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TPS2372: Question on resistor tolerance on REF (pin7)

Part Number: TPS2372

I am using PoE PD module developed by our partner company.

Its PD interface chipset is TPS2372-3 and I am checking the schematic of the module for some reason.

The datasheet says a 49.9Kohm 1% resistor should be connected from REF to VSS but actually 5Kohm 5% resister 

is connected. What would this bring unwanted side effect?

The symptom we are suffering is the capacitors in yellow colored area happen to fail short in 10% defect ratio 

when connecting PSE to DUT during IQC.

BTW, there was no overvoltage in the VIN supplied by PSE when monitored by oscilloscope. The measured maximum

input voltage is around 57.2~57.4V.  (FYI, the GDT is intalled in transformer side for surge protection)

I am trying to figure out the root cause but no clue yet.

Your advisement would be appreciated.

Regards/Jaewon

  • Typo

    5Kohm 5% --> 51K ohm 5%

  • Hello Jaewon,

    Because of the Thanksgiving holiday in the U.S., TI E2E design support forum responses may be delayed this week. Thank you for your patience.

    Regards,

    Rohit Parashar.

  • Hi Jaewon,

    Thanks for reaching out to us!

    1. Regarding to the 51-k 5% resistor, I would suspect that could increase the error of oscillator. For example, some delay timings could have larger error and auto MPS may have larger error of frequency. These error may or may not cause noticeable unwanted features depending on specific design.

    2. Regarding the capacitors in yellow colored area short failures, will these capacitor two terminals were measured short with a voltmeter? Are there capacitors damaged to cause the short or the PD was damaged?

    3. Besides, are C1-C4 2.2-uF ceramic capacitors? For Class-7 case (based on R_classA and R_classB), the recommended bulk capacitance is 68 - 100 uF.

    4. For Class-7 case, TPS2372-4 is recommended compared to TPS2372-3. TPS2372-3 is only recommended for Class 0-6. Compared to TPS2372-4, the TPS2372-3 has (comparisons based on typical values):

    1). lower power rating (51 W to 71 W)

    2). higher pass FET on-resistance (0.3 Ω to 0.1 Ω)

    3). higher thermal resistance (junction-case 40.2 to 38.0 °C/W)

    4). lower inrush current limitation (200 mA to 335 mA)

    5). lower operation current limitation (1.85 A to 2.2 A)

    6). lower recommended current (1.2 A to 1.85 A)

    Best regards,

    Diang

  • Back to your questions above, please refer to below;

    2.  Regarding the capacitors in yellow colored area short failures, will these capacitor two terminals were measured short with a voltmeter? Are there capacitors damaged to cause the short or the PD was damaged?

    => The DC resistance of the capacitor when measured by DMM was 0 ohm. In many cases, bulk capacitors failed short. With one of those DUT, after replacing the malfunction capacitor by a new capacitor, we  input 52VDC to PD module. Then the PD controller, TPS2372-3 burnt as seen in below figure.

    3. Besides, are C1-C4 2.2-uF ceramic capacitors? For Class-7 case (based on R_classA and R_classB), the recommended bulk capacitance is 68 - 100 uF.

    => Yes, they are MLCC (X5R, 100V). The resistance of R38, a class B resistor, is 140ohm, which means class 7. When testing, class 6 PSE is used. Is there a probability this resistance can make the PD controller malfunction?

    Thanks

  • Hi Jaewon,

    Thanks for your updates!

    => The DC resistance of the capacitor when measured by DMM was 0 ohm. In many cases, bulk capacitors failed short. With one of those DUT, after replacing the malfunction capacitor by a new capacitor, we  input 52VDC to PD module. Then the PD controller, TPS2372-3 burnt as seen in below figure.

    ==> I would tend to suspect the downstream circuit have short circuit issue in you case. On a virgin board, can you still measure a short circuit at the capacitor when you remove the CN1 and U6?

    Besides, the TPS2372 should have a current limiting and shut down function for short circuit. Could you get a waveform like the picture below (in TPS2372's datasheet) to see if the protection is functional? 

    => Yes, they are MLCC (X5R, 100V). The resistance of R38, a class B resistor, is 140ohm, which means class 7. When testing, class 6 PSE is used. Is there a probability this resistance can make the PD controller malfunction?

    ==> Theoretically it won't cause the issue. When using Class 6 (Type 3) PSE, the PSE output power limit will be 60 W in your case.

    Best regards,

    Diang