I want to build an adjustable buck converter with uc3843 ic. I built up a circuit in LTSpice for simulating. The problem is that, i want to adjust the output from 3v to near input voltage. With the currenct circuit i can be only set form 14 V because on the FB input 5V is the absolute maximum.For lower voltages it need to go higher in this ciruit. Do you have other solution for this?
The problem is that, i want to adjust the output from 3v to near input voltage.
According to the datasheet, this circuit does not feature a bootstrapped MOSFET driver. Therefore it will be unable to generate the gate drive voltage for a high-side NMOS.
Its output is also clamped to a maximum of 18V.
Allowing for NMOS Vgs, this means the output voltage will not exceed 13-14V, which corresponds to your results.
If you want your output voltage to go gloser to the input voltage, consider using a DC-DC controller chip with real MOSFET drivers, like this one:
(There are many others).
At first: If this IC was used properly, he itself would adjust the final output voltage to such that the voltage at FB is 2.5V This is the idea of the regulation. You have totally missed how the feedback is used to achieve the regulation.
Your circuit cannot function at all because the output cannot turn the mosfet properly on. This IC is assumed to be used in circuits where the cathode of the mosfet is nearly at the ground. You mut add a level shifter between the output of the IC and the mosfet gate if you still decide to use this IC.