Post Go back to editing

LTC3805-5 min duty cycle question


I'm building a flyback using LTC3805-5 and my input voltage conditions are 30V - 300V and have 3 outputs 18, 12 and 5.5V. The total load on my power supply is about 16W.

I wanted to know the what would be the implications of having a duty cycle less than 6% as my initial tests have suggested that the duty cycle could be as low as 1.5%. I've simulated by circuit in LTSpice & already seeing that some Gate Drive signals are being skipped/Blanks - will this be a concern?

Lpri - 56uH

Lsec - 27.7uH

Laux - 13uH

Lter - 2.7uH


  • Hibytewise,

    Quite a wide range of input voltage there. When the part runs into minimum ON time limit of 6%, it will skip pulses. It is normal & nothing that can be done about that. In pulse skipping converter operation, output voltage ripple will be higher than when not pulse skipping. Therefore, you may need much bigger output caps. This is one tradeoff you should consider.

  • Hi, Thanks for the reply - Can you also comment on the equation (on page 9 of datasheet) to select the Rfs (resistor to determine switching frequency)? It seems that the equation don't match up with the resistor values as mentioned in graph ( Oscillator Frequency vs RFS)...