AN296 Silicon_Laboratories, AN296 Datasheet - Page 6

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

AN296
Over the range of 14.5 to 20.5 V, the PD current during the classification stage must be as shown in Table 1.
The classification current for the Si3400/01 is set by the resistor, Rclass (R3 in Figures 3 and 4).
3.3. Powerup
During detection and classification, the PD must isolate the switcher input filter and not apply power to the load.
After completion of this phase, the PSE ramps up to between 44 and 57 V, and PD turns on by closing the internal
hot swap switch.
The PD must turn on at an input voltage of 42 V. After turning on, this voltage can drop to 37 V due to cabling
resistance.
The Si3400/01 hot swap switch has a two-step current limit. The input filter capacitor is first charged to within about
1.3 V of its final value at a typical current of 150 mA. When the filter input capacitor is nearly charged, the current
limit is increased to over 400 mA, and the switcher is allowed to turn on. The hot swap switch operates at the
higher current limit as long as the input filter is charged to about 30 V to allow for any switcher startup transients.
The Si3400/01 is designed so that the hot swap switch current limit is generally not the limitation in terms of the
amount of power the PD can draw. This limit is intended to be set by the switching regulator and load or by the
power sourcing equipment.
However, to limit inrush current as the switcher turns on, the switcher design supports soft-start operation, which is
further described in the detailed switcher descriptions of this application note.
Figure 6 shows the input current and output voltage vs. time when 48 VDC is suddenly applied to the PD circuit.
The initial current spike is due to the charging of the 0.1 µF input capacitor. For this particular device, the filter
capacitor charges up with a current limit of 108 mA in 17 msec. At this point, the current limit increases, and the
capacitor is allowed to fully charge the last 1.3 V, resulting in a brief current spike.
6
Class
0
1
2
3
4
Table 1. Classification Stage PD Current
Minimum
Current
17
26
36
0
9
Rev. 0.8
Maximum
Current
12
20
30
44
4
Units
mA
mA
mA
mA
mA