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Original question:

LM74700EVM: question on TVS, MOSFET Rdson, and

LM74700EVM: TVS+ selection

John Pan
Intellectual 990 points
Part Number: LM74700EVM
Other Parts Discussed in Thread: LM74800-Q1, LM74700-Q1

Hi Kari,

In the data sheet of LM 74700, "The breakdown voltage of the TVS+ should be higher than 48-V jump start voltage, less than the absolute maximum ratings of anode and enable pin of LM74700-Q1 (65V) and should withstand 65-V suppressed load dump." That is why SMBJ58A was chosen. If I have a voltage regulator (or a gate driver) after LM74700, and the voltage regulator or the gate driver has a maximum voltage rating of 50-V, instead of the 65-V rating from LM74700, should I adjust TVS+ to SMBJ45A or keep the SMBJ58A? I hope to protect both LM74700 and the voltage regulator or gate driver from the transient voltage damage. Note SMBJ45A has a breakdown voltage of 50-V.

Thanks and I look forward to hearing from you,

John

  • 0
    TI__Guru** 105361 points

    Hi John,

    Yes, using SMBJ45A is one option but it would be better if a hot-swap FET based solution with LM74800-Q1 is used to clamp the output voltage during Vin transients. This enables to use lower voltage rated components in the downstream. Please refer Figure 11-25 in LM74800-Q1 datasheet

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    Intellectual 990 points

    Hi Rakesh,

    Thank you very much for your quick reply. The purpose of my design is to provide comprehensive immunity to various automotive transient tests on 12-V battery or 24-V battery as per ISO 7637-2 and ISO 16750-2 standard as well as other automotive OEM standards. I think LM74700-Q1 should be fine and LM74800-Q1 exceeds my design's requirement. 

    My another question is about the output capacitor. Table 1 and Figure 19 in page 15 of the LM74700-Q1 data sheet recommends an output capacitor, 1-μF Minimum,47-μF Typical Hold Up Capacitance. If the design has a voltage regulator shown in Figure 19 and the voltage regulator requires an input capacitor, such as 10 uF, my question is

    1) could the output capacitor of LM 74700 and input capacitor of the voltage regulator be combined with only one capacitor, say 10 uF?

    2) or the output capacitor of LM74700 is still required, but could be a capacitor with small capacitance such as 1 uF, followed by a large capacitor (such as 10 uF) for the voltage regulator as an input capacitor?

    3) or output capacitor of LM 74700 should meet LM74700's requirement, say 47 uF, and the input capacitor of the voltage regulator should meet the voltage regulator's requirement, say 10 uF.

    Please explain your choice as well.

    Thanks,

    John

  • 0 in reply to John Pan
    TI__Guru** 105361 points

    Hi John,

    You can use 1uF close to the LM74700-Q1 and 10uF close to the voltage regulator which should be enough.

    Best Regards, Rakesh