Linear Technology DC1680B, Manual

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dc1843bfa
DEMO MANUAL DC1843B
DEMONSTRATION CIRCUIT 1842B OPERATION
Figure 5. DC1842B 8-Port PSE Daughter Card with Digital Isolation Features the LTC4290B and LTC4271
8-Port PSE Daughter Card with Digital Isolation
Demonstration circuit 1842B (Figure 5) features the
LTC4290B/LTC4271 chipset on a compact daughter card
with digital isolation. The LTC4290B/LTC4271 chipset is
an 8-port power sourcing equipment (PSE) controller
designed for use in IEEE 802.3at Type 1 and Type 2 (high
power) compliant Power over Ethernet (PoE) systems. A
transformer isolated communication protocol replaces
expensive opto-couplers and complex isolated 3.3V supply
resulting in significant BOM cost savings. The LTC4290B/
LTC4271 chipset delivers lowest-in-industry heat dissipa-
tion by utilizing low R
ON
external MOSFETs and 0.25Ω sense
resistors, eliminating the need for expensive heat sinks.
Advanced power management features in the LTC4290B/
LTC4271 chipset include: per port 12-bit current monitor-
ing ADCs, DAC programmable current limit, and versatile
quick shutdown of preselected ports. PD discovery uses a
proprietary dual mode 4-point detection mechanism ensur-
ing excellent immunity from false PD detection. Midspan
PSEs are supported with 2-event classification and a two
second backoff timer. The LTC4290B/LTC4271 includes
an I
2
C serial interface operable up to 1MHz.
The DC1842B demonstrates proper LTC4290B/LTC4271
board layout that is approximately the height and width
of a 2 × 4 RJ45 connector. The compact layout is made
possible by the small package size of key components. The
LTC4290B is in a 6mm × 6mm QFN, while the LTC4271 is
in a 4mm × 4mm QFN. Each port has a PSMN075-100MSE
MOSFET in a 3mm × 3mm LFPAK33 package.
The daughter card inserts in the DC1680B motherboard
through J1, a polarized 30-pin connector. Isolated 3.3V
and logic control signals are brought in on this connec-
tor. Also connected at J1 is the PoE V
EE
supply from the
motherboard and 8 PSE controlled outputs.