LINEAR dn397f Data Manual

Advanced TCA Hot Swap Controller Monitors Power Distribution Design Note 397 Mitchell Lee Introduction even Schottky rectifi ers present a serious problem in

Advanced TCA® is a modular computing architecture terms of both voltage drop and power dissipation: a developed by the PCI Industrial Computer Manufacturers conducting pair drop more than 1V and dissipate 6W. Group for use in central offi ce telecom environments. Following the diode manufacturer’s recommendations, PICMG® 3.0 defi nes, among other things, the electrical 8 square inches of board area are needed to satisfy the and mechanical attributes of the backplane, connectors heat sinking requirements.

and removable cards in these –48V systems.

Diode dissipation, voltage loss and board area is reduced Each removable card, or front board, is designed for live in Figure 1 by using MOSFETs as active rectifi ers with insertion into a working system. A power draw of up to the LTC4354 diode OR driver. Total dissipation is cut to 200W per front board is allowed, placing the maximum less than 1W for two conducting “diodes” at maximum load current in the 4A to 5A range. load. Card-centric inrush limiting and quantitative current Zero Volt Transient

and voltage monitoring are highly desirable to sanitize The so-called Zero Volt Transient requirement is a legacy the incoming battery feeds, minimize power plane of earlier telecom equipment standards stipulating uninter- disturbances, allow for budgeting power consumption rupted system operation during the course of a 5ms input and permit failure prediction in an otherwise functional voltage dropout. An energy of 1J is needed to sustain a system. The LTC®4261 Hot Swap™ controller provides 200W load during this interval. these features. Also included is a digital interface for

The accepted method of energy storage to satisfy the 1J

controlling the functions of the LTC4261, and for reading

requirement is a bulk reservoir capacitor which is charged

the current and voltage measurement registers.

through resistors. This technique dictates the use of bulky

Circuit Solutions high voltage storage capacitors, such as 100V (or rare 80V)

Figure 1 shows a complete circuit designed to handle up rated units which can handle the maximum input voltage of to the maximum available power. The LTC4261’s accu- 75V. Since the zero volt transient test commences at 44V, rate current limit is set to provide at least 5.5A under all nothing is gained by storing a higher voltage. Compact conditions, a comfortable margin for 200W, yet trips off 50V capacitors are used instead, by limiting the charging just under 7A to pr eserve fuse integrity in the presence voltage with a simple zener-transistor circuit.

of nuisance overloads. At insertion the LTC4261 allows

The ATCA connector pin confi guration presents a special

contact bounce to settle, then soft starts the load using

design challenge. Here extraction is inferred from the dif-

a ramped current. Inrush current is increased gradually

ference between each ENABLE and its associated VRTN,

to a few hundred milliamperes and held there until the

thereby ignoring input dropouts. A PNP transistor pulls

MOSFET is fully on.

up on EN in the event of an ENABLE disconnect, shutting Current is monitored by the SENSE pin and an 8mΩ shunt down the LTC4261 and permitting safe extraction with resistor. Direct measurement of the current is possible via no connector damage. During a zero volt transient, no the I2C port, with 10-bit resolution and 8A full scale. signal reaches the EN pin; power fl ows uninterrupted to the load when the input voltage recovers.

Cutting Diode Dissipation

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

ATCA’s redundant –48V power feeds are combined

Hot Swap is a trademark of Linear Technology Corporation. on-card with ORing diodes. At 5A current consumption All other trademarks are the property of their respective owners.