Hello,I have built a circuit as shown below.
when signal (GPIO) is 0 then SHDN makes DC-DC shut down and output is discharges to zero.
When signal is logical one then the DC-DC charges to -2.4 and SHDN is zero.
They key here is Vth=2.75.The output charges to -2.4 only because MOSFET with Vth=-2.75 .
What if i would need to charge the output to -5V ,and there would not be a -6V=Vth what could be done to solve the problem?
How can make the MOSFET stay closed and make the output to charge till -5V with my Vth=-2.75V?
How to make the effective Vts of the mosfet to increase ?
Thanks.
LTspice file is attached.

 

You don't need a push-pull output to drive the SHDN input, only one NPN BJ transistor with a resistor load to V+.
That will solve any Vth problems you would have.

 

I presume you mean the Vth of M3.
If you want -5V output, then just remove M3. Why is it there?

 

Hello Ian0,M3 is there because when signal is zero I want both Shan shut down the dc dc and output to discharge to 0.
I need somehow to play with the Vt of M3.
Because if my DcDc whoulf need to be minus 5 then vt of m3 needs to be 6.
So I am looking for some solution other then picking new mosfets all the time .

 

I need somehow to play with the Vt of M3.

Where is M3?

 

Hello Ian0,M3 is there because when signal is zero I want both Shan shut down the dc dc and output to discharge to 0.
I need somehow to play with the Vt of M3.
Because if my DcDc whoulf need to be minus 5 then vt of m3 needs to be 6.
So I am looking for some solution other then picking new mosfets all the time .

So why don't you generate the negative supply permanently and switch its output?

 

Yes , it’s is a good idea .
But still I need somehow to switch it .
And still I would have to find a mosfet with Vth larger then the negative output to make it charge to the needed value .
How do I solar the mosfet Vth problem ?

 

 

Hello Ian,As you can see in the photo below and the attached LTspice file.
The output charges exactly to the value of the Vth.
Could you please show on the example of AO6408 mosfet how i need to update the circuit so it would make the output discharge to -2.4?

 

Discharge to -2.4V now? Yesterday it was down to zero! Make up your mind!

 

Hello , sorry for the missunderstanding When gpio =0 I need the output to be -2.4
It stops at -1 because of Vth.

 

Hello , sorry for the missunderstanding When gpio =0 I need the output to be -2.4
It stops at -1 because of Vth.

See post #8 for circuit.