5
UG-1339 Rev 0
DEMO MANUAL DC2685A
DEMONSTRATION CIRCUIT 2684A DAUGHTER CARD
Figure 4. DC2684A Digital and Analog Isolation
Demonstration circuit 2684A is an IEEE 802.3bt 8-channel
PSE daughter card that features the LTC4292/LTC4291
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 LTC4292/LTC4291 chipset operation, robustness,
and measurement accuracy. It is imperative to fol-
low the layout requirements specified in the LTC4292/
LTC4291 data sheet. The DC2684A demonstrates proper
LTC4292/LTC4291 board layout on a compact daughter
card approximately the height and width of a 2 × 4 RJ45
connector.
Isolation and Power Supplies
The LTC4292/LTC4291 chipset provides communication
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
DD
logic supply and V
EE
PoE
supply is brought in at the 30-pin connector J1.
V
DD33
V
DD
T1
DND
CPD
CND
DPD
AGNDP
V
EE
DNA
V
EE
V
EE
ISOLATION
WÜRTH 7490100143
CPA
30-PIN
CONNECTOR
DC2684 SIDE
MOTHERBOARD SIDE
V
DD
SUPPLY
V
EE
SUPPLY
CNA
DPA
TX
+
RX
–
CT(4)
CT(2)
RX
+
TX
–
TD
+
RD
–
CT(3)
CT
RD
+
TD
–
U2
LTC4291
U1
LTC4292
C23
1µF
C21
0.1µF
R13
100Ω
DC2685A F04
DGND
C19
1µF
100V
D1
SMAJ58A
R14
100Ω
C22
1µF
R15
100Ω
R16
100Ω
R21
100Ω
R35
10Ω
R22
100Ω
R23
100Ω
R24
100Ω
C24, 2.2nF
2kV
TVS
BULK
C
BULK
+–
+–
+
