I believe I did understood you, however we were talking about the input up to the regulator IC. No issues with using center tapped transformer etc..

Okay, I realize there was a flaw in my thinking.
You need a transformer with two isolated output windings, not center-tapped, to do what I suggested using the same regulator design for both.

I can find the circuit for constant negative voltage but not adjustable!.

That circuit, of course, will work with a common positive input supply voltage for both.
And the output voltage is adjustable as determined by R3 and R4 (whose values unfortunately are not shown in the Onsemi spec sheet), the same as the positive regulator.

R1 to ground.

Below is from page 12 of the TI spec sheet:
8.3.3 Inverting Regulator

The circuit in Figure 8-5 converts a positive input voltage to a negative output voltage with a common ground. The circuit operates by bootstrapping the ground pin of the regulator to the negative output voltage, then grounding the feedback pin, the regulator senses the inverted output voltage and regulates it.

This example uses the LM2596-5.0 to generate a −5-V output, but other output voltages are possible by selecting other output voltage versions, including the adjustable version. Because this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the maximum output current greatly depends on both the input and output voltage. Figure 8-6 provides a guide as to the amount of output load current possible for the different input and output voltage conditions.

 

Thank you very much, I really appreciate all the help. I will get the parts and I hopefully will update very soon..

 

Thank you very much, I really appreciate all the help. I will get the parts and I hopefully will update very soon..

Layout is very critical with switching regulators, so be sure you follow the layout suggestions in the data sheets, especially for the high current paths indicated.
Good luck.

 

can find the circuit for constant negative voltage but not adjustable!.

IT.com data sheet adds a diode on the input.
Note the 24V input connections in not clear.
Voltage from +Vin to GND must be less than 40V! In the case of Vout=-18V you exceed the 40V.

 

Voltage from +Vin to GND must be less than 40V! In the case of Vout=-18V you exceed the 40V.

Good point.
The input must be less than 22V for an -18V output.

One suggested source for the input DC would be a surplus laptop supply.
They are often 19.5Vdc output at over 2A.

 

Thank you guys a zillion time..

I don't know how, but I've missed a very important requirement which is the:
"Current limit: around 1A"

I would really appreciate any suggestions to for that?

I have found this circuit, but it's for the XL4015, I believe it should work with LM2596, but how about the Negative/inverting output.
circuit