[SOLVED] picking nmos switch to opperate at low control voltage

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eanema

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I have a hall device that operates at 3.0v with a CMOS output. Its data sheet says that its output control voltage can swing from gnd+0.4v to vdd-0.4v, so 0.4v -> 2.6v.

This device is a low power component that will be used to switch on and off the power of the main circuit. See the schematic below.

The problem I am having is finding a suitable NMOS device that can be switched fully on and fully off by the 0.4->2.6v swing. I've been looking at digikey and there doesn't *seem* to have a category for this type of parameter.

Any and all suggestions would be appreciated

 

If i understand correctly you are looking for a N mosfet with low threshold voltage, you didn't specify the output current specification but
take a look at www.fairchildsemi.com/ds/FD/FDN327N.pdf
Id=2A RDS(ON) = 120 mohm @ VGS = 1.8 V

Alex
 

thanks for the input, this device looks promising. The current should be ~<1A.

I am wondering, from reading the data sheet, how can I estimate Ids when Vgs=0.4v? The data sheet provides the info for when Vgs=0

Zero Gate Voltage Drain Current: Vds = 16v, Vgs = 0v => Ids=1 mA

thanks for you help
 

According to the datasheet the min Gate Threshold Voltage is 0.4v so you may have a small current at that level,
maybe you can try with a diode (or 2 diodes connected in the opposite direction to each other for charge/discharge) to drop the voltage to the gate about 0.6v, this will leave 0v when off and 2.6-0.6=2v when on

Alex

---------- Post added at 02:23 ---------- Previous post was at 02:13 ----------

If you use the diode(s) you should also add a resistor from the gate to the gnd to discharge the gate to 0 because one diode will not discharge the gate at all
and if there is a second diode in reverse it will only discharge the gate to about 0.6v.

Alex
 
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    eanema

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Ah, very nice solution

thanks

 

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