Short circuit current estimation in CMOS inverter

[gkkrish52]

Hi. I am simulating a CMOS inverter. NMOS and PMOS size are 1:3, 40 nm technology. I want to find the short circuit current. Kindly help me how to estimate it through simulation. Thanks in advance.

Best Regards,
Gopal

 

[d123]

Hi,

I don't know. Would knowing/calculating the DS resistances and summing them be a way of finding the short-circuit current?

 

[dick_freebird]

Tie input and output to same supply and take .OP drain
current. Reverse and repeat. All done.

 

[gkkrish52]

Thanks for your help. Have a nice day

 

[ZoOneR]

Hi,

The short circuit current is created when both NMOS and PMOS transistors are conducting, and it depends on the capacitive load and the input signal falling and rising slopes.

The method in which you short the input and output teminals and you do an OP analysis, only gives the maximum short circuit current. Doing this analysis the input and output voltages are settling to VDD/2 in a symetric inverter (NMOS and PMOS are equaly strong).

The short circuit current looks more like a periodic triangle wave, and you cand calculate it by doing a transient simulation and take the mean of the waveform.

 

[dick_freebird]

No, shoot-through current is when both are conducting
(within a single gate).

Short circuit current is when the output is shorted to
something. The most probable being ground, although
it is possible to find the supply or some other output
(in which specific case, NMOS of one and PMOS of
the other is your both-on condition).

Output buffer specs often include short circuit
current. Internal gates, never have I seen this cared
about (if it's shorted, it's inked).

 

[Audioguru]

If you are learning about how to design a Cmos logic IC then isn't maximum current one of its most basic spec's? Ask your teacher.

 

[ZoOneR]

Hi,

From my knowledge it's called the short circuit current because it connects the supply voltage to ground. This means both PMOS and NMOS are conducting.

It is important when you need to compute the power dissipation. You allways need to make this current low, because it's waisted.

To get some more information here's some document i found on the internet. Page 33 and page 36.

**broken link removed**

 

[Audioguru]

If you short an output to ground or to the supply and only one of the two Mosfets is turned on then the current is much higher than if both Mosfets are turned on conducting lower shoot-through current with the output voltage at about half the supply voltage.
More voltage across the single shorting Mosfet results in a higher current than two Mosfets in series and each has only half the supply voltage across it, the difference is about 4 times the current or more.