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LM5575: Pull-up on RAMP pin for additional slope compensation

Oba
Genius 16125 points
Part Number: LM5575

Hello,

My customer is using LM5575 for 24V input and -15V output with the below configuration.

The datasheet, SNVS471G, says that additional pull-up on RAMP pin is required for slope compensation when duty is greater than 50% on P12.
In this customer’s case, the duty should be 15/(24+15) = 39%. Although it is less than 50%, this system requires the additional pull-up on RAMP pin to stabilize.
Without the pull-up, the system is unstable.

 Do you know why does this customer’s system require additional pull-up on RAMP pin although the duty is less than 50%?

 

Regards,

Oba

  • 0
    TI__Expert 8245 points

    Hi,

    50% is a theoretical value considering sub harmonic oscillation. Actually ramp also helps on overall loop control, if the converter is in a bad loop (due to FB compensation circuit), a bigger ramp might help.