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MC34063A: Select inductor for wide range of output current + can the circut work stable over this range?

Sebastian Attinger
Prodigy 10 points
Part Number: MC34063A

Dear TI,

We use the MC34063A for several products and we are fairly satisfied.
Now we try to design the MC34063A in an application where the drawn current has a wide range from 10 mA to 160 mA.
The calculation results have an extreme large range of inductivity from 210 µH at 160 mA to 3290 µH at 10 mA.
Which value is the correct value to use?
Can the circuit work stable over this wide range?

  • 0
    TI__Genius 16135 points

    Hello Sebastian,

    Thanks for reaching out to us via e2e.

    The first thing to consider is the load profile:
    Which current will be drawn most of the time / which is the exception?
    How quickly will the load change?
    Further considerations:
    How quickly will the converter need to follow the change in the load?
    Which maximum ripple would be allowed for which load?

    If you choose a bigger inductor, the ripple will be smaller, but the converter will only react slowly on a load change.
    And the other way round.

    In case of the higher ripple, the peak inductor current will be higher.
    So please make sure to leave enough margin, so that the inductor will not go into satturration - across the whole temperature range.

    Space and weight may also be an argument to consider.

    You will need to find an inductor somewhere in the middle, but optimized towards the most important criteria, as mentioned above.

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    Prodigy 10 points

    Hello Harry,

    > Which current will be drawn most of the time / which is the exception? How quickly will the converter need to follow the change in the load? Which maximum ripple would be allowed for which load?

    Both load profiles are relevant, because when the application is in "standby" there is only a little current consumption, and when the application is fully active, the high load is drawn.

    I have to add following information: The 6V output voltage is not used directly, there are two linear regulators for 5V and for 3,3V behind the switching regulator to supply the µC and additional hardware. These regulators should reduce the voltage ripple.

    > In case of the higher ripple, the peak inductor current will be higher. So please make sure to leave enough margin, so that the inductor will not go into satturration - across the whole temperature range.

    The selected inductor has the inductance value which is calculated for the high current, and the peak current has 1/3 reserve.

    I tested the application with the small inductance value and with a current between 0 mA and 400 mA. (electronic load)

    At 0mA the ripple has about 460 mV and at 400 mA about 240 mV. The output voltage is stable with just 0,1V change,

    So the ripple is higher than expected but relatively steady. Maybe the standard electrolytic capacitors are the reason for that and lowESR would be better.

  • +1 in reply to Sebastian Attinger
    TI__Genius 16135 points

    Hello Sebastian,

    So basically both cases are uncritical in terms of ripple because there are anyway the linear regulators in front of the load. 

    The MC34063A is a very simple part, so you cannot select any light load behaviors.

    To minimize the ripple, you can increase the inductor as much as possible (space, cost), but please make sure that the booster can still follow the load transient, if this is required.  A bigger inductor will make the reaction on a transitions slower.

    I am not convinced that low ESR capacitors will help to reduce the ripple in this cane.

    Best regards,
    Harry