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Old 11-07-2009, 04:16 PM
Piro Piro is offline
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Default Step-down switching regulator 12v to 3.3v

Hello, I am working on a project. And part of it concerns the switching regulator.

I am using the MC34063 from OnSemiconductors.

I plan to use it as a step down converter, 12v to 3.3v with a 200mA output current, so I can power up a microcontroller and other IC's but I have a hard time finding out some of the component values...

I tried using the calculator from the following website,but there are certain values such as the voltage ripple and Minimun frequencies which confuse me a little bit.

I am using the following schematic design (this can be seen in their datasheet)

the values in the schematic are for a 25v input to 5v output (500mA)
I tried adding different values which I calculated from the given formulas in the datasheet and even tried using the calculator, but when I simulated these, it did not work...

I aslo added a switch to allow me or any other use to swtch the microcontroller on and off.

If it helps anyone, I am using the Pic24fj256ga110 from microchip ( the switching regulator will be connected to this u-controller).

Thank you in advance for any advice and help.
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Old 11-07-2009, 04:55 PM
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SgtWookie SgtWookie is offline
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OK, here's the datasheet:
Look on page 7, figure 11, for the basic step-down converter schematic.
Without the optional filter, you'll have around 120mV p-p ripple.
With the optional filter, you'll have around 40mV ripple.
Vout = 1.25(1+ R2/R1)
If you use 910 Ohms for R1 (your R9) and 1.5k Ohms for R2 (your R10), you should get about 3.31v output, with ripple on it; 120mV p-p without the extra filter, 40mV with it.

I don't know what you're trying to do with R44 and SW1, but they won't do much of anything.

Your C12 is far too large; it needs to be 470pF, not 470uF. This is the timing cap for the internal oscillator. With a cap that large, the output would look like a square wave rather than DC with ripple. 470uF = 470,000,000pF

Your R8 is a bit large, too. Reduce it to 0.22 Ohms. 0.75 Ohms will allow for up to 400mA switching current, but 0.22 Ohms will get your C10 charged much more quickly at the cost of somewhat more ripple voltage. Somewhere between 0.22 Ohms and 0.75 Ohms you'll find a happy medium between ripple on the output, and rapid charging of C10.

Your C10 looks like it is 100pF; it should be 100uF instead. Try to place components so that the value fields don't have wires running through them; it makes it difficult to see what the values are. 100pF = 0.0001uF
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.

Last edited by SgtWookie; 11-07-2009 at 06:36 PM.
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Old 11-09-2009, 05:38 PM
Piro Piro is offline
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Join Date: Oct 2009
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Thank you SgtWookie.
It makes much more sence now.
The simulation works and now I just need to order the capacitors that are missing

And I will remember to spread the components so it will be easier to see the values for the next time
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