SN74LS628: Connection and voltage conversion

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 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.

Hi Fukui-san,

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