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Startup issue with HVLED001 demo board (STEVAL-ILL069V1)

Question asked by how.chee_h on May 25, 2016

I am evaluating HVLED001 IC’s demo board (STEVAL-ILL069V1) which I found that such demo board have difficulty to power up if the output is loaded (even as low as 50mA).

To operate the board I have to power up the unit with no load first and then apply the load, which is really impractical.

When such demo board is directly power up with load by probing some signals I can found the followings:

       
  1. The IC is hiccupping
  2.    
  3. The FB pin voltage stay high (> its OFP level of 2.95V) all the time.
  4.    
  5. Vcc pin (hence switching) is oscillating between its ON and OFF threshold of 13.2 and 9.2V.

I tried to extend the softstart time but it didn’t help.

a)      May I know what may help to fix this issue?

b)      Is this phenomenal is only happening with primary side regulation, with loading output Vsec amplitude (Vo) may take longer time to build up hence VZCD can’t rise fast enough before softstart interval elapsed? Hence the VFB is sited above its OFP level all the time.

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