Output oscillations on LNK302 in low-side buck-boost with optocoupler feedback

Posted by: jim.patterson@… on 11/05/2014

Hi,

I am using LNK302D in low-side buck-boost configuration with optocoupler feedback. I took the design from the circuit on p.3 of AN-37. My output is +5V with respect to +Vin.

My diode is an ES1J (35ns trr), optocoupler is TCMT1107. The inductor is a 1200 uH Coilcraft RFB0807-122, as recommended in the application report. The output capacitor is a 30uF MLCC.

I have sawtooth oscillations that are >400mV peak-to-peak at ~1KHz with no load. When the load is connected (~20mA at 5VDC) is connected, the amplitude drops slightly to ~300mV and the frequency increases to ~7KHz.

I really need to eliminate the oscillation, or at least knock the amplitude down to about 1/2 what it is. Any suggestions?

I can provide schematic, layout, and output waveforms if necessary .

Comments

Hi,

Yes, please provide a copy of your schematic and layout and also waveforms so we can support you better.

Regards PI-Sarek

Hi, please see the pdf file:

p1 - schematic

p2 - silkscreen

p3 - top

p4 - bottom

p5 - loaded output

p6 - unloaded output

The noise amplitude I provided initially is wrong, I was working from memory rather than from the scope captures. Unloaded noise is ~300mV peak-peak, loaded is ~400mV

I do not see any attachments with your message. will you please upload again?

Regards