|vlsitechnology.org /IR drop /1W power improved|
Example Calculation of Power Strap Width with 5LM, Same Resistivities and Widths, 1W Core Power
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.
|¹no metal-6 layer; ²no fixed blocks|
Normally we can't calculate L directly because its value depends on p which we don't know. But in this case with m1‑6=0 we can
|=||( 0.62 + 0.8 + 0.8 + 0.8 + 1.6 )|
Step 4: Calculate the power strap allocation percentage p:
|=||(0.430−0.222)×0.216 = 4.50%|
As shown on the right, a spreadsheet can be used to iterate to the answer.
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+4.71%|
The value 4.71% 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.
|¹double bond halves bond wire resistance; ²two supply pads halves pad resistance|
The new solution has allowed the power strap pitch to go up from one every 88µm to one every 244µm. The core side is 769µm less and the core area is 16% less than the first solution, due to double bonding of supply pads, tighter Vddmin spec and wider metal-5 straps.