Performance Summary; Theory Of Operation - Linear Technology DC1788B Demo Manual

DEMO MANUAL DC1788B

performance summary

Table 2. DC1788B Performance Summary
PARAMETER
PD Input Voltage After Start-Up (V
Efficiency
PoE Type Switching Frequency

theory of operation

When an LTPoE ++ PSE is connected to the DC1788B demo
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 R
high current path. This reduces the power and heat dissipa-
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 PWRGD indicator signals the load to start draw-
ing current and the T2P indicator signals the load the type
of PSE powering the PD.
Classification Signaling
The resistors at R and R
CLS
sification signature and sequence. The DC1788B-A in-
cludes user-selectable jumpers to support IEEE 802.3af,
2
) At the PD Ethernet Port
PORT
IEEE 802.3at (Type 2, 25.5W)
IEEE 802.3af (Type 1, 13W)
DC1788B-A, V
DC1788B-B, V
DC1788B-C, V
An LTPoE ++ PSE is Powering a DC1788B-A
in the
DS(ON)
determine the clas-
CLS++
CONDITION
LTPoE ++ 38.7W
LTPoE ++ 52.7W
LTPoE ++ 70W
LTPoE ++ 90W
= 48V, I = 1.1A
PORT LOAD
= 48V, I = 600mA
PORT LOAD
= 48V, I = 350mA
PORT LOAD
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-B includes a preselected R
IEEE 802.3at 2-event classification and this board does not
contain jumper blocks. The DC1788B-C includes multiple
jumpers to support all IEEE 802.3af power levels. Table 3
shows each shunt position and its associated class number
and power level for the DC1788B-A and -C boards. Use
Table 3 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
PoE
ASSEMBLY CLASS
DC1788B-A & -C
DC1788B-A & -C
DC1788B-A & -C
DC1788B-A & -C
DC1788B-A
DC1788B-A
DC1788B-A
DC1788B-A
DC1788B-A
*An LTPoE ++ PD will be classified as Class 4 by an IEEE 802.3at/af
compliant PSE.
VALUE
49.8V to 57V
47.8V to 57V
45.1V to 57V
41.0V to 57V
42.5V to 57V
37V to 57V
99.1% (Typical)
98.9% (Typical)
99.1% (Typical)
840Hz
resistor to support
CLS
POWER LEVEL AT
THE PD INPUT RCLASS JUMPERS
0 13W JP1
1 3.84W JP3
2 6.49W JP5
3 13W JP7
4 25.5W JP9
4* 38.7W JP11
4* 52.7W JP13
4* 70W JP15
4* 90W JP17
JP2
JP4
JP6
JP8
JP10
JP12
JP14
JP16
JP18
dc1788bfa