PM8800ATR datasheet(16/35 Pages) STMICROELECTRONICS

Part #	PM8800ATR

Description	Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source

PDF	35 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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16 / 35 page

PD interface

PM8800A

16/35

PD interface

5.1

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 k

Ω and 26.25 kΩ

PM8800A integrates a 24.5 k

Ω 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 Figure 7) used to disconnect the

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 k

Ω and resistor

disable logic

24.5k

VIN

VSS

11.5V

PM8800A

PM8800ATR Datasheet(PDF) 16 Page - STMicroelectronics