We are designing a Splitter using the TPS23754. The splitter output is 12V DC to power a 24W PD (a low powered integrated LED windows device)
Our prototypes work when we use it a midspan 802.3at injector, but does not work when used with a Cisco 802.3 at switch, the unit turns on briefly and reboots before the OS starts loading. The switch shows that the port is powered at 15.4W
I purchased the TPS23754EVM-383 to check if it is a design flaw. The same symptom exists with TPS23754EVM-383 .
Cisco has the following response:
"Introducing a class 4 device handling in the software. After 1-Event classification is done by the Power Controller the power class of the device is informed to inline power module for power approval. Initially in Type 2 PSE the power budget for class 4 PD shall be same as class 0 until the mutual identification is done through LLDP. After LLDP mutual identification is over then the class 4 PD power shall be provided with Type 2 PD power.
Later depending upon the real power consumption informed through LLDP DTE Power via MDI TLV the budget shall be re-adjusted.
So the device starts out offering only 15.4 watts of power to a device based off of the physical layer classification and then has to use a data link layer classification like LLDP/CDP to further clarify the power level, to allow it to go past 15.4 watts.
So it appears that a 3rd party device needs to get 802.3at power then it need to be cdp/lldp compliant. If not then cisco switch will only provide 15.4 watts. If it’s not 802.3at compliant then you contact the 3rd party vendor to get the changes done at their end. Whereas it is working through other power sources since they communicate only at the physical layer and required power is allocated "
Can we get some design guide lines to ensure that it is compatible with Cisco lldp PoE switches.
WHen there are questions about standard compiance, you should always refer to the IEEE 802.3at standard. http://standards.ieee.org/about/get/802/802.3.html Note that at the next general base-standard update, this will be rolled into clause 33 and others, and *.at will be out-of-date.
The TPS23754 datasheet has some information and advice on designing PDs that are compliant. See especially comments in the Detailed Description section pages 16 and 17.
You would have to do some investigation (look at waveforms) to determine exactly why the switch is turning your device off. My first guess is that it is drawing more than 13W, and either it overloads the PSE (shich shuts it down) in hardware, or the PSE notes that excessive power is being drawn and turns the PD off in software (unofficially called policing).
So - you have discovered the purpose of the T2P output from the TPS23754. See Hardware Classification on P18. The T2P signal is used by the PD's processor to control the load under the scenario you are running into. This scheme is written into the spec to avoid having a type 2 PD plugged into a type 1 PSE, having it overload & shutdown, and be interpreted as a bad unit (returned as defective material). The scheme then does double-duty as a way to handle PSE behavior such as seen in the Cisco router case.
You may find that you have to bring the T2P to the LED device and design the LED device to power manage itself (selectively power circuit blocks). For example, bring the processor and ethernet up to interpret T2P and LLDP messaging before turning displays etc. on.
So - some hardware measures might be necessary in your application to be compatible with some type 2 PSEs.
LLDP messages and PD state machines are discussed in the standard. They operate over infrastrucutre (protocol layer) originally defined in IEEE 802.1 and are a part of your ethernet protocol stack (OSI) software that runs in your device.
If you provide us an email address, we can provide a contact who might have more information about LLDP.
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