Other Parts Discussed in Thread: LM5155
Tool/software: WEBENCH® Design Tools
I used WEBENCH to generate a design for the LM5155 where I am boosting 24VDC up to 100VDC with 0.2A output current. WEBENCH did the design and selected part values. I'm going through the datasheet to verify part selection and am getting stuck on equation 9 for the RSL value selection and equation 13 for Rf and Cf values.
Based on WEBENCH recommendations:
VLoad =100V
Vf=0.62V
Vsupply = 24V
LM = 33uH
RS = 47mOhm
fSW = fRT = 1.97MHz
RSL = 1kohm
Rf = 97.6 ohms
Cf = 1nF
Equation 9 says RSL should be selected to achieve 82% of sensed inductor current falling slope. If I set RSL = 1k as WEBENCH recommends my calculations come out to 127% (which I don't know if is possibe/allowed). If I solve for RSL to achieve 82% I get a resistor value of 173.11 ohms.
Equation 10 calculates the peak inductor current limit in steady state. If I use an RSL of 1k as WEBENCH recommends, the peak current is 1.64A which is within the specifications of the inductor WEBENCH recommends. However if I use a 174 ohm resistor for RSL the peak inductor current is 2.04A which is beyond the limits of the inductor. I don't mind picking a new inductor I just need to know which RSL is correct.
Equation 13 helps determine RC filter values for the CS pin. The Rf and Cf values WEBENCH recommended don't satisfy the required inequality. It's fairly straightforward to pick new values, I'm just not sure why WEBENCH recommended them if they don't work.
