Logic level shifting 3.3V LVCMOS to 5V something hybrid

Discussion in 'General Electronics Chat' started by Marcus2012, Apr 1, 2016.

  1. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
    26
    Hey guys

    I've a bit of a problem and I was wondering if anyone could help me out (again :)) if possible please.

    I'm trying to interface my Raspberry Pi with a 5V logic LED driver display. After looking at the datasheet to the MAX7219 I found it had HIGH range of 3.5-5V so I thought "ok that must be 5V CMOS", then I looked at the LOW range and it was 0-0.8V and then I got confused and thought "err that's a TTL low". So I was a little perplexed by this seemingly hybridized logic.
    Anyway regardless of the low state input for the display driver I realised my 3.3V CMOS would not cut it for a HIGH logic level. So I've been searching for a 3.3V CMOS-5V CMOS level shifter. I did think about a NPN in common emitter topology but for multiple connections this could take up significant room. A search on ebay found modules similar to this but I am not sure on the reliability of these (has anyone bought one before?)
    The following TI application note on the subject of level shifting was very handy but I soon realised that all of the packages mentioned were SMD rather than DIP, which is something I don't have the ability to produce. :(

    Selecting the Right Level-Translation Solution

    So I continued my search and then came across the ON semiconductor level shifters and the MC14504B cropped up and seemed promising as the summary stated 3 to 18Vdc operation for Vdd and Vcc. The datasheet values for HIGH and LOW ranges for CMOS to CMOS didn't go low enough to display any values lower than that for 5V CMOS. So this left two interpretations open to me at this point.

    1)I could potentially drive the inputs with my 3.3V CMOS in TTL-CMOS mode but was unsure as to what voltage to apply to Vcc in this case (the 3.3V or the max TTL, 5V).
    2)Although the datasheet did not mention 3.3V CMOS in the test specifications it does in the summary and Absolute Max etc. So am I expected to assume that the levels for 3.3V CMOS LOW and HIGH are the standard 0.3Vcc and 0.7Vcc respectively. I think the graphical representations reflect this as well but I am unsure, the Operating boundary for CMOS-CMOS mode is seemingly 3V.

    Basically has anyone had any experience using 3.3V CMOS - 5V CMOS level shifters or ready made modules and if so would you have recommendations or would one of my above solution suffice?

    Thanks for your help and time everyone :)
     
  2. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
    26
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