MC33063A: What headroom voltage is possible for MC33063A?

Thank you Jasper!

Hi Jasper,

Actually I have a couple follow-up questions for you. Based on the step-up converter design guide on page 13/14 of the datasheet,

• What does Vsat represent? I have it as 0.7V but I don't understand what saturation of the output switch means in this context.
• If I want a maximum current output of 500mA, will I need to use that value for Ipk? Or would this be Iout(max)?
• Will Ipk flow through Rsc and L? Therefore, I should rate those components for that max current?

Thanks!
Lauren

Hi Lauren,

1. it is voltage drop of the NPN transistor. it is Ok to set it to 0.7V.

2. I would suggest setting 1A Ipk.

3. yes, the inductor current should be higher than 1.5A.

the MC device is very old. and the performance is not good. could you suggest other solution? we can suggest TPS61170 with lower than 18V voltage to the VIN pin. 

Hi Jasper,

Thank you for your reply!

The TPS6110 but that has a maximum input voltage of 18V, they have an input voltage of 24V so I don't think that will work for this application. 

Based on the step-up design, it looks like they are supposed to set Iout which then Ipk is calculated from. I might be misunderstanding. Do they set Iout to be 1A and then calculate Ipk from that, or are you saying to set Ipk to 1A knowing that they want a max output current of 500mA, then calculate Rsc from 1pk being 1A?

Thanks!
Lauren

Hi Lauren,

the TPS61170 can be used to we limit the voltage at VIN pin (note: not the 24V power supply) below 18V. that is why we can add a resistor and zener to generate the 12V for the VIN pin. refer to this link to understand what I mean "http://www.ti.com/lit/an/slyt556/slyt556.pdf "

please calculate the I peak based on the suggestion in the datasheet, then select the Rsc according to the calculation.

Hi Jasper,

They are open to switching to the TPS61170. Is there a reference design or schematic for a 24 Vin to 28 Vout boost circuit we can provide them?

Thanks,

Lauren 

Thank you Jasper!