Hello,
I have used the LT3581 to create a symmetrical DC/DC converter from 11-20V to ±38V, 300kHz based on the application note "1MHz, ±12V Charge Pump Topology Uses Only Single Inductor." 
With L = 68µH, RT = 274k, FB = 442k
It works correctly, but since my load is very low, less than 10mA, it operates in DCM (Discontinuous-Conduction Mode), which creates oscillations of a few MHz. My application is very sensitive to these frequencies, so I tried to eliminate them by adding a snubber circuit with R = 1k and C = 68nF on SW1 pin to GND.
Before Snubber Circuit: 
After Snubber Circuit: 
However, because the Duty Cycle period during conduction is fast, a few MHz, I still have noise. I then increased the inductance to 220µH, the maximum I can use given my footprint, to increase the Duty Cycle period during conduction, which allows me to reduce the Duty Cycle period to 600kHz. Nevertheless, with 220µH I exceed the recommended Lmax in the Datasheet formula.
My questions are:
- Does the Lmax formula apply to this setup, and if so, what are the consequences of exceeding it?
- Is my snubber circuit correct?
- Is there another way to reduce sub-harmonic oscillations?
Thank you in advance for your help!
Best regards,
Jérémy
tag removed
[edited by: JBO at 1:12 PM (GMT -4) on 14 Jun 2024]