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SN74LS156: Problems with OC logic

Cris Harrison
Intellectual 285 points
Part Number: SN74LS156
Other Parts Discussed in Thread: SN7400, , ULN2003A

this may sound stupid but my mind isn't working: Problem how many 14/15ma loads will this device tolerate with a 3.5vdc rail without buffering.  the devices are both 16 segments (alphanumeric) and 7 segments. They are NOT LED but lamps and very expensive. In my mad hairbrained idea, I am using  27 74LS240 for the drive and using the gate control input a PCM signal used for device dimming. this will drive one segment. By using a group of 74LS156 with Open Collector Outputs and a 3.5 vdc as HIGH. So here goes when a segment is driven HIGH by the 240 out and it is not selected via a HIGH on the decoder, Nothing Happens (even with the 240 in TriSate mode).  But if the decider is a LOW you will get current flow. So I have to sink somewhere between 30 ma for a 1 to 240ma for a 16segment Lamp test. 

#2 where can I get a 3.5volt regulator 

  • 0
    TI__Guru* 96842 points
    Hi Cris,
    I can try to help out with the logic side -- for the LDO, I'd recommend posting to the LDO forum.

    I'm curious why you went with the LS family of logic over a more modern family like ABT? The old SN7400, S, and LS families are notoriously inefficient compared with the more modern devices.

    To answer your question - the SN74LS156 is rated to sink 8mA at 0.5V output (max). So, it can support about 1 14-15mA load if you paralleled the two channels.

    If you switch to something like the TPL7407, each output can sink 500 mA, which is about 33 14-15mA loads (per output)

    There's also many devices in the LVC family or HC family that will support 15mA per output. Check out the parametrics table here to find more: www.ti.com/.../products.html
  • 0 in reply to Emrys Maier
    Intellectual 285 points

    about the choice of LS family (well only know C, LS, S, HS, and std families. me OLD school and I am wirewraping it that means the device must be 'N' style plastic dip with legs. but the problem is not the segment drive of 15ma which the 240 drives nicely. Its the OC side and that without buffering on one channel will be about 240ma for 16 segs or 105ma for 7 segs, this only happens during lamp test. As have 5 bits output from my Arduino I want to use 4 x 74xx156's expanded to drive 24+ segments. 

    you can see more at my blog 'www.element14.com' NexGen: CP-1252/ASN-128 Navigation Computer Display: Display Drivers 

  • 0 in reply to Emrys Maier
    Intellectual 285 points
    Must use plastic dip as I am wirewraping this.
  • 0 in reply to Cris Harrison
    TI__Guru* 96842 points
    Sounds like fun - I once spent some time in Iraq without a soldering iron and had to make tiny wire coils for each PDIP leg for a project. It was a massive pain in the rear but worked reasonably well. I did get more noise than I would have liked though (I was building an audio amplifier).

    There's an OC darlington pair driver called the ULN2003A that comes in PDIP and supports sinking up to 2.5A (total across 7 channels). You might also want to take a look at the ABT logic family, for example the SN74ABT541. It's a bi-CMOS process that can handle a lot of output current.