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UC3843A: UC3843A output frequency is different from Oscillator frequency

Part Number: UC3843A

Dear TI

UC3843A is used as flyback, the output voltage is non-isolation, input :11~120VDC

At light load ,increaing input voltage, the output PWM frequency is decreased, less than Oscillator frequency .

for example, the input voltage 60VDC, the PWM frequecy 40KHz ,Oscillator setting frequency is 160KHz, but the output voltage is stable

I want to know why caue this, is the operation mode normal ?