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LM22670-Q1: LM22670-Q1 can support short protect function

Jacking Jin
Expert 1145 points
Part Number: LM22670-Q1
Other Parts Discussed in Thread: LM22670, , LM43603

Dears,

I can't found the LM22670 have short protect function on the datasheet, its only have the frequency current foldback mode, If the circuit is still on short status and the chip whether can heat accumulation, and cause the chip or the inductor to damage. Thanks a lot.

Best regards,

Jacking

  • 0
    TI__Guru 57555 points

    This family of devices uses a frequency fold-back to prevent inductor current runaway during a hard short circuit on the output.

    The SOA curves are shown in the data sheet and are valid as long as the device is not in frequency synchronizing mode.

    These devices do not have a "hiccup" mode, and will tend to heat when a hard short is applied to the output.  The thermal

    shutdown will prevent damage, as long as the SOA curves, and Abs max specifications,  are respected.

  • 0 in reply to Frank De Stasi
    Expert 1145 points

    Dear Frank De Stasi,

    Thanks for your reply!

    At present, I  direct short the output of the circuit, and I can't detect the frequency  decrease to the 1/5 switch frequency, only limited the output current, and I have tested some hous that I found the inductor is burned damage; Could you kindly give me some suggestion for the case? Thanks a lot!

    Best regards,

    Jacking

  • 0 in reply to Jacking Jin
    TI__Guru 57555 points

    If the frequency does not fold-back it is either because hte short is not severe enough to cause fold-back

    or you are using the sync feature.  In either case the device is surviving the short and protecting itself.  If the inductor

    is damaged, you will need an inductor with a larger current rating.  The protection mode will not protect an inductor

    that is too small in current rating.

  • 0 in reply to Frank De Stasi
    Expert 1145 points

    Dear Frank De Stasi,

    Our schematic diagram as below, according the datasheet, I think the circuit no entry SYNC mode, I use a fixed output 5V version. could you kindly help me check it? Thanks!

    And the inductor saturation current reached about 3A. Thanks for your great help!

    Best regard,

    Jacking

  • 0 in reply to Jacking Jin
    TI__Guru 57555 points

    Your schematic looks OK.  The device may only go into short circuit mode at higher input voltages.

    However, it sound like it is protecting itself.  I do not think that we can gaurantee that it will protect the

    inductor as well.  This device has a maximum current limit of a little over 5A. 

    So, your inductor should be rated for this level.  Can you measure the peak current during a short?

  • 0 in reply to Frank De Stasi
    Expert 1145 points

    Dear Frank De Stasi,

    I have measured the current waveform, I don't think that it's cause hot by the saturation current, when short the output the peak current only 0.6A, you can reference the real waveform, and I don't konw why the frequency are not 1/5 switch frequency. When the  short status is lifted and the circuit is working normal.  I want to know why used the frequency current foldback mode? What's the advantage with the tradition control method(if short directly disable the chip )? I hope you can kindly give me  some advice for the issue, and I will quantity production, If this is a inherent problem please kindly help me recommend a P2P automotive grade chip to replace the LM22670-Q1, thanks a lot!

  • 0 in reply to Jacking Jin
    TI__Genius 11085 points
    Hi, the waveform attached doesn't make sense to me. If it is short circuit, the peak current should be at the current limit level. Can you specify what is the method of short circuit? Can you also probe the VIN and VOUT waveform during short circuit as well?

    I agree with Frank that the inductor should be rated upto 5A if it is damaged during short circuit events. It should also be soft saturation type.

    -Yang
  • 0 in reply to YangZhang
    Expert 1145 points
    Dear Yang,
    Thanks for your reply!
    I'm directly short the Vout to GND, and I have test the LM22670 EVM to evaluation, and the inductor is very hot yet. I think the waveform is weird also, So I don't know why the new design choose frequency current foldback mode? Could you kindly help explain difference between this mode with tradition mode (If output short and shut down the switch)?
    And, Could you help me give some advice how to process the issue? Or help me recommend P2P to replace the LM22670, I think the inductor hot is can't solve use LM22670 to design, you can help to test the EVM. Thanks a lot!
    Best regards!
    Jacking
  • 0 in reply to Jacking Jin
    TI__Genius 11085 points
    There are always some customers prefer hiccup current protection, some prefer device keeps running during over current event. There are Pros and Cons on either method. With current foldback, the max DC current is reduced, to keep the device and inductor from getting too hot in case of over current. Output voltage will be lowered by over current, but not necessary down to 0V if it is not a hard short.
    I don't think we have a pin to pin replacement of the LM22670 with hiccup current protection. The LM43603 3A synchronous buck regulator does have hiccup mode.