ltc4259a Linear Technology Corporation, ltc4259a Datasheet - Page 28

• Manufacturer Part Number: ltc4259a
• Description: Quad Ieee 802.3af Power Over Ethernet Controller With Ac Disconnect
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC4259A
components such as the sense resistors (R
bly software running on an external microprocessor,
using the LTC4259A in a PSE does not guarantee 802.3af
compliance. Using an LTC4259A does get you most of the
way there. This section discusses the rest of the elements
that go along with the LTC4259A to make an 802.3af
complaint PSE. Each paragraph below addresses a com-
ponent which is critical for PSE compliance as well as
possible pitfalls that can cause a PSE to be noncompliant.
For further assistance please contact Linear Technology’s
Applications department.
Sense Resistors
The LTC4259A is designed to use a 0.5Ω sense resistor,
R
the sense resistor has been minimized in order to reduce
power loss and as a consequence, the voltage which the
LTC4259A must measure is small. Each port may be
drawing up to 450mA with this current flowing through the
sense resistor and associated circuit board traces. To
prevent parasitic resistance on the circuit board from
obscuring the voltage drop across the sense resistor, the
LTC4259A must Kelvin sense the resistor voltage. One
way to achieve Kelvin sensing is “star grounding,” shown
pictorially in Figure 1. Another option is to use a – 48V
power plane to connect the sense resistor and the
LTC4259A V
28
S
, to monitor the current through each port. The value of
ISOLATED
–48V
ISOLATED
EE
GND
V
EE
pin. Either of these strategies will prevent
10Ω
+
U
CMPZ4702B
1µF
100V
U
910k
W
Si2328DS
510Ω
10µF
16V
Figure 21. –48V to 3.3V Boost Converter
S
), and possi-
U
V
S/S
IN
8
1
LT1619
DRV
GND
7
4
SENSE
GATE
V
C
FB
6
3
47k
4700pF
5
2
voltages developed across parasitic circuit board resis-
tances from affecting the LTC4259A current measure-
ment accuracy. The precision of the sense resistor directly
affects the measurement of the IEEE parameters I
I
ance, use a resistor with 0.5% or better accuracy.
Power MOSFETs
The LTC4259A controls power MOSFETs in order to
regulate current flow through the Ethernet ports. Under
certain conditions these MOSFETs have to dissipate sig-
nificant power. See the Choosing External MOSFETs sec-
tion for a detailed discussion of the requirements these
devices must meet.
Common Mode Chokes
Both nonpowered and powered Ethernet connections
achieve best performance (for data transfer, power trans-
fer and EMI) when a common mode choke is used on each
port. In the name of cost reduction, some designs share a
common mode choke between two adjacent ports. Even
for nonpowered Ethernet, sharing a choke is not recom-
mended. With two ports passing through the choke, it
cannot limit the common mode current of either port.
Instead, the choke only controls the sum of both ports’
common mode current. Because cabling from the ports
generally connects to different devices up to 200m apart,
LIM
100µH
100Ω
, I
CUT
100pF
and I
Si2328DS
0.100Ω
1%
1W
FMMT593
B1100
MIN
. Therefore, to maintain IEEE compli-
1.24k
1%
3.32k
1%
+
FMMT593
4259A F21
100k
47µF
10V
4.7µH
+
ISOLATED
GND
47µF
10V
V
3.3V
300mA
DD
INRUSH
4259afb
,