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From my understanding of having dealt with TPS23753, my answers would be following.

1. When the PD's are connected to the PSE, a start up and detection sequence would occur, in which, the PSE will determine what class/type the PD belongs and accordingly how much power it would consume. The PSE getting a wrong power level of the PD is highly unlikely. According to the result of classification stage, power would be supplied accordingly. 

2. In the start up and detection stage, the PSE would refuse to source power as the PSE can supply 13W, while, your PD is advertising that it needs 25W, though, it may consume less than 13W later, due to whatever reasons. 

I suggest that to understand your own problem thoroughly, go through the application section of the datasheet. In the TPS23753 datasheet, it was explained in detail what exactly happens when the PD is connected to a PSE. I suppose the same would be given in TPS2376-H as well. That will clear all your doubts with all the waveform and detailed explanations given. 

sanchayan maity,Thanks for your reply.

1,for differrent class,I still doutbt the PSE can detect correctly.maybe you don't clearly my design,the attached image is the design diagram.Thanks 

Ok! I hadn't envision it in this way.

Frankly speaking i don't have any experience or idea of what exactly will happen in this case.

To me it seems, it might not work. I cannot understand how both devices are going to get done with the detection and classification that happens when a PD is connected to PSE. Someone else might help you with this. 

Out of curiosity to know, would you mind telling me why are you going or why do you need such a configuration? It might help me also in the future. 

Also on second thought, one PD requires 25W and other requires 13W, at least, it would advertise as so. How can one PSE port with 25W capacity, supply that 38W power? 

OK,it's our customer's requirement,because there maybe some conditions that  the POE Switcher is far away from the PD device,and they need only one twisted-pair with POE line for project convenient. and there  is already a device with the POE powered,now  need an extra device to run cooperate with the orignal device,so my idears come out。

for second thought,the 25W is the total power,and the PD1 is the in front of the PD2,so I think the power class may need the 25W level。

BTW,if there is only one PD，and PSE PD has the same type ,but different class,for example:PSE only can supply 13W class power level,and the PD side is designed for 25W class power level,as your mentioned the class level is not down compatible，so the classification will failed and disconnect,what the PD consumed less 13W or not。right? 

If your PD consumes less than 13W at all times, you would design the PD with a classification resistor of 90.9 ohm in case of TPS23753 (Might be different in case of the IC you are using). For 25W ,it would be 63.4 ohm for TPS23753. 

If your PSE is only of 13W capacity, your PD of 25W won't be supplied with power as it would fail the detection and classification stage. 

For your customer's requirement, can't you use a PoE injector using a local power supply?

A PSE as far as i am aware will never supply more than 25W. Your PD depending on it's requirement should present itself as belonging to one of the five classes possible. 

A PSE of 25W capacity can supply power to any of the 5 classes of PD.

A 13W PSE however cannot power a class4 PD. 

Standard compliant (IEEE 802.3at) implementations do not envision paralleled PDs on one power feed.  This is aligned to data handling.

Please see/read TPS23754 datasheet for some information in the applications section regarding how a type 2 system works. 

I suggest that you consider a power topology where the primary unit is a type 2 full implementation with TPS23754 (h/w type 2 detection & LLDP).  Power the secondary (<13W) unit from the primary units's  Vdd-RTN (see TPS23754 pin definitions) through a second switch (suggest TPS2376). 

The primary unit will need to control its own power usage, and the secondary's as well, to <13W to handle the type 1 PSE / Type 2 PD combinaiton you have mentioned.  This is a normal condition envisioned by the type 2 implementation.

The TPS2376 would be configured without detection or class, and the primary unit would use the uvlo pin as an enable.  In other words the TPS2376 becomes a nice load switch.

Note that there are some safety concerns (IEC60950) with bringing the PoE potential through (out of) the master unit.

I suggest you speak with your TI sales person / FAE for support and news about upcoming new products.

Thanks for your reply.

I have get some information from TI technique supports,the said  one PSE channel, can't support two PD at the same time.

Maybe I make simple things complex.

I can use first POE device to generate an DC voltage to power second device.

