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TPS25947EVM: Startup failure

Koshi Ninomiya
Expert 5455 points
Part Number: TPS25947EVM
Other Parts Discussed in Thread: TPS25947

Tool/software:

Dear Team,

My customer is evaluating the TPS25947EVM, but they have a startup failue and can not continue evaluation.

Please review an attached their settings and measurement waveforms belwo.
TPS25947EVM.xlsx

What is causing thie shutdown?

Best Regards,

Koshi Ninomiya

  • 0
    TI__Mastermind 39025 points

    Hi Ninomiya-san,

    Please ask them to remove Cout (220uF Al cap) and open the dvdt pin (remove jumper). This should allow device startup with 4A load. 

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    TI__Mastermind 39025 points

    Hi!

    Did this resolve the issue? 

    The cause of failure should be thermal shutdown happening during startup due to higher inrush current.

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    TI__Expert 5455 points

    Hi Arush-san,

    They tested to remove Cout (220uF Al cap) and open the dvdt pin, the TPS25947EVM did startup normally with 4A load.
    Please check an attached blow,
    7120.TPS25947EVM.xlsx

    However, 
     1) Remove Cout (220uF Al cap) but not open the dvdt pin --> Startup failed
     2) Not remove Cout (220uF Al cap) but open the dvdt pin --> Startup failed

    In an actual application circuit of them, a maximum Cap of about 330uF is connected to the output stage of the TPS25947 as casa 2) above.
    How should they set the dvdt connector at this application?

    The TPS25947EVM has,
     - U1 TPS259472ARPWT
     - U2 TPS259470LRPWT
     - U3 TPS259470LRPWT
     - U4 TPS259474LRPWT
    They can not use the U1 TPS259472ARPWT because of their Vin=20V, the remaining devices are Latch-Off type. Is this related to the startup failure phenomenon?

    They are thinking to use the TPS259470ARPW (Auto-Retry), but they would like to understand the mechanism of the startup failure phenomenon>

    Best Regards,

    Koshi Ninomiya

  • +1 in reply to Koshi Ninomiya
    TI__Mastermind 39025 points

    Hi Ninomiya-san,

    Let me explain the issue here. Dvdt cap changes the startup speed. (dvdt OPEN is fastest startup condition)

    • If load is resistive or continuous in nature. Then we will reduce the startup time by decreasing dvdt cap. This will reduce the power dissipation
    • If the load is capacitive in nature i.e. inrush current type. We can slow down the startup to reduce the inrush current by increasing the dvdt cap
    • If load is large and have both inrush and continuous current, then device may not be able to support it. In that case, use PG signal of device to turn on the load after device has turned on fully.

    So in the previous test, with 220uF or 330uF and the resistive load, the total power dissipation was very high resulting in thermal shutdown during startup. 

    If the load will start drawing current with a delay, then there should be no concern and for validation, they can perform startup and then turn on the eload (in CR mode).

    Best Regards,
    Arush