HOT-SWAP CONTROLLER SMH4804: (FIGURE 2) Note: See the SMH4804 Data Sheet, AN21 and the SMH4804EV User's Guide [PDF | HTML] for basic design information.   Nuisance Tripping Avoidance Tips Although the SMH4804'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 value pre-programmed into the SMH4804 circuit breaker configuration register. 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 of the SMH4804. 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 SMH4804 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 parasitic capacitances, both static and dynamic, 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 SMH4804 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 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: Where: 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 SMH4804 VGATE output becomes active: Where: 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 1kΩ diminishes its usefulness for snubbing and damping abrupt voltage changes so prevalent in -48V Telecom environments.

00 1 2 3 4 5 6 7 8 9 10 11 12 13 Next --> SUMMIT Microelectronics, Inc. reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. SUMMIT Microelectronics, Inc. assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained herein reflect representative operating parameters, and may vary depending upon a user's specific application. While the information in this publication has been carefully checked, SUMMIT Microelectronics, Inc. shall not be liable for any damages arising as a result of any error or omission. SUMMIT Microelectronics, Inc. does not recommend the use of any of its products in life support or aviation applications where the failure or malfunction of the product can reasonably be expected to cause any failure of either system or to significantly affect their safety or effectiveness. Products are not authorized for use in such applications unless SUMMIT Microelectronics, Inc. receives written assurances, to its satisfaction, that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; and (c) potential liability of SUMMIT Microelectronics, Inc. is adequately protected under the circumstances. Revision 1.1 - This document supersedes all previous versions. Please check the Summit Microelectronics Inc. web site at www.summitmicro.com for updates.   I2C is a trademark of Philips Corporation. SynQor® is a registered trademark of SynQor, Inc. Copyright © 2003 SUMMIT MICROELECTRONICS, Inc. Power Management for Communications™