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LM5164: LM5164-Q1

santhosh ganta
Prodigy 130 points
Part Number: LM5164

Hi sir,

We are using LM5164 converter for step down operation from 55 to 12V with load current of 0.65A.

We selected our filter value and switching frequency as per LM5164 calculator.

The problem is while we verify boundary condition of CCM or DCM by using the formula (load current (Io)>0.5*inductor ripple current) it fails as per theoretical formula.

Formula for CCM    2*L/(R*Ts) >1-D.

For DCM 2*L/(R*Ts) < 1-D.

L is inductor, R = load resistance, D = duty cycle.

Ts = 1/Fs (Fs is switching frequency).

L = 68uH, R = 18.461 , Fs = 300kHz.

2*L/(R*Ts) = 2.21 as per above values. but Duty cycle D range is 0 to 1.

Whether above formula applicable or not we need to know.

  • 0
    TI__Genius 10190 points

    Hi,

    For CCM   2*L/(R*Ts) >1-D; For DCM 2*L/(R*Ts) < 1-D. 

    These two equations are used to calculate the critical inductance value when D is know. They cannot determine D when the inductance is known.

    When R = 18.461, Fs = 300kHz, Vin = 55V, Vout = 12V, if L>24uH, it will be CCM. 

  • 0 in reply to Aurora Tie
    Prodigy 130 points

    HI sir,

    Thanks for your answer.  

    When R = 18.461, Fs = 300kHz, Vin = 55V, Vout = 12V, if L>24uH, it will be CCM.

    But one thing i didn't understand based on what formula you told that above converter operated in CCM.

    How can u distinguish CCM or DCM mode if inductor value is known.

  • +1 in reply to santhosh ganta
    TI__Genius 10190 points

    Hi，

    You already know the duty cycle D when the input / output voltage is known, so you can calculate the critical inductance value. You can compare the known inductance value with the critical inductance value to determine if the circuit should be operating in CCM mode or DCM mode. 

    For example, When R = 18.461, Fs = 300kHz, Vin = 55V, Vout = 12V, the critical inductance value is 24uH. When L= 68uH> 24uH, the converter works in CCM mode. When L<24uH, the converter works in DCM mode.

    I hope my answer will help you.