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PoE Performance Characteristics
5. The VCC regulator powers up during the inrush sequence. During VCC regulator startup, it draws
current on the order of 20mA, but this will likely not be noticed by the user. Once the RTN pin of the IC
drops below 1.5V (referenced to VEE), and the gate of the hot swap MOSFET rises, power good is
asserted by pulling the nPGOOD pin low.
6. Once power good has been asserted, the SS (Soft-Start) pin is released. The SS pin will rise at a rate
equal to the SS current source, typically 10 µA, divided by the SS pin capacitance, CE26.
7. After the soft start is complete, the switching regulator achieves output regulation, and the converter
enters steady state operation. The auxiliary winding will raise the VCC voltage to about 10.5V, thus
shutting down the internal regulator and increasing efficiency.
Figure 5
shows key waveforms during a normal PoE power up sequence. Please note that the PSE used
in the test goes through detection mode, but opts out of classification mode and directly enters full power
application mode.
Horizontal Resolution: 50 ms/div.
Trace 1: PoE input voltage across the VIN and VEE pins. 20V/div.
Trace 2: Voltage across the RTN and VEE pins, namely the voltage across the Hot Swap MOSFET. 20V/div.
Trace 3: The input current. 0.2A/div.
Trace 4: The 3.3V output voltage. 2V/div.
Figure 5. PoE Power Up Sequence with a Midspan PSE
22.2 Auxiliary Input Power-up Sequence
The AUX input power up sequence is simpler:
1. AUX power application quickly charges the input capacitors. The AUX input inrush limit resistors limit
the inrush current and prevent any overshoot of the voltage across the VIN and RTN pins.
2. When the VCC regulator starts up, the PWM controller begins soft start.
3. After soft start, the switching regulator achieves output regulation, and the converter enters steady
state operation. The auxiliary winding will raise the VCC voltage to about 10.5V, thus shutting down the
internal regulator and increasing efficiency.
Figure 6
shows key waveforms during a normal AUX power up sequence.
11
SNOA476C – October 2006 – Revised April 2013
AN-1521 POEPHYTEREV-I / -E Evaluation Board
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