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[ LM5088 ] Instability

Kenichiro
Mastermind 9165 points

[ LM5088 ] Instability

Hi,
According to my customer, they are seeing the instability which cause Vout is getting half of the target voltage.
Here are the conditions.
---
  • LM5088-1
  • Fsw 550KHz
  • 42Vin
  • 8.3Vout/1.5Aout

---

Customer's outcome are...

  • When Vin is relatively high such as more than 40V, Vout is getting half.
  • When customer connect their probe to FB pin, this can be disappeared.
  • When they use 100pF Cff(feed forward C) in parallel with high side feed back resistor, this can be improved.

Still not much information from customer and I'm now working on having more info. and waveforms from customer.

As like other posts, some points I need to check.

  • Current limit could be triggered by noise or other causes.
  • External compensation is not optimized (their Rcomp and Comp 43Kohm/2.2nF are slightly small compare with Webench design)
  • Their Sense resistor (27mohm) is slightly small compare with 

Do you have any other ideas where I should look into?

Also, can you please let me know whether Cff work well for ECM type of device?

Thank you for your support in advance.

Regards,

Ken

  • 0
    TI__Guru 58841 points

    Hi 

    Please check if any change by changing output load. 

    Please check CS-CSG connections are kelvin-connected to current sense resistor. 

    Regards, 

  • 0 in reply to EricLee
    Mastermind 9165 points
    Hi Eric,
     
    Thank you for your reply. When I have the layout from customer, they do not employ the Kelvin Connection for sense resistor (CSG is connected to GND underneath the package).
    Thus, I suggest him to try to follow the guide line in the datasheet and make their board similar with EVM.
     
    Also, with regards to the phase margin, since they have seen some kind of improvement by having Cff, I suggest to have a land pattern for Cff for next board, try to adjust Comp value and make sure the loop stability under certain conditions.
     
    Anyway, little progressed, thank you for your support!
    Thanks,
    Ken