|vlsitechnology.org /IR drop /1W power, 30% fixed blocks|
Example Calculation of Power Strap Width with 5LM, 30% Fixed Blocks, Same Resistivities and Widths, 1W Core Power
This example is the previous one with 30% of the core area blocked to all metal routing because of fixed blocks. This 30% is the core area before adding power straps.
Step 1: Calculate Ipad and Vcore:
|=||1⁄(1.2×16) = 0.052A|
Step 2: Calculate the reference power supply conductance G:
|=||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.
|kan and kcn are 100% because all power straps have the same space allocated and the metal resistivities are the same. kwn is 80% for all metal layers because the power strap widths and spacings are also all the same.|
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.
|=||( 0.44 + 0.56 + 0.56 + 0.56 + 0.56 )|
Step 4: Calculate the power strap allocation percentage p. The solution must be iterated, and the calculation below shows the first iteration.
|=||(0.701−0.156)×0.374 = 20.40%|
As shown on the right, a spreadsheet can be used to iterate to the answer of p=17.27%.
The presence of the fixed blocks has increased the percentage of metal which must be allocated to power straps from 12.53% to 17.27%, an increase of 38%.
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+20.88%|
The value 20.88% is called the IR Drop Adder.
Let us look at a more complex example.
|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.|
|mn||percentage of metal layer n blocked to power straps||30%|
|Vdd||the nominal supply voltage||1.2V|
|Vddmin||the minimum supply voltage, 5% less than nominal||1.14V|
|Vmin||the desired voltage at the centre of the die, 10% less than the nominal||1.08V|
|Npad||number of core Vdd or core Vss power pads||16|
|Rpkg||the resistance of the package leadframe||25mΩ|
|Rbond||the resistance of the bond wire||25mΩ|
|Rpad||the resistance of the bond pad||100mΩ|