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TPS23758: Design Questions

d_zero
Genius 4470 points
Part Number: TPS23758
Other Parts Discussed in Thread: TIDA-010083,

Dear *,

I have few questions regarding TPS23758 and TPS23758 EVM

1) VB pin , in DS is stated it should be bypassed with 0.1uF and a 6.2V zener diode to GND.

In the TPS23758EVM-080 and TIDA-010083 schematic there is no zener diode in schematic.

is the zener required?

2) the same diode 1N4148 is used for D3,D1 and D13 (Ddd , Dg, Dcc) in TIDA-010083, can you tell for the three diodes:

2a) what is the minimal reverse voltage requirement?

2b) what is the minimal forward current requirement?

3) What is the purpose of R16=0R and R34= DNP in TIDA-010083

4) Can we use for Rcs 470mOhm 1W 1% resistor instead of 2x parallel 910mOhm? KRL2012E-M-R470-F-T5

5) Error in schematic symbol of transformer on TPS23758EVM-080 and TIDA-010083, the dots are on wrong pins ( different in DS LDT1018-50  and in 750318525)

Best Regards,

David.

  • 0
    TI__Mastermind 21320 points

    Hello David,

    1. The EVM is being updated and will reflect the changes by tomorrow. Yes it is required if a class resistor is used. 

    2. This diode is what i would consider our standard diode. The forward currents and reverse voltage levels for each one would be different if you wanted to calculate them. D1 is there to allow a fast gate turn off. So the key is the reverse recovery time. D3 is there to rectify the signal on transformer. D13 is there to separate VPD and VDD to establish PoE vs adapter priority. Finding the exact requirements might be more tedious than needed, when these work. I would suggest using these diodes as the minimum values to compare to.

    3. R16 is slope comp. We Always suggest placing a 0ohm resistor here in case more slope comp is needed. R34 is a pullup for the same reason. 

    4. You can but we split them for thermals. 

    5. since it is exactly the opposite, it still works since it is exactly opposite and inductors work both ways. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Genius 4470 points

    Hi Michael,

    thank you for the fast replay.

    5) Regarding transformer, yes i know, but it would be nice to have same notation as in wurth or linkcom datasheet :).

    6) Question regarding R11 R12 and C44 in  TIDA-010083

    6a) is it necessary to have 0805 resistors for R11 and R12 ?

    6b) I'm looking at linkcom das the aux winding can supply 10.5V and 15mA 

    do i use this current and voltage for power rating calculation of the resistors or is there higher peak current and peak voltage used for calculation?

    We would like to use 0402 or 0603 for these resistors.

    6c) also we have standard valuse that we use and we would like to use 4.7ohm for R11 and 4k7 for R12 and 4.7uF or 10uF for C44 is this ok?

    Best Regards,

    David.

  • 0 in reply to d_zero
    TI__Mastermind 21320 points

    Hey David,

    5. The linkcom datasheet is publicly available, I am not sure what you mean?

    https://www.linkcom.com.tw/media/bss/productattachment/LDT1018-50.pdf 

  • 0 in reply to Michael Pahl
    Genius 4470 points

    Hi Michael,

    5) I got the last revision of LDT1018-50 DS from the manufacturer and the dots are on opposite side, also in wurth 750318525 DS are also on opposite side, just saying. 

    LDT1018-50.pdf

    750318525.pdf

    1) Regarding Zener diode is the BZX585-C6V2 ok to use?

    8) another difference between DS and EVM and TIDA, VCC decoupling in DS is told 3.3uF min in table 5.-1. and in the EVM only 1uF is used ? Later in section 6.3 nominal capacitance for VCC is told be1uF as in EVM. 

    9) For the Zener D8 in EVM can we use lower wattage Zener in smaller package?

    10) For the D5 and D( on TIDA we would like to use BZB84-C12,215 is this ok?

    Best Regards,

    David.

  • 0 in reply to d_zero
    TI__Mastermind 21320 points

    Hey David,

    5.) I see what you are saying. The Wurth and Linkcom transformers have different polarities. I tested them myself and can confirm they work on the same board. To add to the confusion, the Altium schematic footprint has a different pinout order than the datasheet, so it appears they are different but they are not. 

    1) Yes that is fine to use, but it is automotive grade. Typically that quality is not needed in PoE designs. You might find automotive grade parts in our designs because our Altium library only has one option for certain components in certain package sizes with certain voltage ratings. So you could probably find something cheaper.

    8.) Could be an error, I will have to check with the datasheet authors on which cap. Please use what is used in the EVM, since that design has been tested.

     9.) No i do not recommend that. 

    10.) No i would not recommend that. the packaging will lead to trace inductance. Additionally it is automotive grade. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Genius 4470 points

    Hi Michael,

    2) Regarding

    "

    This diode is what i would consider our standard diode. The forward currents and reverse voltage levels for each one would be different if you wanted to calculate them. D1 is there to allow a fast gate turn off. So the key is the reverse recovery time. D3 is there to rectify the signal on transformer. D13 is there to separate VPD and VDD to establish PoE vs adapter priority. Finding the exact requirements might be more tedious than needed, when these work. I would suggest using these diodes as the minimum values to compare to.

    "

    Do you maybe know if there is an example for the calculations? I would like to do the calculations :) because

    We use in many projects CDSU4148-HF but this is a 150mA 75V 4ns and not 200mA is it ok to use this one?

    Best Regards,

    David.

  • 0 in reply to d_zero
    TI__Mastermind 21320 points

    Hey David, 

    Please review the following paper for a step by step guide of designing flybacks: https://www.ti.com/lit/an/slva305c/slva305c.pdf?ts=1608145361530&ref_url=https%253A%252F%252Fwww.google.com%252F 

  • 0 in reply to Michael Pahl
    Genius 4470 points

    Hi Michael,

    i already did this and only the Dvc ( DCC)  is covered, and it says it  can use 75V 150mA and this is ok with CDSU4148-HF 

    but there is no D13 ( is there to separate VPD and VDD) and D1 (is there to allow a fast gate turn off)  diode calculation?

    CDSU4148-HF is also 4ns and thus i think ok form this point of view for D1.

    Also 75V is ok for all diodes but i don't know what current do we need for D1 and D13?

    Best Regards,

    David.

  • 0 in reply to d_zero
    TI__Mastermind 21320 points

    David,

    The only difference between these diodes is the max forward current. The Vd breakdown, the diode voltage drop, the reverse recovery time are all the same. 

    The datasheet specifies the current Ivpd-vdd on page 9. 

    You can find the gate current in the FET datasheet.

    This diode will work fine. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments