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MC34063A: Datasheet clarification

Devon Curry
Prodigy 470 points
Part Number: MC34063A
Other Parts Discussed in Thread: MC33063A

I am in need of clarification, the datasheet states on page 11 figure 12 shows a different value for the input than listed in the power supply recommendations

  • 0
    TI__Mastermind 21720 points

    Hi Devon,

    Figure 12 of the datasheet is a layout recommendation (pg17). Can you please attach an image of the figure you are referring to?

    Here is the link to the datasheet that I am looking at.


    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    TI__Prodigy 470 points
    That is the image, the customer wants to know why the recommendations says 1000uF but in the layout it uses a 100uF.
  • 0 in reply to Devon Curry
    TI__Mastermind 21720 points
    Devon,

    The input capacitance is going to depend on the design parameters such as input voltage, current and topology selection (either buck or boost). Figure 12 is just depicting the a reasonable layout for the generic MC34063A design in a buck converter topology.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 20 points

    Garrett,

    The design is configured as a buck converter. 

    The end-use of the device MC33063A is as follows:
    1) input supply range from 9V to 30V
    2) output is 5V
    3) load is up to 700mA, with 500mA typical.

    What is the recommended input side decoupling capacitance? 300uF appears to be adequate. 100uF is not enough, too much back emf is getting to the PS.
    What is the recommended output load capacitance? 440uF is on output. There is still some ac ripple (150mv) on the output and is also not filtering HF noise being coupled from another switching circuit in the system.
    Please get back to me as soon as possible.

    Thanks,
    Clinton

  • 0 in reply to Clinton Hughes
    TI__Mastermind 21720 points

    Clinton,

    In a buck converter the input capacitance must be larger when compared to a boost because the input current is discontinuous. Based on your application 300uF seems reasonable. If you are worried about conducted EMI the capacitor should be larger, or an EMI filter can be added to help reduce high frequency EMI. Here is an article about designing an EMI filter.

    The output capacitance of the buck converter is based on the amount inductor current ripple and the amount of allowable output voltage ripple. This is really independent of IC selection and is based on the design parameters. This application note will walk a designer through some of the more common calculations for a buck converter. 400uF seems like it should be adequate for this application. HF frequency noise can be eliminated by adding ceramic capacitors to the output and can also be impacted by board layout and how the signal is probed. Here is blog that walks though some of the steps for reducing high frequency output noise.

    Thanks,

    Garrett