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TPS23756: TPS23754 vs TPS23756

Part Number: TPS23756
Other Parts Discussed in Thread: TPS23754,

Hi,

I am trying to use TPS23754 with external variable supply. My expected output is 12V.

I have used a resistor divider on PPD pin that should put around 6V on the PPD pin.

Although when I come down to 24V the DC-DC converted stops working. No output.

On the other hand I have TPS23756 EVM that works fine right down till 13V. 

What is the difference between these two chips that one is working and another isn't?

  • Hi Prasanna,

    Thanks for your questions.
    TPS23754 has the same pinout and function with TPS23756. But the converter startup voltage is different.
    TPS23754 is 15V and TPS23756 is 9V. Once the VC pin voltage rises up to 15V/9V, the controller begins to work.

    For your questions,
    1. Make the PPD pin voltage is between 1.55V to 8.3V.
    2. Check whether the TP2 pin is low if you apply a 24V adapter on VDD-VSS.
    3. Check the VC pin waveform of your design during startup. Please send the waveform to us.
    4. Are you testing the TPS23754EVM or your design? If your design please send us the schematic if possible.

    Thank you
    Best Regards
    Jack Chen
  • Hi Jack,

    Thanks for a quick response.

    I have TPS23756 Evaluation board that shows correct transition of VC pin to 9V and around that voltage DC-DC starts working generating 5V output.

    In parallel I am working on my board that uses TPS23754. Unfortunately today morning the PPD pin broke off so I have to switch to APD pin control and auxiliary supply is on VDD1 - RTN rail. But I guess this should still function the same way as you explained in your email.

    Observations -

    1. I am using a variable supply for input and DC-DC is designed for 12V output.
    2. When I start ramping up the input supply, VC pin ramps up accordingly till ~14.9V and then bumps down to 12V and stays fluctuating around that voltage.
    3. Once I cross 24V on power supply input the DC-DC converted starts and puts out 10V.
    4. At this point VC drops to 8.5V.
    5. Then I keep on ramping up the input supply till 30V and output ramps up to 12V. After 30V input it locks to 12V.
    6. At the same time VC also ramps up from 8.5V till 10.23V and stops ramping after input hits 30V.10080753sh01.pdf

    Please find schematics attached herewith. This schematics is almost identical to the TPS23756 evaluation board schematics.

    0285.10080753sh01.pdf

  • Hi Prasanna,

    The problem is that the transformer you used can only support 33V-57V. When you applied a 24V input, the duty cycle is limited so it can not adjust the output to 12V. If you want to work with low input voltage as 24V you need to re-design the transformer. The Nps should be smaller.
    Please refer to this app note for detail.
    www.ti.com/.../slua535.pdf

    Thank you.
    Best Regards
    Jack Chen
  • Hi Jack,

    Yes, I was looking at the transformer spec and then your update came in at the same time. Bang on!
    You got the solution for me. Thanks a lot.
    Have a nice weekend! :)

    BR,
    Prasanna
  • Hi Jack,

    One more question -

    If I put TPS23754 in a mode where PPD pin has > 1.55V will it still arbitrate when the 48V PoE injector is plugged in?

    Thanks,
    Prasanna
  • Hi Prasanna,

    Can you describe more the condition maybe with a block diagram? The reason I ask is because the PPD is the divider between VDD-VSS. If there is a voltage >1.55V does this means there is an adapter connected to VDD-VSS already? And you're asking about what happens if you plug in PoE afterwards? If so, then the adapter voltage already between VDD-VSS will bias the PoE input. The PSE would not be able to pass detection when the injector is plugged in.

    Regards,
    Darwin
  • Hi Darwin,

    Thanks for the reply.

    My application scenarios are as follows -

    We have a display module that we want to operate from non PoE compliant voltage @ 24V as this supply is readily available in our system. We are not using a separate adapter input connection. RJ45 jack brings in this 24V . The TPS23754 is configured in PPD1 mode with >1.55V on that pin. This works fine because the non PoE compliant device won't negotiate and the board comes up.

    In second scenario though where a user may plug in a PoE compliant PD into our display system. Now PD sees that we have configured the TPS23754 in a bypass mode with PPD pin configured with >1.55V.

    In this case will it still negotiate and supply power to display or do we need to do any changes in circuit to accomplish that?

    BR,

    Prasanna

  • Hi Prasanna,

    Now I understand. Yes this should work. As long as the divider current is much less than detection current (megaohms of resistance), then it should be okay. Also note the the class voltage is between 15.5V and 20.5V, you must make sure the PPD voltage range (max/min) does not turn ON around 20.5V. This ensures classification does not get corrupted by a turned ON load.
  • Hi Darwin,

    Thanks for the reply. Now I have more questions :)

    1 - If I set up PPD resistor divider such that the PPD voltage 1.5V won't reach until 21V that will work for 24V non PoE operation. How will the chip respond after that voltage - from 21V to 37V?

    2 - Will the chip work in bypass mode from 21V to 37V?

    3 - And will it switch to PoE compatible device mode by negotiating after 37V?

    Thanks,

    Prasanna

  • Hi Prasanna,

    Above 1.5V, the PD will operate in bypass mode and allow the 21V-57V to pass through to the load.

    After 37V, there is no PoE negotiation. There is only negotiation if a PSE is connected to the PD. With the 24V non-PoE, it'll just turn ON and power the load.
  • Hi Darwin,

    Thanks for the reply. This resolves my query.

    In fact, I have tried putting 390k - 20k divider on PPD pin on the evaluation board of TPS23756 and the board works for 24V external power supply as well as for a PoE injector which puts 52V.

    Shall I continue with the same divider resistors or do you want me to put resistors in a megaohm range?

  • Hi Prasanna,

    410k should be okay. The extra current will make the PSE see a lower detection resistor. However, the PSE has a wide accept range of 19k to 26.5k. With 410k and a 24.9k det resistor, you would still be within this range.

    Regards,
    Darwin
  • OK Darwin thanks for your support.

    BR,

    Prasanna