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LM3478: unstable output when load changes

kyohei suzuki
Prodigy 10 points
Part Number: LM3478

Hi,

In a booster circuit using lm3478 , The output oscillates when the load changes.

Vin is +24V ,Vout is +80V and 0.3A .

The circuit diagram and oscilloscope image are shown.

Please give me advice to solve this problem.

yellow : Vout , green : FB 

yellow : FB , blue : COMP

yellow : Q11 gate , green : Q11 drain

  • 0
    Prodigy 10 points

    There was an error in the first image shown.

    yellow : Vout , green : FB 

    regards

  • 0 in reply to kyohei suzuki
    TI__Genius 15775 points

    Hello Kyohei,
    Thanks for reaching out to us via e2e.

    I have some questions on your schematic:

    Is it correct that C25 and C12 are electrolytic capacitors?
    Are there no bigger ceramic capacitors in the uF range, neither on the input, nor on the output?
    (I only see some 100nF capacitors there, but no bigger ceramic capacitors.)

    What is the maximum load current?
    You wrote +80V and 0.3A which would mean 24W. At an input voltage of 24V you would need an average current of something above 1A / a peak current of below 2A.
    Also, the inductor is rated for 3A. So why do you set the current limit (via R64) to 7.36A?

    I can see some modification / extra circuitry on the upper part of the output voltage feedback divider (R36, R47 and PC6).
    If you want to switch the output voltage, this can be done by modifying the lower part of the feedback divider (R65).
    But you should not modify the upper part because this will have a negative impact on the compensation and can make your circuitry unstable.

    As a first step, I would suggest adding maybe 20uF ceramic capacitors on the input and especially on the output side.
    Also, please make sure that you are using low ESR electrolytic capacitors in your design.

    In addition, please correct the current sense resistor according to the actual peak current (.060 to .075 Ohms for 2A peak inductor current).

    Best regards
    Harry

    All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults” and is subject to TI’s Important Notice (www.ti.com/.../important-notice.shtml).