power routing basics

Power routing basics :

• How to calculate number of core power/ground pads required?

Lets assume 1Watt power is consumed by chip and 1V is ss corner voltage

Power = Voltage*Current

P=V*I 

I = P/V

  =1/1

  = 1 ampere = 1000 milli ampere

Assume each pad can drive maximum of 100mA current.

No.of power pads required (p) = 1000/100=10  pads

No.of ground pads required (g) = p = 10 pads

Ideally 10 ground pads and 10  power pads are sufficient. In most of the cases the power distribution may not be uniform meaning the pads which are placed in the centre of chip are more effective as the sinks at centre might have less resistive paths. So we need to add more pads lets say 30% more pads.

So, no.of power/ground pads = (1+0.3)*10=13

• How to calculate the core PG ring width? Given Jmax (maximum current density of metal layer = 50mA/um)
• Core PG ring width = (Total current)/(No.of sides *Jmax)

             = 1000/(4*50)=5 u

• How to calculate pad to core trunk width?
• Pad to core trunk width = total current/(no.of sides * Jmax)

                                            = 1000/(4*50)=5 u

• How to calculate strap width and no.of straps for a block? Given width of block is W and height of block as H. Total power of block as P
• Let I = P/V
• Total current from top and bottom = (I * W)/(W+H)
• Let current is equally be supplied from top and bottom.
• Itop = Ibottom = (I * W)/(W+H)
• Similarly Ileft = Iright = (I * H)/(W+H)
• With 1% voltage drop at a node in the center
• Reffective (from Vertical) = (0.01*(VDD/2))/Itop
• Reffective (from Horizontal) = (0.01*(VDD/2))/Ileft

• Width of Straps based on EM limits :
• Total width of Vertical strap (Wvstrap-em) = Itop /Jmax
• Total width of horizontal strap (Whstrap-em) = Ileft /Jmax
• Now assume initial width of strap to be 5X of min-width
• Wvstrap = 5*0.07=0.35 (0.07 is the min width for metal6)
• Rstripe-v = (Rpsq * H)/Wvstrap
• Note :  Rpsq =  Rho/thickness
• No.of vertical straps = Nvstraps = Rstripe-v / Reffective
• Now check the total width of vertical straps = Nvstraps * Wvstrap is greater than or equal to Wvstrap-em else increase the Wvstrap accordingly.