Demonstration Circuit 2684A Daughter Card - Analog Devices DC2685A Demo Manual

DEMONSTRATION CIRCUIT 2684A DAUGHTER CARD

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/
C21
0.1µF
V
DD33
CPD
U2
LTC4291
C23
1µF
CND
DPD
C22
1µF
DND
DGND
WÜRTH 7490100143
V
DD
T1
– –
RX RD
R13 R21
100 100
CT(4) CT(3)
R14 R22
100 100
+ +
RX RD
– –
TX TD
R15 R23
100 100
CT(2) CT
R16 R24
100 100
+ +
TX TD
C24, 2.2nF
2kV
V
V
EE
Figure 4. DC2684A Digital and Analog Isolation
DEMO MANUAL DC2685A
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
supply is brought in at the 30-pin connector J1.
DC2684 SIDE
CPA
U1
LTC4292
AGNDP
C19
CNA
1µF
100V
DPA
V
EE
DNA
EE
ISOLATION
logic supply and V
DD
30-PIN
CONNECTOR
MOTHERBOARD SIDE
+
V SUPPLY
–
DD
R35
10
+
C
BULK
D1
TVS
SMAJ58A
DC2685A F04
PoE
EE
+
V SUPPLY
–
EE
BULK
UG-1339 Rev 0
5