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.
Introduction
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
application
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
Qty.
Description
Used
Designation Manufacturer and PN
1
Capacitor, aluminum electrolytic, 47µF, 100V, SM, H16
case, 12mm x 13mm
C1
Panasonic EEVFK2A330P
Digikey PCE3497CT-ND
2
Capacitor, ceramic, X7R, 0.1µF, 10%, 100V, 1206
C2, C4
Vishay
VJ1206Y104KXBAT
1
Capacitor, ceramic, X7R, 1µF, 10%, 10V, 0805
C5
Vishay
VJ0805Y105KXQTW1BC
1
Capacitor, ceramic, X7R, 10µF, 10%, 10V, 1210
C6
AVX 1210ZC106KAT2A
1
Capacitor, ceramic, X5R, 680nF, 10%, 10V, 0805
C3
Vishay
VJ0805Y684KXQTW1BC
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1
Connector, RJ-45, 1 x 2, w/o magnetics, shielded, thru-hole J1
Tyco 406566-2
1
Diode, light emitting, green, 1206
LED1
Lumek SML-LX1206GWTR
Digikey 67-1002-1-ND
1
Diode, zener, 3.3V, SOD-223
D1
Diodes Inc. BZT52C3V3S
1
Diode, TVS, 51V, 600W, unidirectional, SMB
D2
Vishay SMBJ51A
1
Header, 0.1" ctrs, 1 x 3-pin, vertical mount
JP1
1
IC, controller, POE, single, TSSOP-28
U1
MAX5922AEUI
1
NMOS, 100V, 170mA, SOT23
Q1
Fairchild BSS123
1
Resistor, 1/8W, 0.5Ω, 1%, 1206
R2
IRC LR1206-01-R500-F
VishayCRCW1206R499F
3
Resistor, 1/16W, 0Ω, 5%, 0805
R10, R11,
R12
Vishay CRCW0805-000Z
1
Resistor, chip, thick film, 1Ω, 5%, 1/16W, 0805
R4
Vishay CRCW0805-1R0J
4
Resistor, chip, thick film, 10.0k, 1%, 1/16W, 0805
R6, R7, R8, Vishay CRCW0805R13
1002F
1
Resistor, chip, thick film, 150Ω, 5%, 1/16W, 0805
R3
Vishay CRCW0805-151J
2
Resistor, chip, thick film, 9.09k, 1%, 1/2W, 2010
R1, R9
Panasonic ERJ12SF9091U
1
Resistor, chip, thick film, 18.2k, 1%, 1/16W, 0805
R5
Vishay CRCW20101822F
1
Switch, pushbutton, NO, miniature, surface mount
SW1
Omron B3S-1000
Digikey SW415-ND
IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc.
Related Parts
MAX5922
+48V, Single-Port Network Power Switch For Power-OverLAN
Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 3507: http://www.maximintegrated.com/an3507
REFERENCE DESIGN 3507, AN3507, AN 3507, APP3507, Appnote3507, Appnote 3507
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
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