LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 28

• Manufacturer Part Number: LTC4259CGW
• Manufacturer: Linear Technology

Specifications of LTC4259CGW

• Linear Misc Type: Negative Voltage
• Operating Supply Voltage (typ): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
LTC4259
Sense Resistors
The 0.5 resistors (R
a port affect the current limit and all thresholds for a
powered port. An error in the value of a sense resistor can
cause noncompliance to the IEEE specifications for I
I
kept small to minimize power loss, allowing low cost
resistors to be used. As a consequence, the SENSE volt-
ages which the LTC4259 must measure are small. At the
same time up to 450mA may be flowing through the sense
resistors and associated circuit board traces. To prevent
parasitic resistance of the circuit board’s traces from
obscuring the voltage drop across the sense resistor, the
LTC4259 must Kelvin sense the sense resistor voltage.
One way to achieve Kelvin sensing is “star grounding,”
shown pictorially in Figure 1. Another option is to connect
the sense resistor and the LTC4259’s V
power plane in the circuit board. Either or these strategies
will prevent voltages developed across parasitic circuit
board resistances from affecting the LTC4259’s current
sense.
Power MOSFETs
The LTC4259 controls power MOSFETs in order to regu-
late current flow through the Ethernet ports. Under certain
conditions these MOSFETs have to dissipate significant
power. See the Choosing External MOSFETs section 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. Now that two ports pass through the choke, it
cannot limit the common mode current of either port.
Rather the choke only controls the sum of both ports
common mode current. Because cabling from the ports
28
LIM
, I
CUT
and I
MIN
. The value of the R
U
S
) used to sense the current through
U
W
S
resistor has been
EE
pin to a – 48V
U
INRUSH
/
generally connects to different devices up to 200m apart,
a current loop can form. In such a loop, common mode
current flows in one port and out the other, and choke will
not stop it because the sum of the currents is zero. Another
way to view this interaction between the paired ports is
that the choke acts as a transformer coupling the ports
common modes together. In nonpowered Ethernet com-
mon mode current results from nonidealities like ground
loops; it is not part of normal operation. However Power
over Ethernet sends power and hence significant current
through the ports; common mode current is a byproduct
of normal operation. As described in the Choosing Exter-
nal MOSFETs section and under the Power Supplies
heading below, large transients can occur when a port’s
power is turned on or off. When a powered port is shorted
(see Surge Suppressors and Circuit Protection), a port’s
common mode current may be excessive. Sharing a
common mode choke between two ports couples start-
up, disconnect and fault transients from one port to the
other. The end result can range from momentary noncom-
pliance with 802.3af to intermittent behavior and even to
excessive voltages that may damage circuitry (in both the
PSE and PD) connected to the ports.
Detect, AC Blocking and Transient Supressor Diodes
During detection and classification, the LTC4259 senses
the port voltage through the detect diodes, D
16. Excessive voltage drop across D
LTC4259’s detect and classification results. Select a diode
for D
0.4mA and less than 1V of forward drop at 50mA.
The AC blocking diodes can interfere with AC disconnect
sensing if they become leaky. If the AC blocking diode, D
in Figure 16, begins leaking, it contributes to the Ethernet
port impedance, potentially bringing the impedance low
enough to draw I
port powered. More likely, leakage through the AC block-
ing diode will cause shifts in the AC disconnect threshold
that are not large enough to make the PSE noncompliant.
Generally, diode leakage is caused by voltage or tempera-
ture stress. Diodes that are rated to 100V or more and can
DET
that will have less than 0.7V of forward drop at
ACDMIN
from the DETECT pin and keep the
DET
will corrupt the
DET
in Figure
4259i
AC