LM5071HEEVAL National Semiconductor, LM5071HEEVAL Datasheet - Page 3

• Manufacturer Part Number: LM5071HEEVAL
• Manufacturer: National Semiconductor

Specifications of LM5071HEEVAL

• Lead Free Status / Rohs Status: Not Compliant

Connection and Proper Test Methods
Figure 2 shows the connections for the LM5071 evaluation
board.
The LM5071 evaluation board has three ports for connec-
tions. The RJ45 receptacle connector is for PoE input, the
PJ102A power jack for AUX input (also accessible with posts
P3 and P4 located right behind the jack), and the 3.3V output
port accessible with posts J4 and J5.
For the PoE input, two diode bridges BR1 and BR2 steer the
current to the positive and negative supply pins of the
LM5071. For the AUX input, the high potential of the AUX
input voltage should feed into the center pin of the PJ102A
jack.
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 size not smaller than AWG #18 is
required when connecting the source supply and load. Also,
monitor the current into and out of the unit under test.
Monitor the voltages directly at the board terminals, as re-
sistive voltage drops along the connecting wires may de-
crease measurement accuracy. Never rely on the lab supply
voltmeter if accurate efficiency measurements are needed.
When measuring the dc-dc converter efficiency, the con-
verter input voltage should be measured across C4 or C5, as
this is the input of the converter stage. When measuring the
evaluation board overall efficiency, both input and output
voltage should be read from the terminals of the evaluation
board.
Source Power
To fully test the LM5071 evaluation board, a DC power
supply capable of at least 60V and 1A is required for the PoE
input. For the AUX source power, use a DC power supply
capable of 1.5A. Use the output over-voltage and over-
current limit features of the bench power supplies to protect
the board against damage by errant connections.
FIGURE 2. LM5071 Evaluation Board Connections
3
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 current through the PoE connector exceeds
390mA (nominal). 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.
Power Up
It is suggested to apply PoE power first. During the first
power up, the load should be kept reasonably low. Verify 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 600 µA at 15V as 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 the 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
(+3.3V) and J5 (3.3V RTN), will allow direct measurement of
the 3.3V output line. Because the output voltage is isolated,
it should not be measured by a meter referenced to the
bench power supply ground. If the 3.3V output voltage is not
observed within a few seconds, turn the power supply off
and review connections.
A final check of efficiency is the best way to confirm that the
unit is operating properly. Efficiency significantly lower than
80% at full load indicates a problem.
After the PoE operation is verified, apply the AUX power. It is
recommended that the application of the AUX power follow
the same precautions as that of the PoE. If no output voltage
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