Datasheet

LTC4265
12
4265fa
APPLICATIONS INFORMATION
OVERVOLTAGE LOCKOUT
The LTC4265 includes an overvoltage lockout (OVLO)
feature (Figure 5) which protects the LTC4265 and its load
from an overvoltage event. If the input voltage exceeds the
OVLO threshold, the LTC4265 discontinues PD operation.
Normal operations resume when the input voltage falls
below the OVLO threshold and when C1 is charged up.
THERMAL PROTECTION
The IEEE 802.3af/at specifi cation requires a PD to withstand
any applied voltage from 0V to 57V indefi nitely. However,
there are several possible scenarios where a PD may
encounter excessive heating.
During classifi cation, excessive heating may occur if the
PSE exceeds the 75ms probing time limit. At turn-on, when
the load capacitor begins to charge, the instantaneous
power dissipated by the PD Interface can be large before
it reaches the line voltage. And if the PD experiences a
fast input positive voltage step in its operational mode
(for example, from 37V to 57V), the instantaneous power
dissipated by the PD Interface can be large.
14
13
12
1
2
3
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
COILCRAFT
ETHI - 230LD
4265 F07
1
7
8
5
4
10
9
11
5
6
4
D3
SMAJ58A
TVS
BR1
HD01
BR2
HD01
TO PHY
GND
LTC4265
C1
V
IN
V
OUT
V
OUT
SPARE
–
SPARE
+
C14
0.1µF
100V
Figure 7. PD Front-End with Isolation Transformer, Diode Bridges,
Capacitors, and a Transient Voltage Suppressor (TVS).
The LTC4265 includes a Thermal Protection feature which
protects the LTC4265 from excessive heating. If the
LTC4265 junction temperature exceeds the over-tempera-
ture threshold, the LTC4265 discontinues PD operations
and power-good becomes inactive. Normal operation
resumes when the junction temperature falls below the
over-temperature threshold and when C1 is charged up.
EXTERNAL INTERFACE AND COMPONENT SELECTION
Transformer
Nodes on an Ethernet network commonly interface to the
outside world via an isolation transformer. For PDs, the
isolation transformer must also include a center tap on
the RJ45 connector side (See Figure 7).
The increased current levels in a Type-2 PD over a Type-1
increase the current imbalance in the magnetics which
can interfere with data transmission. In addition, proper
termination is also required around the transformer to
provide correct impedance matching and to avoid radiated
and conducted emissions. Transformer vendors such as