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LM34936: question of design calculation

Part Number: LM34936

Hello Experts,

I have question as follows, Would you please answer it?
1. Equation 22, what does "D" mean? Does it mean the output PWM duty cycle? Also what kind of relation between Cin and Icin(rms)? In other words, how to select Cin from Icin(rms)?
2. Regarding FMOD, do you have any recommended frequency? 1/10*Fsw is reasonable?

Best Regards,

  • Hello Yasushi,

    1. D in this formula means the duty cycle of the power switch on the buck side. In Figure 25 this is QH1. The input capacitors need to be able to handle big ripple current, so this formula gives a first order estimation for the ripple current the input capacitor needs to handle. As the worst case ripple happens in a buck converter when the duty cycle is 50%, 50% is used as calculation for the RMS ripple. Now the input capacitors need to be selected the way that they are not damaged by the high ripple current. In addition, with this ripple current you can estimate the input voltage ripple on the capacitor and calculate the minimum needed input capacitance. Normally this is a lot smaller than the one needed for handling the ripple current.

    2. Fmod shall be as low as possible to not interfere with the stability of the converter. So 1/10th of the switching frequency is typically too high. In general I would recommend to start the design without Dithering and add the capacitor on the DITH pin after the system is working properly. Normally the EMI measurements then define how big it needs to be.

    Best regards,