• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 547 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MC33063ADR Component Switching For Boost Configuration

David Munzer
Prodigy 70 points
Other Parts Discussed in Thread: MC33063A

For the MC33063ADR, we have a lot more questions. We want a Vout of 9V, so based on the equation provided R2/R1 = 6.2. However, we were not sure what range of values to use. Going off of the application schematic, we were assuming between 2-100k ranges. Based off of that, an early guess would be R1= 2.2 kΩ and R2=13.6 kΩ. Another issue was how to properly to determine Vf and Vsat. Lastly, we weren’t sure what would be good values for the minimum switching frequency or the ripple voltage. Obviously, the ripple should ideally be low so we must use a bigger capacitor for Co. However, Ct, Lmin, and ton are all dependent on the switching frequency and their function in the circuit is harder to predict, making it harder to choose values for them. Lastly, we wanted to check the purpose of the optional LC filter; we are not sure if it is present to further reduce ripple and noise or if it serves any other purpose.

  • 0
    TI__Mastermind 24975 points
    David,

    Will get back to you soon. I apologize for the delay.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    TI__Mastermind 24975 points

    Hey David, 

    First of all, you didn't mention what topology you will be using, either step-up (boost) or step-down (buck). Im sure you know, but I couldn't figure out which you were doing based on your post.

    Please find my answers to your questions below:

    1. We are not sure what range of values to use for resistors.

    Your guess is appropriate. Resistors in that range are acceptable. Therefor R1 = 2.2k and R1 = 13.6k will work fine.

    2. How to determine Vf and Vsat.

    Vf is the forward voltage of the schottky diode. The actual forward voltage will depend on what schottky diode you choose, but will typically be in the range of 0.2 - 0.4V. Just look at the datasheet for the 1N5819 for this information if you plan on using that device.

    Vsat is the saturation voltage of the Q1 transistor within the MC33063A device. This information is contained in section 7.6 Electrical Characteristics - Output Switch. This Vsat voltage will depend on the configuration you use. If you are doing a buck converter, then you will need to look at the Vsat for the Darlington connection. If you are doing a boost converter, then you will need to look at the Vsat for the non-darlington connection. Using the typical value for the calculations will do.

    3. Not sure of good values for minimum switching frequency or ripple voltage.

    The short answer is - Well, it depends on what you want for your system, its really up to you. What ripple voltage do you find acceptable.

    The longer answer is - A higher switching frequency allows you to use smaller inductors, thereby reducing board space. Consequently, a higher switching frequency can lead to higher switching losses, thereby decreasing the efficiency of your converter. As for ripple voltage, its up to you to say. Maybe 50mV or 100mV is acceptable. As you mentioned you will need a larger output capacitor to reduce the ripple.

    4. Components are all dependent on switching frequency, so other components are harder to predict.

    You have to make a decision up front of what switching frequency you want, what output voltage you want, what output current you want to be able to provide, and what output ripple is acceptable.

    Maybe a good starting point is 50kHz, 9V, 150mA, and 50mV. 

    The calculations in the datasheet are setup so that you go in order from top to bottom, and this makes it pretty easy to calculate through all of the values. 

    You should pick something, calculate the values, see if the component values are acceptable, and tweak accordingly.

    5. Purpose of the LC filter

    It is by no means required, but will help to further reduce ripple on the output. Simple as that.

    Final Comments:

    Fortunately, for quick prototyping, this device comes in PDIP package. So my suggestion would be that you just grab one of these and a breadboard as well as other components and start with a design. You can tweak the values really quickly on the breadboard and that will help you to see how the design changes with small changes in components. Its pretty easy to put a quick design with this chip together on a breadboard.

    I hope this helps! Let me know if you have any other questions.

    Best,
    Michael