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Buffer IC to connect two different voltage level devices

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Junior Member level 3
May 25, 2001
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About Buffer IC

Hi everyone:

I need the buffer IC to connect two difference voltage level device.One is altera CPLD Max3256 (3.3 Voltage),the other is Digital Input Output Card(5 Voltage).

I had tried the logic IC ,such as 7404,74244,and 74245.
My design works at 20MHz,AND the test result is not good.
I think the current of the buffer drive IC is not high enough to drive 5 Voltage device (Digital Input Output Card).

So, if you have any idea ,or any other buffer IC you used, please tell me.
Let me try.

I really need your help.

Thank you very much.

let's go to see datasheet of the VHC familly...
looks the differents level (input low and hight, output low and hight ), you must look its carrefully and take the better bad configuration. I don't think that it's a current pb!!

hi,i know IDT have the buffer you need,so do pericom.something like pi74lpt244,input canbe 3v or 5v,output can be 3v or 5v or mixed ,can sink and source 24ma.(from pericom).i can not remember the chip's name from IDT and TI,you can go these yourself.
hope this can help you.

Both TI & Fairchild have dual voltage level translators in 8 & 16 bit versions.

It depends if you need only level shift (drive to 3v) and have 5v tolerant I/O (single supply voltage corresponding to lowest level -3v)
If you need a proper BiDir level translator so that with 3v in, you can drive 5v out (Dual supply voltage 3v & 5v).
These parts are quite different.

Make a search on the TI/Fairchild website and d/l some of their application notes as well which will help with part selection.

Let's have a look at your issue: you have to connect a 3.3V outpu to 5 V logic chip input and opposite.

The specification of a 3.3 CMOS output should be good enough for both VIL and VIH for a 5V (TTL compatible - that is the usual case) input. I do not know any logic chip input which should demand too much current - except if the card has terminating resistors. In that case a LV buffer (244 or similar) might help. But if you suspect too high load, measure it! You can see the pulse amplitude with an oscilloscope, and even use a multimeter to check currents (put a mA meter it between that input and GND to see how big is the "sink" current, and between input and 3.3V for "source" current)

I think you might have some other issue tha the loading.

For direction 5V output to 3.3V input a level converter might be useful, except if the input is "5V tolerant" - then nothing is needed. See the data sheet of that CPLD. Actually, often a series resistor and a (schottky) clamp diode to 3.3V is good enough and fast enough when placed close to the 3V CMOS chip. Input capacitance is usually quite low (with clamp diode maybe 15-20 pF?) so that the speed is not impaired horribly much with some 200-300 ohm series resistor (which together with a schottky clamp should protect the receiver, and maybe draw a few mA from the driver in worst case. But check with oscilloscope again.

Good luck,


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