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Original question:

LM7480-Q1: Reason for bidirectional vs unidirectional TVS diode on the input side

LM7480-Q1: Unidirectional vs Bidirectional TVS

Ant S.
Prodigy 40 points
Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM7480

Tool/software:

Hi,

I would like to know why is requiered a bidirectional TVS diode at the input of LM7480 to protect against reverse transients instead of an unidirectional TVS diode. I mean, if my application is intended to work at 24V DC I understand that I have to protect the circuit if I have a voltage greater than my working voltage but with negative transients if I use a unidirectional TVS the voltage seen by the LM7480 would be -0,7V. What is the point of using a bidirectional TVS which would clamp the negative transient in a more negative voltage? To summarize, What is the advantage of using SMBJ33CA instead of SMBJ33A in this example application.

Thank you in advance for your support.

  • 0
    TI__Mastermind 47835 points

    Hi Ant,

    If a unidirectional TVS is used, we would just short the battery and have no control on flow of current. Resulting in damage of TVS.

    This is true even if we use it in the other direction and short circuit will happen during reverse polarity.

    The TVS- is used for clamping ISO pulses like pulse 1 whereas TVS+ is used just to block the power path (no clamping function required) and avoid any short circuit.

    Regards,

    Shiven Dhir