AdvancedTCA Hot Swap Controller Monitors Power Distribution Design Note 397 Mitchell Lee Introduction AdvancedTCA ® is a modular computing architecture developed by the PCI Industrial Computer Manufacturers Group for use in central office telecom environments. PICMG® 3.0 defines, among other things, the electrical and mechanical attributes of the backplane, connectors and removable cards in these –48V systems. Each removable card, or front board, is designed for live insertion into a working system. A power draw of up to 200W per front board is allowed, placing the maximum load current in the 4A to 5A range. Card-centric inrush limiting and quantitative current and voltage monitoring are highly desirable to sanitize the incoming battery feeds, minimize power plane disturbances, allow for budgeting power consumption and permit failure prediction in an otherwise functional system. The LTC ®4261 Hot Swap™ controller provides these features. Also included is a digital interface for controlling the functions of the LTC4261, and for reading the current and voltage measurement registers. Circuit Solutions Figure 1 shows a complete circuit designed to handle up to the maximum available power. The LTC4261’s accurate current limit is set to provide at least 5.5A under all conditions, a comfortable margin for 200W, yet trips off just under 7A to preserve fuse integrity in the presence of nuisance overloads. At insertion the LTC4261 allows contact bounce to settle, then soft starts the load using a ramped current. Inrush current is increased gradually to a few hundred milliamperes and held there until the MOSFET is fully on. Current is monitored by the SENSE pin and an 8mΩ shunt resistor. Direct measurement of the current is possible via the I2C port, with 10-bit resolution and 8A full scale. Cutting Diode Dissipation ATCA’s redundant –48V power feeds are combined on-card with ORing diodes. At 5A current consumption 09/06/397 even Schottky rectifiers present a serious problem in terms of both voltage drop and power dissipation: a conducting pair drop more than 1V and dissipate 6W. Following the diode manufacturer’s recommendations, 8 square inches of board area are needed to satisfy the heat sinking requirements. Diode dissipation, voltage loss and board area is reduced in Figure 1 by using MOSFETs as active rectifiers with the LTC4354 diode OR driver. Total dissipation is cut to less than 1W for two conducting “diodes” at maximum load. Zero Volt Transient The so-called Zero Volt Transient requirement is a legacy of earlier telecom equipment standards stipulating uninterrupted system operation during the course of a 5ms input voltage dropout. An energy of 1J is needed to sustain a 200W load during this interval. The accepted method of energy storage to satisfy the 1J requirement is a bulk reservoir capacitor which is charged through resistors. This technique dictates the use of bulky high voltage storage capacitors, such as 100V (or rare 80V) rated units which can handle the maximum input voltage of 75V. Since the zero volt transient test commences at 44V, nothing is gained by storing a higher voltage. Compact 50V capacitors are used instead, by limiting the charging voltage with a simple zener-transistor circuit. The ATCA connector pin configuration presents a special design challenge. Here extraction is inferred from the difference between each ENABLE and its associated VRTN, thereby ignoring input dropouts. A PNP transistor pulls up on EN in the event of an ENABLE disconnect, shutting down the LTC4261 and permitting safe extraction with no connector damage. During a zero volt transient, no signal reaches the EN pin; power flows uninterrupted to the load when the input voltage recovers. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Data Sheet Download www.linear.com (408) 432-1900 1630 McCarthy Blvd., Milpitas, CA 95035-7417 Linear Technology Corporation dn3mlf LT/TP 0906 305K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 ● For applications help, call (408) 432-1900, Ext. 2452 –48V_B –48V_A ENABLE_A ENABLE_B VRTN_B VRTN_A 7A 7A MEDIUM LONG MEDIUM SHORT 1M 100k SHORT 1M 100k VDB– 1μF VDB+ SHORT LONG 10A LONG 10A VDA– 1μF VDA+ FDS3672 GB VSS VSS FDS3672 FDS3672 DB GA 2k 2k 10k SMBT70A 1μF D: 1N4148WS HZS5C1 1μF NC 22nF 100V 91Ω 100nF SENSE GND 100k 100nF VFB VCC ITH/RUN NGATE LTC3803ES6 22k 10.2k 100nF 137k D 33Ω • 2.49k 330nF VIN 1.1k 1.1k 1.1k 1.1k 8mΩ 47nF 1μF IRF1310NS 1M 1k + 2.4k ×7 33nF 10nF 100V PG SCL SDAI SDAO ALERT PGIO PGI ADIN FMMT591 PDS5100H GATE DRAIN RAMP 10k 10Ω SENSE LTC4261CGN TMR VEE VDB– 1μF VDB+ VDA– 1μF VDA+ 330nF UVH UVL ADIN2 OV ON INTVCC FLTIN EN ADR1 ADR0 SS D D 100nF • • PULSE P0926 107k ZVN3320F 100Ω 100nF 100V 100Ω Figure 1. AdvancedTCA Hot Swap Controller with I2C Current and Voltage Monitor DA 2k LTC4354CS8 FMMT5401 D 100k VCC GA DB DA FMMT5401 D 100k D 2k LTC4354CS8 VCC FDS3672 GA DB DA VSS VSS VSS VSS D VCC LTC4354CS8 8.2k D 1mF 50V ×4 DN397 F01 –48VOUT BZT52C47 –48VRTN(OUT)