electronics- logic gates: measuring current limitsdenethor.wlu.ca/pc320/lectures/lgccurmbeam.pdf ·...

Logic gate operating currentsDetermining current limits

ElectronicsLogic Gates: Measuring Current Limits

Terry Sturtevant

Wilfrid Laurier University

September 4, 2013

Terry Sturtevant Electronics Logic Gates: Measuring Current Limits

Logic gate operating currentsDetermining current limits Operating current limits

Current convention

By convention, current into a chip is positive, so current out of achip is negative.

Current convention

By convention, current into a chip is positive, so current out of achip is negative.

Operating current limits

Gates have current limits as well as voltage limits.There are limits to the input and output currents of eachindividual gate.In addition, there is some current required by the chip itself,as long as power is applied.

Gates have current limits as well as voltage limits.

There are limits to the input and output currents of eachindividual gate.In addition, there is some current required by the chip itself,as long as power is applied.

Gates have current limits as well as voltage limits.There are limits to the input and output currents of eachindividual gate.

In addition, there is some current required by the chip itself,as long as power is applied.

Gates have current limits as well as voltage limits.There are limits to the input and output currents of eachindividual gate.In addition, there is some current required by the chip itself,as long as power is applied.

Measuring current limits; Fan–inMeasuring current limits; Fan–out

Floating inputs

What happens with floating inputs?

Floating inputs

If input floats HIGH,output will be LOW.

Floating inputs

If input floats HIGH,output will be LOW.

If input floats LOW,output will be HIGH.

If the input floats HIGH, then current will need to be drawn toground to make it LOW.

Input floating HIGH

Vi > VILmax

A large resistor won’t allow enough current to make the input LOW.

Input floating HIGH

Vi . VILmax

Some resistance will allow enough current to make the input VILmax .

Input floating HIGH

Vi < VILmax

A low enough resistance will allow enough current to make the inputbelow VILmax ; i.e. LOW.

Input floating HIGH

=VILmax

Vi = VILmax

The current limit is when Vi = VILmax .

If the input floats LOW, then current will need to be drawn fromVcc to make it HIGH.

Input floating LOW

Vi < VIHmin

A large resistor won’t allow enough current to make the input HIGH.

Input floating LOW

Vi & VIHmin

Some resistance will allow enough current to make the input VIHmin .

Input floating LOW

Vi > VIHmin

A low enough resistance will allow enough current to make the inputabove VIHmin ; i.e. HIGH.

Input floating LOW

=Vcc−VIHmin

Vi = VIHmin

The current limit is when Vi = VIHmin .

In order to measure the input requirements for a gate, youcould wire up the first circuit of the previous figure.

An oscilloscope could be used to measure Vin and Vout.Since one end of R is tied to a known voltage, and the otherend is connected to the oscilloscope, then the voltage acrossR is known, and thus the current through R can be calculatedusing Ohm’s Law.

In order to measure the input requirements for a gate, youcould wire up the first circuit of the previous figure.An oscilloscope could be used to measure Vin and Vout.

Since one end of R is tied to a known voltage, and the otherend is connected to the oscilloscope, then the voltage acrossR is known, and thus the current through R can be calculatedusing Ohm’s Law.

In order to measure the input requirements for a gate, youcould wire up the first circuit of the previous figure.An oscilloscope could be used to measure Vin and Vout.Since one end of R is tied to a known voltage, and the otherend is connected to the oscilloscope, then the voltage acrossR is known, and thus the current through R can be calculatedusing Ohm’s Law.

With nothing connected to the inputs of a gate, so there is nocurrent in, you may observe its output.

If the inputs of a device “float” so that they appear to be in aparticular state with nothing connected, then by definition youdon’t need to supply any current to keep them in that state!

With nothing connected to the inputs of a gate, so there is nocurrent in, you may observe its output.If the inputs of a device “float” so that they appear to be in aparticular state with nothing connected, then by definition youdon’t need to supply any current to keep them in that state!

Actually it can be more complicated than that.

