|vlsitechnology.org /IR drop /2W power, 30% RAM, 20% analog|
Example Calculation of Power Strap Width with 6LM, 30% RAM, 20% Analog, Different Resistivities and Widths, 2W Core Power
The horizontal power straps have half the allocation (or twice the pitch) of vertical ones; metal resistivities are different; core power consumption is 2W with 32 core Vdd and 32 core Vss pads; 30% core is RAM blocked to metal-4; 20% is analog blocked to all layers. It is the same as a previous example but with RAM and analog blocks.
Step 1: Calculate Ipad and Vcore:
|=||2⁄(1.2×32) = 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.
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.
|=||( 0.12 + 0.4 + 0.2 + 0.4 + 0.32 + 4.48 )|
Step 4: Calculate the power strap allocation percentage p. The solution must be iterated, and the calculation below shows the first iteration.
|=||(1.403−0.086)×0.169 = 22.23%|
As shown on the right, a spreadsheet can be used to iterate to the answer of p=19.69%.
If the designer sets the power strap pitch, then the supply allocation for each metal layer n is pitch×kan×p⁄2 and the supply width is pitch×kwn×p⁄2. An example is shown in the spreadsheet on the right where we have chosen a vertical power strap pitch of 250µm.
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+17.53%|
The value 17.53% is called the IR Drop Adder.
The presence of the fixed blocks has increased the percentage of metal which must be allocated to power straps from 14.92% to 19.69%, an increase of 32%.
|Pnom||core power consumption||2W|
|ps||fraction of metal-1 in the standard cells used for power supplies||22% (for vsclib)|
|r1||resistivity of metal layer 1 in ohms per square||0.09Ω per sq.|
|r2-5||resistivity of metal layers 2-5 in ohms per square||0.07Ω per sq.|
|r6||resistivity of metal layer 6 in ohms per square||0.02Ω per sq.|
|m1-4||metal layers 1-4 blocked to power straps||50%|
|m5-6||metal layers 5-6 blocked to power straps||20%|
|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||32|
|Rpkg||the resistance of the package leadframe||25mΩ|
|Rbond||the resistance of the bond wire||25mΩ|
|Rpad||the resistance of the bond pad||100mΩ|