Power Supply Routing resistance approach
The resistance approach: In this case the power network may look much as
it does in the root approach, but the tapering is based on a calculated resistance
from the supplying pad to the specific block in the chain. The amount
of resistance that is tolerable is determined by an acceptable voltage drop
through the power supply line as calculated by Ohm’s law.
• The width of the supply lines is based on the resistivity of the metal used
for routing the supply.
• Number of vias is very carefully calculated to help reduce the total resistance
or to ensure that via resistance is not a limiting factor.
• The choice of metal should be based on lower resistivity values
• Routed metal should be implemented such that changing layers are
M1 » M2 » M1 instead of M1 » M2 » M3 » M2 » M1.
• In some specialized and fast chips, there are metal layers dedicated to
power supply level only in order to reduce power supply resistance and
ensure a consistent power supply level to all parts of the die.