LM3485 high current step-down regulator

[mrmp17]

Hi everyone!

I'm working on high current switching regulator and I have a few problems to solve.

Regulator will be powered from rectified 450W, 24V transformer. I need around 300W on output at around 16V (voltage must be adjustable with a pot). I'm now in breadboard testing stage and things don't work quite well 

My controller is LM3485 (Hysteretic PFET Buck Controller) datasheet: http://www.ti.com/lit/ds/symlink/lm3485.pdf
PFET - IRF5305 datasheet: http://www.redrok.com/MOSFET_IRF5305_-55V_-31A_6mO_Vth-4.0_TO-220.pdf
Diode - BYV32-200 datasheet: http://www.onsemi.com/pub_link/Collateral/BYV32-200-D.PDF
Inductor - 68uH, 3A (just for testing)
Low ESR in/out caps


I assembled everything on breadboard (see Typical Application Circuit in LM3485 datasheet) and on the first look it was working just fine (12V source, 5V output), but when load was connected (around 2A) voltage dropped for almost a volt ! Also mosfet got really hot, even with heatsink. I measured everything and got very poor efficiency, I don't remember exactly, it was aroud 60%.

I also have problems with current limit. I haven't had connected the Isense and ADJ pins on anything. I don't know how to calculate the ADJ resistors. Can anyone teach me how to calculate the resistors? When I connected the Isense wire to drain of mosfet and LM3485 went in smoke - didn't have ADJ resistors connected yet (my fault, I had the circuit connected to power). So I have to solder a new one on adapter to get testing again.

So, anyone have an idea why I have so poor efficiency?