|vlsitechnology.org /IR drop /2W core power|
Example Calculation of Power Strap Width with 6LM, Different Resistivities and Widths, 2W Core Power
Horizontal power straps are half the width (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.
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.
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
|=||( 0.24 + 0.8 + 0.4 + 0.8 + 0.4 + 5.6 )|
Step 4: Calculate the power strap allocation percentage p:
|=||(1.403−0.173)×0.121 = 14.92%|
As shown on the right, a spreadsheet can be used to iterate to the answer.
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||= x+12.70%|
The value 12.7% is called the IR Drop Adder.
This represents a more "normal" power busing scenario (but not necessarily a better one), where vertical power straps are preferred over horizontal ones. The extra use of metal-6 for power straps makes sense (a) because it is a long way from the transistors and so the impact on routing density should be less; and (b) because it is a thick metal layer with better conductivity.
Note the value of L=8.24 of which 5.6 comes from metal-6.
|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.|
|mn||percentage of metal layer n blocked to power straps||0%|
|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|