LM5072EVAL National Semiconductor, LM5072EVAL Datasheet - Page 3

• Manufacturer Part Number: LM5072EVAL
• Description: BOARD EVALUATION LM5072
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5072EVAL

• Main Purpose: Special Purpose DC/DC, Power Over Ethernet
• Outputs And Type: 1, Isolated
• Power - Output: 9.9W
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 38 ~ 60V
• Regulator Topology: Flyback
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5072
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Connection and Proper Test Methods
Figure 2 shows the connections for the LM5072 evaluation
board.
The LM5072 evaluation board has the following four ports for
connections.
For the PoE input, two diode bridges (BR1 and BR2) steer
the current to the positive and negative supply pins of the
LM5072. For both FAUX and RAUX inputs, the higher po-
tential input voltages should feed into the center pins of the
PJ102A jacks, or to P1 and P3, respectively. It should be
pointed out that P2 and P4, the returns for the FAUX and
RAUX inputs, should not be interchanged because they do
not represent the same potential in the circuit. The RAUX pin
is not reverse protected, and an additional reverse blocking
diode will be required for complete RAUX input reverse
protection.
For the output connection, the load can be either a passive
resistor or active electronic load. Attention should be paid to
the output polarity when connecting an electronic load. Use
of additional filter capacitors greater than 20 µF total across
the output port is not recommended unless the feedback
loop compensation is adjusted accordingly.
Sufficiently large wire such as AWG #18 or thicker is required
when connecting the source supply and load. Also, monitor
the current into and out of the circuit board. Monitor the
voltages directly at the board terminals, as resistive voltage
drops along the connecting wires may decrease measure-
ment accuracy. Never rely on the bench supply’s voltmeter
or ammeter if accurate efficiency measurements are desired.
When measuring the dc-dc converter efficiency, the con-
verter input voltage should be measured across C4, as this
is the input to the converter stage. When measuring the
evaluation board overall efficiency (which is more relevant),
both input and output voltages should be read from the
terminals of the evaluation board.
• J1, the RJ45 connector for PoE input
• J2, a PJ102A power jack, for Front Auxiliary (FAUX) input
• J3, the other PJ102A power jack, for Rear Auxiliary
• The 3.3V output port accessible with posts J4 and J5
(also accessible with posts P1 and P2 located immedi-
ately behind the jack)
(RAUX) input (also accessible with posts P3 and P4
immediately behind the jack)
FIGURE 2. LM5072 Evaluation Board Connections
3
Source Power
To fully test the LM5072 evaluation board, a DC power
supply capable of at least 60V and 1A is required for the PoE
input. For the auxiliary source power, either FAUX or RAUX,
use a DC power supply capable of 3A. Use the output
over-voltage and over-current limit features of the bench
power supplies to protect the board against damage by
errant connections.
Loading / Current Limiting
Behavior
A resistive load is optimal, but an appropriate electronic load
specified for operation down to 2.0V is acceptable. The
maximum load current is 3.3A. Exceeding this current at low
input voltage may cause oscillatory behavior as the part will
go into current limit mode. Current limit mode is triggered
whenever the average current through the PoE connector
exceeds 440 mA (default nominal). The current limit can be
programmed to any desired level up to 800 mA by selecting
a resistor value for R23 (see the LM5072 datasheet for
further details). If current limit is triggered, the switching
regulator is automatically disabled by discharging the soft-
start capacitor C26 through the SS pin. The module is then
allowed to restart, but the unit will operate in an automatic
re-try (hiccup) mode as long as the over-current condition
remains. Switching regulator shut down during a fault condi-
tion such as current limit can be delayed by adding additional
filtering capacitance to the nPGOOD pin.
Power Up
It is suggested to apply PoE power first. During the first
power up, the load should be kept reasonably low. Check the
supply current during signature and classification modes
before applying full power. During signature mode, the mod-
ule should have the I-V characteristics of a 25 kΩ resistor in
series with two diodes. During classification mode, current
draw should be about 700µA at 16V; the RCLASS pin is left
open, defaulting to class 0. If the proper response is not
observed during both signature and classification modes,
check the connections closely. If no current is flowing it is
likely that the set of conductors feeding PoE power have
been incorrectly installed.
Once the proper setup has been established, full power can
be applied. A voltmeter across the output terminals J4
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