proper charging and discharging of dc-dc using mosfet

[xvr]

LTC1261 required capacitor on its output to operate. This is NOT load capacitance (see recomended value in datasheet). Discharge current for THIS capacitor will be significant (much more than 2mA). Unfortunately nobody can help there😕 Just add resistor into discharge net to limit current by maximum allowed for SST

[john23]

Hello , I think that it i open g1 then at any mosfet its highZ its pure capacitative load.
The current that they write i think is I=CdV/dt.
so i think there is no resistance in the load.
Where is the error in my logic?
Thanks.

[xvr]

No, it's has resistance load. 4mA is a pure resistance load. Capacitance load is not taken in account at all.

[john23]

Hello, I am confused.you say that the current will be much higher then 2mA.
So how it wont burn the component?
Where is the reccomended values in the datasheet?I cant see the section.
Thanks.
https://www.minicircuits.com/pdfs/PMA5-83-2W+.pdf

[xvr]

This current don't have any connection with component at all. This current will be ONLY in circle comprised output capacitor and discharge SSR. It will not affect amplifier.

[xvr]

Once again - I suggest treat Vg1 not as power rail but as control voltage (2mA of load allow to do so). Remember - you need some facility to adjust this voltage in range -2V till -0.5V. With DC/DC it will be another round of hacks to do so.

So, you will need stable negative voltage, without any adjustment or enable/discharge circuits and one opamp.


Here:

• DAC output 0 to +3.3V mapped to Vg1 output 0 to -2V
• C1 is a blocking capacitor (take a look in amplifier datasheet which one is needed)
• +V is a positive power rail for opamp. It can be 0 (ground) if opamp is RRIO
• -V should be not less than 2V or more, it depends of output voltage swing range of opamp.
• R5 used to isolate opamp output from capacitor load to suppress possible oscilation.
• opamp should be able to provide at least 2mA of output current

[john23]

Hello XVR, An important issue is the load.From the datasheet we have two hints shown in the photos below.
The basic caracteristic says its -0.8V with 15 micro amp which meeans that the resistance at Vg1 is almost highZ.

So we start with -2V and increase till we reach  15uA.
What load capacitor you reccomend to use?

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

[xvr]

DS keep silence about load capacitance of VG1 & VG2. In any case recomended circuit from DS includes 3 capacitance on each VG? pin with total capacitance about 4.7uF

[xvr]

So we start with -2V and increase till we reach  15uA.

Until we reach 400mA. We change VG1 and track Idd (not Ig1)

[john23]

Hello XVR, I am still confused regarding the 250mA over the capacitor.
Suppose the gate capacitance is 100pF i put in parralel 10nF capacitor .
when i charge this gate to get -2V .
I saw 250mA going threw that capacitor, why you are saying its not going to touch the gate capacitance?
my 4mA rating is very fragile. could you say what exact load i should use?
Thanks.

[xvr]

You will get 250mA discharge flowing through your 10nF capacitor, and much less through gate capacitor. Put both of them in LTSpice and draw current through gate capacitor only. Current of external blicking capacitor (10nF) will not flow neither through gate capacitor nor through VG1 in general

[john23]

Hello, I have tried to impement your idia in lt spice using AD8034 as shown below.
I know that OPAMP is a comparator.
But it has feedback also.
What is the formula for the output of the opamp system you showed so I could see where dod i go wrong?
Thanks.

[xvr]

I know that OPAMP is a comparator.

No, it's a opamp. So it's ok to use it here.

As for schematic - V2 polarity is wrong. Swap its pins.

What is the formula for the output of the opamp system you showed so I could see where dod i go wrong?

Opamp with negative feedback will keep potentials of its input at the same value. Positive input has zero potential, so negative also will be zero.
By Kirchhoff's law currents through R1 and R2 are equal by module and has different signs.
So V1/R1 = Vout/R2

You can scale both R1 and R2 by the same value, it will change current from DAC and Opamp output to feedback circuit. Now it's 1 mA