Hello,
While following the application note "How to Design a Boost Converter Using LM5123" (SNVA991, Oct. 2022), I noticed a discrepancy in section 2.3: Current Sense Resistor Selection (page 5). The calculations show RCSslope ≤ 2.86 mΩ and RCSpower ≤ 1.8 mΩ. With these in mind, the final paragraph on that page is confusing:
Sub-harmonic oscillation can occur in a design where the RCSslope value is greater then than the RCSpower value. If this condition occurs, the ripple ratio of the inductor needs to be decreased and the procedure in Section 2.2 should be revisited. By decreasing the ripple ratio the effective slope compensation is increased. The slope compensation is adequate for this design example, and current sense resistor value is selected as a standard value of 1.5 mΩ.
The example values show RCSslope > RCSpower, which according to the description means the design is at risk for sub-harmonic oscillations, and the initial ripple ratio should be decreased. However, RCSslope is proportional to inductor value, and decreasing ripple generally requires a higher inductance, which just further exacerbates the condition where RCSslope > RCSpower. Is the final paragraph on page 5 a simple typo, and it should actually read "...where the RCSslope value is less than the RCSpower value"?
Thanks in advance for your help!
Best regards,
Derek
