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LMP7709: 100m ohm to suppress the large output cap ringing

Uttam Reddy Pailla
Expert 6020 points
Part Number: LMP7709
Other Parts Discussed in Thread: OPA4350, OPA350

Hi Team, 

I have a query related to LMP7709, one of our C2000 customer has moved from OPA5350 to LMP7709. Could you comment if a 100m ohm highlighted is needed from op-amp perspective. We had such need with OPA5350. I assume this was to suppress any potential output ringing due to relatively large cap load across the op amp.  

Let me know if its good to have or not? 

Thanks & Regards,

Uttam

  • 0
    TI__Guru* 77501 points

    Hi Uttam,

    Do you mean OPA4350 instead of OPA5350?

    If OPA350 is driving large capacitive load at its output, the following closed loop compensation technique for each op amp's output is recommended. LMP7709 has a different loop compensation requirements. what capacitive load is the replaced LMP7709 driving? I need to check out the LMP7709's loop stability from its output to the GPIO pins.  Please let me know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Expert 6020 points

    Hi Raymond,

    my mistake that was OPA4350, Here is the schematic attached from one of our EVMs (sheet-4) for your reference.

     /cfs-file/__key/communityserver-discussions-components-files/14/F2837x_5F00_180controlCARD_5F00_R1_5F00_3_5F00_SCH_5F00_02Oct2015.pdf

    Here MCU and load caps are same, could you please check and let me know about 100m ohm resistor need for LMP7709?

    Thanks & Regards,

    Uttam

  • 0 in reply to Uttam Reddy Pailla
    TI__Guru* 77501 points

    Hi Uttam,

    Based on the enclosed schematic, there is no or very low capacitive load at the output of OPA4350 op amp. The slide switch only operates statically and its parasitic capacitance should be low. 

    OPA350 capacitive load 03182024.TSC

    Without capacitive load at the output of OPA350, non of these 100mΩ and 22uF are required, and OPA350 has plenty phase margin at 67 degrees. 

    If this assumption is correct, the phase margin of LMP7709 is very poor, and it is not unity gain stable. LMP7709 op amp is decompensated op amp and will require gains > 6V/V in order to be stable. 

    Why do you want to use such old part for the buffer? Please advise. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Guru* 77501 points

    Hi Uttam,

    We have many op amps that will meet the customer's need, if this is the cost issues. 

    Raymond Zhang1 said:
    Without capacitive load at the output of OPA350, non of these 100mΩ and 22uF are required, and OPA350 has plenty phase margin at 67 degrees. 

    If this is dynamic transient instability issues, I would recommend to remove 100mohm and/or 22uF, disconnect the snubber compensation, the OPA350 should be stable (assume that there is little or no capacitive load at the buffer's output). 

    I am going to close this inquiry for now. If you have other questions, please let me know. 

    Best,

    Raymond