2
dc1788bfa
DEMO MANUAL DC1788B
Table 2. DC1788B Performance Summary
PARAMETER CONDITION VALUE
PD Input Voltage After Start-Up (VPORT) At the PD Ethernet Port
LTPoE++ 38.7W 49.8V to 57V
LTPoE++ 52.7W 47.8V to 57V
LTPoE++ 70W 45.1V to 57V
LTPoE++ 90W 41.0V to 57V
IEEE 802.3at (Type 2, 25.5W) 42.5V to 57V
IEEE 802.3af (Type 1, 13W) 37V to 57V
Efficiency DC1788B-A, VPORT = 48V, ILOAD = 1.1A 99.1% (Typical)
DC1788B-B, VPORT = 48V, ILOAD = 600mA 98.9% (Typical)
DC1788B-C, VPORT = 48V, ILOAD = 350mA 99.1% (Typical)
PoE Type Switching Frequency An LTPoE++ PSE is Powering a DC1788B-A 840Hz
performance summary
theory of operation
WhenanLTPoE++PSEis connected tothe DC1788Bdemo
board RJ45 connector, J1, via a CAT5e or CAT6 Ethernet
cable, the PSE initiates the detection process. During this
process, the discrete active bridge acts as a standard
diode bridge and the PD solution presents a 25k resistive
load to the PSE. After the PD has passed the detection
process, the PSE uses an IEEE 802.3at/IEEE 802.3af or
LTPoE++ classification method to determine the power
level the PSE can deliver to the PD. The PSE turns on the
port and provides power to the PD if the classification is
successful. During turn on, the MOSFETs of the discrete
active bridge are fully enhanced with low RDS(ON) in the
highcurrentpath.Thisreducesthepowerandheatdissipa-
tion compared to the conventional diode bridge. Once the
LT4275 detects the PSE has turned on the port, it drives
an external N-channel MOSFET to ramp up the output
voltage. After a successful PD controller output turn on,
the LT4275 drives a power good indicator (PWRGD) and
a PSE Type indicator (T2P) (DC1788B-A and DC1788B-B
only).The PWRGDindicator signals the loadto startdraw-
ing current and the T2P indicator signals the load the type
of PSE powering the PD.
Classification Signaling
The resistors at RCLS and RCLS++ determine the clas-
sification signature and sequence. The DC1788B-A in-
cludes user-selectable jumpers to support IEEE 802.3af,
IEEE 802.3at, and LTPoE++ power levels from 3.84W to
90W. Refer to Table 3 for the power levels. If an LTPoE++
power level is selected, an IEEE 802.3af or IEEE 802.3at
compliant PSE will classify this PD as a Class 4 PD. The
DC1788B-BincludesapreselectedRCLS resistortosupport
IEEE802.3at2-eventclassificationandthisboarddoesnot
contain jumper blocks. The DC1788B-C includes multiple
jumpers to support all IEEE 802.3af power levels. Table 3
showseachshuntpositionanditsassociatedclassnumber
and power level for the DC1788B-A and -C boards. Use
Table3 as a selection guide to choose a suitable power
level for the load.
Table 3. DC1788B-A and DC1788B-C Shunt Positions for PoE
Power Levels
ASSEMBLY
PoE
CLASS
POWER LEVEL AT
THE PD INPUT RCLASS JUMPERS
DC1788B-A & -C 0 13W JP1 JP2
DC1788B-A & -C 1 3.84W JP3 JP4
DC1788B-A & -C 2 6.49W JP5 JP6
DC1788B-A & -C 3 13W JP7 JP8
DC1788B-A 4 25.5W JP9 JP10
DC1788B-A 4* 38.7W JP11 JP12
DC1788B-A 4* 52.7W JP13 JP14
DC1788B-A 4* 70W JP15 JP16
DC1788B-A 4* 90W JP17 JP18
*An LTPoE++ PD will be classified as Class 4 by an IEEE 802.3at/af
compliant PSE.
