PM8800 STMicroelectronics, PM8800 Datasheet - Page 17

• Manufacturer Part Number: PM8800
• Description: Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source
• Manufacturer: STMicroelectronics

PM8800A
5.2
Classification
Classification process in the IEEE 802.3af standard is optional for the powered device. This
feature allows PSE to plan and allocate the available power to the appliances connected to
the PoE network. IEEE 802.3af specification groups the need for power of the PD in 5
classes, one is reserved for future use. After successful detection, the PSE sets a constant
voltage between 15.5 V and 20.5 V for a maximum duration of 75 ms and senses the current
flowing through the cable to determine the PD's class.
The relevant thresholds in PM8800A are 11.5 V and 23 V, with a turn off hysteresis of 1.4 V.
To support the classification function, an equivalent programmable constant current
generator has been implemented. The following figure depicts a principle schematic of the
classification circuit. Just after the detection phase has been successfully completed, the
voltage of the RCLASS pin is set to the 1.4 V voltage reference and a pass transistor
connects the VIN pin to RCLASS pin.
Figure 8.
Classification resistor can be detached by three main causes:
Designers can set the current by changing the value of the external resistor according to the
following table.
–
–
–
An auxiliary power source (front or rear) has been connected (see
The device is in thermal protection
The classification has been successfully completed.
PM8800A: reference schematic of the PoE classification logic
R
CLASS
RCLASS
VSS
VIN
PM8800 A
EN
+
-
1.4V
Thermal_alarm
Class_enable
Aux_enabled
Section
PD interface
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