Post Go back to editing

LTM8074 LTPowerCAD Model Issue


I've recently started using LTPowerCAD and found a discrepancy that's either with me or the tool (model). The design is straightforward: create a 3.3V supply with 750mA of output current capability from a 5V input using the LTM8074. LTPowerCAD very quickly allowed me to simulate it, tweak the feedback capacitors to get a good transient response and then export it to LTSpice. During the low-load 100mA condition, the "Iout, iL (A)" current is shown as oscillating between about +375mA and -180mA which averages out to about 100mA - all good and demonstrating what I understand to be continuous conduction mode (CCM) providing a faster recovery when the load reduces from 750mA to 100mA (at 100A/us).

However, in LTSpice the CCM doesn't happen and the LTM8074 datasheet mentions that "The LTM8074 does not operate in forced continuous mode regardless of SYNC signal." nor do I see any mention that the component actually supports CCM.

Is there a way to activate CCM that I'm just not seeing or is there a boo-boo in the model or is there some other way to achieve that same good voltage response to current transients?

Thanks in advance.

  • Hi SRob80,

    Looking at the datasheet, I think it is clear that LTM8074 does not support CCM operation. LTpowerCAD only has computations for CCM operation but the actual part does not support it.

  • Since posting that question, I've noticed some other minor things associated with the tool and overall I'm really appreciative to have it. It would be nice if there were some sort of "YMMV" indication provided, IMHO. The devil is always lurking in the details and there is an argument to be made that bad information is worst than no information. Thanks

  • Hi SRob80,

    Based on the datasheet of LTM8074, it has four (4) operating modes only such as Burst Mode, Pulse-Skipping Mode, Spread Spectrum Mode and Synchronization Mode. Unfortunately, LTM8074 does not support Force Continuous Mode (FCM) instead it will switch to Pulse Skipping or Burst Mode at light load. For applications that requires fast transient response, we recommend other micro modules with Force Continuous Mode (FCM) such as LT8640S-2/LT8643S-2.

    LTM8074 operation modes LT8640S-2/LT8643S-2 operation modes
  • Hi SRob80,

    Rey is correct and I was mistaken. Forced Continuous mode (FCM) is not supported but the device supports CCM operation. LTpowerCAD only has computations for CCM operation.

  • Rey, thank you for the comprehensive and helpful response. One difference between the two parts is that one is a self-contained module ("LTM") while the other requires the inductor, etc. as well as proper layout (with possible impact to the stackup) ("LT"). This isn't intended to be discouraging and I'm truly appreciative of your efforts and reply.

    Using LTSpice, I've convinced myself that the LTM8074 can work for this application, I was just surprised that the LTPowerCAD model behavior wasn't as "true" as I expected.

    Stay well.