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

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

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

Hi Jimmy,