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TPS548B23EVM: TPS548B23EVM Output Rising Waveform Issue

Part Number: TPS548B23EVM
Other Parts Discussed in Thread: TPS548B23

Tool/software:

Hi TI team,

When testing the TPS548B23 EVM, I have the following two questions. We replaced the Lout on the EVM with 0.82µH, set the switching frequency (Fsw) to 600kHz, and Vout setting to 3.3V.

  1. Regarding the definition of Soft Start (SS) time, is it measured from 10% to 90% of Vout or from 0% to 100% ?

Currently, we found that when measuring SS time set to 2ms, if Vout is rising from 0% to 100%, the measured time is 2.799ms (pic1), which exceeds the specified SS time range (1.4ms~2.6ms).

 

  1. When measuring the output during turn-on with a 16A load, we observed a glitch in Vout during its ramp-up (pic2).

Further investigation showed that this is related to a missing switching pulse on SW during Vout ramp-up (pic3).

Could you help identify the possible cause of this issue?

 

Pic1.

Ch1:Vout

Vout:3.3V , Iout:0A.

 

 

Pic2.

Ch1:Vout

Vout:3.3V , Iout:16A.

 

 

Pic3.(Zoom in glitch)

Ch2:SW , Ch3:Vout

Vout:3.3V , Iout:16A.

I hope you can help clarify my doubts. Thank you!

  • Hi Wei Chun, 

    1. Regarding the definition of Soft Start (SS) time, is it measured from 10% to 90% of Vout or from 0% to 100% ?

    Currently, we found that when measuring SS time set to 2ms, if Vout is rising from 0% to 100%, the measured time is 2.799ms (pic1), which exceeds the specified SS time range (1.4ms~2.6ms).

    The soft start time is based on an internal SS reference signal that measures the time it takes to reach 500 mV. However, the loop regulation and feedback voltage (VFB) may be slower due to factors like output capacitance. This is why you see a longer soft start time than expected. We may need to clarify this specification in the final datasheet. I will work with the team and update you next week on our plans for the datasheet changes. I apologize for the delay, as some team members are on spring break this week. I’ll keep you posted.

    1. When measuring the output during turn-on with a 16A load, we observed a glitch in Vout during its ramp-up (pic2).

    Further investigation showed that this is related to a missing switching pulse on SW during Vout ramp-up (pic3).

    Could you help identify the possible cause of this issue?

    The waveform looks like a load transient response since the output voltage rises while the switch node is low. Check to see if there are any changes in load current during that period. If you are using an electronic load, this may be your compliance voltage.

    Thank you,
    Tomoya

  • Hi Tomoya,

    Your response helped me solve the problem. I would like to ask if there are any further updates regarding the clarification on SS in the datasheet.

    Thank you!

  • Hi Wei Chun,

    Glad to hear that! I've aligned with the team regarding the soft start spec. In the next revision of the datasheet, we will be change the test condition to "VFB from 10% to 90%, tSS=2ms".

    Thank you,
    Tomoya