PM8800 STMicroelectronics, PM8800 Datasheet - Page 16

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

PD interface
5
5.1
16/35
PD interface
Detection
In power over Ethernet systems, the PSE senses the connection to detect whether an IEEE
802.3af compatible device is plugged to the cable termination by applying a small voltage
(2.7 to 10 V) on the Ethernet cable and measuring in two successive steps the equivalent
resistance. During this phase, the Powered Device must present a resistance between
23.75 kΩ and 26.25 kΩ
PM8800A integrates a 24.5 kΩ signature resistor to simplify the design of PoE powered
Device appliances and to reduce the overall component count.
Signature resistor is in series to a pass transistor (see
resistor itself upon completion of the detection phase.
The value of the integrated detection resistance has been selected taking into account also
the diode bridges typical voltage drop.
During detection, most of the circuits inside the PM8800A are disabled to minimize the
offset current.
Figure 7.
PM8800A: reference schematic of the integrated 24.5 kΩ and resistor
disable logic
VSS
VIN
11.5V
PM8800 A
+
-
Figure
24.5k
7) used to disconnect the
PM8800A