Source Power
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10
Source Power
To fully test the evaluation board, either a high power PSE able to supply 30W or a lab bench DC power
supply capable of at least 60V and 1A is required for the PoE input. For the AUX source power, use a 24V
AC adapter or a DC power supply capable of 30V and 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.
11
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 7.3A. Exceeding this current at low input voltage may cause
oscillatory behavior as the circuit will go into current limit mode. Exceeding this current at high input
voltage may force the DC-DC converter to run into cycle by cycle peak current limit. Current limit mode is
triggered whenever the average current through the PoE connector exceeds 800 mA (setting is
determined by RE23; see LM5072 Integrated 100V Pwr Over Ethernet PD Interface & PWM Cntrl w/Aux
Support Data Sheet (
SNVS437
for details). The circuit then runs into a retry mode (hiccups). Cycle-by-
cycle peak current limit mode narrows the duty cycle and hence the output voltage loses regulation and
enters an under voltage condition. In both current limit modes, the circuit will not be latched off and normal
operation will be automatically restored after the removal of the fault condition.
12
Power Up
For the first time power up, it is recommended to apply PoE power first. The load should be kept
reasonably low (under 25% of full load). 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 RE22 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, full power can be applied. A voltmeter
across the output terminals JE5 (3.3V) and JE6 (3.3V RTN) will allow direct measurement of the 3.3V
output line. If the 3.3V output voltage is not observed within a few seconds, turn off the power supply and
review connections. A final check of efficiency is the best way to confirm that the circuit is operating
properly. Efficiency being significantly lower than 80% at full load indicates a problem.
After proper PoE operation is verified, the user may apply AUX power. It is recommended that the
application of AUX power 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.
13
PD Interface Operating Modes
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.
See LM5072 Integrated 100V Pwr Over Ethernet PD Interface & PWM Cntrl w/Aux Support Data Sheet
(
SNVS437
) for details.
14
Signature Detection
The 25 k
Ω
PD signature resistor is integrated into the LM5072 IC. The PD signature capacitor is realized
by CE29, a 100 nF capacitor. During AUX power operation, CE29 also improves the noise immunity of the
IC substrate (interconnected to the VEE pin) by providing a low impedance path to the COM node.
It should be noted that when AUX power is applied first, it will not allow the PSE to identify the PD as a
valid device because the AUX voltage will cause the current steering diode bridges BR1 and BR2 to be
reverse biased during detection mode. This prevents the PSE from applying power, so the evaluation
board will only draw current from the AUX source.
8
AN-1521 POEPHYTEREV-I / -E Evaluation Board
SNOA476C – October 2006 – Revised April 2013
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