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TPS54240 recommendation for RT/CLK pin setting

Ho Cheol Song
Prodigy 130 points
Other Parts Discussed in Thread: TPS54240, TPS54260

Dear Team,

I have one question about RT/CLK pin of TPS54240.

Our customer wants to use both 400KHz and 700KHz switching frequency to avoid RF interference from DCDC switching noise.

So adding the below circuit at RT/CLK pin. (Reference resister value will be changed by FET ON/OFF condition.)

 

But This circuit has some problem. At FET turn-on condition, It makes DCDC output voltage unstable such as jitter. As a result of verification, the voltage of RT/CLK pin is unstable. So we add the CAP to make it stable as the below circuit.

My question is whether this circuit can occur some problem or not. If this circuit has any problem, please let me know it.

Also if you have other solutions to solve our issue, please advise me what I should do.

Thanks and regards.

  • 0
    TI__Guru**** 191445 points

    The RT/CLK connection is sensitive.  We recommend tightly coupling the RT resistor to RT/CLK and GND close to the device.  So I am not surprised that you are having difficulties.  We have not tried any type scheme that you propose, so i do not know if there are any additional issues.  What is the value of the capacitor?

  • 0 in reply to JohnTucker
    Prodigy 130 points

    The value of CAP is 102~103(1nF~10nF).

    If any idea, please let me know it.

    Thanks.

  • 0 in reply to Ho Cheol Song
    TI__Guru**** 191445 points

    We are going to run a simulation to see if there are any issues. 

  • 0 in reply to JohnTucker
    Prodigy 40 points

    The data sheet for the TPS54260 indicates that the RT pin will have a DC voltage on it, so I wasn't careful with it, and my resistor is about 8 mm from the pin. Unfortunately, the pin is insanely sensitive to noise. The part becomes unstable if I bring my finger near (10 mm) the RT resistor and as I increase load current, the part seems to pop back and forth between RT clocking and syncing to it's own switching noise.

    On my layout, I could weasel a small capacitor from the RT pin to the pad copper sneaking out from under the part. I tried 0.015uF piggybacked on the RT resistor, but the regulator really didn't like this. How much capacitance can I hang on the RT pin to ground before the internal amplifier driving the pin goes coo-coo?

    Had the data sheet mentioned sensitivity of the RT pin, I would have put the resistor right there with no effort. The IC ground is, of course, on the opposite corner of the package. Is the ground pin of the package tied to the pad internally?

    Thanks