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Understanding digital level shifting

Posted: 06 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:CMOS? TTL? resistor? digital level translation? MOSFETs?

Schottky diodes are preferred so that they will turn on before the device's internal diodes. Be careful of the input current (limited by the input resistor) that you are forcing though the diode into the supply rail. If it exceeds the current needs of the total circuit, the supply voltage can go up and destroy some of the circuitry.

A long-forgotten technique is to use CD4049 or CD4050 chips. These devices do not have internal diodes to the supply, and so can withstand voltages up to the breakdown voltage (20V max) of the IC, which could be well outside the supply voltage. There are also some other pretty old level translators that could prove useful, like the CD40109 and CD4504, which will allow higher and non-standard voltage shifts.

Figure 3: Standard input protection against overvoltage.

In some cases, your input voltage may not be all that well regulated. In the HVAC industry, for instance, the input levels are derived from a nominal 24VAC transformer. My approach is to half-wave rectify, smooth, and then use a resistor divider followed by diode clamp protection, thereby combining several of the techniques mentioned above.

We all think of opto-isolators only in the context of providing isolation, but they can actually be used as level translators as well. Provided there is enough drive in the source, you can interface to almost any input voltage and then!using a pull-up resistor at the open collector output (or a pull-down at an open-emitter output)!convert it to whatever voltage is required.

Oftentimes, outputs require conversion from logic levels to high voltage and/or high current to drive something like a relay or solenoid. The voltage shift can be handled with a transistor output or dedicated drivers. There used to be quite a number of serial-in shift registers with high voltage outputs, but the industry trend seems to be moving away from them now. In a recent blog!Where are all the (MCU) outputs?!I bemoaned the paucity of options and I suggest you look at it whether this is the type of driver you need.

An open-collector (or open-emitter, open-source, open-drain) is frequently used for simple level shifting, especially where there is a common rail, and some of the approaches described above use this technique. Here are some more. Going back a long way, but still available, is the TTL favourite 7406 (and 7407), which will handle up to 30V and sink 40mA on its open-collector outputs. If you wanted higher current, there are the ULN2803 (8 channel) and the ULN2003 (7 channel) drivers, although!being Darlington drivers!their outputs will not go down to zero.

I'm sad to say that these drivers are getting a little long in the tooth now, and it appears that the number of manufacturers is diminishing. However, TI has brought out a device that is plug-replaceable with the ULN2003, but that uses MOSFETs instead of BJTs. Someone with a sense of the past must have given this new device its number: TPL7407L.

Going back even further into the mists of time, we shouldn't forget that a relay is sometimes the ideal way of level shifting!with isolation thrown in!for either inputs or outputs.

But just to prove that not everything is as old as the hills, both TI and Maxim have interesting devices (click here and here, respectively) that will take eight high-voltage (30VDC) inputs, convert them to 5V, and serialize them. In addition, serving as level shifter in terms of a pull-up or pull-down as well as through-isolation are the members of the Infineon ISOFACE family of parts for both input and output.

Do you need to take your signals to the next level? What techniques do you use?

About the author
Aubrey Kagan is an Engineering Manager at Emphatec.

To download the PDF version of this article, click here.

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