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PMP11257: DEN Resistor value of TPS 2378

Nuri Aslan
Prodigy 100 points
Part Number: PMP11257
Other Parts Discussed in Thread: TPS2378, TPS2372-4EVM-006, LM76005, , LM5145, LM5116

In this design (11257), DEN  resistor seems to be 27.4K, However Datasheet of TPS2378 states that TI  recommends a resistor of 24.9K. Could you please clarify?

  • 0
    TI__Intellectual 2440 points

    Hi,

    This design uses a half FET bridge on the front end to provide better efficiency.  The resistors used to bias the bridge (R3, R8, etc) are in parallel with the DEN resistor during detection.  The value of the DEN resistor has to be increased to account for this extra parallel impedance.  Increasing the DEN resistor to 27.4k brings the nominal value of the resistance the PSE sees back close to 25k.

    If you are using an all diode bridge then the DEN resistor would be 24.9k.

    Thanks,

    David

  • 0 in reply to David Strasser
    Prodigy 100 points

    I have the following circuit, POE rectifer is taken from TPS2372-4EVM-006 Evaluation Module (Rev. A),  POE of TPS2378 is from PMP11257 and Buck converter of LM76005 is from webench. An Adaptor of 24 VDC is foreseen with priorty, Output is 5V/4.25A. It will be powering an IoT camera.

    Can you have a look at it?

    POE with ADP 24 VDC (5V,4.25A).pdf

  • 0 in reply to David Strasser
    Prodigy 100 points

    I have the following circuit, POE rectifer is taken from TPS2372-4EVM-006 Evaluation Module (Rev. A),  POE of TPS2378 is from PMP11257 and Buck converter of LM76005 is from webench. An Adaptor of 24 VDC is foreseen with priorty, Output is 5V/4.25A. It will be powering an IoT camera.

    Can you have a look at it?

    4111.POE with ADP 24 VDC (5V,4.25A).pdf

  • 0 in reply to Nuri Aslan
    TI__Intellectual 2440 points

    Nuri,

    Here are my comments on the circuit:

    1.  The LM76005 is rated for 60V continuous and 65V transient.  Even though the PoE steady state voltage is 57Vmax, transients can exceed 65V.  The 58V TVS can clamp as high as 95V.  C15 will help absorb the transients.  It would also help to have an inductor between C15 and CIN to block transients.  I usually use 3.3uH.  I think the inductor will also be needed to pass EMI.  I would recommend using a device with a higher transient rating.

    2.  The bridges U1 and U2 need to be turned off when an adapter is used.  The manufacturers website has an app note showing how to do this.

    3.  Ground on J7-8 should be on the other side of L8 at the L8/C16/R29 connection.

    4.  R27 should be 24.9k when using the integrated FET bridge.

    5.  U7 pin 5 should not connect to U7 pins 4 and 9.  U7-5 should be connected to ground.  The internal hot swap FET of the TPS2378 is between pins 5 and 4/9.

    6.  COUTX pin 2 should not be VSS1...it is ground.

    7.  U5 pin 18 (EN) needs a pull up resistor to VDD.

    8.  PFBT should have a place holder for a parallel cap in case it is required for loop compensation.

    9.  RFBB should be 24.9k.

    10.  Probably need two CIN caps to reduce ripple to acceptable level for PoE requirements and to pass EMI.

    11.  Also, adding an inductor between C15 and CIN will probably be necessary to pass EMI and to keep ripple current through C15 within its rating.  Again, I usually use a value of 3.3uH.  It also helps to have a high frequency cap (0.1uF) in parallel with C15.

    Thanks,

    David

  • 0 in reply to David Strasser
    Prodigy 100 points

    Dear David,

    Thanks a lot. Your comments are apreciateed.

    Regards,

    Nuri

  • 0 in reply to Nuri Aslan
    Prodigy 100 points

    Dear David,

    I can not trace a device with higher input voltage which has similar efficinecy of LM7005. Since the power input is limited, can you advise another device which has similar efficiency?

  • 0 in reply to Nuri Aslan
    TI__Intellectual 2440 points

    Nuri,

    To get a higher voltage rating at 4.25A you will need to use a controller with external FETs.  PMP11257 uses the LM5116.  It has a 100V rating, both steady state and transient.  There is also the LM5145 that has a 75V steady state rating and 105V transient rating.

    Thanks,

    David

  • 0 in reply to David Strasser
    Prodigy 100 points

    Dear David,

     

    I have a couple of more questions regarding PMP11257 and LM5116;

    1. When using and adaptor of 24VDC alongside with POE TPS2378 and LM5116, would you think that I should employ some protection devices for reverse polarity, high current, etc.?

    2. In PMP11257, after TPS2378, there is an inductor of L3 (3.3uH) and 4 x 2.2 uF capacitor on High Side FET (Q5). However, in Webench design there is no such inductor and capacitors. Could you please clarify it for me?

     

    Thanks,

    Nuri 

  • 0 in reply to Nuri Aslan
    TI__Intellectual 2440 points

    Nuri,

    1.  D9 in PMP11257 provides reverse polarity protection on the adapter input.  It also blocks the PoE voltage from the adapter.  Although not shown on PMP11257, a 58V TVS device (same as D11) should be used after the filtering on the anode of D9 to GND.

    2.  There are two requirements for noise and voltage ripple on the input.  The IEEE802.3bt specification has four frequency ranges for maximum peak to peak ripple listed in Table 145-29.  At this power level the four capacitors and the inductor should reduce the peak to peak ripple to a level that meets the IEEE802.3bt requirements.  In addition, it is likely the EMI requirements of CISPR32 (or some other EMI specification) will have to be met.  A common mode inductor on both the PoE and adapter inputs is often needed to meet the EMI requirements.  It would be inserted to the right of L1/L2 and L4/L7 on the PMP11257 schematic.

    Thanks,

    David