High current MC34063 step down converter

[boylesg]

Does anyone have a specific example of this?

I am looking at the example in the datasheet but there are no specifics regarding the transistor:

Do you do the MC34063 calculation for base current of your selected transistor at saturation?

Usually the base to GND resistor is 10x the value of the series base resistor. But in this case there is no series base resistor so what value do I multiply by 10?

Is the output capacitor in the NPN schematic the same as Co in the non boosted schematic? Or, in the NPN boosted schematic, is Co unnecessary and C is determined by a different calculation?

 

[ud23]

http://www.intusoft.com/onsemipdfs/AN920-D.pdf

 

[boylesg]

http://www.intusoft.com/onsemipdfs/AN920-D.pdf

I found that but none of these circuits match the one in the datasheet with the transistor base on pin 2.

 

[wade_hassler]

Value of Co is determined by the ripple-voltage you want on the output, not the type/topology of switch.
The NPN's base-pull-down resistor is high enough to not affect rise-time and low enough to shut the switch off decisively.
(what a weasel)

 

[boylesg]

Value of Co is determined by the ripple-voltage you want on the output, not the type/topology of switch.
The NPN's base-pull-down resistor is high enough to not affect rise-time and low enough to shut the switch off decisively.
(what a weasel)

Oh well they make it confusing by referring to the capacitor as Co in one schematic and just C in the other. To my inexperienced eyes it looks as though they are different capacitors altogether.

Any hints om how to calculate the value of the resistor or what it generally should be?

 

[wade_hassler]

Oh well they make it confusing by referring to the capacitor as Co in one schematic and just C in the other. To my inexperienced eyes it looks as though they are different capacitors altogether.

Any hints om how to calculate the value of the resistor or what it generally should be?

... 10K to 100K? Must admit I never think too hard on this until/unless something goes wrong.