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LM5118 or LM25118 Ramp Capacitor and Resistor Calculations

Keith Keller
Expert 7260 points
Other Parts Discussed in Thread: LM5118, LM25118

Hello,

Could you please share more details on how to calculated the RAMP capacitor and resistance value for the LM5118 and LM25118 devices?
In the datasheet says if Vin_max > 20V and Duty Cycle > 50% requires RAMP cap and resistor.

Regarding the resistor calculation, we see that we want the R_ramp resistor to apply 50uA.
On some designs we have seen 127K ohm on Ramp to VCC.

We are actually biasing VCCX off a generated 12V bias from another regulator.
We’re wondering if we should tie RAMP cap and resistor to VCCX in this case?

Thank you for your help, Keith