Level Shifter

Level shifter is a special cell in low power design that assists in the transition of signal flowing from one voltage domain to another voltage domain.
Top level blocks in a chip may consist of multiple blocks inside it which have different functionality, and each of these blocks may have different power supplies too. This brings us to the concept of power domains in a design. In such a case, a signal in the chip passes from one power domain to another power domain. There may be as such twp different possibilities:

• Low voltage domain to high voltage domain.
• High voltage domain to low voltage domain.

Now let's make an analysis if we really require a level shifter in the fist case i.e, Low voltage domain to high voltage domain

Signal flowing from Low to high voltage domain

Here in Fig 1 we can see the signal passing from 3.3V voltage domain to a 5V voltage domain.

Fig2 CMOS Inverter

Now in fig2 we have considered an inverter. Let us assume that:

• Vin is output from the power domain A i.e. 3.3V
• Supply voltage Vdd is 5V (let’s assume this inverter is a part of domain B).

Now let’s see what is the output Vout in this case with different input.

case 1 When Vin = 0V, the pmos is pulled up to Vdd and the output is Vout= 5V

case 2 When Vin =3.3V, the nmos is pulled down to ground but here pmos is not completely turned off and the output hasn't reached fully 0V and Vout is somewhere in between 0 to 1.6V.

This is the reason that we have issues when a low voltage domain signal passes to a high voltage domain because it doesn’t reach the required voltage level and having such an issue is not preferable and so we use a cell called level shifter to solve this issue. Fig3 below shows the level shifter.

Level Shifter low to high

Now coming to the working of a level shifter

Let’s consider the same assumptions as in inverter, VDD being 5V and input is from power domain A. In this level shifter the pmos M3 and M4 are weak (having lesser oxide thickness).

Case1: input =0, M1 is on ,M2 is off , at this point the point A is pulled low and M4 is on, because A is low ,now VDD (5V) passes to output and point B is pulled high.

Case2: input =1, M1 is off ,M2 is on ,at this point the point B is pulled low and M3 is on, because B is low, now 0 at point B passes to output and point A is pulled high.

Now let's see if we will need a level shifter in case of signal traveling from high to low voltage domain

Signal transmission from high to low voltage domain

We saw how shifting works for low to high voltage domain in the discussion above. Now let’s work on the transition for signal traveling from high to low voltage domain which has been shown in fig 4 above. When a signal moves from a 5V voltage domain to a 3.3V voltage domain, we don’t need a level shifter as using a level shifter would simply increase the number of transistors to the design. So instead of using 8 transistors in a level shifter it is sufficient to use a simple cascaded inverter with two transistors which does the work bringing down the signal transition to a low level.