Datasheet

7–167
Partial Power Down
Partial power down and live insertion are becoming a major issue in today’s system designs. Many new standards have included
this as part of their specification. The plug-and-play feature is beginning to dominate the PC market and the telecom industry
has been using it for a long time. When a system is partially down, the unpowered device is expected to go into a
high-impedance state so the device does not disturb or disrupt the data on the bus. When using standard CMOS devices, there
is a path from either the input or the output (or both) to V
CC
. This prevents partial power down for such applications as hot-card
insertion without adding current-limiting components. This is not the case with ABT, LVT, LVC, GTL, and BTL, as these paths
have been eliminated with the use of either blocking diodes or current-blocking circuitries. Figure 37 shows
functionally-equivalent schematics of the input and output structures for these families. Refer to Figures 5 and 10 for more
detail on the input and output behavior under these conditions.
Consider the situation shown in Figure 38. The driving device is powered with V
CC
= 5 V while the receiving device is powered
down (V
CC
= 0). If these devices are either LV, ALVC, or ALB, the receiver can be powered up through the diode, D2 and D3,
when the driver is in a high state. ABT, LVT, LVC, GTL, and BTL devices do not have a comparable path and are thus immune
to this problem, making them more desirable for this application. The electrical characteristics table in the data sheet has a
specification called I
off
. This specification shows the test condition and the maximum leakage a device can source or sink when
V
CC
is off. Refer to the individual data sheets for more details.
a) ALVC, LV, AND ALB EQUIVALENT I/O STRUCTURE b) ABT, LVT, LVC, GTL, AND BTL EQUIVALENT I/O STRUCTURE
V
CC
D2
D1
Input
LV and ALB Only
•••
D4
Output
V
CC
D5
Input Output
•••
D6
Figure 37. Simplified Input Structures for CMOS and ABT Devices
V
CC
= 5 V V
CC
= 0
Figure 38. Example of Partial-System Power Down