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LM2611 issues at low load

Other Parts Discussed in Thread: LM2611

Hi,

We're experiencing issues with the LM2611 when running at low output loads (~10mA) where after a period of minutes or hours, the IC suddenly draws large amounts of current and unless protected by the current limit of a supply is destroyed. The IC is being used in the following configuration from the datasheet. Due to the low output load, the IC is constantly running in pulse skipping mode, but provides a reasonable output until the failure occurs.

I wonder if Cff of 330pF is suitable for very low load conditions, but there is not a great deal of detail on choosing Cff in the datasheet. Any ideas are appreciated.

Best regards,

James.

  • Hi James,

    At very low load conditions, the LM2611 will be in pulse-skipping (hysteretic) mode since the energy stored in the Cuk capacitor is more than the energy required by the load.  The Cfff will have no effect on the control loop, so this value will not make a difference in such low currents.  You can decrease the values of the resistor divider network (RFB1 and RFB2) by a factor of 10x (so that RFB1 = 2.94K and RFB2 = 1K) and increase Cff by the same factor (3300pF) to provide the same loop response when back at full load.  Alternatively, can your application tolerate the use of a permanent load RL resistor to pull some extra load current?

    Best Regards,

    -Juan

  • Hi James,

    We'll also like to review your PCB layout to make sure the input capacitor is placed close enough to the IC since increased parasitic inductance on this path is undesirable.  Please upload schematic and layout for review.  

    Best Regards,

    -Juan

  • Greetings,

    Is there a recommended minimum current for a 12V to -5V circuit?

    Thank you.

    Best Regards,

    Dan Barnett

  • Hello,

    I don't believe we have a minimum value for the load. You will most likely have to try it on the application to see what is the reasonable value.

    Regards,
    Akshay