Hello,
I'm working on a boost converter using the LM5123, and I've run into some discrepancies between the equations given in the App Note "How to Design a Boost Converter Using LM5123" and the equations used to generate the transfer functions on the Loop Modeling portion of the quickstart calculator. There are two variables in particular that make a massive difference in the stability of the system: KD and sn.
For KD, the app note has a rather involved equation, followed by a footnote that reads "KD is approximately equal to 2." In my design, I have calculated KD to be in the range of 0.5-0.8 (depending on input voltages). In the quickstart calculator, the app note equation is used, but 2 is added to the result, so KD ends up being in the range of 2.5-2.8. So there are three competing values, and I'm unsure which to use: the estimate of 2, the app note equation, or the quickstart equation. The difference is noticeable but not critical: It affects the DC gain, but the stability is not at risk from this alone.
The other more concerning discrepancy is in sn (sensed rising inductor slope). According to the app note equation, this is proportional to ACS (current sense amplifier gain, equal to 10 V/V). However, the quickstart calculator leaves this variable out of the equation, resulting in a factor of 10 difference. This has a significant effect on the quality factor, which in turn affects one of the high-frequency poles, and in my situation is the difference between a stable plant transfer function and one with some significant issues.
Can you please advise me on which equation is correct for these two variables? Thanks in advance!
Best,
Derek
