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LM25141-Q1: Inductor and capacitor design

Wenyi Zhao1
Prodigy 255 points
Part Number: LM25141-Q1
Other Parts Discussed in Thread: LM5143-Q1, LM5141-Q1

Dear,

I have some questions about the application information in the datasheet.

1.When designing the inductor using the equation below, why does the duty lose or ignored?

2.When designing the output capacitor using the equation below, what does the Istep mean and what does the equation come from?

3.When i want to add the frequency dithering, how to choose the modulation frequency?

Thanks .

Wenyi

  • 0
    TI__Guru 72702 points

    Hi Wenyi,

    See answers included below.

    Regards,

    Tim

    Wenyi Zhao1 said:

    Part Number: LM25141-Q1

    Dear,

    I have some questions about the application information in the datasheet.

    1.When designing the inductor using the equation below, why does the duty lose or ignored?

    [TH] Please use the inductance calculated in the LM25141-Q1 quickstart calculator: http://www.ti.com/tool/LM25141DESIGN-CALC. This quickstart calculator file uses the same equation for inductance as that provided in the LM5143-Q1 datasheet. That expression derives the correct inductance to establish the ideal slope compensation based on one times the inductor downslope based on the chosen shunt resistance.

    2.When designing the output capacitor using the equation below, what does the Istep mean and what does the equation come from?

    [TH] Istep is the load transient step requirement (e.g. the delta from light load to full load in terms of the load current change).

    3.When i want to add the frequency dithering, how to choose the modulation frequency?

    [TH] Choose the modulating frequency approximately equal to the resolution bandwidth (RBW) specified by the applicable EMI standard. This is 9kHz for most EMI specifications, e.g. CISPR 22 and CISPR 25. For more detail, see this white paper: www.ti.com/lit/slyy136  

    Thanks .

    Wenyi

  • 0 in reply to Timothy Hegarty
    TI__Prodigy 255 points

    Hi Tim,

    Thanks for your answer!

    I already know the slope compensation but i still don't know how to get the expression because i think there should be a duty in the expression.

    As well, i know the meanthing the Istep is but i stll can't get the expression from the explanation the LM25141-Q1 datasheet give.

    In all, if i just want to get the parameter, i can do it by quickstart calculator or expression, but i still don't know why.

    Regards,

    Wenyi

  • 0 in reply to Wenyi Zhao1
    TI__Guru 72702 points

    Hi Wenyi,

    In terms of inductor value, we seek to match the inductor current downslope (scaled by the shunt resistance) to the internal slope compensation ramp. Equation (10) in the LM5143-Q1 datasheet is also applicable to the LM5141-Q1. This equation is derived by equating the downslope to the internal slope comp amplitude that's proportional to switching frequency (557mV/us at 2.2MHz).

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Prodigy 255 points

    Hi Tim,

    Thanks for your answer and i have understood how to determine the inductor with your help.  

    However, what's the meaning of the Equation (27) in the LM25141-Q1? Does it use the equation I=Cdu/dt and Vin-Vout=Ldi/dt to get the Equation (27)?

    Regards,

    Wenyi

  • 0 in reply to Wenyi Zhao1
    TI__Guru 72702 points

    Hi Wenyi,

    Yes, it considers the energy delivered at minimum input voltage to the change in output capacitor energy due to the voltage dip during a load-on transient event.

    Alternatively, consider equation (18) in the LM5143-Q1 datasheet that sizes the output cap based on the peak voltage deviation during a load-off transient.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Prodigy 255 points

    Hi Tim,

    Sorry, when analysing the internal slope comp amplitude that's proportional to switching frequency (557mV/us at 2.2MHz), the calculated internal slope comp amplitude  by equation (10) in the LM5143-Q1 datasheet when frequency is 2.2MHz is 52.8 which doesn't equal 557mV/us you give. What's the problem?

    Although i can get the inductor from the expression you give in the LM5143-Q1 datasheet, why can i use the  Equation (15) in the LM5141-Q1 datasheet which you haven't explain? if it uses Vout=Ldi/dt, does the delta t equals  1-dutymin multiply the period instead of just the period? 

    Regards,

    Wenyi

  • 0 in reply to Timothy Hegarty
    TI__Prodigy 255 points

    Hi Tim,

    Why are the expressions  during a load-on transient event and  a load-off transient event so different? I mean in my opion, the difference occurs only between Vout+deltaV and  Vout-deltaV? And, does the energy change only between the inductor and the capacitor? Where are the source and load?

    Regards,

    Wenyi

  • 0 in reply to Wenyi Zhao1
    TI__Guru 72702 points

    Hi Wenyi,

    The LM5143-Q1 equation is correct, and the LM25141-Q1 datasheet will be updated shortly to correct that. You can use the LM25141-Q1 datasheet equation with the 1 - D factor if you prefer. Please contact me offline and I can send the derivation for the output cap equation.

    Regards,

    Tim