Keywords: PoE, power-over-ethernet, IEEE 802.3af, power-over-lan, power-sourcing-equipment, PSE, PD,

Keywords: PoE, power-over-ethernet, IEEE 802.3af, power-over-lan, power-sourcing-equipment, PSE, PD,
Maxim > Design Support > Technical Documents > Reference Designs > Hot-Swap and Power Switching Circuits > APP 3507
Keywords: PoE, power-over-ethernet, IEEE 802.3af, power-over-lan, power-sourcing-equipment, PSE, PD,
Power Device, IP Phone, Wireless LAN
REFERENCE DESIGN 3507 INCLUDES: Tested Circuit Schematic BOM Description Layout
IEEE 802.3af-Compliant Single-Port, PowerSourcing-Equipment Application
Jun 07, 2005
Abstract: Power-over-Ethernet (PoE) technology enables power-sourcing equipment (PSE, a switch/router or
midspan system) to deliver up to 15W of power to IEEE® 802.3af-compliant powered devices like IP phones
and wireless LAN access points. This application note describes how to design a single-port PSE system to
meet the IEEE 802.3af signature, classification, and power-management requirements. The single-port PSE in
this example is designed using the MAX5922A IC.
Power-over-Ethernet (PoE) technology enables switch/router and midspan systems, collectively named Power
Sourcing Equipment (PSE), to deliver up to 15W of power over the data cable. The power-receiving system is
called a powered device (PD). To meet the IEEE 802.3af standard, which governs how the PSE and PD
interact, the PSE must detect and classify the PD correctly before powering it. Once the PD is powered, the
PSE must sense the PD disconnection and power down the cable promptly. The PSE must also protect the
cable and the PD from overcurrent and short-circuit conditions.
Single-Port Circuit for a PSE System
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By integrating all required functions, the MAX5922 makes designing a PSE system simple. The circuit in
Figure 1 below illustrates a complete PSE design.
Figure 1. Circuit diagram of the MAX5922 power injector.
The MAX5922A circuit has the following features:
IEEE 802.3af compliance
RJ-45 data input jack (J1)
RJ-45 data and power output jack (J1)
+32V to +60V wide operating input range
28V (±7%) turn-on voltage
0.45Ω integrated power switch
Configurable power device (PD), detection and classification
424mA (±6%) programmable current limit.
Configurable zero-current detection with status output
Classification status output
Detection-collision avoidance feature for midspan PSE application; configurable for endpoint PSE
Power-good status output with green LED indicator (LED1)
Overcurrent and overtemperature protection with status outputs
Current-limit foldback with timeout and duty-cycle control
Selectable latch or autorestart fault management (JP1)
Single pushbutton reset (SW1)
Immune to ESD and cable discharge
The MAX5922A circuit is configured as a midspan PSE and used as a single-port, midspan power injector.
The J1 RJ-45 connector input receives data from an Ethernet router/switch and directly routes the data to the
output. The circuit injects 48V into the spare pairs (pins 4, 5, 7, and 8) of the J1 RJ-45 connector output. For
an endpoint PSE application, connect the DCA pin to DGND and the output power to the signal pairs with
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PoE magnetics.
Once a valid PD is detected and classified correctly, this circuit automatically performs PD detection and
classification, and turns on the power to the output. CL0-2 reports the classification result. To disable the PD
classification function, connect the CLASS pin to DGND.
Output current to J1 is limited to 424mA, ±6%. Reducing the value of the current-sensing resistor, R2, will
increase the current limit accordingly, and vice versa. When the output is overloaded or shorted, the circuit
acts as a current source, limiting the output current to 424mA. The circuit will turn off the output if the
overload/short-circuit condition persists for longer than 60ms. The circuit will restart automatically if JP1 is
strap-high (positions 2 and 3), or will latch off if JP1 is strap-low (positions 1 and 2).
Once the output is fully on, POK is asserted and the green LED (LED1) turns on. The circuit continuously
monitors the output load current for load disconnection (Zero Current Detection). If the load current drops
below 7.5mA (typical) for longer than 350ms (typical), the circuit will turn off the output, assert the active-low
ZC signal, and then resume detection and classification. Connect ZC_EN pin to DGND to deactivate the loaddisconnect detection function.
C4 and D2 are protective devices to absorb ESD and cable discharge energy applied to J1. They are also
used to clamp any inductive kickback current when the MAX5922 turns off the output after an output shortcircuit event. Figure 2 shows the PCB layout for the MAX5922 and Table 1 is the bill of materials.
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Figure 2. Circuit PCB layout information.
Table 1. MAX5922 Circuit Bill of Material
Designation Manufacturer and PN
Capacitor, aluminum electrolytic, 47µF, 100V, SM, H16
case, 12mm x 13mm
Panasonic EEVFK2A330P
Digikey PCE3497CT-ND
Capacitor, ceramic, X7R, 0.1µF, 10%, 100V, 1206
C2, C4
Capacitor, ceramic, X7R, 1µF, 10%, 10V, 0805
Capacitor, ceramic, X7R, 10µF, 10%, 10V, 1210
AVX 1210ZC106KAT2A
Capacitor, ceramic, X5R, 680nF, 10%, 10V, 0805
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Connector, RJ-45, 1 x 2, w/o magnetics, shielded, thru-hole J1
Tyco 406566-2
Diode, light emitting, green, 1206
Lumek SML-LX1206GWTR
Digikey 67-1002-1-ND
Diode, zener, 3.3V, SOD-223
Diodes Inc. BZT52C3V3S
Diode, TVS, 51V, 600W, unidirectional, SMB
Vishay SMBJ51A
Header, 0.1" ctrs, 1 x 3-pin, vertical mount
IC, controller, POE, single, TSSOP-28
NMOS, 100V, 170mA, SOT23
Fairchild BSS123
Resistor, 1/8W, 0.5Ω, 1%, 1206
IRC LR1206-01-R500-F
Resistor, 1/16W, 0Ω, 5%, 0805
R10, R11,
Vishay CRCW0805-000Z
Resistor, chip, thick film, 1Ω, 5%, 1/16W, 0805
Vishay CRCW0805-1R0J
Resistor, chip, thick film, 10.0k, 1%, 1/16W, 0805
R6, R7, R8, Vishay CRCW0805R13
Resistor, chip, thick film, 150Ω, 5%, 1/16W, 0805
Vishay CRCW0805-151J
Resistor, chip, thick film, 9.09k, 1%, 1/2W, 2010
R1, R9
Panasonic ERJ12SF9091U
Resistor, chip, thick film, 18.2k, 1%, 1/16W, 0805
Vishay CRCW20101822F
Switch, pushbutton, NO, miniature, surface mount
Omron B3S-1000
Digikey SW415-ND
IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc.
Related Parts
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Application Note 3507:
REFERENCE DESIGN 3507, AN3507, AN 3507, APP3507, Appnote3507, Appnote 3507
Copyright © by Maxim Integrated Products
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