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A fuzzy area I want to clear up on the MC34063AP is Duty Cycle.

In the MC34063AP datasheet, toff an ton are a result of 1 / f, which is obvious.

toff is calculated by: 1 / f / ton / toff + 1

ton is calculated by: ( 1 / f ) - toff

The timing Capacitor CT is given by: 4.0 x 10 to -5 ton

What I can not find, and seems there is no logical direction, is what component, Resistor Capacitor or Inductor determines the Duty Cycle.

If, for example, I had an existing driver, and I wanted to modify the duty cycle, how would I go about it?

Configuration: Step Down

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Look up the AN920A/D and slva252b.pdf (Texas Instruments) for all the gory details on how this chip works. There are several free MC34063 calculator programs available that help out with the component values.

It's not a super trivial process to do by hand.

The maximum Ton:Toff ratio is 6:1, assuming no current limits are reduce that and are controlled by the charge capacitor and the chip current sources, reference sections 1.2-1.4 of the TI note.

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  • \$\begingroup\$ Make sure you test the current overload shutdown characteristics, the way the current limit works is it adds an additional charge path to the capacitor, however since this is also preset, there's no real "shut it off NOW" on this thing, and if the current is high enough it'll damage the chip before it goes into the "off" phase. \$\endgroup\$
    – user201365
    Commented Oct 22, 2018 at 2:55

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