Datasheet

7–179
Slow Input Edge Rate
With increased speed, logic devices have become more sensitive to slow input edge rates. A slow input edge rate, coupled with
the noise generated on the power rails when the output switches, can cause excessive output errors or oscillations. Similar
situations can occur if an unused input is left floating or is not actively held at a valid logic level.
These functional problems are due to voltage transients induced on the device’s power system as the output load current (I
O
)
flows through the parasitic lead inductances during switching (see Figure 4). Because the device’s internal power-supply nodes
are used as voltage references throughout the integrated circuit, inductive voltage spikes, V
GND
, affect the way signals appear
to the internal gate structures. For example, as the voltage at the device’s ground node rises, the input signal, V
I
′, appears to
decrease in magnitude. This undesirable phenomenon can then erroneously change the output if a threshold violation occurs.
In the case of a slowly rising input edge, if the change in voltage at GND is large enough, the apparent signal, V
I
′, at the device
appears to be driven back through the threshold and the output starts to switch in the opposite direction. If worst-case conditions
prevail (simultaneously switching all of the outputs with large transient load currents), the slow input edge is repeatedly driven
back through the threshold, causing the output to oscillate. Therefore, the maximum input transition time of the device should
not be violated, so no damage to the circuit or the package occurs.
V
CC
V
I
V
I
′
I
O
L
GND
V
GND
Figure 4. Input/Output Model
Floating Inputs
If a voltage between 0.8 V and 2 V is applied to the input for a prolonged period of time, this situation becomes critical and
should not be ignored, especially with higher bit count and more dense packages (SSOP, TSSOP). For example, if an 18-bit
transceiver has 36 I/O pins floating at the threshold, the current from V
CC
can be as high as 150 mA to 200 mA. This is
approximately 1 W of power consumed by the device, which leads to a serious overheating problem. This continuous
overheating of the device affects its reliability. Also, because the inputs are in the threshold region, the outputs tend to oscillate,
resulting in damage to the internal circuit over a long period of time. The data sheet shows the increase in supply current (∆I
CC
)
when the input is at a TTL level [for ABT V
I
= 3.4 V, ∆I
CC
= 1.5 mA (see Figure 5)]. This becomes more critical when the
input is in the threshold region as shown in Figure 6.
These characteristics are typical for all CMOS input circuits, including microprocessors and memories.
For CBT or CBTLV devices, this applies to the control inputs. For FB and GTL devices, this applies to the control inputs and
the TTL ports only.