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Ask Michael: How Can I Simplify My Digital Input Design? and Optical Isolators?

February 04, 2020

Michael Jackson  By: Michael Jackson
 Principal Writer, Maxim Integrated 

Hi, Michael:

My boss has asked me to design an optically isolated digital input for a programmable logic controller (PLC) I/O module. Trying to keep the current anywhere close to 2mA for the Type 3 specification over such a wide input voltage range is wrecking my head (never mind trying to meet the switching thresholds). That, combined with the Schmitt trigger circuit on the output node, is making the whole thing very "unwieldy" (to put it politely). Any suggestions to help this frustrated hack?

John, CA

Hi, John:

I think you're probably talking yourself down, but, hey, we've all been that soldier at different times. Sounds like your patience is wearing thin, so I'll get straight to the point…the good news is that I have something that will do the job perfectly for you. I suggest you check out the MAX22191 parasitically powered digital input (Figure 1).

MAX22191 digital Input IC block diagramMAX22191 digital Input IC block diagram

The MAX22191 is an IEC 61131-2 compliant device that translates a 24V digital industrial input to a current for driving optical isolators. It meets the Type 3 spec for voltage thresholds and current (2.7mA maximum). You didn't say if you need to sink or source current, but that doesn't matter because this part will do either. It will also work with 48V inputs, with some external resistors (and safely withstand voltages from -60V to 60V). Just a couple of other things: it's fast (250ns response time) and it doesn't need a separate supply voltage (it's powered from the digital input signal), which might help simplify your board layout. Mind you, its tiny 6-pin SOT-23 package won't take up much space anyway. The IC is ideal for applications such as current sourcing inputs, individually isolated inputs, industrial automation, motor controls, and process automation. For some details on the part have a read of this article, "Worried About Heat in Your I/O Module? No Need to Sweat."

Hope this makes life a bit easier for you, John.

All the best,