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On what factors does the voltage requirement of an ic depends?

rishi2628

In electronics, we have set certain standards of voltages like most ic and microcontrollers run on 5 volts ,others like arm run on 3.3 volts.My question is, what are the factors that help us decide these voltages. I mean what factors are preventing us from using lower voltages like for example 1 volt to power the microcontrollers/microprocessors. Is it the complex circuitry, or the breakdown voltages of semiconductors. Please give your views

  • It's to do with semiconductors and hole, electron energy and fermi levels, so (reasonably deep) physics lessons will cover this.

    Also, it's to do with standards (otherwise interoperability between components suffers) and discriminating logic levels within noise.

  • Hi Rishi,

     

    The main issue about source voltage has to do with the current loads placed upon the device.

    At lower voltages, less current flow is needed to establish the digital states of the device.

     

    The down side is that each output has a very low current drive capability.  This limitation is usually only of concern if you need more current to drive the next device in line to the output.

     

    If you establish a load that exceeds the current drive capability, then the device usually fails, depending upon the internal circuitry used to prevent this overdrive current problem.

     

    So you pick your devices after you identify what output devices you need to drive.  There are lots of options to deal with specific issues, but it all boils down to understanding what you want to do and selecting the devices you need to keep everything working.

     

    There are valid reasons why some of us spent a lot of class room time learning about this stuff.  It does matter.

     

    DAB

  • Generally, as you lower the supply voltage you use less power, which is an advantage.  However, the devices also switch slower, which is a disadvantage.  As shabaz noted, you lose noise margin and the devices start having input errors.

     

    FPGAs typically use a very low voltage for internal logic (e.g., 1.2V) but use higher I/O voltages (e.g. 1.8V, 2.5V, or 3.3V) so that you have decent noise margins for talking to other devices.  This way you get the power advantages of low voltages in the chip without the disadvantages of low-voltage I/Os.  Some FPGAs have parts that go down to 1.0V, but they're slower than the 1.2V parts in the same family.

     

    Another factor in supply voltage is that as an IC becomes more dense, the transistors are smaller and the oxide that separates the transistor gates from the substrate gets thinner.  If you put a higher voltage across a thin gate oxide, the electric field strength (voltage / distance) becomes high enough to zap the transistors.  So as transistors shrink in size, you have to reduce voltage.