Datasheet

7–177
Introduction
In recent years, CMOS (AC/ACT, AHC/AHCT, ALVC, CBT, CBTLV, HC/HCT, LVC, LV/LV-A) and BiCMOS (ABT, ALVT,
BCT, FB, GTL, and LVT) logic families have further strengthened their position in the semiconductor market. New designs
have adopted both technologies in almost every system that exists, whether it is a PC, a workstation, or a digital switch. The
reason is obvious: power consumption is becoming a major issue in today’s market. However, when designing systems using
CMOS and BiCMOS devices, one must understand the characteristics of these families and the way inputs and outputs behave
in systems. It is very important for the designer to follow all rules and restrictions that the manufacturer requires, as well as
to design within the data-sheet specifications. Because data sheets do not cover the input behavior of a device in detail, this
application report explains the input characteristics of CMOS and BiCMOS families in general. It also explains ways to deal
with issues when designing with families in which floating inputs are a concern. Understanding the behavior of these inputs
results in more robust designs and better reliability.
Characteristics of Slow or Floating CMOS Inputs
Both CMOS and BiCMOS families have a CMOS input structure. This structure is an inverter consisting of a p-channel to V
CC
and an n-channel to GND as shown in Figure 1. With low-level input, the p-channel transistor is on and the n-channel is off,
causing current to flow from V
CC
and pulling the node to a high state. With high-level input, the n-channel transistor is on,
the p-channel is off, and the current flows to GND, pulling the node low. In both cases, no current flows from V
CC
to GND.
However, when switching from one state to another, the input crosses the threshold region, causing the n-channel and the
p-channel to turn on simultaneously, generating a current path between V
CC
and GND. This current surge can be damaging,
depending on the length of time that the input is in the threshold region (0.8 to 2 V). The supply current (I
CC
) can rise to several
milliamperes per input, peaking at approximately 1.5-V V
I
(see Figure 2). This is not a problem when switching states within
the data-sheet-specified input transition time limit specified in the recommended operating conditions table for the specific
devices. Examples are shown in Figure 3.
ABT DEVICES
Drops
Supply
Voltage
Inverter
D1
Q1
Q
p
Q
n
V
CC
To the
Internal Stage
Input
Inverter
Q
p
Q
n
To the
Internal Stage
Input
LVT/LVC DEVICES
V
CC
Figure 1. Input Structures of ABT and LVT/LVC Devices