PM8803 STMicroelectronics, PM8803 Datasheet - Page 18

• Manufacturer Part Number: PM8803
• Description: High efficiency integrated IEEE 802.3at PoE-PD interface and PWM controller type 2 PSE indicator, plus support for forward active clamp topology
• Manufacturer: STMicroelectronics

PD interface
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4.1
4.2
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PD interface
Detection
In Power over Ethernet systems, the Power Sourcing Equipment (PSE) senses the Ethernet
connection to detect whether the Powered Device (PD) is plugged into the cable termination
by applying a small voltage (2.7 V to 10 V) on the Ethernet cable and measuring the
equivalent resistance in at least two consecutive steps. During this phase, the PD must
present a resistance between 23.75 kΩ and 26.25 kΩ.
The signature resistor must be connected between the DET and VDD pins. This resistor is
in-series to a pass transistor (see
current is flowing through the signature resistor for the rest of the operative phases
(classification and turn-on).
The value of the detection resistance has to be selected also taking into account the typical
drop in voltage of the diode bridges. The typical value that can be used in most case is
24.9 kΩ.
During detection, most of the circuits inside the PM8803 are disabled to minimize the offset
current.
Classification
The classification phase in a PoE network is the feature that allows PSE to plan and allocate
the available power to the appliances connected to various Ethernet ports.
The PM8803 complies with both IEEE802.3at 1-event and 2-event classification schemes.
1-event classification in IEEE802.3at is the same as specified in the IEEE 802.3af standard,
which divides the power levels below 12.95 W into 5 classes (Class 0 to Class 4).
While Class 4 was reserved in IEEE802.3af, in IEEE802.3at Class 4 identifies Type2 PDs
requiring up to 25.5 W.
A Type2 PD is a PD that provides a Class 4 signature during Physical Layer Classification,
understands 2-event classification and is capable of Data Link Layer classification.
Figure 8
2-event classification. A Type2 PD will present in both classification events a Class 4 current
while during the so called “mark-event”, between the 2 classification fingers, the PD will
present an invalid signature resistance.
To support the classification function, an equivalent programmable constant current
generator has been implemented.
circuit. Following the successful completion of the detection phase, the voltage of the CLS
pin is set to the 1.4 V voltage reference and a pass transistor connects the VIN pin to the
CLS pin.
represent the voltage at the input of the PD when connected to a PSE performing
Doc ID 018559 Rev 1
Figure
Figure 6
3) enabled only during the detection phase. No
depicts a primary schematic of the classification
PM8803