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MC34063 Ct Thresholds

Johnny Roberts
Intellectual 350 points
Other Parts Discussed in Thread: MC34063A

Hi all, I have a working inverting regulator using the MC34063 configured as shown in Figure10 of the TI datasheet.  External timing capacitor Ct should ideally charge and discharge between 0.75 and 1.25 V thresholds, yielding  a voltage delta of 0.5V but I'm seeing  it discharge all the way to 0V which gives a voltage delta of ~1.25V.  Has anyone else seen this behavior?  *the chip is actually an OnSemi variant, my apologies in advance*

My design specs are below.  Component values is in agreement with those given by the nomad design tool.

Vin = well regulated +15VDC.

Vout = -5VDC

Iout = 50mA

Fsw = 50kHz

  • 0
    TI__Guru**** 166666 points

    Hello Jdr,

    How did you determine that the timing capacitor voltage should stay between 0.75V and 1.25V?

    Regards,
    Ron Michallick

     

     

  • 0 in reply to Ron Michallick
    Intellectual 350 points

    Hi Ron, I determined this from Application of the MC34063 Switching Regulator, section 1.2:

    "The upper threshold is 1.25 V, which is same as the internal reference voltage, and the lower threshold is
    0.75 V. The oscillator runs constantly, at a pace controlled by the value of CT."

    A 0.5V swing is also used in the derivation for the CT equation:

     

  • 0 in reply to Johnny Roberts
    TI__Guru**** 166666 points

    Thanks Jdr,

    This drawing will help with my explanation. The waveform and formula are accurate but not complete. The internal oscillator threshold comparator (with hysteresis) has a propagation delay that is not part of the formula. After the CT voltage hits the threshold voltage (0.75V or 1.25V) the comparator will switch the capacitor current polarity but not until the propagation delay is complete; during this delay the capacitor will continue to charge or discharge. So in addition to the prop delay, the capacitor overshoots the threshold making the delta-V greater further slowing the oscillator. A truly accurate formula would be very complex, but figure 1 in the data sheet shows the relationship between Ct on Ton. This chart is the best method of choosing Ct; much better than the simplified formula.

    In your captured waveform, some of the Ct signal has higher peaks. This is the peak current limit (Rsc=0.3V) increasing the Ct charge rate and shortening Ton during that pulse.

    Regards,
    Ron Michallick

     

     

     

     

  • 0 in reply to Ron Michallick
    Intellectual 350 points

    Hi Ron,

    Thanks for the explanation, that makes sense.  I currently have a 220pF Ct and am seeing ~83kHz oscillator frequency.  This should be no problem because inductor selection was based on a 50kHz frequency.

    Should I be concerned with exceeding the part's maximum frequency due to charge / discharge current variance between parts?  (the variance is huge!)

  • 0 in reply to Johnny Roberts
    TI__Guru**** 166666 points

    Jdr,

    I do not foresee any problem. The most important consideration with the MC34063A is simply not to over heat it. If you have any doubts, check the top of case temperature under full load.

    Regards,
    Ron Michallick