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The circuit looks like it makes use of the driver "Q1" of the MC344063 as both a driver for the buck PNP (in this case MPS-U51A) and also as the boost "Q2" output transistor.
It shouldn't be difficult to find another low voltage high speed PNP, but I don't think that would completely solve the problem. Observe that the app note schematic uses the MPS-U51A which is already good to 2A yet the circuit surrounding it is only specified to 120mA. I recommend dong the math on the whole circuit before expecting that simply substituting a transistor will extend the circuit performance. And, the max switch current in the MC34063 is 1.5A.
The TIP42 could handle the current, but it might not be fast enough. You would be changing Rbe and Rb as well.
The circuit looks like it makes use of the driver "Q1" of the MC344063 as both a driver for the buck PNP (in this case MPS-U51A) and also as the boost "Q2" output transistor.
It shouldn't be difficult to find another low voltage high speed PNP, but I don't think that would completely solve the problem. Observe that the app note schematic uses the MPS-U51A which is already good to 2A yet the circuit surrounding it is only specified to 120mA. I recommend dong the math on the whole circuit before expecting that simply substituting a transistor will extend the circuit performance. And, the max switch current in the MC34063 is 1.5A.
Yes, I forgot to tell that I also wanted to put a PNP/P Channel MOSFET which Base/Gate would be driven by Q2 transistor (somehow like D45VH4 in figure 22, page 23).
I think you should draw us a schematic. In figure 18, Q2 is after the inductor, which is key to its boost function. You can't do that if you're using Q2 as a PNP driver as in figure 22.
Or maybe you're trying to build Figure 22 without the second output? If that's it, just remove the second winding and the 1N5819.
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