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LM34927: Questions about the design of the LM34927

Part Number: LM34927

I would like to ask you two questions about the LM34927.
Question ①
See No. 8.2.1.2.1 on page 17 of the data sheet
"If the secondary (isolated) output voltage is significantly higher or lower than the primary output voltage, a turns
ratio less than or greater than 1 is recommended. The primary output voltage is normally selected based on the
input voltage range such that the duty cycle of the converter does not exceed 50% at the minimum input voltage.
This condition is satisfied if VOUT1 < VIN_MIN / 2."
Is there a description, why? And what happens when it exceeds 50%?
Question ②
Older version datasheet (SNVS799A-APRIL 2012-REVISED NOVEMBER 2012)
fsw = Vout / (10 ^ -10 × Ron)
However, the latest new version of the data sheet (SNVS799H – APRIL 2012 – REVISED NOVEMBER 2017) says
fsw = Vout / (K × Ron)  K = 9 × 10 ^ -11
It is described as. Why is it different?

Thank you for your reply.

  • Hi YO,

    Q1: The primary inductor require V*S balance which means (Vin-Vout)*Ton<=Vout*Toff,  Vout=Vin*Ton/(Ton+Toff), this will deliver the requirement Vout<Vin_min/2, otherwise the primary inductor will be saturated.

    Q2:  this is modified for better accuracy to estimate Fsw.

    B R

    Andy