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LMS3655: BIAS-pin

Takashi Yamaguchi
Genius 12060 points
Part Number: LMS3655

Hi team,

My customer is considering to use the LMS3655. 

The 9.2.1.2.4 BIAS in the d/s describes that "Because this input is connected directly to the output, it must be protected from negative voltage transients. Such transients may occur when the output is shorted at the end of a long PCB trace or cable".   I believe that the negative voltage transients are caused by inductance of long PCB trace or cable and output capacitors(LC resonance). Is my understanding correct? 

Also, is there an equation to calculate the current out of the BIAS pin during negative voltage transients?

Regards,

Yamaguchi

  • 0
    TI__Mastermind 23835 points

    Hi Takashi-san,

    I believe that this is just a precaution if you have a long cable. and shorted output and causing the BIAS pin to be negative beyond -0.3V.

    What we recommend on the datasheet are as follow:

    Use a 3-Ω to 5-Ω resistor between the output and BIAS if the load is far from the output of the converter or inductive shorts on the output are possible

    You can see the implementation below

    I dont think there is an equation yet to calculate this negative transient

    Thanks

    -Arief