I am designing a carrier board with 16 POE PSE channels in addition with 4*4 ports marvell 88E1548 phy. For the moment, i don't know if the distant devices will draw more than 12.95W so i considered the LTC4266 POE+ PSE as a start.
I have read some of the specifications about POE, but i still don't get some of the features.
Let's assume that i want to give a certain margin to my PD device and i use a class 4 PD (LT4275B) . My questions are :
Sorry for the silly questions, i'm not familiar with POE.
Thank you in advance.
As usual i answer my own questions when i decide to read the datasheet in details and not the scattered informations on google.
But correct me if i am wrong :
-I want to use a LTC4266 with a 54V/200W external power supply
-The distant device (<12.95W) uses a LT4275 PD with the correct RCLASS for a class3 signature(802.3 af)
-At start-up, the LTC4266 performs auto classification (AUTO pin = 3.3V) and detects a class 3 devices (p18 figure 13). The power injected for classification is around 30mA*30V = 900mW
-When the 4 ports are classified, the LTC4266 outputs 54V on all of its outputs
Simple as that?
You're pretty close to understanding the basics of PoE. IEEE 802.3at follows these steps: Detection, Classification, and Turn On.
Detection is a low voltage (<10.1V) low energy (<1mA) probing by the PSE to determine if the PD is valid.
Classification follows Detection, where the PSE and PD mutually identify one another's power levels. The PSE applies a Class voltage between 15V and 20V, and the PD draws a current between 0mA and 40mA. The current drawn by the PD, known as its class signature, indicates how much power it is requesting, i.e 40mA for 25.5W and 28mA for 13W.
The PSE will then reduce the voltage to just below 10V, this is called mark. A Class and Mark form a classification event. The PSE may provide a second Class pulse to recheck the current drawn by the PD. A PSE that allocates 13W provides a single classification event, while a PSE that allocates 25.5W provides two classification events.
After classification, if the IC is in AUTO mode it will power on the port with a controlled inrush.
To answer your questions, during classification the PSE consumes (54V - 20V) * 30mA = 1.02W, but only for 12ms. The PD consumes 20V * 30mA = 0.6W, but again only for 12ms. The power drawn from the power supply is 54V * 30mA = 1.62W. If four valid PDs are connected, after the PSE has classified each then all four ports will be powered to 54V.
i was very far from the truth : )
Thank you for the explanation!
I have another question about the power injection.
I see in datasheet that it uses a ETH1-230LD transformer to feed the power to RJ45 port. I am using a 7499111001A port with integrated magnetics.Can I route the power directly to RJ45 magnetics or do i have to add the ETH1-230LD?
edit : i did a mistake. the transformers are ETH1-230LD not HCPL (sorry for the confusion)
Looking at the datasheet for 7499111001A I don't see PoE mentioned anywhere. There is no rated DC current either. It seems like this part will not work with PoE.
You would need to connect to the highlighted center taps of the Ethernet transformer, which appear internally connected. The magnetics, i.e. the transformer and common-mode chokes would also need to be rated for your expected power range.
Many manufactures make Ethernet transformers and even integrated RJ-45 and magnetics for PoE, I suggest you find one of those to replace your current part.
Ok Eric, thank you for the answer. My design is now complete!