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SN6505A: SN6505A for USB 5V isolation

Khoi Nguyen83
Expert 1180 points
Part Number: SN6505A
Other Parts Discussed in Thread: TPS25200, LP3878-ADJ

1) I need a 5V, 0.5A , 4kV isolated DC-DC from my 5V power to drive USB 2.0 VBUS pin.

This is for medical application, hence 4KV is needed but the board space is limtied.

Can I use a smaller LDO at the output of transformer instead of TPS76728-EP from the SN6505 eval board?

I assume 4kV is tested between the Ground so LDO at the output should be OK.

2) Since we need a current limit of 0.5A, I'm using PTC and TPS25200.

Would it be better to put TPS25200 on the system side (isolated), instead of the line side.

OR we can use LDO with output current limit.

  • 0
    Expert 1180 points
    Since LDO input needs to be be > 10V, I am thinking of using LP3878-ADJ.
  • 0 in reply to Khoi Nguyen83
    TI__Intellectual 2325 points
    Hello Khoi,

    Thank you for using our E2E forums.

    1) Yes, you can use a physically smaller LDO at the output of the transformer. The LDO provides additional regulation at the output but it is the transformer that provides galvanic isolation from input 5V supply to output 5V supply domain. So choosing different LDO is fine and will not affect the isolation ratings.


    2) Good news here is that you do have multiple choices on the technical implementation side. One thing to keep in mind is to see approximately how fast you need your current limit needs to kick in. This, along with size and cost can help you decide which option you like to pick.

    PTC fuses trigger based on heating when high currents flow in and so this can generally take a few milli-seconds; the actual reaction time is spec'd in the PTC datasheet and you can pick parts as needed.

    Also, you can indeed use an LDO with a built- current limit. This is likely to be faster to react to current limit events ( micro-second range).

    I looked at the LP3878-ADJ; Vin goes to 16V (good), quiescent current is high but there is a shutdown pin to get it to < 10uA pretty easily so that helps as well. it can provide 800mA current output with a typical current limit of 1.3A (short circuit).
    To cover UB 2.0 operation (500mA needed), you can pick any LDO which provides >= 500mA during normal operation and then with the limit slightly above this.

    Regards,
    Abhi