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TPS25982: How to set the inrush current spec?

Frank Xiao
Genius 12055 points
Part Number: TPS25982
Other Parts Discussed in Thread: TPS25942A

Hi team,

How to set the inrush current spec, and why we set the spec to 1A on the datasheet? whether based on the SOA?

Thanks.

  • 0
    TI__Guru** 104881 points
    Hi Xiao,

    The datasheet design considers 1A as an example. The capacitance on dVdt pin (CdVdt) sets the Vout slew rate and thereby controls the inrush current. Equations 11 and 12 in the preliminary datasheet can be used for calculations.
    For eFuse devices, we do protection based on over temperature which indirectly does SOA protection.

    However, in actual, the inrush current value should be selected such that the device does successful startup without hitting thermal shutdown due to power stress during startup. It should be similar to example(10.2.1 Design Requirements in TPS25942A datasheet). We are targeting to make detailed calculation for the final TPS25982 product release

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Genius 12055 points
    Hi Rakesh,

    based on the calculation，The TPS25982 may have problem. for example, Vin=12, Iinrush=1A, PD=12*0.5*1=6w, So Tj=TA+6*ja， much larger than 150°C, coudl you please help double check the Iinrush current?

    thanks.
  • 0 in reply to Frank Xiao
    TI__Guru** 104881 points
    Hi Xiao,

    Tj=TA+6*ja is only applicable for steady state condition. Startup time lasts for few 'ms' where the device supports higher power stress due to thermal time constant.
    For example, refer Figure 30. Thermal Shutdown Time vs Power Dissipation in TPS25942A datasheet. The device supports 6W for 100ms at TA=85C.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Genius 12055 points

    Thank you for your help about the TPS25982.

     

    I have two more quick questions below. Could you please help?

     

    1. How to decide to select circuit breaker or active current limiter?
    2. What’s the function of LDSTRT? I cannot fine the advantage based on my understanding.

     

    Thanks.

  • 0 in reply to Frank Xiao
    TI__Guru** 104881 points
    Hi Xiao,

    1. In general, circuit breaker is preferred to allow transient over current to the load for a defined time interval (set by fault timer) . Circuit breaker feature essentially avoids system downtime by allowing pulsed overloads at the output for an interval of fault period.
    where as current limiting function is used to tightly keep the fault currents below the supply rating so that supply does not get stressed.
    Which one to select depends on application use case and end equipment.

    Can you please let me know your application details.

    2. The LDSTRT pin provides a mechanism for the downstream load circuit to signal a handshake to the TPS25982 that the load powered up successfully. If the downstream load does not send signal to pull down LDSTRT pin before reaching 1.2V, the TPS25982 switches OFF the output.
    if this feature is not required, connect LDSTRT pin to GND.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru** 104881 points
    Hi Xiao,

    Please let me know if you have any follow up questions else Please click "This resolved my issue" button to close this thread.

    Best Regards,
    Rakesh