Discrete COT Controller

[24vtingle]

Hi
Just doing a discrete Constant off time controller as
attached. FET should get latched off when hit the current sense
threshold. Then the RC off timer gets released and C charges up.
When the off timer RC has charged to the threshold, the off time
is up and the fet should get turned back on and the latch should
get reset.Not had time to complete it yet, any tips please forward
if possible.

 

[KlausST]

Hi,

I see no error description.
In which regard do you need assistence?

Klaus

 

[FvM]

The switcher schematic reminds me of my comment in this previous thread obviously related to the same design problem: https://www.edaboard.com/threads/low-power-sepic-smps.414729/post-1791727

Component costs can be a reason not to use a dedicated low-power switcher, however knowing your typical designs with dozens of additional parts, I seriously doubt that overall costs of your design will undercut a slim LMR36500 circuit.

Thus my tip: Either use a dedicated low power switcher or one of the real simple solutions suggested in the previous thread.

 

[kd502]

You can make this work. But I don't think that it is cheap. Assembly is not for free.

With such primitive voltage regulator, regulation is poor and ripple is high.

Improved without thinking:

 

[24vtingle]

Thanks, though i note that yiour excellent effots are not COT control.

..soo...
The COT controller is nearly working but not quite. (as attached LTspice and png)
I did an "Imitation" current sense signal ramp...and it does one cycle correctly, but does not
persist in constant off time control.
Any corrections much appreciated.

 

[kd502]

Thanks, though i note that yiour excellent effots are not COT control.

Why do you think so?

Simulator works painfully slow with your circuit so I simplified it. I don't see reason to make monostable with bistable.

Your circuit is also not constant of time control if your monostable can't make constant delay.