AN296 Silicon_Laboratories, AN296 Datasheet - Page 7

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

AN296
Figure 6. Typical Startup Waveform with 2.5 Ω Load
3.4. Maintain Power Signature
The PSE detects the dc current to the PD by either looking for the low ac impedance of the input filter or making
sure that it is drawing current.
For this reason, the input filter capacitor must be >5 µF, and the load must be such that the input current is >10 mA.
Since the Si3400/01 is designed to be very efficient and dissipate very little power, there is a minimal load current
requirement of 250 mW (50 mA at 5 V output) so as to draw >10 mA from the input supply. It has also been
observed that if the switcher is operated with no load, the switcher tends to pulse on and off, which may be
undesirable. For this reason, it is recommended that a 250 mW load always be present.
3.5. Turn Off
As the PSE reduces input voltage, the PD is required to turn off at 30 V. Failure to maintain power signature or a
system-initiated denial of power to the PD will result in the system cycling back through the detection and
classification phases.
The Si3400/01 has approximately 4 V of hysteresis between turn-on and turn-off with respect to the voltage across
the switcher input filter capacitor so that inrush current at startup will not cause the part to turn off.
Additionally, the Si3400/01 has an early power loss feature where the voltage on the cable side of the diode bridge
is sensed. If this voltage drops to between 25 and 30 V, the power loss signal (PLOSS) is asserted. This allows for
early detection of power removal while the switcher input capacitor is still charged.
Rev. 0.8
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