ltc4266a Linear Technology Corporation, ltc4266a Datasheet - Page 25

• Manufacturer Part Number: ltc4266a
• Description: Quad Poe/poe+/ltpoe++ Pse Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
V
Because it supplies a relatively large amount of power and
is subject to significant current transients, it requires more
design care than a simple logic supply. For minimum IR
loss and best system efficiency, set V
amplitude (57V), leaving enough margin to account for
transient over- or undershoot, temperature drift, and the
line regulation specs of the particular power supply used.
Bypass capacitance between AGND and V
portant for reliable operation. If a short-circuit occurs
at one of the output ports it can take as long as 1μs for
the LTC4266A/LTC4266C to begin regulating the current.
During this time the current is limited only by the small
impedances in the circuit and a high current spike typically
occurs, causing a voltage transient on the V
possibly causing the LTC4266A/LTC4266C to reset due to
a UVLO fault. A 1μF , 100V X7R capacitor placed near the
V
Isolating the Serial Bus
The LTC4266A/LTC4266C includes a split SDA pin (SDAIN
and SDAOUT) to ease opto-isolation of the bidirectional
SDA line.
EE
EE
pin is recommended to minimize spurious resets.
is the main supply that provides power to the PDs.
0.22μF
V
EE
100V
C77
10μF
C76
63V
10Ω
10Ω
R58
R60
+
C78
0.22μF
100V
2200pF
C79
R54
56k
4.7k
R51
1%
EE
1
3
Figure 15. Positive V
near maximum
ITH/RUN
V
R53
4.7k
EE
SUMIDA CDRH5D28-101NC
1%
FB
EE
is very im-
LTC3803
supply and
GND
V
CC
5
2
100μH
L3
NGATE
SENSE
10μF
C75
16V
6
4
DD
R57
1k
IEEE 802.3 Ethernet specifications require that network
segments (including PoE circuitry) be electrically isolated
from the chassis ground of each network interface device.
However, network segments are not required to be isolated
from each other, provided that the segments are connected
to devices residing within a single building on a single
power distribution system.
For simple devices such as small PoE switches, the isola-
tion requirement can be met by using an isolated main
power supply for the entire device. This strategy can be
used if the device has no electrically conducting ports
other than twisted-pair Ethernet. In this case, the SDAIN
and SDAOUT pins can be tied together and will act as a
standard I
If the device is part of a larger system, contains additional
external non-Ethernet ports, or must be referenced to
protective ground for some other reason, the Power over
Ethernet subsystem (including all LTC4266A/LTC4266Cs)
must be electrically isolated from the rest of the system.
Figure 16 shows a typical isolated serial interface. The
SDAOUT pin of the LTC4266A/LTC4266C is designed to
drive the inputs of an opto-coupler directly. Standard I
SMBus devices typically cannot drive opto-couplers, so U1
is used to buffer the signals from the host controller side.
Boost Converter
Q15
FDC2512
R59
0.100Ω
1%, 1W
B1100
D28
100μF
6.3V
C74
2
FMMT723
C/SMBus SDA pin.
4266AC F15
LTC4266A/LTC4266C
Q13
SUMIDA CDRH4D28-100NC
R52
3.32k
1%
R55
806Ω
1%
10μH
L4
R56
47.5k
1%
Q14
FMMT723
10μF
6.3V
C73
3.3V AT 400mA
25
4266acfa
2
C/