Wide voltage logic input design

I need to provide an interface to a logic device (3.3V) from an input logic signal which can be anything from 5V to 30V. I've come up with a few ideas to do this with the least board area but I'd be interested to hear anyone else's views.

TIA.

John

Reply to
John
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It rather depends. For example, where does the input come from - is it some other clean-edged signal or is it a sensor that might have slow or noisy transitions? How noisy is the signal in the first place? Do you need to control or vary the threshold voltages? Does it need hysteresis? Are there any ESD-type issues with the input? What switching frequency do you need to cope with? Is it actually a logic input or is it a close-contact type with a pullup or down or an open collector type output? Opto-isolation?

Depending on the answer to these you could get by with something as simple as a transistor with a couple of base resistors to a comparater with a threshold voltage controlled by a DAC output. Primarily if you need a well defined input switching characteristic at low frequencies you'll probably end up with a comparator of some sort.

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Nobby Anderson
Reply to
Nobody Here

series resistor and zener clamp to ground

Reply to
steve

Probably the easiest way is to limit input current to some value like 20ma with a series resistor (30v - 3.3v / .020) = 1.3k then clamp the limited signal to the 3.3v rail with a diode.

You'll have to verify that the current requirements of your input logic are low enough to be an non- issue. They most likely are.

Reply to
Jim Stewart

A npn transistor with a base series resistor (perhaps 10k) and and equal base-emitter and an 1 k pull-up at the collector to VCC should to the job.

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Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
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Reply to
Uwe Bonnes

And if the device you are connecting to has clamping diodes built in, you can even do without the external diode. But I wouldn't advice this approach if the 5-30 volt signal is an external signal.

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Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)
Reply to
Stef

Well then, if it's truly not noisy you could, for example, choose to have a switching level of about 2.5V. Use a 4k7 resistor to a base, and then choose the base-emitter resistor so that the base voltage is about

0.7V when the input is about 2.5V ie about 1k8, i'd probably use a couple of K or 2.2, assume base not conducting just before switchon to simplify the maths. This limits the base current to a bit more than 6mA if the input voltage is 30V. Use a highish gain transistor with a beta of say 400 and the transsitor will be on with a base current of about 8uA with a 1k load at 3.3V.

That assumes your drive can supply 6mA. Your requirement for ESD protection indicates to me that you expect the signal might be noisy, or do you just mean that the circuit should be able to pass ESD tests as in for CE marking or whatever? I'd use different protection mechanisms depending on if the signal comes from outside the box (and outside my control) or it's all contained inside the box.

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Nobby Anderson
Reply to
Nobody Here

Yikes! - and pray that your 3.3V regulator is able to SINK current to keep the rail held at 3.3V.

ie Doing clamping to your internal Vcc is NOT a good idea !!

-jg

Reply to
Jim Granville

A (normally reverse-biased) diode is required across the E-B junction.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

It's commonly done. Granted, I made the assumption that 20ma is a small fraction of what he's drawing from the 3.3v rail. If he's only drawing 20ua, then yes, it would not be a good idea.

He did ask for a minimal parts count solution and it would work.

Reply to
Jim Stewart

5kHz, clean signal, ESD protection. Is this a single line? How much current can be supplied by the input source? What kind of delay can be accepted for the edges, rising and falling? (And can you accept different delays for rising and falling edges? -- I'm thinking about "speed-up" caps to minimize delays and differences.) How much below 3.3V at the output of the circuit can you accept (would 20-30mV be okay, for example?) Inverting okay? Etc. What ideas have you considered and what did you think of them?

Jon

Reply to
Jonathan Kirwan

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