Example Calculation of Power Strap Width with 5LM, Same Resistivities and Widths, 1W Core Power

Step 1: Calculate Ipad and Vcore:

Ipad =	Pnom Vdd × Npad
=	1⁄(1.2×16) = 0.052A

Vcore =	Vddmin(1−2× Ipad ×(Rpkg+Rbond+Rpad))   Vdd
=	1.164×(1−2×0.052×(0.025+0.0125+0.05)⁄1.2
=	1.155V

Step 2: Calculate the reference power supply conductance G:

G =	7 4×r2
=	7 ⁄ (4 × 0.07) =	25 mhos

Step 3 is to set out the values of k_an, k_wn, k_cn and m_n for each metal layer, and use these to calculate the value of L.

metal layer 1 2 3 4 5 6  kan 100% 100% 100% 100% 100%   0%¹ power metal allocated coefficient  kwn  80%  80%  80%  80%  80%   0%¹ power metal used coefficient  kcn 100% 100% 100% 100% 100%   0%¹ conductivity coefficient  mn²   0%   0%   0%   0%   0%   0% core area blocked

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 m_1‑6=0 we can

L =	kw1kc1(1-ps)(1-m1(1-ka2p)(1-ka3p))+
	kw2kc2(1-m2(1-ka2p)(1-ka3p))+
	kw3kc3(1-m3(1-ka2p)(1-ka3p))+
	kw4kc4(1-m4(1-ka2p)(1-ka3p))+
	kw5kc5(1-m5(1-ka2p)(1-ka3p))
=	( 0.62 + 0.8 + 0.8 + 0.8 + 1.6 )
=	4.62

Step 4: Calculate the power strap allocation percentage p:

p =	{ Vddmin×Pnom −kc1×ps } ×  1  (Vcore−Vmin)×Vdd2×G L
=	{ 1.14×1 −1×0.22 } × 1 (1.125−1.08)×1.22×25 4.62
=	(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%
	√(((1-ka2p)(1-ka3p))		√0.9552		0.955

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.

	p	allocation met‑1‑4	allocation met‑5	pitch	core side
old	12.53%	5.5µm	5.5µm	88µm	9.146mm
new	4.50%	5.5µm	11µm	244µm	8.377mm