PD Interface Power Up Sequence
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In the case that the RAUX power source may vary over a wide range from under the minimum startup
input voltage of the DC-DC converter to above the maximum rated voltage of the VCC pin of the DC-DC
converter’s controller, the previously mentioned linear regulator should be used.
It should be pointed out that the RAUX power option bypasses the LM5073 hot swap MOSFET and feeds
power directly into the input filter of the DC-DC converter. In order to improve the LM5073’s noise
immunity under RAUX power option, C2 of 0.047µF is used which provides a low impedance path from
the IC substrate the RAUX source return. This explains why both C1 and C2 are used on the board to
realize the required PD signature capacitance.
14.5 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 of the on board DC-DC converter is 3.0A. Higher output power can
be obtained by using an external DC-DC converter. The maximum current through the LM5073 PD
interface’s hot swap MOSFET is 800mA. Exceeding this DC current may cause oscillatory behavior as the
circuit will go into current limit mode. The current limit can be programmed to suit the application, the
setting is determined by R7, see the LM5073 datasheet for details. Current limit may also be achieved by
the DC-DC converter, which will reduce the duty cycle, causing the output voltage to lose regulation. In
both current limit cases, normal operation will be automatically restored after the removal of the fault
condition.
14.6 Power Up
For the first time power up, it is recommended to apply PoE power first without the DC-DC converter
connected. Check the supply current during signature detection and classification modes before applying
full power. During detection mode, the module should have the I-V characteristics of a 25 k
Ω
resistor in
series with two diodes. During classification mode, the current draw should be about 40 mA at 16V, which
is determined by R6 of 31.6
Ω
. This sets the evaluation board to Class 4, which is “reserved for future use”
per IEEE 802.3af, namely the high power application. If the proper response is not observed during both
detection 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, connect the DC-DC converter and full power can be applied. A voltmeter across the output
terminals of the DC-DC converter will allow direct measurement of the output line. If the output voltage is
not observed within a few seconds, turn off the power supply and review connections.
After proper PoE operation is verified, the user may apply FAUX and RAUX power. It is recommended
that the application of Aux powers follow the same precautions as those for PoE power application. If no
output voltage is observed, it is likely that the Aux power feed polarity is reversed. After successful
operation is observed, full Aux power testing can begin.
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PD Interface Power Up Sequence
When connecting into the PoE system, the evaluation board will go through the following operating modes
in sequence: PD signature detection, power level classification (optional), and application of full power.
Signature Detection
The 25 k
Ω
PD signature resistor is integrated into the LM5073 IC. The PD signature capacitor is realized
with both C1 and C2, each of which is 0.047 µF.
Note that when either FAUX or RAUX power is applied first, it will not allow the PSE to identify the PD as
a valid device because the auxiliary voltage will cause the current steering diode bridges BR1 and BR2 to
be reverse biased during detection. So the PSE will not apply power, and the evaluation board will only
draw current from the auxiliary source.
Classification
PD classification can be implemented with R6, which is not populated on the evaluation board and meant
to take the default Class 0. Refer to the LM5073 datasheet to select R6 value.
Depending on the PSE in use, the circuit may or may not go into classification.
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AN-1574 LM5073 Evaluation Board
SNVA214A – April 2007 – Revised April 2013
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