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TPS23754: The TPS23754 fails to be powered on due to excessive peak current.

Part Number: TPS23754
Other Parts Discussed in Thread: TPS23881


  We use TPS23754 as PD equipment, powered from  PSE device.   However, the peak value of the power-on current is too large, causing startup failure.

  1. PSE powers on the self-developed PD device. The current and voltage are shown in Figure 1 below. 


The peak value of PSE chip initial current ranges from 420 mA to 450 mA, and the actual test value is 442mA. At this time, power supply failure occurs, which is in line with expectations.

2. PSE powers on the purchased equipment. The current and voltage are shown in Figure 2 below.


The maximum measured instantaneous current is 279mA, less than 420mA, and the load is successful, in line with expectations.


The standard maximum current of PD is 400mA. The load of outsourced PD meets the standard, while the load of self-developed PD exceeds the standard.

The schematic as attachment,


  • Hi,

    Thank for reaching out.

    It seems that PSE VDD has been turned off when pwm converter starts to work.

    Could you try to increase the capacitance value of C55?

    It can help to increase start-up time period.



  • Hi Fengbin,

        We tried to adjust it, but it didn't seem to work, either by turning the capacitance value up or down

  • Hi ,

    How many capacitance  do you add at Vc (44uF)?

    A PSE limits output current to from 400 mA to 450 mA for up to 75 ms after power up. If it continues to clamping the current limit, it will shut down the PSE port.  I suspect the pwm controller works too early from the waveform you show. 

    To add the Cvc could let the pwm converter work late. It should improve the result. Thanks.



  • Hi Fengbin

    I tried to change C52 from 1uF to 2.2uF and 4.7uF, and found that the larger the capacitor, the higher the peak value of the starting current. Changing this capacitor is actually the same as changing C55 and C42 last time (because these three capacitors are all connected in parallel).

    Please help to see if there are other solutions.

  • Hi Sir/Madam,

    The DC/DC startup time depends on the time VC_RTN needs to rise to 15 V. The PSE can has a current power limitation per channel from 400 mA to 450 mA for up to 75 ms after power up (normally it is 60 ms). If you have a high power load or startup current the PSE may turn off the port at the end of 75 ms if the current keeps high. 

    You can either reduce your load current within 75 ms or you can increase capacitance of VC-RTN higher (may be try ~ 50 uF) to delay the DC/DC start up time. Below is an example of a 4-pairs PD inrush failure due to TPS23881 PSE turn off the port at high current (PSE inrush time was set to 120 ms for the testing purpose, while it should be 60 ms normally), and purple is the current.

    Best regards,


  • Hi Sir/Madam,

    Since we have not received your response for a relative long period, I will close this thread for now. Please reply or open a new thread if you have any further questions.

    Best regards,