MC33063A minimum voltage input requirement for buck converter

[seyyah]

For a 12V output with this simple switcher in buck configuration what should be the minimum input voltage and how to determine it? Thanks.

 

[FvM]

Input output difference > Output switch saturation voltage + current limiting resistor threshold, about 2 V total.

--- Updated Jan 13, 2022 ---

Correction, according to a AN920D, ton/(ton+toff) can't exceed about 0.86, this would increase the minimal input voltage for 12 V output to about 16 V.

https://www.onsemi.com/pub/Collateral/AN920-D.PDF

 

[KlausST]

Hi,

only a few people will have the complete datasheet information in mind, thus for such questions it´s always a good idea to give a link to the datasheet. https://www.onsemi.com/pdf/datasheet/mc34063a-d.pdf

Another solution is to use:
Page11 --> claculation --> Step-down --> t_on/t_off formula.

The lower the input voltage the higher t_on/t_off.
I guess the maximum duty cycle is 100%, thus you can go simple and just set the
term below the dividing line = 0

--> V_in(min) -Vsat -V_out = 0
solved to: V_in(min) = V_out + V_sat
(ignoring the voltage drop at the shunt)

Alternatives:
* Look for application notes or design notes with a V_in(min) calculation
* Maybe the manufacturer provides selection tools or calculators for the MC633063A. Play with them to find the V_in limit.


Klaus

 

[seyyah]

Input output difference > Output switch saturation voltage + current limiting resistor threshold, about 2 V total.

--- Updated Jan 13, 2022 ---

Correction, according to a AN920D, ton/(ton+toff) can't exceed about 0.86, this would increase the minimal input voltage for 12 V output to about 16 V.

https://www.onsemi.com/pub/Collateral/AN920-D.PDF

Thanks, I could not see DCMax= 0.86 information in the datasheet and my practical tests did not comply. Now it makes sense. Thanks again.