Power DI-88 Poe detection and classification (class 0-3) interface circuit Datasheet

Design Idea DI-88
®
DPA-Switch PoE Detection and
Classification (Class 0-3) Interface Circuit
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
PoE/VoIP
DPA423G
6.49 W
36-57 VDC
3.3 V
Flyback
Design Highlights
•
Simple interface for Power Over Ethernet (PoE) Powered
Devices (PDs)
Includes PoE detection and classification circuits for all
classes (0, 1, 2 and 3)
Compliance to IEEE802.3af PoE standards verified by
University of New Hampshire Interopability Consortium
(UNH-IOC)*
Includes under voltage lockout (42 VDC on, 34 VDC off)
•
•
•
Classification occurs as the input voltages rises from 14.5 to
20.5 VDC. The sending device determines the class of the PD
by monitoring the amount of current drawn by the PD during
this phase. For classes 1, 2 and 3, the classification current (ICL),
is programmed by resistor R34, as per the table.
PoE Detection and Classification
PoE is becoming widely adopted for networking and VoIP
telecom applications. A typical PD solution is shown in
Figure 1 and has a PoE interface circuit and a DPA-Switch
DC-DC converter block (see EPR-68 for full details of
operation of the DC-DC converter).
The PoE specification requires the PD to implement
three functions: detection, classification and pass-switch
connection.
Detection occurs as the input voltages rises from 2.5 to
10 VDC. Resistor R31 within the PD presents the detection
impedance to the PSE.
Ethernet
(RJ-45)
D101
Connector DL4002
POUT
(min)
POUT
(max)
ICL
(min)
ICL
(max)
R34
W
W
mA
mA
Ohms
0
0.44
12.95
0.5
4
-
1
0.44
3.84
9
12
133
2
3.84
6.49
17
20
69.8
3
6.49
12.95
26
30
45.3
Class
Components R32, R33, R34, R35, Q31, Q32, Q33, Q34, VR31
and U31 are not installed for Class 0 applications, and the
classification current is programmed by R31 (24.9 k).
Operation
Resistor R31 provides the detection impedance. In order to allow
correct operation over the detection voltage range, Zener diode
C4
1000 pF R7
1500 V 10 kΩ
L1
10 µH
1A
PoE Interface
(1,2)
R32
100 kΩ
D105
DL4002
(3,6)
DL4002
D106
D107
DL4002
(7,8)
Q31
R33
2 kΩ
1%
Q33
LMV431A1M
DL4002
D104
MMBTAO6L
Q32
R36
51 kΩ
U31
1%
R34
69.8 Ω
1%
R4
1 MΩ
1%
C1
1.5 µF
100 V
VR32
27 V
MMBTAO6L
(4,5)
VR31
11 V
MMST3904
D103
DL4002
R31
24.9 kΩ
1%
MMBTAO6L
DL4002
D102
9,10
1
R5
649 kΩ
1%
C8
330 µF
6V
C7
330 µF
6V
C10
1 µF
J2-1
10 V 3.3 V, 2 A
L2
1 µH, 2 A
+
C9
330 µF
6V
6,7
2
VR3
SMAJ
150A
D1
SL42
J2-2
RTN
4
3
D2
BAV21
5
C2
1.5 µF
100 V
D
C11
1 µF
50 V
R11
1 kΩ
R10
75 Ω
R21
R23
DPA-Switch 174 kΩ 10 kΩ
1%
U1
DPA423G
Q20
L
MMBTS R22
3906 10 kΩ
CONTROL
R12
34 kΩ
1%
U2
PC357
N1TA
C
R37
220 kΩ
R35
100 kΩ
S
X
F
C5
0.1 µF
C13
0.1 µF
R9
51 Ω
U3
CAT431L
Q34
R6
8.66 kΩ
1%
VR33
15 V
DL4002
D108
C6
22 µF
10 V
Q35
100 V
0.250 mΩ
C12
0.33 µF
R13
20.0 kΩ
1%
PI-4042-111005
Figure 1. PoE Interface Circuit – Class 2.
*UNH-IOC test reports are available on the PI website www.powerint.com/poe
DI-88
www.powerint.com
November 2005
DI-88
of the classification circuit to approximately 20 mW at
high input line (57 VDC) plus detection resistor (R31)
dissipation of 130 mW.
•
•
•
•
For Class 0, remove components VR31, R32, R33, R34,
R35, Q31, Q32, Q33, Q34 and U31.
R34 values: Class 1, R34 = 133 Ω; Class 2, R34 = 68.9 Ω;
Class 3, R34 = 45.3 Ω.
It is possible to use either bipolar transistor or MOSFET
pass-switches (Q35). A bipolar transistor is less expensive,
but a MOSFET gives higher pass-switch efficiency. See
design idea (DI-70) for details.
The bias current source (Q31) is used to allow the classification
current source to be turned-off for minimal power loss once
input voltage exceeds 28 VDC. This limits the dissipation
Current (mA)
0.60
10
12
14
PI-4044-111005
16
20
18
22
Voltage (V)
Figure 3. Classification Current (Class 1:
R34 = 133 Ω).
Class 3 Max
30
23.75 kΩ
0.20
10
Class 2
Class 3
Detection
Voltage
Range
0.40
Classification
Voltage Range
8
Current (mA)
Class 1
Class 2
Class 3
Detect Min
Detect Max
Class 1 Max
12
Class 1 Min
PI-4043-080905
0.80
Class 1
Classification
Voltage
Range
26
PI-4045-111005
Key Design Points
14
Current (mA)
VR31 inhibits the classification circuit at input voltages below
11 V. Components Q32, Q31 and R32 form a 350 µA bias current
source programmed by resistor R33 working in conjunction
with the base-emitter voltage of Q31. Transistor Q33 forms
the classification current source programmed by resistor R34
working in conjunction with the 1.24 V voltage reference U31.
Transistor Q34 disables the classification current source when
Zener diode VR32 conducts (when the input voltage exceeds
approximately 28 V).
Class 3 Min
22
Class 2 Max
18
26.5 kΩ
Class 2 Min
14
0.00
0
2
4
6
8
10
10
12
12
14
16
18
20
22
Voltage (V)
Voltage (V)
Figure 4. Classification Current (Classes 2 & 3:
R34 = 69.8 Ω and R34 = 45.3 Ω).
Figure 2. Detection Impedance V-I Curve.
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