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LM5123-Q1: Current limit resistor

Arie Li
Expert 3390 points
Part Number: LM5123-Q1

Hi experts,

Is there any minimum required value of the current limit resistor? Customer want to use 1.5mohm for 40A current limit. The 1.5mohm is very small, will it cause any risk here? Do you suggest to add an RC filter? What value do you suggest? Thank you.

Arie

  • 0
    TI__Mastermind 31015 points

    Hi Arie,

    I would make sure current limit resistor can handle power dissipation (I^2*R = 2.4W), so I would suggest at least 3W. 

    You can add RC filter and initially start with 100 Ohm in series with CSN pin and 100pF parallel between CSP and CSN to differentially filter out noise. If common mode filtering is needed, need to form a RC network with resistor in series with both CSN/CSP pins and capacitor to GND. However, you need to be careful in selecting value to avoid offset error.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Expert 3390 points

    Hi Richard,

    I'm concerned about the loop stability if Rs is too small. Considering the Vos of the internal amplifier, small Rs would reduce the current sensing accuracy. Will it cause any issue on the peak current mode control? What's your suggested minimum Rs value? Thank you.

    Arie

  • 0 in reply to Arie Li
    TI__Mastermind 31015 points

    Hi Arie,

    1.5mOhm is fine and should not cause issue with peak current mode control. The LM5123 has internal slope compensation at the CSP pin in order to assure that part will not enter subharmonic oscillation under current limit condition. 

    What is the maximum peak current that the customer expects to see? Depending on additional margin, current sense resistor value can be increased if necessary. 

    Thanks,

    Richard