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MC33063A: Frequency change in short circuit

Part Number: MC33063A

Hi team,

My customer designing a buck converter on MC33063A. Schematic i like this:

But the customer foun strange bevaior: IC usually have freqency about 73kHz, but at short circuit event, the frequency rises up to 147kHz.

Can you please explay why this happens?

There are scopes before a short circuit and during the short circuit.

Blue - voltage on Ct.

Yellow - voltage on pin 2.

  • Hi,

    at normal operating condition, the Ton is approximately 6 time of the Toff. the charging current of the Ct is 1/6 of the discharge current.

    in the short condition, the charging current increases to reduce the Ton, but the discharge current is unchanged. i think this is why the frequency increase.