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SN74LS628: Connection and voltage conversion

Part Number: SN74LS628
Other Parts Discussed in Thread: SN74LS629

Hello team,

I have a question about connection and voltage conversion of SN74LS628.

Is it possible to directly connect the Z and Y pins of SN74LS628 (DC5V input to VCC) to 3.3V IO (FPGA, etc., absolute maximum rating of 3.85V)?
If it is not, it may be necessary to use a voltage level conversion IC, etc. We would appreciate it if you could tell us the recommended IC in that case.

Best Regards,
Ryotaro Fukui

  • Hello,

    The easiest way to be sure that the SN74LS628 doesn't output too high of a voltage would be to add a load resistor at the output (from each output to ground).  Here's a plot of the LS family output voltage vs load current:

    The highlighted curve is for the 'high' output state.  You can see that when the output current is very small, the voltage can increase up to the supply value. As long as you have at least a small load (even 1mA would work) the output will be 3.5V or less.

    This behavior comes from the darlington pair structure of the output:

    With modern CMOS devices, their input current may be small enough that the darlington pair will no longer be biased and the output could drift up to ~VCC.

  • Hello Maier-san,

    Thank you for your answers and reference materials. I finally got a reply from the customer.

    In the answer, "With modern CMOS devices, their input current may be small enough that output could drift up to ~ VCC."

    Is this modern device refers to this SN74LS629? Or is it the device that connects to the output?

    Best Regards,
    Ryotaro Fukui

  • Hello Fukui-san,

    I was referring to modern CMOS inputs, for example, the inputs to a typical FPGA. The SN74LS629 is neither modern nor CMOS -- it is a very old design (about 50 years old) and uses bipolar logic.

  • Hello Maier-san,

    The customer is going to input this signal to 3.3V IO FPGA like following so I guess this will be no problem.

    SN74LS628 Z,Y pin === 3.3V IO FPGA pin

    Thank you for your advice. I would like to share this information to our customers additionally.

    Best Regards,
    Ryotaro Fukui

  • Hi Fukui-san,

    Yes, the addition of a pull-down resistor will ensure the the output remains at or below 3.5V.