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TPS54628: TPS54628 VIN startup rise time and shutdown fall time

tateo yamashiro
Guru 10890 points
Part Number: TPS54628

Hi,

We got a question from customer about TPS54628.
Could you help us?

[Question]
- Is there the recommended VIN startup rise time and shutdown fall time range?

Best Regards,
tateo

  • 0
    TI__Genius 13700 points
    Hello Tateo,


    Regarding how to decide how to configure the rise time. It is very application dependent. That said there are key parameters that you want to pay attention to.
    -Know the cumulative capacitance on the output net of the TPS54628 and design your soft-start rise time in such a way that you do not hit the current limit of the TPS54628.
    -Another important facet of selecting you rise time is to ensure that it is slow enough to not result in significant ringing on the output net that could damage any devices that are connected to that net.
    -It is also important to look at the input source to the TPS54628. If the soft start time is too short it could be possible that the input supply that is feeding the TPS54628 through drawing to much current. It is important to look at your input supply limits and design your soft start to comply with the input supplies abilities.

    When it comes to minimum rise time configurations it is important to look at what is connected to the output net of the TPS54628. Ideally you would configure you rise time to be very quick to avoid time spent in unknown UV/Brownout cases for any parts powered by this rail.

    All that to say selecting the right soft-start time for a converter is a holistic question that needs to take into account the whole output net of the converter and the ability of what ever the input source is to support the start up current.

    Regarding the discharge time of the output net of the TPS54628. The only way to change this is by looking at the RC time constant that is created but the total output capacitance on the output net of the TPS54628 and the output discharge resistor inside the TPS54628 which has a typical preset value of 500ohm and a potential max value or 800ohms.
    A similar situation regarding UV/Brownout is important during shutdown as well. Review the characteristics of the devices attached to the output net of the TPS54628 and ensure that when the output discharge feature is use that you account for potential unexpected operation of devices as they enter UV and brownout.

    Let me know if there are any other questions.
  • 0 in reply to Ryan May
    Guru 10890 points

    I'm sorry for the long delay in my reply.
    Thank you for your support.

    Could you provide test condition of startup rise time range?
    They need this spec to certify their standard parts.

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Guru**** 191445 points
    While the points that Ryan makes are valid, I think what you are looking for is the input voltage ramp up and ramp down minimum and maximum specification. So far as I know, that parameter is not characterized. I can offer some general guidelines though. TPS54628 can accept reasonably fast VIN rise and fall times, faster than 500 usec. You do not want to "hot plug" or apply the input voltage instantaneously without proper protective circuitry to prevent transient overshoot that could damage the TPS54628. As far as longer rise time for VIN, that is application dependent as Ryan points out and will depend on your output capacitance, output voltage and SS time. For longer VIN ramp rates, there are two specific conditions you need to be aware of. When the rise time is very slow near the UVLO threshold, there may be a toggling of the UVLO circuit. There should be enough internal hysteresis that you would never see it, but for a loosely regulated input voltage it is theoretically possible. The more likely scenario is the for higher output voltage like 5 V, extremely slow rise time on VIN may not allow the output voltage to get into regulation before the end of the SS time. You can account for that with appropriate SS capacitor selection. Extremely slow VIN ramp down may also cause your 5V output voltage to not regulate properly as VIN decreases towards the UVLO point.