AN296 Silicon_Laboratories, AN296 Datasheet - Page 16

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

AN296
Note: Input current at 50 mA/div and output voltage at 1 V/div.
Figure 13. Isolated Mode Startup with 5 W Load
5. Surge
The Si3400/01 has an input clamp that will protect it against surges as spelled out in IEEE 802.3af.
IEEE 802.3af specifies a 1000 V surge with 0.3 µsec rise time and 50 µsec fall time applied to each conductor
through a series resistance of 402 Ω. Because this pulse is generally applied to all conductors, the differential
current at the input is generally very limited. The Zener clamp itself can withstand about 1 A of surge for 50 µsec.
The Si3400/01 is designed to handle a 50 µsec, 5 A pulse that would result from applying the surge to either both
Tx or Rx pairs and grounding the other pair. This is accomplished by turning on the hot swap switch while disabling
the switcher if current flows in the input clamp. During the 50 µs transient, a large portion of the input energy is
redirected to the switcher input capacitor. For this reason, a 15 µF minimum input capacitor is recommended.
The Si3400/01 is also required to survive the application of telephony ringing voltage. 802.3af specifies 56 V dc +
175 V peak ringing applied through 400 Ω source impedance at a frequency of 20 to 60 Hz. In this case, the
switcher could turn on during the ringing application, which would be very undesirable and could cause damage to
the switcher FET. To prevent this from happening, the switcher is actively shut down when there is any current
>1 mA flowing through the clamp.
Continuous application of such a large ringing signal will damage the Si3400/01 (although it will not cause a safety
hazard). However, such a large ringing signal should also cause a "ring trip" or apparent off-hook indication at the
central office within 200 msec. It has been found that the Si3400/01 can withstand application of telephony ringing
for over one second before damage occurs; so, in general, telephony ringing will not cause damage.
Refer to “AN315: Robust Electrical Surge Immunity for PoE PDs through Integrated Protection Output” for more
information on surge test immunity results.
In some applications, up to 16 kV of system-level ESD immunity is required. The standard Si3400/01 EVB designs
meet this requirement when the input is not powered. However, when the input is powered and the Si3400/01 is
producing an output through the dc-dc converter, damage may occur to the input diode bridges for ESD events
above 4 kV when applied to the output terminals if C10 to C17 are not used. Capacitors C10 to C17 allow passing
system-level ESD events in excess of 16 kV.
For isolated applications that require a high level of system-level ESD immunity, the capacitors are recommended.
For non-isolated applications, it is generally not possible for an ESD event (at the output supply) to occur because
the output terminals of the dc-dc converter are generally not accessible while input power is applied. However,
even for non-isolated designs, there is a possibility large ESD events may reach the power supply terminals, in
which case capacitors (C10 to C17) are also recommended.
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