Datasheet
7–184
TI also offers the bus-hold circuit as a feature added to some of the advanced-family drivers and receivers. This circuit is similar
to the stand-alone circuit, with a diode added to the drain of the second inverter (ABT and LVT only, see Figure 12). The diode
blocks the overshoot current when the input voltage is higher than V
CC
(V
I
> V
CC
), so only the leakage current is present. This
circuit uses the device’s input stage as its first inverter; a second inverter creates the feedback feature. The calculation of the
maximum number of components that the bus-hold circuit can handle is similar to the previous example. However, the
advantage of this circuit over the stand-alone bus-hold circuit is that it eliminates the need for external components or resistors
that occupy more area on the board. This becomes critical for some designs, especially when wide buses are used. Also, because
cost and board-dimension restrictions are a major concern, designers prefer the easy fix: drop-in replaceable parts. TI offers
this feature in most of the commonly used functions in several families (refer to Table 1 for more details).
V
CC
V
CC
Input
Input Stage
Bus Hold
1 kΩ
V
CC
V
CC
Input
Input Stage
Bus Hold
1 kΩ
ABT/LVT Family ALVC/LVC Family
Figure 12. Input Structure of ABT/LVT and ALVC/LVC Families With Bus-Hold Circuit
Figure 13 shows the input characteristics of the bus-hold circuit at 3.3-V and 5-V operations, as the input voltage is swept from
0 to 5 V. These characteristics are similar in behavior to a weak driver. This driver sinks current into the part when the input
is low and sources current out of the part when the input is high. When the voltage is near the threshold, the circuit tries to switch
to the other state, always keeping the input at a valid level. This is the result of the internal feedback circuit. The plot also shows
that the current is at its maximum when the input is near the threshold. I
I(hold)
maximum is approximately 25 µA for 3.3-V
input and 400 µA for 5-V input.