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CSD87350Q5D: Switching frequency

Part Number: CSD87350Q5D

Hi,

I find recommended switch frequency should more than 200KHz in datasheet. 

But customer want to know if they can choose switch frequency at 20KHz, what will happen? And why?

Thanks a lot!

  • Hi Rock,
    Thanks for your questions. You are correct. The minimum recommended switching frequency in the datasheet is 200kHz. However, it should be OK to operate at lower switching frequency, such as 20kHz. Is this for a motor drive application or a buck converter? Please keep in mind, that motor drive applications typically operate around 50% duty cycle. This power block has been optimized for low duty cycle, synchronous buck converter applications meaning the FETs are sized differently (small high side & large low side). Another thing to consider is the bootstrap circuit. Typically, the bootstrap capacitor is 0.1uF with an optional, low-value series resistor (<10ohms). You may need to increase the value of the bootstrap capacitor for lower frequency operation and take care at startup to insure it has adequate time to charge.
  • Hi John,

    Thanks a lot for your reply.

    And what does "FETs are sized differently (small high side & large low side)" mean?

    Best Regard,

    Rock

  • Hi Rock,
    In a low duty cycle (10% - 20%) buck converter, the high side MOSFET is on for a very short (10% - 20%) portion of the switching period while the low side MOSFET is on for the remainder (80% - 90%) of the switching period. Because of this, switching loss is dominant in the high side FET and conduction loss is dominant in the low side FET. Therefore, the die size for the high side FET is relatively small to minimize charge and switching loss since rds(on) is not as critical. Likewise, the die size for the low side FET is larger to minimize rds(on) and conduction loss. For an application operating around 50% duty cycle, the FETs would be symmetric with the same rds(on) and gate charge. Let me know if you have any additional questions.