Now I  want to confirm  if 12.95W PSE can work with 25W design PD(the PD may consumed less than 12.95W)

the following is just my understanding:

1,the classification is base on one pair AC line,am I right? 

So if the class 3 is confirmed,the PSE will delievered max 12.95W。
2,For 25W PD design,it just use 2 pair AC lines,so it will delievered 2X12.95W,right?

3,So,if the PSE is only can delievered 12.95W,that means it only support one pair output,the spair pair or data pair。

but the classification process will still successed with class 3,whatever one pair or two（For reason 1）.

the problem is if the PD consumed more than 12.95W,the PSD may overload and shutdown。

if PD less than 12.95W, I think it will work.

On Page 18 of TPS23754 datasheet, under hardware classification, it clearly mentions

A type 1 PSE will treat a class 4 device like a class 0 device,allotting 13 W if it chooses to power the PD.

"Now I  want to confirm  if 12.95W PSE can work with 25W design PD(the PD may consumed less than 12.95W)

the following is just my understanding:

1,the classification is base on one pair AC line,am I right?

So if the class 3 is confirmed,the PSE will delievered max 12.95W。"

I am not sure why you refer to "AC line,"  power is delivered as "phantom power" over 2 pairs (4 pairs are in standard cable).

Type 1 PSEs inject up to 15.4W over 2 pairs, type 2 PSEs inject up to 30W over 2 pairs.

2,For 25W PD design,it just use 2 pair AC lines,so it will delievered 2X12.95W,right?

See 1.  PD power draw is 13W or 25.5W, the difference from PSE injected power is an allowance for cable loss.

3,So,if the PSE is only can delievered 12.95W,that means it only support one pair output,the spair pair or data pair。

but the classification process will still successed with class 3,whatever one pair or two（For reason 1）.

See 1. 

the problem is if the PD consumed more than 12.95W,the PSD may overload and shutdown。

if PD less than 12.95W, I think it will work.

______________________________________________________________________________________________

From the TPS23754 datasheet (P16):

Type 2 PSEs are required to do type 1 hardware classification plus a (new) data-layer classification, or an

enhanced type 2 hardware classification. Type 1 PSEs are not required to do hardware or data link layer (DLL)

classification. A type 2 PD must do type 2 hardware classification as well as DLL classification. The PD may

return the default, 13W current-encoded class, or one of four other choices. DLL classification occurs after

power-on and the ethernet data link has been established.

From P17:

A compliant type 2 PD has power management requirements not present with a type 1 PD. These requirements

include the following:

1. Must interpret type 2 hardware classification

2. Must present hardware class 4

3. Must implement DLL negotiation

4. Must behave like a type 1 PD during inrush and startup

5. Must not draw more than 13W for 80ms after PSE applies operating voltage (power-up)

6. Must not draw more than 13W if it has not received a type 2 hardware classification or received permission

through DLL

7. Must meet various operating and transient templates

8. Optionally monitor for the presence or absence of an adapter (assume high power).

As a result of these requirements, the PD must be able to dynamically control its loading, and monitor T2P for

changes. In cases where the design needs to know specifically if an adapter is plugged in and operational, the

adapter should be individually monitored, typically with an optocoupler.

_________________________________________________________________________________________________

So the big picture here is that if your primary unit is a fully compliant type 2 PD, it will only run with type 1 power draw until the conditions above (type 2 h/w class or LLDP agreement).  This is the compatibility mode built into the standard.

When the type 1 PSE sees the h/w class 4, it will power it to a 13W limit (PSE always has the option to not power PD).  Type 2 PD behavior provides compatibility to at least provide power mismatch indication to user or reduced functionality operation.

The 13W draw is something that you build into your device's hardware/software.  It cannot be implemented directly into the PD IC.  For example, if your PD normally wanted 20W during full operation, you would use (processor) controlled load switches or software to load shed enough power to be under the 13W limit until the high power classification was acknowledged.

I have suggested above that  you use a TPS2376 as a load switch to the secondary unit in order to control PSE power draw as a part of the overall type 2 PD power control strategy.  When operating from a type 2 PSE, this configuration should meet the 25.5W maximum power draw.