Datasheet
LTC4267-1
18
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
Transient Voltage Suppressor
The LTC4267-1 specifies an absolute maximum voltage of
100V and is designed to tolerate brief overvoltage events.
However, the pins that interface to the outside world can
routinely see excessive peak voltages. To protect the
LTC4267-1, install a transient voltage suppressor (D3)
between the input diode bridge and the LTC4267-1 as
shown in Figure 9. A SMAJ58A is recommended for typical
PD applications. However, a SMBJ58A may be preferred in
applications where the PD front-end must absorb higher
energy discharge events.
Classification Resistor Selection (R
CLASS
)
The IEEE specification allows classifying PDs into four
distinct classes with class 4 being reserved for future use
(Table 2). An external resistor connected from R
CLASS
to
V
PORTN
(Figure 4) sets the value of the load current. The
designer should determine which power category the PD
falls into and then select the appropriate value of R
CLASS
from Table 2. If a unique load current is required, the value
of R
CLASS
can be calculated as:
R
CLASS
= 1.237V/(I
DESIRED
– I
IN_CLASS
)
where I
IN_CLASS
is the LTC4267-1 IC supply current during
classification and is given in the electrical specifications.
The R
CLASS
resistor must be 1% or better to avoid de-
grading the overall accuracy of the classification circuit.
Resistor power dissipation will be 50mW maximum and
is transient so heating is typically not a concern. In order
to maintain loop stability, the layout should minimize
capacitance at the R
CLASS
node. The classification circuit
Figure 10. PD Front End with Isolation Transformer, Tw o Schottky Diode Bridges
1
3
2
RX
–
SPARE
–
6
RX
+
3
TX
–
2
TX
+
J2
IN
FROM
PSE
T1
RJ45
1
7
8
5
4
6
8
7
TXOUT
+
OUT
TO PHY
TXOUT
–
SPARE
+
RXOUT
+
RXOUT
–
16
14
15
11
9
10
NOTES: UNLESS OTHERWISE SPECIFIED
1. ALL RESISTORS ARE 5%
2. SELECT R
CLASS
FOR CLASS 1-4 OPERATION. REFER
TO DATA SHEET APPLICATIONS INFORMATION SECTION
C2: AVX 1808GC102MAT
D9 TO D12, D14 TO D17: DIODES INC., B1100
T1: PULSE H2019
R
CLASS
1%
R
CLASS
V
PORTN
D13
MMSD4148
C11
0.1µF
100V
D6
SMAJ58A
R30
75Ω
C24
0.01µF
200V
R31
75Ω
C25
0.01µF
200V
R1
75Ω
C7
0.01µF
200V
R2
75Ω
C3
0.01µF
200V
C2
1000pF
2kV
D10
B1100
D12
B1100
10Ω
D9
B1100
D11
B1100
D17
B1100
D16
B1100
D15
B1100
D14
B1100
V
PORTP
LTC4267-1
42671 F10