How to calculate the IR drops ?

[analayout]

hi guys
hw can i calculate the IR drops
assume I=100uA metal width=5um length=20um
can anybody help me
regards
analayout

 

[rajkumaru]

Re: IR drop

Hi,
In my view, Voltage Drop V=IR
Here given that-
I= 100uA, So, we need to know the value of R. For that we check in design manual/foundary_provided_document that what is the value of metal resistance for unit area. Then we can use formula R_metal = metal length*metal width*metal per unit area resistance.
Suppose it is given that R=5uohm/um*um then R_metal=5um*20um*5= 500uohm

So, Now V=IR=100uA*500ohm= 50000uV=0.05V

If, I am wrong then please viewers let me correct. Thanks in advance!

Raj

 

[MSSN]

Re: IR drop

So, we need to know the value of R. For that we check in design manual/foundary_provided_document that what is the value of metal resistance for unit area. Then we can use formula R_metal = metal length*metal width*metal per unit area resistance.

I think the more common in the foundaries' documents is the resistance per square(square resistance) and what u do then is that u simply divide the length by the width to get the number of squares and then calculate the total resistance by multiplying the number of squares by the square resistance.

 

[srieda]

Re: IR drop

I agree. You will usually find the Rs (Sheet Resistance) given in terms of ohms/square.
So R = Rs * L / W

 

[pvnk]

Re: IR drop

Hi,
In my view, Voltage Drop V=IR
Here given that-
I= 100uA, So, we need to know the value of R. For that we check in design manual/foundary_provided_document that what is the value of metal resistance for unit area. Then we can use formula R_metal = metal length*metal width*metal per unit area resistance.
Suppose it is given that R=5uohm/um*um then R_metal=5um*20um*5= 500uohm

So, Now V=IR=100uA*500ohm= 50000uV=0.05V

If, I am wrong then please viewers let me correct. Thanks in advance!

Raj

Here is the calculation,
suppose R is the ohm/square for your metal layer ( refer FAB electrical specs.)
L is the length if your rout
W is the width of your rout,
I is the current flowing ( take slightle higher than your avg I )

Now to calculate no. of sqrs, N = L/W

total resistance Rtot = N * R

IR dorp = Rtot*I volts ( if I is in amps )

Hop it's clear. it's ok if you have a 10mV drop for digital lines and 1mV for analog generally. for analog, always ensure that your disigner knows what is the drop on important nets.

thanks

if you are working on a Cadence platform, you can get this ohms/square by dumping your technology file ( provided your tech file contains this info) go to icfb > tools > technology file manager ( something similar ) > dump > select electrical rules and give file path to dump > click ok.
i
or mail your FAB and they will be happy to give you this info.