Step 1: Calculate Ipad and Vcore:

1⁄(1.2×16) = 0.052A
Vcore =
1.164×(1−2×0.052×(0.025+0.0125+0.05)⁄1.2
1.155V

Step 2: Calculate the reference power supply conductance G:

G =
 7 4×r2

7 ⁄ (4 × 0.07) =  25 mhos

Step 3 is to set out the values of kan, kwn, kcn and mn for each metal layer, and use these to calculate the value of L.

metal layer 1 2 3 4 5 6
kan 100% 100% 100% 100% 200%   0%
power metal allocated coefficient
kwn  80%  80%  80%  80%  80%   0%
power metal used coefficient
kcn 100% 100% 100% 100% 100%   0%¹
conductivity coefficient
mn  30%  30%  30%  30%  30%   0%
core area blocked
¹no M6 layer

The value of L depends on p which we don't know. We iterate to the solution and use p=0 for the first estimate.

 L = kw1kc1(1-ps)(1-m1(1-ka2p)(1-ka3p))+ kw2kc2(1-m2(1-ka2p)(1-ka3p))+ kw3kc3(1-m3(1-ka2p)(1-ka3p))+ kw4kc4(1-m4(1-ka2p)(1-ka3p))+ kw5kc5(1-m5(1-ka2p)(1-ka3p)) = ( 0.44 + 0.56 + 0.56 + 0.56 + 1.12 ) = 3.24

Step 4: Calculate the power strap allocation percentage p. The solution must be iterated, and the calculation below shows the first iteration.

 m1′ = m1×(1-ka2p)(1-ka3p)
p =
 { Vddmin×Pnom −kc1×ps(1-m1′) } × 1 (Vcore−Vmin)×Vdd2×G L
 { 1.164×1 −1×0.22×(1-0.3) } × 1 (1.155−1.08)×1.22×25 3.24
(0.430−0.156)×0.309 = 8.49%

As shown on the right, a spreadsheet can be used to iterate to the answer of p=7.70%.

Step 5: Calculate the new core size. If the initial core size estimate without power straps is x, then with power straps the core size becomes x

 x′ = x = x = x = x+8.34% √(((1-ka2p)(1-ka3p)) √0.92302 0.9230

The value 8.34% is called the IR Drop Adder.

The width of the power straps is set by the pitch, strap allocation or width chosen by the user and the value of p just calculated. An example is shown in the table below, where we set the supply strap allocation to 5.5µm and compare it to the old solution.

p allocation   pitch   core side
met‑1‑4 met‑5
old 17.27% 5.5µm 5.5µm  64µm 9.671mm
new  7.70% 5.5µm 11µm 143µm 8.667mm

Design Attribute Value
Pnom core power consumption 1W
ps fraction of metal-1 in the standard cells used for power supplies 22% (for vsclib)
rn resistivity of metal layer n in ohms per square 0.07Ω per sq.
kan
 user defined   ratio of metal layer n allocated to power metal-2 allocated to power
100%
kwn
 user defined   ratio of metal layer n used for power metal-2 allocated to power
80%
mn percentage of metal layer n blocked to power straps 30%
Vdd the nominal supply voltage 1.2V
Vddmin the minimum supply voltage, 3% less than nominal 1.164V
Vmin the desired voltage at the centre of the die, 10% less than the nominal 1.08V
Rpkg the resistance of the package leadframe 25mΩ
Rbond the resistance of the bond wire 12.5mΩ¹