EPR-86 Datasheet, PDF (7/28 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP 86 - 6.6 W Multi-Class Powered Device (PD) for Power over Ethernet (PoE) Using DPA-Switch-(DPA423G)

13-Apr-2006
EP-86 – 6.6 W, 3.3 V, 2 A PoE Powered Device
compensation is required to ensure stable operation of the supply and optimum response
to line and load transients. Capacitor C12 performs a soft-finish function that prevents
the output voltage from overshooting the regulation set point during initial startup of the
converter.
4.5 PoE Interface Circuit Description
See DI-88 for a full description. Resistor R26 provides the correct impedance for the
detection phase of PD operation.
Figure 4 – Detection Impedance V-I curve.
The classification circuit is enabled when Zener diode VR6 conducts (above 11 VDC).
Transistor Q9 controls the bias current source programmed to approximately 350 µA by
resistor R21. This bias current source provides the minimum operating current to voltage
reference IC U6. The main classification current flowing through R20 generates a
voltage that is referenced to the internal reference (1.24 VDC) of U6 and that later closes
the loop by controlling the base drive of Q7. The value of the classification current
source is determined by the value of the voltage on the reference pin of U6 divided by the
value of R20 in ohms.
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