3.3V-5V level shifters

74LVC07

BTW, I thought you are from Germany, not from India.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

In the rare occasions in the past I've looked at it, LVCs looked a good bet. There are some other LV parts too IIRC.

What's wrong with the specs ?

Graham

I third the LV parts, should be fine either direction. I vaguely remember something about Fairchild's tiny logic too.

Cheers

That would be open drain ...

Well, yeah, why? I am just not a digital guy. Just wanted to know what's popular with the others.

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Regards, Joerg

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Thanks.

Well, for example the example I posted. It's horrible or as you guys say, horrid.

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The Fairchild and OnSemi Tiny parts are mostly high-voltage tolerant on their inputs (as in well over Vcc) so work fine in the 5-to-3.3 direction.

Going the other way, they'll work but get warm, unless you use the Schmitt version, and then the levels might get iffy.

John

We had that argument before. Driving the 5V HCT inputs from 3.3V levels is the standard practice; the input gate cross conduction is about 1ma.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

The 74LVC1T45 could be a good idea, too. I'm using this chip and I know another company who uses it without problems. It converts a single signal, only, but if you'll buy three of it, it might be less expensive than one

74LVC4245, and maybe easier for routing because it is smaller than a 24-TSSOP, if you need just three of it for one direction. But placement costs might be higher.

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I have some thermal images around here somewhere that are interesting... a cute little row of hot spots. Some of the faster parts can get downright toasty at 3.3 in, Vcc=5. The lower power/slower ones are usually fine.

I don't want to argue; I just want to warn people to be careful about this.

John

Single gates on a chip you mean ? Have you seen the prices they charge for them !

Graham

I expect they mean the driver on the bed of nails tester setup.

It has to overpower the pulldown to turn off the tri-state drivers in the chip so its other hand can drive the outputs of the chip.

If I didn't need that for testing, I'd use a 0R pulldown. (I use ones that don't show up on the BOM. The board vendor gives them to me for free.)

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That's exactly one reason why I asked here. There is a flurry of devices out there and for some of them even the specs are iffy. Like one of them that I almost picked (the TXB0104). It requires a "stiff" drive of 2ma min. So it need a 1K pull-up when open and since the other guys insist on series terminators .... meeeep ... out. Whew.

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Thanks, Frank. But it's >15c, ouch. Maybe ok on this design though but the LVC4245 is only around 30c. Placement costs are not such an issue out here.

On this section my layouter is probably just going to hit the autoroute button and then have a coffee :-)

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Hey, 6:43pm means 3:43am in Germany. When do you guys over there sleep ... ?

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My inventory report is showing parts from 12 cents to 44 cents each, depending on quantity purchased and function. There are plenty of times when being to plop down 1 or 2 gates somewhere is real handy.

NL37WZ16 is a remarkable part for 27 cents. It's a fierce, sub-ns edge rate triple buffer/driver in a US8 package. It easily violates US export restrictions for fast pulse generators.

John

Joerg schrieb:

Any simple 74HCT gate will do the level shifting from 3.3 to 5 V. I have used 74HCT08 in such cases (noninverting). Really cheap also, plus many sources.

For converting 5 to 3.3V, just use a 5V tolerant 3.3V logic family -

74LVC is fine for this, for three signals I would again use a single 74LVC08. Or simply add (large enough) series resistors if the 3.3V input impedance is high and speed is not critical...

However, you can of course also use single transistors for level shifting (common base circuit). Works fine, but with the additionally required resistors (and assembly expense) it's not cheaper than using the ICs mentioned above. However, you'd have more flexibility for placement and routing.

Tilmann

How about the Fairchild 74LVX3245?

Currently it is a bit more work. The marketing of one of our clients has produced some flyers for a product, which will be presented at a tradeshow next monday and sent it to their end customers. I'm the programmer of the firmware and I was amazed to see all the nice features. Unfortunately only a subset was defined in the requirements specification. Now it's getting a bit more expensive :-)

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Frank Buss, fb@frank-buss.de
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Going from 5v to 3.3v is easy with Fairchild 74LCX541 and other parts in the family. I've found that Fairchild seems to have more interesting parts in jellybean digital, power FETs, and analog switches.

Mark