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LM61440-Q1: loop stability test at light load failed

guangmin zuo
Prodigy 110 points
Part Number: LM61440-Q1

Tool/software:

Hello expert ,

we used LM61440AASQRJRRQ1 in our project , currently we tested the loop stability of this circuit , as the load is disabled so we test  at light load (current =0A) the result is failed the output voltage is OK. 

here is our circuit and  the test results ; 

can you help  review the schematic and give your advice ? 

if we need to adjust the value of  Rff and Cff, is there any guide document  and formula . many thanks 

  • +1
    TI__Mastermind 22300 points

    Hello,

    The 10uH inductor is oversized for this 4A device operating at 500kHz. I calculate 5.7uH when 30% current ripple is assumed. When selecting the inductor, you want to design for 20%-40% current ripple because a peak current mode buck regulator requires a certain amount of current ripple to regulate properly.

    Since the load is disabled, your device will be operating in PFM mode. Generally, bode plots are not valid at the PFM condition. For light loads in PFM mode, it is recommended to assess stability using a load transient test.

    Best regards,

    Ridge