Wow, thanks for the reply guys! This is why I love this forum, one can get real world practical advice from those who have more experience. Simulations are all well and good, but they aren't sitting on a PCB hooked up to multimeters
Ok, so more info. I have two prototypes, one on single sided FR4, all surface mount. Probably should have ground plane on the other side, but I didn't have any doublesided PCB material at the time. A picture of this is attached.
Second prototype was, believe it or not, on solderless breadboard (I can hear people spitting their coffee at their screen in laughter right now). A mosfet, the LM3485, diode, and inductor were all sodlered to adapters made form stripboard/veroboard, and the connections were made using single core wire as short as possible. I only made this once the above design on the PCB failed to meet my specs, and constantly desoldering/soldering different parts started to get to me.
Some answers to questions:
1) Value for Radj. I used 24k initiailly, but changed this to 47k when the output dropped (assuming it was curent limit kicking in), made no difference. I'm confident the way of calculating it in the datasheet using either the RDSon, or an external current sense resistor is fine.
2) Computed values for switching loses? no :/ I did check the input (gate) capacitance of the MOSFET I was using, noticing is was quite high, significantly higher than the one used in the reference design. Although, it was under 2000pf, so I was hoping it was be 'ok'.
3) Computing frequency was difficult, since I used SMT electrolytic caps from my 'bits box', but thankfully I found the datasheet for them, so I could get a 'rough' estimate of frequency. I also measured it with a frequency meter (which obviously fluttered about) on the MOSFET gate. This reading 300-400kHz.
CafeLogic: I was aware of my MOSFET's gate charge/capacitance, at 1440pf, (instead of ~700pf for the reference design), but it wasn't until you mentioned the output capactiance, as well turn on/off times that I checked the datasheet. You're absolutely spot on!!.... seems it takes an age to turn off. This *also* explains why the LM3845 itself gets rather hot... having to cope with the large gate capacitance of the FET.
I'm afraid I wasn't aware (I should have thought it through) of this specification... as you said, at 300khZ, thats a fair bit of switching, which seems to be beyond what this particular MOSFET is designed for. The only other availabe MOSFET I have is the IRF7205:
https://www.electro-tech-online.com/custompdfs/2010/08/irf7205.pdf
Which has better (ie lower) input and output capacitance, but alas, still has quite a large turn off time.
I originally picked this chip because I wanted to use a small inductor, that is 10-47uH. Some of the 'simple switchers' (as well as the dreaded mc34063A) require up to 220, possibly even 330uH inductors, which, when running for an output of up to 1Amp, can take up a lot of board space. Plus I have quite a few 22uH sheilded 1.5A inductors around, very small and low profile. Also, since the output load can drop as low as 30mA, I wanted my converter to remain relatively efficient at these loads. All in all this chip sounded ideal - no internal switch, but that just means I get to learn more (which I seem to be doing lol).
So, I guess the main problem with my setup is the FET? specifically its turn on and off times, as well as total gate charge. Seems I require a readily available alternative, which can manage a much higher switching frequency.
As an FYI, here's the PCB/schem. Note its single sided, with a couple of jumpers on the other side...far form ideal.
Thankyou all for your replies, as always, any more information/reference designs woudl be much appreciated.
- Blueteeth