DPA422-426
D41
BAV19WS
20 V, 10 mA
7.5 V, 0.4 A
C41
4.7 µF, 35 V
VR41
6.8 V IN4148
D42
Ethernet
(RJ-45)
4
3
6
D101
D31
L1
Connector
DL4002
PoE Interface
20CJQ060
1 µH 2.5 A
1
3
8
(1,2)
7
C31
100 µF
10 V
Q22
Si4804
7
6
C25
VR51
28 V
L2
16 µH 4 A
DL4002
D102
C22-C24
100 µF 5 V
R4 1 µF
C21
2.2 nF
5 V, 2.4 A
RTN
160 Ω
10 V
R1
649 kΩ
1%
5
2
8
R21
10 Ω
R22
7
4
R52
20 kΩ
D6
10 Ω
BAV
D103
DL4002
5
19WS
Q21
Si4804
VR21
D21
SL13
15 V
T1
(4,5)
C1
1 µF
100 V
C2
1 µF
100 V
R23
174 k
1%
C6
4.7 µF
20 V
R23
10 kΩ
R51
24.9 kΩ
DL4002
D104
1% 1/4 W
R21
10 k
U2
Q20
MMBTS3906
R11
10 kΩ
R16
R22
10 kΩ
10 kΩ
D105
DL4002
U2
PC357
N1T
1%
(3,6)
DPA-Switch
C51
1 nF
50 V
C12
R12
U1
100 nF
D51
150 Ω
DPA424PN
L
D
DL4002
D106
BAV19
R13
11 Ω
CONTROL
C
D11
BAV19WS
D52
BAV19
R3
R14
1 kΩ
S
X
F
C11
2.2 µF
10 V
C13
68 nF
VR1
SMAJ
150
1.0 Ω
D107
DL4002
C4
220 nF
R53
20 kΩ
R54
20 Ω
C5
47 µF
10 V
U3
R15
(7,8)
R2
LM431AIM3
10 kΩ
13.3 kΩ
1%
1%
Q51
PI-3824-031008
TIP29C (100 V/1 A)
or MMBTA06
DL4002
D108
Figure 27. PoE Interface Circuit Using a Bipolar Transistor Pass-Switch and DPA424P.
controlled pass-switch. By adding this circuitry to the front end
of a DPA converter, a low cost and low component count PoE
powered device (PD) power supply can be implemented.
Figure 27 shows a typical PD solution.
Once the three start up phases have been successfully
completed, the DPA-Switch is allowed to function as a forward
converter (described in Figure 25 and accompanying text).
Key Application Considerations
The PoE specification requires the PD to provide three
fundamental functions: discovery, classification, and pass-
switch connection.
DPA-Switch Design Considerations
Power Table
This section provides a description of the assumptions used to
generate the power tables (Tables 1 and 3 through 6) and
explains how to use the information provided by them.
When input voltage is applied to the PD, it must present the
correct discovery signature impedance in the voltage range of
2.5 VDC to 10 VDC. This impedance is provided by R51 in
Figure 27.
All Power tables: Tables 1 and 3 through 6
•ꢀ Maximum output power is limited by the device internal
current limit. This is the peak output power which could
become the continuous output power, provided adequate
heat sinking is used.
•ꢀ Data assumes adequate heat sinking to keep the junction
temperature at or below 100 °C and worst case RDS(ON) at
TJ = 100 °C.
•ꢀ The use of P and G packages are recommended for device
dissipation equal to or less than 1.5 W only due to package
thermal limitation. For device dissipation above 1.5 W, use
R package.
The second “classification” phase occurs at input voltages
15 VDC to 20 VDC. The PD must draw a specified current to
identify the device class (Class 0 specifies 0.5 mA to 4 mA).
This is again accomplished by resistor R51.
In the third phase, the bipolar pass-switch (Q51 in Figure 27)
connects the input voltage to the power supply at voltages
above approximately 30 VDC (28 V+VR52). Zener diode VR51
conducts, driving the current through resistor R52 to the base
of Q51. Resistor R53 prevents turn-on under other conditions.
Once the Power supply has started, components D51, D52,
C51 and R54 enhance the base-current drive by coupling
power from the power supply bias winding.
16
Rev. T 12/12
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