Minimum on time calculation in DCM mode

[vaibhavwaman]

Hello,

I am working on Buck converter design using LMR14006 converter. In this at light load it enters into DCM (discontinues conduction mode).

So for this I wanted to understand how to calculate minimum on time of swich in this DCM mode ?

Thank you.

Best Regards,
Vaibhav

 

[mtwieg]

The datasheet says:

Minimum on time isn't something you can calculate.

 

[BradtheRad]

how to calculate minimum on time of swich in this DCM mode ?

Ampere level through switch must go higher (by 10 to 50 percent) than desired Ampere output.
The switch-On time is based on the inductive time constant L/R.
...where R is impedance through the current loop at that moment. You must leave the switch On for as much time as it takes for Amperes to rise through the inductor.

Notice a reservoir capacitor at the output offers little impedance... However it opposes current in the sense that its voltage charge opposes voltage coming from the system.
Since you state the load is light, you don't need much On-time. Your reservoir capacitor discharges little during idle times of the cycle. This makes it hard to calculate On-time.

 

[Easy peasy]

For zero load the min on time is about 0.1% of the period, usually 50 - 80nS