MOSFET logic level question

[Morcego]

Hi,

What determines if a Mosfet is logic level or not?
Is the VGS or the Gate Threshold Voltage?

For instance the IRF7314 from IRF as a VGS of +/-12V
and a Gate Threshold Voltage (minimum) of 0,7V. (maximum is not show on the datasheets)

This device is not advertise as logic level. But I belive it is.

Help please?

Thanks

 

[itsmoked]

"Logic level" means you can turn it on with only 5V. That's all. Native run-of-the-mill fets need about 10V.

 

[IRstuff]

Logic level implies something about noise immunity, which actually requires a minimum threshold compatible with logic signals, e.g., the threshold cannot be below about 1V or so. Obviously, there's not much unanimity in the industry or you'd know for sure.

You could probably use your device in a circuit, and it would probably work, but that would only be due to the fact that most circuits have ample noise margin.

But, you could get burned if the circuit design is marginal or overly noisy, particularly at high ambient temperatures.

TTFN

 

[itsmoked]

Morcego; Be very careful! Just because a FET starts conducting at "logic" levels [0-5V] doesn't mean you want to use it!! As you start to turn on a FET it has a lot of resistance and will dissipate a lot of power as you will be running it in it's linear region. Look closely at the Vgs-resistance curve. Check the power dissipation at your Vgs!

 

[benta]

It depends a bit on the supplier how this is specified. The important thing is that the on resistance is specified at a gate voltage that you can supply.
Example: ON Semiconductor calls MOSFETs that are specified at a gate voltage of 5 V "logic level".
Fairchild specifies gate voltages of 2.5 and 4.5 V as "logic level".
Standard MOSFETs are normally specified at 10 V gate voltage.
The threshold voltage is not a useful parameter here.

Regards.

 

[Warpspeed]

A lot depends on what particular characteristics of the mosfet are most important to you in the particular circuit.

There are really two points of interest, the initial gate threshold where it begins conduction, and the gate voltage at which it will reach it's lowest specified Rds on. Temperature may sometimes be significant too.

A low gate threshold will be an advantage if you only have five volts to drive the gate, and you need be able to reach a very low turn on resistance for heavy loads.

But a low gate threshold will make it slower to turn off.

The only real advantage of logic level mosfets is that all the figures on the spec sheet are guaranteed with only a five volt gate drive signal. If you need full speed and power capability, the logic level mosfets are the safe way to go. But in low power, slow, or non critical applications, ordinary garden variety mosfets will often be perfectly o/k.

 

[nbucska]

A general engineering principle is that you can exceed
the specification if you know what you are doing.

E.G. I often used as ECL pull-down 10 to 20 Kohm to
- 15 V.( it acts as a pulse stretcher, speeds up rising and
delays falling edge.)


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Morcego]

Thanks all for the anwsers. Live and learn!

I think that I will change my circuit to use IRL series of mosfets (logic level). Better safe than sorry. Two SOT 23 mosfets are cheaper(!) than one pair in SO8 package.
It's amazing how a tiny SOT23 package can deliver 4.3 Amps
in 4.5V ! (I will use only 2A).

I will go for the IRLML6401 (VGS +/-8v).

Thanks all.

Morcego

 

[itsmoked]

Yes Morcego! It's all thru the miracle of low_on_resistance. If the resistance was zero you could run all the current in the world thru it. The resistance is lowered by making the transistor larger hence the lower the resistance the more they tend to cost. Normally you buy only the resistance you need.