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TPS24750: Importance of Plim and its roll in controlling hot swap

Part Number: TPS24750

Hi team,

Can you please help with the below query:

we are using TPS24750 hot swap for powering QSFP modules.

 

Per design resistor values and datasheet formulas, Ilim = 10.9A and Plim = 23W.

 

Operation is stable till 9.4A on 3.3V (31W). We observed that HS is shutting down around 9.5A.

 

Here what is the importance of Plim and its roll in controlling hot swap, since we see the power is exceeding the set limit?

 

Value

Rset

42.2

Rsns

0.002

Rprog

59000

Rimon

1300

Plim

23.211375

Ilim

10.955769

Using formula mentioned in the Page 16 and 17 of Datasheet

  • Hi Mayank,

    When is the shutdown happening ? Is it during steady state or during device turn-on? What are the input and output voltages? It would be nice if you can share the waveform of input voltage,output voltage,drain pin voltage,timer pin voltage,output current,FLT pin voltage during the shutdown event. The reason of shutdown can be overcurrent or due to overtemperature?

    The role of PLIM is that during the hot swap ,power dissipation across device increases rapidly which can cause the device temperature to cross the maximum internal fet junction temperature and cause shutdown . To prevent this a constant power PLIM is maintained across device device during hotswap so that device temp is well below the max junction temperature and device operates in safe operating area.

    Regards

    Kunal Goel

  • Hi Kunal,

    Can you please help to address below further quaries:

    So is it like PLIM takes cares during Hot-Swap and ILIM during steady state operation ?

     

    Shut down happening during steady state operation, when current crosses 9.4A (electronic-load testing, ILIM = 10.9A)

    Max current we are expecting for optic modules is 7A, we are not seeing any issue so far during power-on, (switching from 0A to 7.5A).

     

    Input and output are same 3.3V

    Waveforms requested by you needs to be captured.

  • Hi Mayank,

    Until the current reaches ILIM value PLIM takes care that device power is maintained constant and after current crosses ILIM then the circuit breaker and fast trip comes into picture.

    Is customer using electronic load in CC mode or CR mode? CR mode should be used . CC mode does not give accurate results.

    When current is crossing 9.4 and less than ILIM ,the reason of shutdown can be overtemperature . But the waveforms will give a better picture.

    Regards

    Kunal Goel