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TL2845B: the frequency will dither but it's a constant frequency device

Howard Zou
Genius 12365 points
Part Number: TL2845B

Hi, my customer is using TL2845B in a flyback converter, they will observe the device RC frequency dither at after start-up, stable working, and before shut down. The output load is constant, and the output voltage is still stable. The frequency dither doesn't bring any side effect to the system, but it's hard to explain.

Start-up: YELLOW- VCC, GREEN- RT/CT.

stable working: yellow- RT/CT, green- OUTPUT

before shut down: green- VCC, yellow- RT/CT.

  • 0
    TI__Mastermind 31580 points

    Hi Howard,

    The frequency of the TL2845B should be constant and it should not vary.

    I suggest the customer take the waveforms at a faster timebase like 2 us or 5us a division and see how much the frequency is changing by. The main suspect is that some form of noise is is corrupting the signal on the RT/CT pin and causing the frequency to change.

    Can you ask the customer to check the layout to ensure the RT and CT are placed close to the IC Pin and that their ground trace goes straight back to the GND pin of the IC.

    Regards

    Peter