PTB48540 Datasheet, PDF (5/12 Pages) Texas Instruments – 10-W Power-Over-Ethernet Isolated Power Module Assembly

Application Notes
PTB48540 Series
Operating Features of the PTB48540
Series Power-Over-Ethernet Modules
Overview
Figure 1-1 shows the block diagram of the PTB48540
series of Power-over-Ethernet (PoE) modules. Input
power to the module can be supplied through either the
Data Line A/B or Spare Line A/B associated Ethernet
connections. A diode bridge associated with each of these
input pairs allows the input source to be supplied in ei-
ther polarity. A transient suppressor, located across the
common output of the diode bridges, protects the module
against power surges.
The input power to the internal DC/DC converter is
controlled by the TPS2375 IC. This IC is a power inter-
face switch, specifically designed for use with PoE powered
devices. The IC provides the device detection, classification,
and current limiting control that is necessary for a powered
device (PD) to comply with the IEEE 802.af Standard.
The DC/DC converter input circuit includes an EMI
filter, which maintains the module in compliance with
CISPR 22 (EN5022); class ‘B’ for radiated, and class ‘A’
for conducted emissions.
PD Detection
Prior to power up, the PoE power sourcing equipment
(PSE) must detect a 24.9-kΩ “discovery” load resistance
from the PD. This default value of resistance is neces-
sary for the PD to be recognized as a “valid device” by
the PSE. A 24.9-kΩ resistor is provided internally to the
PTB48540 modules. It is located across the ‘Detect A’
(pin 5) and ‘Detect B’ (pin 6) terminals. By placing an
external resistor across these pins (in parallel with the
internal 24.9-kΩ resistor), the module can be made to
communicate a “non-valid device” signature to the PSE.
A non-valid device is recognized by a resistance of less than
12 kΩ. Connecting a 16.9-kΩ external resistor between
the ‘Detect A’ and ‘Detect B’ pins creates an equivalent
resistance of 10 kΩ. This is sufficient to communicate a
non-valid device signature. The external resistor should
not be less than 16.9 kΩ as this increases power dissipa-
tion in the power interface IC.
PD Classification Signature
The PSE uses a classification current to determine the
maximum supply current that the PD is allowed to draw.
The classification current is sensed by the PSE when the
supply voltage to the PD is between 15 V and 20 V. The
classification current is set on the PTB48540 by a pro-
gramming resistance. Table 1-1 gives the resistance values
Figure 1 1; Block Diagram of the PTB48540 Series Modules
Output Inhibit 13
Input Ref 4
Detect A 5
External
RDETECT
Detect B 6
Data Line A 10
Data Line B 9
Spare Line A 8
Spare Line B 7
Class B 11
External
RCLASS
Class A 12
PD Detect
24.9 k
(Valid Device)
Class Select
4.42 k
(Class 0)
TPS2375
PoE Interface
1
ILIM
VDD ⁸
2 _CLASS
NC ⁷
3 _{DETECT EN_DC} 6
4 _VEE
RTN ⁵
Input EMI Filter
1500 VDC
Isolation
DC/DC
Converter
Inhibit
Adjust
+VIN
+VOUT
2 VOUT Adj
3 + VOUT
–VIN
–VOUT
1 –VOUT
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