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L6699 incorrect startup




I have an issue with the L6699. The controller initiates the start-up sequence but after a few pulses it stops permanently.

I describe the application first:

  • Vin=385V
  • Vout=39 to 60 V
  • Transformer magnetization inductance Lm=127uH
  • Resonant tank inductor: 42uH
  • Resonant tank capacitor: 60nF

The programming of the IC is as follows:

  • RFmin 8.5k
  • CF 330pF
  • Rss 15.3k
  • Using internal bootstrap circuitry

This gives an startup frequency of around 200kHz and a minimum frequency of 75kHz.


Once the LINE input is set, the controller does correctly the first LVG pulse of 10us, waits around 50us as expected and innitiates the switching, but after a few cycles, something goes wrong and a transition of the LVG from ON to OFF does not occur at the expected time. The next cycle the switching is permanently stopped.


I have no clues of what is going on. The only fact that bothers me is that the Vcc is above 17V, so the internal zener is having some current. Can this be a problem? The temperature of the IC is normal, so I understand that this is not the issue.

Another suspicion is that the high side voltage driving level is 3.7V less than low side because of the internal bootstrap circuitry, can this be a problem?


I attach an caption where the ISEN, and the low side and high side drivings are shown.


I would like to understand why the transition of the low side drive does not occur at the expected time.


I really appreciate your support.