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MC33063A: Vout - a little higher

Andres Gomez
Intellectual 430 points
Part Number: MC33063A

Tool/software:

Hi.

I have a question about the MC33063A converter. We designed a buck converter 3V3@60mA and it worked well. However, we want to use the same controller to generate 3V3@150mA in the output. Our parameters is Vin = 9-30V, Iout = 150mA. Vout = 3.3V, Vout_ripple = 1% (33mV) and Fsw = 50Khz.

The voltage across the diode when is ON Vf = 0.5V

Following this table

Ipeak = 0.3mA => Rcs = 1ohms

Ton/Toff(Vout+Vf)/(Vi_min - Vf - Vout) = (3.3+0.5)/(9-0.5-3.3) = 0.778

Ton + Toff = 1/50khz => Toff = 11.25uS and Ton = 8.75uS, which is Ton_max as I am using Vi_min

Ct  = 350pF

Lmin = (Vin_min-Vsat-Vout)*Ton_max/(Ipk)=(9V-0.82V-3.3V)*8.75uS/0.3A = 143uH. L= 160uH

Co = Ipk/((Fsw)*(8Vripple)) = 0.3/((50khz)(8*33mV) = 22.75uF => Co = 22uF

R2 = 18.25K, R1 = 11K => Vout = 3.323

In the PSPICE simulation and the design test, we are having the expected outcome at full load Iout = 150mA

however, the issue is with a light load (3.3mA), the voltage is rise upto 3.41V. Is there something wrong with our calculation?

  • +1
    TI__Guru 65170 points

    Hello Andres Gomez,

    Every converter has a minimum on-time, so it seems that in your design, the energy that the converter delivers to the output at minimum on-time is more than needed during the low output current time, so the IC will increase the output voltage until it reaches the time when it stops switching. I recommend to check the output voltage and the switch node with an oscilloscope to get a better understanding what is going on.

    If your system is not ok with this voltage rise, you might need to add some dummy load.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Intellectual 430 points

     Switching node, does it mean Switching emitter pin?

    Brigitte said:
    I recommend to check the output voltage and the switch node with an oscilloscope to get a better understanding what is going on.

    Our system can survive with these values as it will power a MCU and some optocouplers. But I was wondering why I was getting 2.4% higher. 

  • 0 in reply to Andres Gomez
    TI__Guru 65170 points

    Hello Andres Gomez,

    The switch node is the connection of the transistor, diode and inductor. It depends on the usage of an external transistor, which exact point it is in the schematic.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Intellectual 430 points

    Hi Brigitte. 

    I could see that Ton was a little higher than our system needed. By dropping Ct from 220pf to 85pF, I reduced Vout error. Another question, do you know how I calculate Ton_max, because DS shows than Ton is set by Ct and it is constant. Or Ton_max = Vout*Ts/Vin_min

  • 0 in reply to Andres Gomez
    TI__Guru 65170 points

    Hello Andres,

    The Ton_max should be the on calculated when using Ct, but as far as I know, it is not very accurate.

    Best regards,
    Brigitte