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LT8390 Light Load issue at Buck-Boost-Mode

Question asked by swagn on May 10, 2018
Latest reply on May 10, 2018 by KSz

Hello everyone,

 

I have designed a DC/DC-converter with the LT8390. Actually I have three boards, same layout, for test purpose. My feedback network is designed for a constant-voltage output of 24V

 

Two issues have appeared:

 

  • At light load (~<10mA) the output voltage sometimes goes down to 20V or even less, depending on the input voltage. But only in a range of approx. 24V...28V_in. Also at this state I can measure an input current of 0.2A! Don't know why.
  • When I increase the load (e.g. 0.4A) output voltage Begins oscillating between approx. 20V and 24V with a frequency of approx. 15Hz. The waveform looks like a discharge curve or similar. Sometimes after a few seconds output voltage discontinues oscillating and becomes stable, 24V. The problem occurs also in a range between approx. 24V...28V. But when I start with an input of 30V the output is immediately stable. When I reduce it now under 28V it remains stable, no oscillation. 

 

Suprisingly one board makes no problem, 24V output voltage everytime. The other ones have the described issues but also slightly different.

 

Any ideas? Thanks!  

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