Replacing multiple devices with one? That’s good logic.

Jan 16, 2015

Logic ICs are handy devices engineers sprinkle in for quick fixes in their designs. However, simple fixes along the way sometimes complicate things even more. There are many reasons why an engineer would choose to use a logic device in their application.

Perhaps the signal needs an extra boost because the I/O peripheral on their microcontroller is too weak, or the output voltage is too high when trying to drive a “low” signal. To their disappointment, engineers are surprised to learn when designing a motherboard/daughter card system, there is a large amount of capacitance caused by long traces and heavy loads that need to be separated to improve signal integrity.

There are many other examples of problems that logic devices can solve. For example, if the engineer needs to verify that two power supplies have ramped to the proper levels, an OR gate will do the trick!

A CMOS inverter is part of a low-cost clocking solution, and it can also reverse the direction of an inverted signal caused by incompatible output-enable polarities.

Great! You have now found the logic devices you need and are ready to design them into your system. In electronic design, logic devices are the “nuts and bolts” of the system. The logic has just solved whatever design problem you were facing, such as weak outputs, long traces or heavy load separation. We often used logic devices for quick fixes, but simple fixes can sometimes introduce complications.

In the frenzy to fix the signal issues, you’ve accidentally overlooked a major problem. You’ve fed a 1.8-V signal into the logic buffer using a 1.8-V V_CC. Consequently, you will get a 1.8-V output going into the microcontroller on the other side. The only problem is that the microcontroller accepts a minimum of 3 V (V_IH) to be considered a high input.

Quick – find a voltage level translator! Suddenly you have to re-route your board to get a second 3.3-V V_CC to attach to your translator. You now find yourself re-routing signal lines to fit the new part. Thankfully, you had just enough board space for the additional part.

TI offers the SN74LV1T family of devices that can replace multiple logic devices with one to resolve translation issues in your design. Most of the SN74xx devices in TI’s portfolio represent standard logic. The first few letters after the SN74 represent the logic family. Examples of these families include LVC, AUC, AUP, AHC, LV1T and more. The letters help to determine the devices’ performance specs, including V_CC range, current drive and sinking capability, speed, and power consumption. Each family is suited for different applications.

That’s where the SN74LV1T family comes to the rescue! The LV1T family of logic devices acts as single-supply translators and logic in the same part. To translate up or down, the designer only needs to set the V_CC. The V_CCinput level sets the device’s output level. There’s also an LV4T quad buffer if you need more bits.

Moreover, the logic family consists of seven functions: buffer, inverter, AND, OR, NOR, NAND, and XOR. The LV1T family combines a wide V_CC range with a wide V_IH range. The result yields a nearly universal logic level translator. The following list shows examples of how the LV1T/4T family can translate between standard logic levels:

Up-translation possibilities:

1.2 V to 1.8 V
1.8 V or 2.5 V to 3.3 V
2.5 V or 3.3 V to 5 V

Down-translation possibilities:

5 V to 1.8 V, 2.5 V, or 3.3 V
3.3 V to 1.8 V or 2.5 V
2.5 V to 1.8 V

As an added bonus, LV1T devices save space on your board with their small footprint (as low as 2.5 mm²), reduce the need for external translators and eliminate the need for pull-up resistors on the output. The devices are characterized to operate up to 50 MHz at 3.3-V V_CC.

Here is a comparison of the of LV1T combo devices vs. other logic families: