Hi:
I've made a circuit for +20V to -80V with LT8365.
The application is for design that requires low noise, so I added a [1uH inductor] where in reference design has [20ohm resistor].
The above change was tested in simulation with LT8365 LT spice model resulting in less voltage spike at output.
Schematic shown below:
with this design, at no load the Vout is smooth, yet at 8mA load, the device is bursting at 1.2kHz, and switching is around 140kHz.(It was set to 400kHz with 100Ω resistor at RT pin ).
Waveform from scope shown below.
Waveform of SW node:
Waveform of Vout:
Does anyone know what causes the burst mode to create such large Vp-p? any way to reduce this?
Thanks!
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Edit 2:
V_FB pin circuit is as such:
The scope reading from observing the V_FB:
The noise on top of V_FB seems to be around 400kHz(switching frequency), and
This issue might be due to poor layout, which allowed the switching noise to appear on fb pin via capacitive or inductive coupling.
(Mostly capacitive i guess?)
Temporary patching solution:
Adding 1nF parallel capacitor C12 across R6 as shown above.
With added capacitor, the output noise is reduced, yet this affects the stability of the system.
Edit Notes
Edit 2: more images will be added soon[edited by: Jeffwm at 7:09 PM (GMT -5) on 6 Nov 2024]
