Hysteretic Buck converter circuitry?

[eem2am]

Hello,
The below is the schematic of my attempt at doing a simulation of a hysteretic buck converter.(LED driver)

The idea is that one comparator will turn off the FET when the inductor current reaches the peak, and the other comparator will turn the FET ON when the inductor current reaches the trough.
However, unfortunately, it doesn't work, as a little "glue" logic is needed, and I haven't quite got this right yet.

Do you know how I can make this work?

schematic and LTspice simulation attached

 

[Arouse1973]

Not sure what you want this circuit to do, control the current through the LEDs is it? But just having a quick look it looks like you have AAA(GND) connected to the 0.2R resistor and the inductor on the output, is this correct.
Also you would be better using the correct 0V(GND) symbol for your diagram as this is much easier to work out.
Adam

 

[Arouse1973]

Sorry just notice your one GND connection at the bottom of the page. I really am confused now what is AAA supposed to be because you have it connected to your LEDs through the inductor.
Adam

 

[eem2am]

Yes my bad, that's not a good ground type symbol, ive changed it in the below schem. -that ground has to be there because the fet driver has to be grounded to near the source of the fet.

What I need is to control led current , yes. -I wish to have the inductor current (ie the sense signal of it) banging up and down between the two comparator references, and as such I control the inductor current ,, and hence the average output current.


Here is a fairly working version, but it is noisy, with sometimes uneven switching.
Do you know how to get rid of the noise in this new circuit below? (otherwise it works quite well)
The comparators have "L" inputs which latch their output.

Simulation and schematic attached

 

[(*steve*)]

The normal way to do this is with a single comparator with a small amount of hysteresis

It turns the mosfet on until the current rises above the limit, then turns it off. Once the current falls below the limit it turns the mosfet back on.

Adding a little hysteresis to this prevents the circuit from operating in a linear fashion, and reduces the frequency.

 

[eem2am]

http://www.diodes.com/datasheets/ZXLD1366.pdf

..As you know, as in the ZXLD1366 ,if you can have two comparator ref voltages, and "bat" up and down between those, then that is more accurate than a single comparator with hysteresis...specially when vin changes and iout is required to stay the same

 

[(*steve*)]

It looks to me like you have arranged your comparators something like a window comparator. This is *not* what you want.

What you probably want is a pair of comparators driving the inputs of an SR flip-flop (or something functionally equivalent). This is also pretty common and allows you to independently set each of the thresholds. You see this type of construct inside a 555 (albeit with voltage dividers rather than voltage references).