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TPS548A20: PWM Frequency Selection

KIM Andy
Expert 2530 points
Part Number: TPS548A20


The customer applied TPS548A20RVER to configure 12V-> 1.2V.
On the schematic, Rmode is connected to PGOOD PIN and 150Kohm and is set to FCCM Mode.
The customer said there was a problem with the switching frequency.


The customer's opinion is that the switching frequency is from 395kHz to 437kHz as shown in the figure below.
But it seems to me that the frequency does not move much. I think it seems to be a measurement problem.
I talked about trigger condition changes and probe measurements.
But customers are saying the same result.
What do you think about the content?

  • 0
    TI__Expert 5910 points
    Kim,

    i think it looks okay and it maybe just the effect of the jitter that you see and the single sequence capturing different switching frequencies.

    Regards,
    Mathew
  • 0 in reply to Mathew Jacob1
    TI__Guru**** 191445 points
    Kim,

    TPS548A20 uses DCAP3 control. For DCAP3, there is no internal oscillator. DCAP3 uses adaptive constant on time and hysteretic off time to approximate a constant switching frequency. For normal PWM jitter you will see a fixed rising edge and some variability of the falling edge of the switch node waveform. For DCAP3 you can see variability in both the rising and falling edge of the SW node voltage as both the on time and off time can change slightly on a cycle by cycle basis. So what you observe is normal for TPS548A20.