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MC33063A Component Question

Phillip Mobley
Prodigy 20 points

Good afternoon everyone,

I am building a DC/DC converter which will take +12V and convert to +24V. Looking at the datasheet, I see that in Figure 6, there is a resistor labeled Rsc = 0.22 ohms. In my application, I am using a 0.24 ohm resistor because it is more commonly available.

My question is this, when I pick out the component, what should I expect the power through this resistor be? For example, should I get a 6 W resistor?

Thank you for your time and I hope someone will reply soon.

  • 0
    TI__Guru**** 191445 points

    That resistor is used to sense your peak switch current.  W = Isw(RMS)^2 * Rsc.  For 1.5 A peak switch current 1 W should be plenty safe.

  • 0 in reply to JohnTucker
    Prodigy 20 points

    Thank you very much. Can you point in the datasheet (or app notes) where it says that? I was looking for something like that yesterday but I couldn't turn up anything.

    Also, thank you for moving the topic to a more appropriate board.

  • 0 in reply to Phillip Mobley
    TI__Guru**** 191445 points

    The datasheet does not really give much information.  But as a long time power apps engineer, I was able to figure it out.  If you look at the table on page 10, you can see the Rsc = 0.3 V / Ipk.  Internal logic will reset the R input in the PWM latch when the voltage drop across Rsc reaches 300 mV to act as current detection.  The absolute max switch current is 1.5 A so you should size Rsc to limit switch current below that (unless you are using an external switch).