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MC34063A: Output Current Calculation

TI-CSC
Genius 11120 points
Part Number: MC34063A
Other Parts Discussed in Thread: LM5155, LM5156, LM5158, LM5157

Hello E2E Experts,

Good day.

I am trying to design a circuit using the MC34063A. I need a 5V, 1.5A output using between 7-26V input. I am not good at this sort of thing and I don't quite get the datasheet. I was hoping for assistance in selecting values. due to the chip shortage, a regulator we currently use is not available. I am looking to design a sort of drop-in substitute.

I calculated R1 and R2 easily enough, R1 being 1.2K and R2 being 3.6 or 3.8K, but I don't understand what is determining the output current. The inductor, caps, and output current values are kinda where I am stuck.

I don't know if I need 1.5A, exactly, that's just what the current one does. I know I need 500mA minimum upwards to 1.5A max. If this works, we will probably need a version that's 3.3V out too, but I assume not much beyond R1 and R2 will change.

I need assistance understanding it. I already have the datasheets. I found a bunch of online calculators, but none of them show me how the output current is configured.

Thank you in advance for your support.

Regards,

CSC

  • 0
    TI__Mastermind 49361 points

    Hi CSC,

    thank you for using the E2E forum.

    The output current is typically not calculated as this is a parameter which needs to be provided as spec parameter for the power supply.

    Then the power supply is designed to be able to provide this current. This highly influences the Inductor and power switch selection.

    Here I would like to recommend using one of the LM515x devices (LM5155/LM5156 with external Switch or LM5157/LM5158 with internal Switch)

    For these devices there are also the Excel QuickStart calculators available (can be found on the product page) which helps you to dimension the external components.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Genius 11120 points

    Hello Stefan,

    Thank you for the response

    I calculate Rsense to be either 0.22 for 1.5A or 0.33 for 1A.

    How do I determine Vsat? Vout will be 5V, Vin will be a minimum of 7V (upwards exceeding 24V) and Vf is 0.55V if I use a 1N5819 as suggested.

    Another question is that I need the regulator to supply a minimum of 1A for some applications. Is this even possible? I tried using an online calculator and it said the Ipk switch current exceeded the limitations. Because the peak current is 2 times the output current. So is the maximum actual output current limited to only 750mA?

    Regards,

    CSC

  • 0 in reply to TI-CSC
    TI__Mastermind 49361 points

    HI CSC,

    For that I recommend to use the Power Stage Designer 

    POWERSTAGE-DESIGNER Design tool | TI.com

    Entering the parameter for the power stage and then clicking on the inductor shows the current wave form.

    The saturation current of the selected inductor should be well above the max current. Also take care that the saturation current in the inductor Datasheet are often given where the inductance dropped already by 30%. So it is a good practice to stay then 30% below that current.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Genius 11120 points

    Hello Stefan,

    Do the values I have listed even work with this device? This tool doesn't take the device into account.

    Regards,

    CSC

  • +1 in reply to TI-CSC
    TI__Mastermind 49361 points

    Hi,

    the power stage designer only considers certain power topologies but does not consider dedicated devices.

    So yes, this is generic tool and this also works for this device.

    Best regards, 

     Stefan

  • +1 in reply to Stefan Schauer
    TI__Mastermind 49361 points

    Hi ,

    I have not seen an update for the last 9 days, so I assume the questions are/or answered and the issue is solved.
    I close this thread now. If there is still something open reply and the thread will get open again.
    If you have any other question or of the thread gets locked, please open a new one.

    Clicking the Resolved Button also helps us to maintain this forum.

    Best regards,
    Stefan