lm5072mhx-80 National Semiconductor Corporation, lm5072mhx-80 Datasheet - Page 12

• Manufacturer Part Number: lm5072mhx-80
• Description: Integrated 100v Power Over Ethernet Pd Interface And Pwm Controller With Aux Support
• Manufacturer: National Semiconductor Corporation

www.national.com
Detection Signature
During detection mode, a PD must present a signature resis-
tance between 23.75 kΩ and 26.25 kΩ to the PoE power
sourcing equipment. This signature impedance distinguishes
the PD from non-PoE capable equipment to protect the latter
from being accidentally damaged by inadvertent application
of PoE voltage levels. To simplify the circuit implementation,
the LM5072 integrates the 24.5 kΩ signature resistor, as
shown in Figure 4.
FIGURE 4. Detection Circuit With Integrated PD Signature
Figure 5 shows the LM5072’s implementation of PD classifi-
cation using an external resistor connected to the RCLASS
pin. During classification, the voltage across the VIN and VEE
pins is between 13V and 23.5V. In this voltage range, the
class resistor RCLASS is engaged by enabling the 1.25V
buffer amplifier and MOSFET. After classification is complete,
the voltage from the PSE will increase to the normal operating
voltage of the PoE system (48V nominal). When V
0 (Default)
Class
1
2
3
4
Resistor
FIGURE 5. PD Classification – Fulfilled With a Single External Resistor
TABLE 2. Classification Levels and Required External Resistor Value
Reserved
0.44W
0.44W
3.84W
6.49W
From
PD Max Power Level
Reserved
12.95W
12.95W
3.84W
6.49W
20184618
To
IN
rises
12
During detection mode, the voltage across the VIN and VEE
pins is less than 10V. Once signature mode is complete, the
LM5072 will disengage the signature resistor to reduce power
loss in all other modes.
Classification
Classification is an optional feature of the IEEE802.3af spec-
ification. It is primarily used to identify the power requirements
of a particular PD device. This feature will allow the PSE to
allocate the appropriate available power to each device on the
network. Classification is performed by measuring the current
flowing into the PD during this mode. IEEE 802.3af specifies
five power classes, each corresponding to a unique range of
classification current, as presented in Table 2. The LM5072
simplifies the classification implementation by requiring a sin-
gle external resistor connected between the RCLASS and
VEE pins to program the classification current. The resistor
value required for each class is also given in Table 2.
above 23.5V, the LM5072 will disengage the RCLASS resis-
tor to reduce on-chip power dissipation.
The classification feature is disabled when either the front or
rear auxiliary power options are selected, as the classification
function is not required when power is supplied from an aux-
iliary source. The classification function is also disabled when
the LM5072 reaches the thermal shutdown temperature
threshold (nominally 165°C). This may occur if the LM5072 is
17 mA
26 mA
36 mA
From
0 mA
9 mA
ICLASS Range
12 mA
20 mA
30 mA
44 mA
4 mA
To
20184619
RCLASS Value
LM5072
71.5Ω
46.4Ω
31.6Ω
Open
130Ω