• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 61 subscribers
  • Views 395 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM20333 oscillation at light loads

Lada Lostak
Intellectual 330 points
Other Parts Discussed in Thread: LM20333

Hello, I have problem with LM20333 DC-DC. At light loads, it starts oscillating. When load is applied, it starts work well. When load step is applied 1A -> 0A, thjere is 50% chance, it will start oscillating or works without problem. While oscillating, it consumes ~50-70 mA. After startup (poweron) it usually works properly and starts oscillaing randomly (without any external event).

Screenshots taken when Vin=13.2V

Iout (noload) = ~10-20mA

Iout (load) = 1A

DC-DC powered from clean AC/DC source.

Following image shows oscillation at the output (L3 input) when there is no load condition

When 1A load applies, output looks:

And finally, 1A -> 0A step non-oscillations looks:

Schematics (copied from webbench)

  • 0
    Intellectual 330 points
    Thank you for any hint :) I can take any scope plot/measuring.
  • 0
    Intellectual 330 points

    Looking at FB pin indicates, OVP protection starts protecting :) It also explaines additional current drawn for PS when oscillating. So, the only question is: is there any probl;em in design (eg. loop) or LM20333 need some minimal current to be drawn from output ? If so, it is comutable to be protected against manufacturing tolerances (inductors, resistors, ...).


    See FB pin (1:10 probe without setting proper probe in SW). Upper rule=950mv, lower rule=630mV. It corresponds datasheet: OVP should start somewhere around 880mV at feedback pin.

    There is note in datasheet about minimal ON time: For low duty cycle operation, when the on time of the switch node is less than 200ns, an additional capacitor
    (C C2 ) should be added from the COMP pin to AGND. The recommended value of this capacitor is 20pF. So, will help increasing C25 in this case ?

    Thx :)