Jecksons Ben,

If you try to build this from your drawing you have some big problems ahead:

1) The MC34068A part is being applied in a manner where the switching element onboard the IC is being used. When in this configuration the device can be used to make a converter where it switches up to 1.5A maximum. To get higher current capability it is necessary to use an external switching transistor that has a higher rating.

2) These parts use a bipolar transistor technology and the output switch has a saturation voltage of 1.4 to 1.6 volts. This has a huge impact on the power dissipation in the device package. Note that this high VCE is used to keep the transistor from going into hard saturation which would can shorten the OFF time too much at higher switching frequencies.

3) The output current from the regulator will be limited in large part due to thermal limitation of the part. The max rating is 1W for the DIP package so if no heat sink is used on the part there has to be an appropriate derating in maximum current output to keep the chip from burning itself up.

4) <s>/A /f/a/c/t/o/r /c/o/n/t/r/i/b/u/t/i/n/g /t/o /t/h/e /p/o/w/e/r /b/e/i/n/g /d/i/s/s/i/p/a/t/e/d /i/n /t/h/e /p/a/r/t /i/s /t/h/e /m/a/g/n/i/t/u/d/e /o/f /t/h/e/ /i/n/p/u/t /v/o/l/t/a/g/e/ /u/s/e/d/. /Y/o/u/r/ /p/r/o/p/o/s/a/l /t/o /r/a/i/s/e /t/h/i/s /b/y /a/l/m/o/s/t /t/h/r/e/e /t/i/m/e/s /b/r/i/n/g/s /a /r/i/s/k /o/f /t/h/e/r/m/a/l /o/v/e/r/l/o/a/d /t/o /t/h/e /d/e/s/i/g/n/.</s> I had to strike this because of my initial confusion of just that the exact circuit topology actually was.

5) You are missing the pin 4 connection to the MC34063A which is the chip GND connection.

6) The comparator input divider resistors you show at 22K / 2.5K. These values would produce an output of 12.25 volts. Not the 33V output that you propose.

7) I would strongly advise you to label what is the input and what is the output terminals of the circuit.

8) It is conventional to draw circuits with the inputs on the left and the outputs on the right. You have this flipped around backwards leading to confusion as to what the circuit topology actually is.

9) It would be foolhardy to try to build this circuit without any input capacitance. Switchers need suitable capacitance at the input to be able to supply the high current surges required to charge the inductor.

10) It is really silly to use standard silicon diodes as the catch component in a switcher design. You should use a good Schottky type so that that the design can be as efficient as possible. I know it is common to see cheap converters that use this chip to apply equally cheap standard silicon diodes. The loss of efficiency is passed off as acceptable when you consider how crappy these parts are from an efficiency standpoint due to the 1.5V saturation voltage of the on-board switch transistor.

11) The part uses the series resistor that feeds from the input voltage rail to the inductor as the means to measure the peak input current. The value shown as 0.5 ohms limits the maximum input current at 0.6A. This may not be at all suitable to the design intent.

12) Switchers of this type should usually have an additional series inductor at the output feeding into another downwind filter capacitor. This reduces ripple and switching noise in the output. The component values should be computed based upon the switching frequency of the regulator.

13) Forgive me for saying this so bluntly but it is ultimately stupid to switch up to 33V from a 5V source to then turn around and use a heat generating series regulator to step back to down to +12V. A far more sane approach is to build two step-up converters, one from 5V -> 12V and the other from 5V -> 33V.

Michael Karas