and either active-
high or active-low can be selected.
The output section consists of two output stages (the Y side and Z side), each of which has the same sink
current and source current capacity.
Input is TTL compatible, and an output current limiter circuit is built into the output stage as shown in
figure 2.
1A
1Y
1Z
2A
2Y
2Z
3A
3Y
3Z
4A
4Y
4Z
Enable G
Enable G
Figure 1 HD26LS31 Block Diagram
The output current limiter circuit consists of transistor Q₁ and resistance R₁, and operates when the voltage
drop on both sides of R₁ reaches approximately 0.7 V. At this time the current, i, is as follows:
i = 0.7 (V) / 9 (Ω) ≈ 78 (mA)
When a current greater than this flows, Q₁ is turned on, the Q₂ base current flows to the output side, and the
flow of an excessively large output current is prevented.
However, since this type of current limiter circuit has the characteristics shown in figure 3, the output stage
power dissipation is large.
Therefore, when the output is shorted, this should be limited to a maximum of one second for one pin only.
The I_OL vs. V_OL characteristic for low-level output is shown in figure 4.
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