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TPS2378: In-Rush Current Issues.

Pang
Expert 2415 points
Part Number: TPS2378

Dear Ti,

I have faced some issues on in-rush current from your TPS2378 and would like to clarify further with you on the below.

1. From the values of the datasheet for TPS2378, what are the major differences of the current limit and inrush current limit data? I assume that current limit is the current during the working condition and the inrush current is the input current drawn by the device during the initial turn on.

 
2. This inrush current limit is generated by the PD interface IC itself ? Or it means that the PD interface IC has an ability to handles the inrush current from external circuit?
3. What is the major cause of the in rush current keep increased? 
 
4. If the inrush current limit is >180mA what will happen? 
 
5. Is there any maximum period for this inrush current limit? For example the typical is 140mA and how about the withstand time? 
 
6. Found that if the larger of Cbulk capacitance will prolong the inrush current time period, tried capacitor 100uF inrush current prolong period is around 50ms, 470uF is 250ms....Will the prolonged period cause any problem on the PD interface IC? 
 

E2ERESOLVE

  • 0
    TI__Intellectual 1695 points

    Hi Incognito,

    Thanks for your questions. 

    1. Inrush current limit is that when PSE power on the PD, there will be a period that the  PSE will charge the bulk cap at the back end of PD.

    During this time, the current shouldn't be larger than the inrush current threshold of PSE. Here it is limited by PD internal hotswap FET.

    Current limit is used to protect the circuit during normal operation which means the PD has passed the inrush stage and worked in fully on.

    2. The inrush current limit is accomplished by internal circuit controlling the hotswap FET. This current is flowing from VDD - bulk cap -RTN- VSS.

    3. The inrush current is due to charge the bulk cap. If the cap is increasing, the inrush current will be limited to 180mA max and the inrush period will be increased. So basically it shouldn't increase during inrush because it is limited by PD

    4. Under normal operation, the inrush current will be limited to about 140mA (180mA max). 

    5. From IEEE802.3at standard, the inrush period should be smaller than 75ms. And the minimum delay from inrush to normal operation should be larger than 80ms.

    6. If the bulk cap is very large, the inrush period will be long which will make PD hotswap FET heat up. But PD has thermal protection and will shutdown the device to cool down. It is not recommended that add too much bulk cap.

    Thank you.

    Best Regards

    Jack Chen