1
dc1788bfa
DEMO MANUAL DC1788B
Description
LT4275
LTPoE++, IEEE 802.3at/
IEEE 802.3af Compliant
PD Controller
Demonstration circuit 1788B features the
, a fourth
generation powered device (PD) controller for Power over
Ethernet (PoE) applications.
The DC1788B is available in DC1788B-A, DC1788B-B, and
DC1788B-C versions to meet the power level required by
the PD application. The DC1788B-A features the LT4275A
PD controller. This controller supports the IEEE 802.3at
(Type 2, PoE+), IEEE 802.3af (Type 1, PoE) and LTPoE++™
specification. LTPoE++ adds four power levels to the ex-
isting IEEE standard with 38.7W, 52.7W, 70W, and 90W
of delivered PD power at the RJ45 jack. The DC1788B-B
features the LT4275B PD controller and is compliant with
the IEEE 802.3at and IEEE 802.3af specifications. The
DC1788B-C features the LT4275C PD controller and is
compliant with the IEEE 802.3af specification.
The DC1788B includes an RJ45 Ethernet port, a high
power Ethernet magnetic, a discrete active bridge recti-
fier, a surge protector, an external N-channel FET, and the
LT4275. The discrete active bridge rectifier is used to
achieve higher efficiency than a conventional diode bridge
rectifier. Many of the main features from the previous gen-
eration PD controller are included in this generation PD
L
, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
LTPoE++ and Hot Swap are trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
controller. These include a power good indicator, a power
sourcing equipment (PSE) type indicator, and support
for an auxiliary power input. The major difference from
the previous generation PD controller is that the LT4275
drives an external N-channel Hot Swap™ FET at the PoE
high side voltage rail. This allows the user to choose a low
R
DS(ON)
N-channel MOSFET to maximize power efficiency,
reduce heat dissipation, and ease thermal design. An LED
status indicator is included to indicate the Hot Swap FET
is fully turned on and the PSE is powering the PD. A suf-
ficient load to sink more than 10mA is also included to
assure the PSE maintains power to the PD and to meet
the DC maintain power signature current required by the
IEEE 802.3at/IEEE 802.3af specification.
Simply connect the output of the DC1788B to the DC/DC
converter that is right for the application. Linear Technology
offers a variety of DC/DC converter solutions that can be
used with the DC1788B (eg DC894, DC1317, etc).
Design files for this circuit board are available at
Table 1. Summary of Features Supported by the DC1788B Assemblies
ASSEMBLY
PoE STANDARD
MAXIMUM POWER
LEVEL
POWER GOOD
INDICATOR (PWRGD)
PSE TYPE INDICATOR
(
T2P
)
AUXILIARY SUPPLY
SUPPORT
DC1788B-A
LTPoE++
PoE+
PoE
90W
Yes
Yes
No
DC1788B-B
PoE+
PoE
25.5W
Yes
Yes
Yes
DC1788B-C
PoE
13W
Yes
No
Yes
