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TPS23730EVM-093: PPD does not react to Vppden

Jan BIlek
Prodigy 20 points
Part Number: TPS23730EVM-093

Hi,

I'm now playing with the TSP23730EVM-093. I'm trying to verify functionality of the PPD pin. I would like to use this IC in a daisy-chained PoE application, where voltage can drop even below 30V due to resistive losses on the line. 
The datasheet claims that "Raising VPPD-VSS above 2.5 V enables the hotswap MOSFET, activates TPH and TPL and turn class off." However, I'm not observing this behavior on the EVM. Output load (DC-DC converter) is always turned on/off around 35V (Vdd-Vss) regardless of PPD setting (tied to VSS or to certain voltage from the divider). 

In particular, I measured following:

PPD configuration voltage when load was turned on
Vdd-Vss		 voltage when load was turned off
Vdd-Vss
PPD tied to VSS (J5 left open) 35.7V 35.4V
J5 shorted, 100k-15k default divider 35.6V 35.5V
J5 shorted, 100k-22k divider not turned on in order to protect 
PDD from exceeding 6V		 not turned on in order to protect 
PDD from exceeding 6V
J5 shorted, 100k-11k divider 35.7V 35.5V

Note 1: With the 100k-15k and with the 100k-11k dividers the TPL and TPH went low according to the datasheet (diodes D7 and D8 turned on). So it somehow detected voltage present on the PPD pin and pushed TPL and TPH low but did not adjust the UVLO accordingly.

Note 2: With the 100k-22k divider, voltage on the PPD pin approached 6V when Vdd-Vss was around 33V. I would probably approach abs max rating of this pin very closely if I would approach turn-on/off voltage of 35V. So I rather did not further increase the voltage. 

For sure, the load is not turned on/off around 2,5V on the PPD pin. 

Am I doing something wrong? Could you provide me your explanation, please?

One more important observation. The PPD pin shall not experience voltage larger than 6.5V. However, with the 100k-15k divider on the EVM and with the upper voltage threshold of 57V it can experience even 7.4V! I understand that it is designed for lower voltage, but then you should not claim "42-57V" at the J3 connector (see schematic).

Best regards,
Jan.

  • 0
    TI__Mastermind 21320 points

    Hello Jan,

    The datasheet claims that "Raising VPPD-VSS above 2.5 V enables the hotswap MOSFET, activates TPH and TPL and turn class off." Then you observed:

    "Note 1: With the 100k-15k and with the 100k-11k dividers the TPL and TPH went low according to the datasheet (diodes D7 and D8 turned on). So it somehow detected voltage present on the PPD pin and pushed TPL and TPH low "

    So the only measurement you are missing is confirming if the hotswap FET closes when PPD is above 2.5V.

    Please understand, the hotswap FET turning on and the "load is turned on" are very different behaviors. The indication the hotswap FET is on is that VSS and RTN are connected. The load turning on is related to the transformer turns ratio between the primary and auxiliary, the DCDC design, and lineUV.

    PPD is not meant to adjust the UVLO so the entire design works at lower input voltage. Otherwise the design would work at 19V input. 

    Thank you for the feedback. The J3 connector is the auxiliary power input connector, and it does work between 42V-57V, so it is valid. 

    PPD is supposed to enable a LOWER input voltage, so the thinking was the divider is set for an input range of 19V-30V (centering around a 24V input). That way, users can understand that they can turn on the hotswap FET with this lower input voltage. as 24V input for auxiliary is common. Then they can design their project based on that information. This design is not intended to  use a 57V input for PPD, as 57V is the highest input voltage. Do you understand the contradiction?

    Please understand, this board is meant to display / evaluate the behavior of the IC. If you can confirm the behavior of PPD with the input voltage, but it does not require the DC_DC to work with an input voltage range of 24V-57V. This is considered a Wide-Vin design, and we decided to do a traditional PoE input range for our DC-DC. When evaluating PPD, please apply a voltage that is within the range of the part (~19V- 46V).

    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
    Prodigy 20 points

    Hello Michael,

    thank you very much for your response. 

    You are right, I forgot to look at the conditions under which the DC-DC starts. LineUV is one of them that was apparently causing the DC-DC to start at 35V and I forgot to look at this parameter.

    And you are also correct, when looking at RTN-Vss to see when the hotswap switch turns on it is really around 2.5V at the PPD pin.

    I understand that the MOSFET turn-on and DC-DC turn-on are separate events. 

    Now, when I adjusted LineUV I'm able to turn-on the DC-DC even below 30V which is something I wanted to achieve.

    Thank you again for your support.

    Best regards,

    Jan.