
A-8
Appendix A Interfacing the L64005 to 5-V Signals
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Resistor Loads — The 5-V protection network does not affect the
choice of pull-up resistors for open drain outputs and bidirectional buff-
ers. The resistor load present on the output determines the output low
DC current consumption. The DC IOL current at VOL output at low state
is:
(VDD5 - VOLmin)/REFF = (5.5 - 0.2V)/REFF.
Large DC currents may be present if the resistive load value is too small.
A.3
Mixed Voltage
System Design
Considerations
In general practice, electronic systems with multiple power supplies
require analysis. In the case of mixed 3.3-V and 5-V systems, if the
power supplies are independent, or if the 3.3-V supply depends on the
5-V supply, then you must focus special attention on power failure and
power-on sequencing.
A.4
Engineering
Practice for
Mixed Voltage
Systems
LSI Logic’s 5-V compatible I/O design reduces the need to take precau-
tions when mixing 3.3-V and 5-V systems. LSI Logic engineering has
examined the following areas of potential risk: latch-up, gate oxide break-
down, cumulative gate oxide deterioration, junction breakdown, and tran-
sistor BVD breakdown. If you follow good engineering practices when
designing your mixed voltage system, these areas of potential risk will
not affect the operation or long term life of the L64005 with 5-V compat-
ible I/Os from LSI Logic. Additional precautions to protect the mixed volt-
age ICs are not necessary.
A.4.1
Precautions
During Power
Sequencing
Latchup is not a concern during power sequencing.3.3-V and 5-V sup-
plies do not require sequencing during power up. However, before driving
a signal at a level greater than or equal to 3.3 V, bring the 3.3-V supply
applied to the L64005 to a level of VDD greater than or equal to 3.0 V to
minimize over-voltage stress during system startup. To limit over voltage
stress during power up, power on the 3.3-V supply either prior to or
simultaneously with the system 5-V supply, or hold all 5-V signals to a
logic LOW state until VDD reaches 3.0 V. Regardless of the method cho-
sen to limit over-voltage stress during power up, the L64005 must not be
exposed to more than 6.5 VIN input voltage.