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Questions of LM2731 : Discontinuous operation, Duty Cycle, In/Out Voltage difference

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LM2731
Hello,
 
I'm using LM2731 and have some questions.
Please kindly advice to me about this device.
 
 1.Discontinuous Operation
Please kindly let me know the demerit of Discontinuous operation.
(Data sheet reccomend Continuous Operation)
 
 2.Minimum Duty Cycle
Please let me know Minimum Duty cycle.
There  is Maximum Duty cycle only in data sheet.
Does this mean there is no limitation to Minimum Duty cycle?
 
3.In/Out Voltage difference
Please let me know the minimum differential voltage between Input and Output.
Can I use at the condition at  5Vin to 5Vout ?
 
(I think that we should add Vf of  Diode and Drop voltage of Inductor to Vin.
It mean ,we can't use at the condition at 5Vin to 5Vout.)
 
Best Regards,
Ryuji Asaka
 
 
 
 
  • 0
    TI__Prodigy 370 points

    Hello Ryuji

    1)    The ratio of peak inductor current to output current, Ipeak/Iout, is larger in discontinuous current mode (DCM) compare to continuous current mode (CCM). Thus the part will run into current limit at much lower output current in DCM. Greater explanation of the relation of peak inductor current to output current can be found in page 12 to page 23 of the datasheet.

    2)    Usually a minimum ON time exists in current mode control. Unless in very special case, the minimum ON time would not impact the design of a boost converter. I don’t have the minimum ON time of the LM2731 handy. I’ll ask and let you know.

    3)      Provided Vin will not be higher than Vout and depends on the range of output current requirement, the LM2731 may work for 5Vin to 5Vout. Transient load response may not as good as wish, though. I would encourage you to order an evaluation board to test if it meets your application requirement. Refer to BOM shown in page 4 of the application note AN1280 to configure the evaluation board for 5V output.

     Let me know if any further assistance I could provide. I’d appreciate if you could share with me your test result.

    Best regards,

    Jimmy Poon

  • 0 in reply to Jimmy On-Wing Poon
    Guru 20090 points
    Hello Jimmy ,
     
    Thank  you for your prompt reply!
     
    I would like to know more demerit of DCM (difference between CCM and DCM) if there are.
    For example, low efficiency , oscillation etc...
     
    If demerit is large peak inductor current only, It is no problem for me.
     
    Best Regards,
    Ryuji Asaka
      
  • 0 in reply to Ryuji
    TI__Prodigy 370 points

    Hi Ryuji,

     For a given load current I-peak in DCM will be higher than that of CCM, more power dissipation will be expected in the Mosfet, the inductor and the diode. Because right half plane zero does not appear in DCM, loop stability would less likely be impacted.

     However, for 5Vin to 5Vout the converter will be working at the edge of minimum ON time limit, it is normal to observe some duty cycle jitter, even missing few duty cycles when  load current is reduced to very low level.

     Best regards,

    Jimmy Poon

  • 0 in reply to Jimmy On-Wing Poon
    Guru 20090 points
    Hi Jimmy,
     
    Thank you.
    Are  below my understanding correct ?
     
    *DCM  (compared with CCM)      
    -peak current : high 
    -dissipation : high 
      (due to the peak current is high)     
    -loop stability : high 
      (due to RHP zero does not appear in DCM. If RHP zero appear, loop stability is not good.) 
     
    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Prodigy 370 points

    Hi Ryuji,

    Yes, your understandings are correct for boost converter in general.

    Best regards,

    Jimmy Poon 

  • 0 in reply to Jimmy On-Wing Poon
    Guru 20090 points

    Hi Jimmy,

    Thank you !
    I understanding difinitely.
     
    Best Regards,
    Ryuji Asaka