Hello,
Regarding to the Ct threshold on MC33063A, my customer is asking a question..
They use it as boost and Ct voltage is exceeded threshold(1.25V) at peak current.
According to thread in E2E, about Ct threshold is described as following.
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.
Also, other thread in E2E, it is is described as following.
If the current exceeds the desired maximum, then the CT charging current will increase, which will quickly end the current Ton pulse (turn off the switch).
In the Boost configuration, the R sense resistor will always be able to sense the inductor current, so if the current limit is exceeded then no Ton pulse will occur. If no Ton pulse occurs, then the switch will not turn on and the Inductor will continue to draw current. In this case the output current will not be maintained but will drop (as the inductor discharges) until the R sense current is no longer exceeded, at which point the Ton pulses will resume.
In this case(boost), which thread should I refer about behavior of exceeding Ct threshold?
Regards,
Tao2199
