Re: Buck circuit questions.
I'm not sure I understand what is going with the zener diode in your picture. My intent of using one is to prevent the device from turning on during the classification stage of PoE which would give it a classification of 0. The only way to do that is to connect it to a switch to control the main loop, or to put it in series with the IC Vin.
This is an area where you'll need to see how well it works. Does the IC have a built-in time delay? Does it check conditions at all pins before it starts producing pulses?
A series zener by itself is not likely to add a time delay. What it will do is conduct when incoming PoE is high enough to overcome its threshold. Below that it will not conduct. Thus if PoE rises slowly, the classification stage might occur while your IC is off. It may result in doing what you want.
If you need a longer delay, however, then you may need to put a capacitor in there somewhere. Or add another circuit to provide power after a delay.
As for my post #21, it shows typical use of a zener to limit supply V.
Since the IC must not receive greater than 40V supply, I reckon 35V regulation is adquate. My two schematics show what happens when incoming PoE is highest (57V) and lowest (40V).
It may require that you find out what supply V works with your IC, to find out which is the best way to configure the zener.
I do get what you are saying about the resistors being too high though. What do you think about ['URL='this setup] where the 300ohm resistor represents the IC?
It may work. The zener subtracts 27V.
Then notice that with the 270 ohm resistor you are automatically dividing voltage by 2 (approx). So the supply might range from 7V to 16V.
Question: Can you be certain it produce sufficient voltage to drive the mosfet?
The current limit is on the internal transistor, although the external FET in my previous drawing did have a gate max voltage of 30. The problem then is getting the voltage low but keeping the current just right. I think the way I split it int he link above would work.
Yes, these considerations are all important to getting proper performance. Some adjustments will happen automatically, of course.
One factor that may become crucial, is whether you can count on 350 mA always being available from PoE. Therefore it may be wise to minimize the instantaneous current you draw from the PoE.
Better a longer On-time at less current, than shorter On-time at more current.
To optimize this, the transformer ratio may need to be 1:5, or 1:10, or a value in between.
It would be great if you could purchase a few different ratios, and use the transformer that works best. A simulator can help, but its results are only tentative.
I have been adjusting your latest simulation, making a priority to draw minimal current, yet power the load even when PoE supply is 40V.
I find the transformer ratio should be 1:8, 11 mH primary winding, driven at 11 kHz, and 56 percent duty cycle. Again this is only theoretical.
The 1:8 ratio of course calculates to step down 40 V to 5 V. However when PoE is at 57V, the duty cycle must be reduced.
The transistor is driven to saturation. A mosfet can be done likewise.
https://tinyurl.com/9fwuqqv
I installed LTspice, but I'm at a loss on how to get the regulator in there. Should I just pick one from Linear that is as close as I can find, or is there some place to download the IC into the program?
I haven't used LTspice enough to know for sure, but I've heard of cases where someone makes a data file so a simulator can run this or that component.