proper charging and discharging of dc-dc using mosfet

[john23]

Hello,tc1261lcs8 needs to be both turn on and off using esp8266 3.3V GPIO pulse and the output needs to charged and discharged with this GPIO pulse.When my GPIO =0V output off when GPIO=3.3V output is ON.
Which is the opposote of what the SHDN does as shown in the data sheet below.I have made an inverter as shown below .
I want to discharge the negative voltage to ground using NMOS.
Vg=5V and Vs=-2V so the NMOS is open.

However the output only goes from 0 to -1 instead of 0 to -2.4 expected by the formula below as it was withoud the SHDN switching.
Why its not charging back to -2.4V?
another problem is that i want the output to be discharge at logical 1 (5V) but here i got the opposite)
Is there something that could be done?
Ltspice model is attached.
Thanks.
https://eu.mouser.com/datasheet/2/609/1261lfa-3124642.pdf

[xvr]

I want to discharge the negative voltage to ground using NMOS.
Vg=5V and Vs=-2V so the NMOS is open.

When you 'close' MOSFET and set 0V to its gate you still have -2V on source, so MOSFET still open. Only when Source drops below Vth MOSFET will be closed.
This is a reason of -1V output.

[john23]

Hello XVR , When Vg=5V output discharges very good to 0V.Vth=1V.
But when the Vg=0V the mosfet suppose to close in order to allow the output capcitor to charge to -2.4V.
As you said the Vgs<Vt only works till -1.
Vs from 0V it start to lower it self till the mosfet will open. 
Vg-Vs<Vt
0-Vs<1
So Vs<-1 is the boundary to which the mosfet could lower itself without opening the mosfet.
So if it tries to further lower itself beyong -1V suppose -1.5V
Vs=-1.5V Vg=0V Vgs=0--1.5>1 and -1.5 meets the GND potential :-) so basickly we are right on the border of the MOSFET opening.This is backly my thoughts no question.

I solved it  only because I found a 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?
Thanks.

[xvr]

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?

Than you need to change discharge schema. The simplest one will be small SSR instead of MOSFET.

[john23]

Hello,SSR is a very new concept :-) i only know how to charge and discharge using mosfets.
Could you reccomend me a scematics i could try to implement?
Thanks.

[xvr]

Hello,SSR is a very new concept :-) i only know how to charge and discharge using mosfets.

It is basically MOSFET controlled by LED (all in one package)

Take a look at:

https://www.digikey.ie/en/products/filter/solid-state-relays-ssr/183?s=N4IgjCBcoCwdIDGUBmBDANgZwKYBoQB7KAbRAGYAGAdjBkpAKuvICYInzqY4QBdAgAcALlBABlYQCcAlgDsA5iAC%2BBMFSihkkdNnxFSIegFYAnADZTjI5XPma-IaMgTp8pavCmAHFYTbdXAJiSDIGPk8AWlZNJChpAFd9ELJjfii-LXipJODDCAjPGNCJcQAlRxBzWJkAEzFIsEoOEBExa2EAT0EcMTQsZGVlIA

Could you reccomend me a scematics i could try to implement?

Just connect MOSFET part of SSR over load to discharge (may be with current limiter series resistor), LED part to control signal (through series resistor). That's all.

[john23]

Hello , is there are some component i could use for simulation and see how it works?
Thanks.

[xvr]

You can use CPC1017 for test

[john23]

Hello , I was suggested another way which i am trying to undestand its logic.
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  show on the example of AO6408 mosfet how i need to update the circuit so it would make the output discharge to -2.4?
So I could understand the intuition of this method?

[xvr]

This is not discharge scheme but power switch. For discharge you should plug M1 transistor directly to output voltage source (use original MOSFET from your first schematic instead M1). It should work

[john23]

Hello XVR, I have succseeded in using SSR for my purposes ans shown in the plots below .
Two questions:
1. how do i know the proper  functionality of this SSR? how much current it needs or "gate" voltage?
In the attached table I see the caracteristics but there are LED forward switch on and LED forward only .
There 4 states and i dont know which apply to my case how do i open or close is like in a case of mosfet.
2. the charging time is much mush higher then what i have with mosfet , is there a way to speed it up?
Thanks.

[xvr]

SSR controlled by LED, so MOSFET gate characteristics is not important. LED current should be about 10mA.Your slow discharge rate is a sequence of low LED current.

