Linear ANALOG DEVICES DC2685B Demo Manual page 5

DEMONSTRATION CIRCUIT 2684A-B DAUGHTER CARD
Demonstration circuit 2684A-B is an IEEE 802.3bt 8-chan-
nel PSE daughter card that features the LTC4291-1/
LTC4292 chipset. This daughter card mates with a 4-Port,
4-Pair PoE motherboard for use in IEEE 802.3bt Type 3 or
4 compliant Power over Ethernet PSE systems.
Board Layout
Parts placement, sense resistor block and Kelvin sens-
ing, copper fill, trace routing, and spacing are crucial
for LTC4291-1/LTC4292 chipset operation, robustness,
and measurement accuracy. It is imperative to follow
the layout requirements specified in the LTC4291-1/
C21
0.1µF
V
DD33
CPD
U2
LTC4291-1
CND
DPD
DND
DGND
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V
DD
T1
– –
RX RD
R13
100
CT(4) CT(3)
C23
R14
1µF
100
+ +
RX RD
– –
TX TD
R15
100
CT(2) CT
C22
R16
1µF
100
+ +
TX TD
C24, 2.2nF
WÜRTH 7490100143
2kV
V
EE
Figure 4. DC2684A-B Digital and Analog Isolation
DEMO MANUAL DC2685B
LTC4292 data sheet. The DC2684A-B demonstrates
proper LTC4291-1/LTC4292 board layout on a compact
daughter card approximately the height and width of a 2
× 4 RJ45 connector.
Isolation and Power Supplies
The LTC4291-1/LTC4292 chipset provides communica-
tion across an isolation barrier through a data transformer
(Figure 4). All digital pins reside on the digital ground
reference and are isolated from the analog PoE supply on
the daughter card layout. A V
supply is brought in at the 30-pin connector J1.
CPA
R21
U1
100
LTC4292
AGNDP
R22
100
C19
CNA
1µF
100V
DPA
R23
100
R24
V
100 EE
DNA
V
EE
ISOLATION
logic supply and V
DD
30-PIN
CONNECTOR
DC2684 SIDE MOTHERBOARD SIDE
+
V SUPPLY
–
DD
R35
10
+
C
BULK
D1
TVS, 58V
DC2685B F04
PoE
EE
+
V SUPPLY
–
EE
TVS
BULK
Rev A
5