Datasheet
7–183
As mentioned previously in this section, TI offers the bus-hold capability as stand-alone 10-bit and 16-bit devices
(SN74ACT1071 and SN74ACT1073) with clamping diodes to V
CC
and GND for added protection against line reflections
caused by impedance mismatch on the bus. Because purely ohmic resistors cannot be implemented easily in CMOS circuits,
a configuration known as a transmission gate is used as the feedback element (see Figure 10). An n-channel and a p-channel
are arranged in parallel between the input and the output of the buffer stage. The gate of the n-channel transistor is connected
to V
CC
and the gate of the p-channel is connected to GND. When the output of the buffer is high, the p-channel is on, and when
the output is low, the n-channel is on. Both channels have a relatively small surface area — the on-state resistance from drain
to source, R
dson
, is about 5 kΩ.
V
CC
V
CC
Figure 10. Stand-Alone Bus-Hold Circuit (SN74ACT107x)
Assume that in a practical application the leakage current of a driver on a bus is I
OZ
= 10 µA and the voltage drop across the
5-kΩ resistance is V
D
= 0.8 V (this value is assumed to ensure a defined logic level). Then, the maximum number of
components that a bus-hold circuit can handle is calculated as follows:
N +
V
D
I
OZ
R
+
0.8 V
10 mA 5kW
+ 16 components
The 74ACT1071 and 74ACT1073 also provide clamping diodes as an added feature to the bus-hold circuit. These diodes are
useful for clamping any overshoot or undershoot generated by line reflections. Figure 11 shows the characteristics of the diodes
when the input voltage is above V
CC
or below GND. At V
I
= –1V, the diode can source about 50 mA, which can help eliminate
undershoots. This can be very useful when noisy buses are a concern.
30
20
10
0
5.5 6 6.5 7 7.5
40
50
60
8 8.5 9
–30
–40
–50
–60
–2 –1.75 –1.5 –1.25 –1 –0.75
–20
0
5
–0.5 –0.25 0
V
I
– Input Voltage – V V
I
– Input Voltage – V
–10
5
15
25
35
45
55
–5
–15
–25
–35
–45
–55
I – Forward Current – mA
F
I – Forward Current – mA
F
V
CC
= 5 V
V
CC
= 5 V
Upper Clamping Diode Lower Clamping Diode
Figure 11. Diode Characteristics (SN74ACT107x)
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