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LM46002 - question regarding switching between DCM and CCM

Jeff Coletti
Expert 8755 points
Other Parts Discussed in Thread: LM46002

HI Support team

I have a customer with a question where they need to understand the threshold in the LM46002, section 8.3.2 of the data sheet, between switching between DCM and CCM. Basically they want to keep the device in CCM mode for EMI reasons. Vin is from 8 to 40v, and vout is 3.3v.

Is there a method to keep the device in CCM mode, and or can you tell us what the conditions are that cause the switch so we can design the circuit such that it does not hit the threshold.

Thanks

Jeff

  • 0
    TI__Genius 11085 points

    The threshold of CCM and DCM operation depends on the ripple of inductor current. The boundary condition is when the valley of the inductor current ripple become zero. At the boundary condition, the DC output current equals 1/2 of the peak-to-peak inductor current ripple. The part will operate in DCM with DC current lower than the boundary and in CCM when output current is higher than the boundary.

    The ripple current can be estimated by ripple = (VIN-VOUT)*(Vout/Vin) / L /Fs. (See equation 13 on the DS)

    This part does not have a forced CCM mode. The only way to keep CCM is to choose large inductance and higher frequency to reduce the ripple current. But higher frequency will reduce the efficiency. Too small current ripple is not good for signal to noise ratio of the design, and may affect stability.

    You mentioned the customer wants to keep in CCM for EMI. Just to clarify, in DCM operation, the switching frequency does not change. Only at very light load, when the part operates in PFM (pulse frequency modulation) mode, the frequency will reduce.

    What is the minimum load current of this customer's application?

    We can review the customer's design if desired.

    Best,

    Yang