AN296 Silicon_Laboratories, AN296 Datasheet - Page 5

• Manufacturer Part Number: AN296
• Description: Using THE Si3400 AND Si3401 POE PD Controllers IN Isolated AND Non-isolated Designs
• Manufacturer: Silicon_Laboratories

3. Basic Detection, Classification, and Power Sequencing
The circuit configurations in Figures 3 and 4 have the same operation during detection, classification, and power
sequencing. A full power cycle is shown in Figure 5.
As will be described in more detail below, a low voltage is used to detect a valid PD; a higher voltage is used to
classify the power level of the PD, and full power operation starts at a voltage of 42 to 57 V.
3.1. Detection
During the detection phase, the PSE applies two voltages between 2.8 and 10 V dc and measures the current (with
a current limit of 5 mA). The slope of the I-V characteristic of the PD must be between 23.75 and 26.25 kΩ. This
slope is set by the resistor, RDET (R4 in Figures 3 and 4). Additionally, the input capacitance must be between 50
and 120 nF, which is set by the capacitor, CDET (C1 in Figures 3 and 4).
The low voltage and current applied in the detect phase as well as the requirement for specific values of resistance
and capacitance makes it unlikely that non-PoE enabled equipment will be recognized if plugged into PSE
equipment that supports PoE.
3.2. Classification
During the classification phase, the PSE applies a voltage between 15.5 and 20.5 V, current-limited to 100 mA, and
determines the maximum output power requirement.
20.5 V
15.5 V
2.8 V
30 V
57 V
42 V
10 V
Class
0
1
2
3
4
Detection
Switcher must turn on
Switcher must turn off
Figure 5. Full Power Cycle
Power Level that the PSE Must Support
Classification
Reserved (Treat as Class 0)
Rev. 0.8
15.4 W
15.4 W
4.0 W
7 W
Apply Power
Turn off
Time
AN296
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