What formula of Short-Circuit Current Limiting of MC34063A?

Bob,

The MC34063 current limit only limits the switch current and it has to turn the switch on to check the current. Therefore there will be a small on time regardless of the inductor current. With a short circuit on the output, the coil discharges into a very low voltage so it does lose much current during the switch off time. This can lead to higher then expected inductor and output current especially if the timing capacitance is low. 

Dear Ronald Michallick,

Thank you very much for answer.

I will change the question because question did not communicated.

I use Fig11(b) schematic.
When Short-Circuit Current Limiting?

Can use Rsc=0.3/Ipk(switch)?

Regard.

Bob Lee

Bob,

Yes, that is the correct formula.

Dear Ronald Michallick,

Thank you very much for your answer.

I measured switch peak current.

There was a switch peak current difference in the measurement and formula.
How much margin need to formula?

Regard.

Bob Lee

Bob, 

The data sheet has a range for the "0.3V" (in the formula), it can be as low as 0.25V and as high as 0.35V.
Once the programmed current level is reached the internal comparator sets the switch off, but there is some propagation delay. During this delay switch peak will continue to increase. Normally the increase is small.

Dear Ronald Michallick,

Thank you very much for answer.

Current limit sense voltage 0.35 and Rsc (Rsc×0.95%) were calculation.

But difference in the measurement and formula.

How much propagation delay?

Could you teach propagation delay?

Regard.

Bob Lee

Bob,

Propagation delay depends on a few things with the most important being CT capacitance.

Note that the current limit controls the oscillator, when the current limit is reached, the oscillator will charge the capacitor much faster to end the cycle early. 

Measure the time between pin 7 crossing VCC - 300mV and the output switch turning off. This is the propagation time for the application.

Dear Ronald Michallick,

Thank you very much for your answer.

Regard.

Bob Lee