Although the part's programmable filter delay times are generally adequate to handle most overcurrent transients, a simple R-C filter placed across the current sense resistor prevents nuisance tripping in very harsh "48V environments. Using a telecom-like printed circuit board, the values of R22 and C12 were determined empirically by switching in a separate "48V feed (-48VB) 10% higher in voltage than that already powering the board. The time constant of R22 and C12 is:
This time constant adds to the pre-programmed delay in the circuit breaker configuration register located in the device.
Another concern raised by board insertion or the addition or removal of a "48V feed is the change in voltage experienced at the DRAIN SENSE pin. This pin senses the drain voltage of the MOSFET during both the live-insertion and steady state intervals. An abrupt voltage increase on this pin causes the hot swap controller to immediately de-assert the PG# outputs and shutoff the MOSFET gate drive. Adding a small valued capacitor of 0.01uF-0.1uF from this pin directly to the VSS pins prevents nuisance tripping and unwarranted downtime without otherwise jeopardizing normal operation.
External Slew-Rate Limiting Component Selection
The static and dynamic parasitic capacitances of the N-Channel MOSFET require the addition of a few external components particularly since it is driven by a current source (100uA for the SMH48xx Family). When a card is live-inserted, the VGATE output is high impedance until adequate voltage (>9V) is present on the VDD pin. This condition can allow the MOSFET to turn on if the bus produces enough gate voltage to exceed the turn-on threshold voltage of the MOSFET. This is prevented by adding 2 capacitors: one across the drain to gate terminal (C2), another across the gate to source terminal (C3). The resistor (R27) in series with C2 forms a negative feedback network with C2 to control the slew-rate current and total soft-start interval. Choose C2 to limit the inrush current during the soft-start interval:
CBulk is the total capacitance on the drain side the MOSFET (include that present in the DC-DC converters).
IVGATE is the nominal gate drive current from the VGATE pin (100uA).
IInrush is the maximum desired current passing through the MOSFET during the soft-start interval.
Note: The time required to charge the bulk capacitance is inversely proportional to the inrush current.
Choose C3 to prevent the MOSFET from turning on before the VGATE output becomes active:
VIN(MAX) is the maximum bus input voltage.
VGS(th) is obtained from the MOSFET data sheet.
Choose the maximum value of R27 that does not interfere with the soft-start action resulting from IVGATE and C2:
Reducing R27 below 1k-ohm diminishes its usefulness for snubbing and damping abrupt voltage changes so prevalent in "48V Telecom environments.
EMI filters are used to suppress any noise riding on the "48 Volts supply and/or any noise coming from the card supplies that may disturb the card itself or another card in the system. Often times these filters are required to meet conducted (and radiated) emissions standards. They are placed where the "48V supplies come onto the card and also between the DC/DC converters and the hot swap controller.
The advantages of split EMI filtering may be undermined by the introduction of excessive inrush currents into the filter residing on the "48V bus (see scope photo, Figure 6B). The cause of the inrush current is the CX (line to line) capacitor, Figure 5. To minimize the inrush current choose a filter with the lowest possible CX capacitor value. To ensure the inrush current does not disturb the "48V bus, either add a capacitor with a value 5 times that of the filter's CX capacitor nearby the system card on the motherboard or backplane. Another option is to remove input filter and only use switched side DC/DC filter. Figure 6A shows the Inrush current without the input EMI filter.
Figure 5 " Internal schematic of the EMI Filters.
Figure 6A No EMI Filter before the SMH4804, 5A/div.
Figure 6B "20A EMI Filter, 5A/div.