Constant-on time and constant-off time BUCK DC-DC

[Ayman Essam]

Hi
I am required to fully understand both of constant-on and constant-off time, and to understand the boundary that a BUCK converter can encounter if using either one

 

[BradtheRad]

To adjust output voltage of either type, change switching frequency.

Also try a forum search of "constant on" and "constant off".

 

[treez]

constant off time doesnt need slope compensation.
Constant on time is like the LM3402.....it is much rarer

 

[Easy peasy]

constant ON time = variable freq, running to very low freq's

constant off time, assumption is on time can go to very low values for regulation, not such wide freq range...

 

[treez]

Here is a constant on-time buck converter controller, the LTC3833
http://cds.linear.com/docs/en/datasheet/3833f.pdf


….it waits till the inductor current has fallen back to a certain value before the buck fet is turned on again…….it seems amazing, but it doesn’t actually have a current limit for its on time when the inductor is being “charged”…. One of the advantages is that since you are not current sensing in order to terminate the on time, you can have really really small on times..in this case down to 20ns.
This is because the “switch on current spike” doesn’t matter when you are not actually sensing current during the inductor charging interval.