Sometimes the inputs float to voltage which is in theindeterminate region, so that sometimes they will appear tobe high and other times they will appear to be low.

Actually it can be more complicated than that.Sometimes the inputs float to voltage which is in theindeterminate region, so that sometimes they will appear tobe high and other times they will appear to be low.

Starting with the lowest current possible, in the first circuit above,if you monitor the value of Vout as current is increased, you candetermine when the output voltage just reaches the specified valueof VOLmax and thus determine IIHmin

Keep in mind the earlier note about whether a limit is called a“maximum” or a “minimum”, it will be referred to here as aminimum.

If you wire up the second circuit above and starting with the lowestpossible current, monitor the value of Vout as current is increased,you can observe when the output voltage just falls below thespecified value of VOHmin . and thus determine IILmin .

Fan–out

When one gate feeds into many gates, as in the figure below, thethe first gate must be able to source (or sink) enough current forall of the gates it feeds.

Fan–out

The fan-out of a gate is the number of gates of that familywhich can be fed by a single gate.

Keep in mind that the fan-out will be limited by the lower ofthe number for sourcing and sinking, since both must bepossible.

The fan-out of a gate is the number of gates of that familywhich can be fed by a single gate.Keep in mind that the fan-out will be limited by the lower ofthe number for sourcing and sinking, since both must bepossible.

CMOS fan–out

For CMOS, adding more gates will change the speed limit of thecircuit.

CMOS fan–out

For CMOS, adding more gates will change the speed limit of thecircuit.

Output low

Output should be LOWVcc

Output low

Suppose we tie it HIGH

Output low

Vcc VccIo

Vo < VOLmax

A large resistor won’t allow enough current to keep the output frombeing a valid LOW.

Output low

Vcc VccIo

Vo . VOLmax

Some resistance will allow enough current to make the outputVOLmax .

Output low

Vcc VccIo

?Vo > VOLmax

A low enough resistance will allow enough current to make the out-put above VOLmax ; i.e. invalid as a LOW.

Output low

Vcc VccIo = IOLmax

=Vcc−VOLmax

Vo = VOLmax

The current limit is when Vo = VOLmax .

Output High

Output should be HIGH

Output High

Suppose we tie the output LOW

Output High

Vo > VOHmin

A large resistor won’t allow enough current to keep the output frombeing a valid HIGH.

Output High

Vo & VOHmin

Some resistance will allow enough current to make the outputVOHmin .

Output High

Vo < VOHmin

A low enough resistance will allow enough current to make the out-put below VOHmin ; i.e. invalid as a HIGH.

Output High

Io = IOHmax

=VOHmin

Vo = VOHmin

The current limit is when Vo = VOHmin .

In order to measure the output current limits of a device, youcan wire up the first circuit above.

The oscilloscope can be used to measure Vin and Vout.Since one end of R is tied to a known voltage, and the otherend is connected to the oscilloscope, then the voltage acrossR is known, and thus the current through R can be calculatedusing Ohm’s Law.

In order to measure the output current limits of a device, youcan wire up the first circuit above.The oscilloscope can be used to measure Vin and Vout.

Since one end of R is tied to a known voltage, and the otherend is connected to the oscilloscope, then the voltage acrossR is known, and thus the current through R can be calculatedusing Ohm’s Law.

In order to measure the output current limits of a device, youcan wire up the first circuit above.The oscilloscope can be used to measure Vin and Vout.Since one end of R is tied to a known voltage, and the otherend is connected to the oscilloscope, then the voltage acrossR is known, and thus the current through R can be calculatedusing Ohm’s Law.

Using the value of the output sinking current IOLmax from thedata sheets, you can determine the maximum value of currentwhich keeps the output in the low state and thus IOLmax .

If you set up the second circuit above, and obtain the value of theoutput sourcing current IOHmax from the data sheets, then youcan determine the maximum value of current which keeps theoutput in the high state and thus IOHmax in a manner similar tothe one previously described.

electronics- logic gates: measuring current limitsdenethor.wlu.ca/pc320/lectures/lgccurmbeam.pdf ·...

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