TPSM33615: Switching Frequency and Low Load Ripple

Part Number: TPSM33615


Hello everyone,

We are currently using the TPSM33615RDNR as a step-down converter from 24 to 14 volts at low load. In our standard circuit, a jumper is provided on the RT pin to ensure that the controller runs at 2.2 MHz, as recommended.
However, this results in approximately 300 mV of ripple with frequencies between 6 and 10 kHz at the input and output, as well as a noticeable coil whine.

We therefore connected the RT pin to VCC for testing, which resolved the issue. The controller now runs at 1 MHz, with a maximum ripple of 30 mV.

However, we have encountered an unexpected outcome. When we allowed RT to float, the controller exhibited the same good behavior. According to the data sheet, however, this is not recommended practice.

Please explain why this is the case and why the behavior does not match the data sheet.
I have never encountered a similar issue with TI.

Best regards,

  • Thank you for the post and apologies for not being able to address it today.

    Please wait for the next business day and the an appropriate engineer will help service this post.

  • Hello Fabian,

    It is generally not recommended to have the RT floating.

    The functional safety information document mentions that this is a Class C if the RT pin is left floating.

    In this case, "frequency is undefined" 

    When we allowed RT to float, the controller exhibited the same good behavior

    Did you also check to see what switching frequency the device was in when RT is floated and device was regulating Vout?

    If you want to operate the device in 2.2MHz, the RT pin should be just hard shorted to GND.

    For 1MHz, RT pin should be connected to VCC. 



  • Thank you for your answer.

    Yes, I know that floating is not recommended, however, both VCC and GND on the RT pin resulted in coil whine and indefinable switching frequency. Only floating RT provides about 1 MHz switching frequency and acceptable ripple and thus an usable operating condition.

    There are therefore recommended conditions in which the DCDC converter does not work and non-recommended conditions in which the DCDC converter does work.

  • Hello,

    Please share the schematic and PCB layout of the application and an apps engineer can help assist the review.

    It sounds like there may be something in the schematic (inductor or design instability) that may be causing this issue.