What is the minimum controllable on-time for these parts? Datasheet doesn't specify it for any of these. LM20323 datasheet mentions that for on-time <= 200ns operation extra 22pF capacitor should be added to the feedback loop, I take that to mean that 200ns is on the low end for all of these devices.
The Cc2 capacitor adds extra pole on the feedback loop (as pointed out in LM20242 and LM20343) so it's not exactly "meant" for low on-time compensation.
I'd like to use 20242 part for 24->3.3V conversion, I take it I should keep it at 500kHz or less to stay on the good side of the Ton limits?
We recommend no less than 150 ns. for steady-state operation. You should be fine up to roughly 900 kHz.
The extra pole on the feedback loop helps filter noise on the comp. pin at very high frequencies, even if the pole is outside of the loop bandwidth. This filtering becomes critical at lower on-times.
Thanks. Maybe this 150ns figure should be added to the relevant datasheets?
there is also one other thing: In the spreadsheet for LM20242 you can find 300ns as a min. value for the pulse width
This is a little bit confusing...
Thanks and regards
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