2022-11-10 15:24 Tags: # Logic Level-shifter Modern electronics usually operate with many different supply voltages. It is often necessary to translate logic signals between different supply voltage levels. Translating from higher to lower isn't an issue, but lower to higher requires some thought. ## Common Circuit ![](attachments/Logic%20Level-shifter.png#invert) Cross coupled PMOS are driven to ground by two NMOS with complementary gate signals. The A inputs to the NMOS are driven by a lower supply voltage than the high supply. ![](attachments/Logic%20Level-shifter-1.png#invert) ### Operation Consider starting from A Low, A_bar high, so Y_Bar is high, Y low. When A changes (A_bar a little later), Y will still be at a low voltage. Then what drives the $\overline{Y}$ output is an NMOS turned on and a PMOS turned on. You must ensure the NMOS will win irrespective of how hard the PMOS will pull up. This will then start to turn on the other PMOS which will pull the Y output up. This makes it easier for the NMOS to pull $\overline{Y}$ down and we end up where $\overline{Y}$ is at GND and $Y$ is pulled up to the high voltage. ![](attachments/Logic%20Level-shifter-2.png#invert) ### Sizing Both NMOS and PMOS are on at the same time - different to [Static CMOS Logic Gates](Static%20CMOS%20Logic%20Gates.md). When both NMOS and PMOS are on with the full gate source voltage the NMOS draws more current. i.e. saturation current of the NMOS must be larger than the saturation current of the PMOS. $\frac{1}{2}(\mu C_{OX})_N \frac{W_N}{L_N}(V_{DDL}-V_{THN})^2>\frac{1}{2}(\mu C_{OX})_P \frac{W_P}{L_P}(V_{DDH}-V_{THP})^2$ The power supply voltage for the NMOS can be much lower than the PMOS - be careful of this equation! The digital transistor models are insufficient to model this essentially digital circuit! It may even be necessary to make the PMOS longer than minimum to satisfy the equation. ### Complementary Circuit Can use cross coupled NMOS and go from higher voltage to lower voltage but this is more difficult to implement due to the equations and so is not often done. [^1] --- # References [^1]: [vr-4602-wk09-sc02-gateusefunction](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W9/vr-4602-wk09-sc02-gateusefunction.mp4)