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Original question:

LM26420 / Phase margin and crossover frequency

LM26420-Q1: Loop Response

Vidhya Jeyapaul
Intellectual 780 points
Part Number: LM26420-Q1
Other Parts Discussed in Thread: LM26420

HI Team,

LM26420Q1XSQ/NOPB part for 3.3V (0.2A actual consumption) and 1.2V (0.45A actual consumption) generation on board.

1.2V Gain margin fails. Kindly share your feedback to improve this.

3.3V designed for 0.3A and 1.2V designed for 1.6A output current. 

Vin is 5V.

3.3V: 3.3uH with a 22uF cout

1.2V: 1uH with 44uF CoutLM26420.pdfPFA loop stability reports with 100mV injection.

  • 0
    Intellectual 780 points

    Note: I have added RC snubber (6.8Ohms and 6.8nF) on both 3.3V and 1.2V Switch nodes. 

  • 0
    TI__Expert 3135 points

    Hi Vidhya Jeyapaul,

    the test result seems not very smooth, could you help please redo the test under high sampling rate and lower injection voltage(50mV/30mV) ?

    Best Regards,

    Kevin

    If this answers your question, Please click "This resolved my issue" button.
    ____________________________________________
    TPS62810-Q1 – New Automotive 6Vin, 2.25MHz buck family with 1% accuracy and SYNC-pin. 1-2-3-4A options.
    Smallest 2x3QFN package with wettable-flanks for optical PCB inspection.

  • 0 in reply to Kevin-Cheng
    Intellectual 780 points

    Hello Kevin,

    I tried with 50mV injection voltage and performance is still the same.

    PFA plots for reference. I tried with extra load also. Gain margins are always failing at different load conditions too.

    1. with 50mV injection voltage.

    2. with 50mV injection- additional 0.4A external load

    3. With 50mV injection- Additional 1A external load. 

    Kindly revert.

    Thanks and Regards,

    Vidhya

  • 0 in reply to Vidhya Jeyapaul
    TI__Expert 3135 points

    Hi Vidhya,

    two suggestios:

    1. you can try to change the feedback resister R114 to 5K, R151 to 10K

    2. you can add feedforward cap to adjust the margin, the design guide is:http://www.ti.com/lit/an/slva289b/slva289b.pdf

    Best Regards,

    Kevin

    If this answers your question, Please click "This resolved my issue" button.
    ____________________________________________
    TPS62810-Q1 – New Automotive 6Vin, 2.25MHz buck family with 1% accuracy and SYNC-pin. 1-2-3-4A options.
    Smallest 2x3QFN package with wettable-flanks for optical PCB inspection.

  • 0 in reply to Kevin-Cheng
    TI__Expert 3135 points

    Hi Vidhya,

    Since I have not received your reply for several days, I will turn off this item. If you have further question, you can reply it again.

    Best Regards,

    Kevin

    If this answers your question, Please click "This resolved my issue" button.
    ____________________________________________
    TPS62810-Q1 – New Automotive 6Vin, 2.25MHz buck family with 1% accuracy and SYNC-pin. 1-2-3-4A options.
    Smallest 2x3QFN package with wettable-flanks for optical PCB inspection.