PMOS Voltage control Switch

[satiz]

Hi There,

I want to switch ON my 2Amps when the input voltage is >11V. Vin 9V to 24V
To do that I am planning to use the following circuit.
While simulating PMOS is ON. But I have a doubt, SQJ457EP datasheet saying VGS(th) Min =-1.5 Typ=-2.0 Max=-2.5 which one I should consider.

Please correct me if I am doing something wrong here.

Attached is simulation model reference.

View attachment Pmos voltage control switchv1.txt

 

[srizbf]

Vgs(typ) is recommended for analysis.

 

[KlausST]

Hi,

A Mosfet is no ideal ON/OFF switch.
It has an analog region between ON and OFF.
This analog region will drift:
* from part to part
* with temperature
* ... and it depends on load

And V_GSth tells you at which level the mosfet starts to get conductive, mind the tiny drain current.

If you want it to hard switch between ON and OFF you need some extra circuit.
Some kind of amplifier with positive feedback (hysteresis).
You may use a comparator IC for this (more precise), or you may use an additional transistor ( more simple, more difficult to calculate exact level, drifts with temperature).

If you want to control the 11V limit more accurate than you need a reference voltage.

To prevent it from "chattering" (ON OFF ON OFF ...) you need to consider a hysteresis. This depends on the source impedance.
The problem is, that when you switch ON the load then the input voltage may drop a little and it will become below 11V .... causing to switch OFF .... then the voltage will rise above 11V again ... and it will switch ON .... repeating..
A hysteresis makes to switch ON at a given input voltage. Lets say 11.0V ... but then it will stay ON until the input drops to a lower value, let's say 10.5V.

I've given some options ... so it's in you which way to go.

You need to decide how accurate you want it to operate.
..how exact (which range) the ON threshold should be
.. and the OFF threshold range.

Klaus

 

[danadakk]

Your application is called "high side drive", basically switching loads
in their "hot" side, loads that have a permanent ground.

The considerations for this are show in section 5.1 -

https://www.ti.com/lit/ml/slua618a/slua618a.pdf?&ts=1589195063227



Regards, Dana.

 

[d123]

Hi,

Electrical specifications section is good for understanding minimum on voltage (for 250uA, probably) but the graphs lower down in the datasheet tell a lot more about actual required VGS for fully on and/or for x amount of current.

I agree with Klaus, a 'power good' subcircuit is needed such as a comparator and stable reference to switch on at 11 Vin. I did one like yours and it's a nuisance as Vout creeps up as Vin rises, MOSFET is 'dumb' device and only compares source voltage to gate and if gate is lower than source it knows to be on... I used comparator and 1.25V reference to sense resistive divided down 12V to 1.25V for comp to go low and turn PMOS on hard.