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LM5576: Ripple current

Ryo Akashi
Genius 9470 points
Part Number: LM5576

Hello experts,

I got question about ripple current.

In datasheet P19, there is desription about Iripple should be less than 0.5Ap-p.
I'm not sure why Iripple<0.5Ap-p.
Is it decided by device's limitation?
Thanks and best regards,
Ryo Akashi
  • 0
    TI__Expert 4555 points

    Hello Ryo,

    Please let me confirm this internally, and get back to you as soon as possible.

    Best Regards,

    Ankit Gupta

    Application Engineer

  • 0 in reply to Ankit Gupta95
    TI__Genius 9470 points

    Hello Ankit,

     

    Thank you for your reply.

    Once you can confirm, please let me know.

     

    Thanks and best regards,

    Ryo Akashi

  • 0
    TI__Mastermind 22299 points

    Ryo,

    This is not a device limitation, it is related to the current mode topology.

    This device is a current mode controller, the control loop works best when when device is continuously conducting.

    In buck converters at any point the output current is the 1/2Iripple + Ipk(-) . 

    The peak to peak ripple current Iripple is controlled by the VIN and VOUT voltages, switching frequency, and inductor.

    The Continuous conduction mode (CCM) boundary is exactly when the load current Io == 0.5 * Iripplepk-pk.

    This is also exactly when valley current I_pk- is 0A.

    If the output current Io falls below 0.5 * Iripple, then the valley of Iripple(I_pk-) would go negative, below 0A.

    However this device has a lowside diode instead of a mosfet, and cannot conduct negative current, so the ripple current is cannot go below zero.

    To avoid overcharging the output in light load conditions, instead the SW will just not turn on in the next cycle, and the device will operate in a discontinuous conduction mode (DCM), where the switching frequency is reduced.

    The recommended ripple current is 500mA because the designed minimum output current is 250mA, that way the device runs in continuous conduction mode and fixed frequency when the output current is at least 250mA.

    Running in DCM is not bad, however the switching frequency will not be as designed, and if the peak current is much larger than the output current you could see larger output voltage ripple.

    If you have further questions let me know,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Genius 9470 points

    Hello Orlando,

    Thank you for your kind explanation.

    0.5 Ap-p is the requirement for Minimum load current = 250mA. Is it correct? (If minimum load current is 100mA, I ripple should be less than 200mA?)

    I may have misunderstood that 0.5Ap-p is the ripple current limit applied for all design.

    Also, thank you for your advise about DCM operation.

    Do you have recommendation of app note of DCM operation at light load?

    Customer wants to know what will occur if valley current reach zero. 

    Thanks and best regards,

    Ryo Akashi

  • 0 in reply to Ryo Akashi
    TI__Mastermind 22299 points

    Ryo,

    0.5 Ap-p ripple is the requirement for minimum load current = 250mA for the converter to always be in CCM.

    The converter is still working if the load current falls below 250mA, just in DCM.

    Most applications do not require the converter always in CCM, and sizing for CCM would be impractical for applications with small minimum load currents.

    Instead I suggest you size your ripple current to be 40% of the MAX output current.

    So if you are using the full 3A output that would be a 1.2A ripple current.

    Please see http://www.ti.com/lit/an/slva057/slva057.pdf for how DCM works.

    Once the inductor current reaches zero, diode conduction stops, and with 0 inductor current, SW node voltage will decay to VOUT voltage (with LC ring).

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Genius 9470 points

    Hi Orlando,

    Thank you for your reply.

    I understand that 0.5 Ap-p ripple is the requirement for Iload_min=250mA and the converter to always be in CCM.

    Also, thank you for suggestion about maximum ripple rate.

    I will also check the app note.

    Thanks and best regards,

    Ryo Akashi