[john23]

Hello XVR,  I need to do similar thing to provide 5V for PMA5-83-2W+
I can do this turn on and off trick with l3505 DC-DC and i can do it with regulator like LM7805 or LT1761.

I can supply both voltages to PMA5-83-2W+ using these .
What is the key difference between DC-DC  and a regulator with performance?
https://www.sparkfun.com/datasheets/Components/LM7805.pdf
https://www.minicircuits.com/pdfs/PMA5-83-2W+.pdf
https://www.analog.com/media/en/technical-documentation/data-sheets/1761sff.pdf
https://www.qorvo.com/products/d/da007774
https://www.analog.com/en/products/lt3505.html

[xvr]

For +5V you can use linear regulator with Enable input (take a look in your other topic - I'd give you part number there).
Difference between DC/DC and linear here is simplicity and low noise level of Linear. Drawback of Linear (high power loss) is not relevant in your application because of low current - power loss proportional to current, so it will be neglectiable.

[john23]

Hello, Suppose I have a general bias of +15V gnd -15V.
For  5V  Vg2 i will use LT1761 LDO.
What device you reccomend to use for -2V ? the  low noise option would be great :-)

What device you recommend for 12V step?
https://www.analog.com/en/products/lt1761.html

[xvr]

What device you reccomend to use for -2V ? the  low noise option would be great :-)
What device you recommend for 12V step?

https://www.eevblog.com/forum/projects/dc-dc-recomendations-for-pma5-83-2w-sequencer/msg5543047/#msg5543047
https://www.eevblog.com/forum/projects/dc-dc-recomendations-for-pma5-83-2w-sequencer/msg5542941/#msg5542941

[john23]

Hello  ,I have tried to upgradethe lt1761-5  a circuit so i could connect INPUT to SHDN when pulse is 3.3V  and to discharge the output when pulse is 0V.
I see the error in the circuit below where whr mosfet discharges when pulse is 3.3V so i tried to reverse the opamp inputs to make the mosfet be closed .

But the bigger problem is that SHDN not equal Vin when i open the M2.there is 2V gap between them.
How do you reccomend to solve this issue?

Thanks.
LTSPICE file attached

[xvr]

Top level of SHDN pin is 2V maximum. So there is no need to bring it up to 5V. 3V us enough

[john23]

Hello , for the -2V i used LTC1261L ,I dont see that an LDO the less noisy version in the datasheet below.

For G1 I need -2V and 4mA according to the datasheet shown below.
But as you can see in the plot i get 400mA peaks of current.

How can i improve this -2V circuit so i will meet the required demands on the datasheet for Vg1?
LTSPICE and relay simbols of the relay compnent are attached.
DC_Dc_relay_m2p4.asc

https://eu.mouser.com/datasheet/2/609/1261lfa-3124642.pdf

[xvr]

400mA is your discharge current. You don't have load at all.

[john23]

Hello XVR,
Vg1 is a gate so i assume its a highZ resistance.
I put 0.5uF capacitative load as shown in the schematics above.
What load capaciltance should i put?

Thanks.

https://www.minicircuits.com/pdfs/PMA5-83-2W+.pdf

[xvr]

Discharge current mostly determined by R4 and turn on resistance of U1. Load capacitance could affect mostly discharge time, not initial current. And discharge current taken from load capacitance, it not affect regulator.

[john23]

Hello XVR, Its really hard to reach the 4mA mark :-) and it ruind my sqare signal.
I have tried to play with the capacitors and resistors.
Is there any more productive way?
Thanks.

[xvr]

4mA is a load current (not capacitive but resistive). You don't have any resistive load in scheme, so 4mA not relevant for it. I don't know what 0.5uF in your circuit means. If it real capacitor, then it will be real discharge current but it has nothing in common with 4mA of load current. If it model of Gate capacitance of amp then it TOO high - MOSFET capacitances ordinary have a tens of nF maximum

[john23]

Hello XVR , Yes you are tottaly correct. I put 0.5UF as a load with no thought at all.
4.5mA is the maximal rating so i prever 2mA so it wont burn :-)
So given -2V and  2mA then load resistance is 2/2mA =1Kohm
suppose my gate capacitance is 100nf and  i put  series resitance as 1Kohm

i get a very bad Vout signal (its jumping a lot)
but the good news that the current on the load capacitor is around 1-2mA.
How can i fix the voltage signal?