MC33063 buck 5V to 3.3V

See page 8 for calculations.
Your output should be 3,25V with the given resistors.

Try to increase the inductance of you inductor to e.g. 68uH.

http://www.ti.com/lit/ds/symlink/mc33063a.pdf  

Otherwise use the LM2674-3.3 from National/TI.
As I recall I use 68uH for 3.3V output from 11V in. 

http://www.ti.com/lit/ds/symlink/lm2674.pdf

Thanks for response.

I will test for 67uH.

I calcucated refering to data sheet.

It seems to be OK for component value as the above schematic attachedPlease advise me about what's wrong about my calculation refering to attached excel sheet.

You could see calculation equation when click each cell.

except Vsat is unclear.

And please let me know the value of Vsat about attached schematic.

Best regards.

SunSUng

4377.LM33063Calc.xls

1234.MBRS2040LT3-D.pdf

SunSung,

The peak current of the MC33063A is 1.5A, so the output current must be less than 1.5A
TI has other buck regulator with a higher peak current rating.

Regards,
Ron Michallick

Thanks for response

I recalucated components value. for 0.5A output current.

Please review whether or not my calculation is correct.

4300.LM33063Calc_Test.xls

Regards,

SunSung

SunSung, although the MC33063 is an active product, I hesitate to recommend it for your application.  Texas Instruments has many newer products that provide superior performance.  TI's Power Quick Search tool or WEBENCH tool are great places to select a power solution.  If you choose to use the MC33063, it will be important that you build and test your solution to validate your design.  If you choose to continue with the MC33063, I would be interested to understand your system requirements that result in the need to use this IC.

SunSung,

While looking at your values, I noticed a serious problem. Not in your values, but in the MC33063A drive circuit.

The MC33063A has a Darlington NPN output that can drop almost 2V in certain conditions.
Your input is 5V, possibly lower with tolerances. The output needs to be 3.3V. So the nominal difference is just 1.7V

You either need to use a buck regulator with a much lower voltage loss to get good efficiency and current drive or you can use a linear regulator and get up to 66% efficiency.

Sorry that I did not notice this sooner.

Ron Michallick

I want 3.3V output (@300mA current) from 4.25V, using MC33063a.

Sandeep,

The TPS62237 or TPS62007 are better solutions with fewer external components.
To use the MC34063 would require an external low loss switch..

Thanks Ronald!

But my input voltage range is from 4.25V to 28V and output voltage is 3.3V. So what is the suggestion?

Sandeep,

Generated from Webench on main ti.com page.

Schematic

BOM