Power Electronics

Hot-Swap Power Controllers Continue To Make Progress

Hot-swap power controllers have usually protected circuit boards and telecom/networking systems from damage when cards or boards are either removed or installed under live condition. Their ability to limit inrush current or break circuits has permitted users to safely insert or pull out boards from live -48V backplanes, PCI bus or redundant power systems without having to shut down the system power.

Such power controllers have continued to evolve. Consequently, manufacturers are further improving these devices and adding more features and functionalities to provide new advancements to address myriad needs of telecom, networking and other applications. To meet the growing demands of designers, lately suppliers such as Texas Instruments Inc., Motorola, Linear Technology Corp., Maxim Integrated Products and Micrel Semiconductor have all unveiled new members in their respective hot-swap product lines.

While Texas Instruments has announced an intelligent hot-swap controller that eliminates the need for OR-ing diodes in redundant -48V systems, Motorola's newest member integrates on-chip power MOSFETs. And Linear Technology's latest I2C hot — swap controller allows a board to be safely inserted and removed from a live CompactPCI bus slot. Likewise, Maxim's version offers high reliability and greater fault protection. Micrel Semiconductor, on the other hand, has implemented power supply tracking and sequencing to tackle power supply requirements of today's complex digital boards.

Besides eliminating the OR-ing diodes, TI's TPS2350 hot-swap power manager also achieves higher efficiency and protection for redundant -48V telecom power supplies. Aimed at distributed power systems such as wireless base stations and central office switching, this simple-to-program device also minimizes system power dissipation and voltage drop. The TPS2350 hot-swap power manager operates with supply voltages ranging from -12V to -80V, and can withstand spikes up to -100V.

Permitting precise control of current and voltage during turn-on and operation, the TPS2350 uses two power field-effect transistors (FETs) as low-voltage drop diodes to efficiently select between two redundant power supplies. It incorporates a third FET to provide hot-swap load current slew rate control and peak magnitude limiting that is programmed by a single resistor and a capacitor. The controller shown in Fig.1, uses less than 10 external components. Traditional diode OR-ing methods for connecting redundant power supplies often result in power losses and uncontrolled inrush over-current.

Allowing a significantly less amount of power loss, TI's controller integrates — on a single chip — programmable features such as undervoltage and overvoltage limits, circuit breaking and a fault timer. The low loss of power achieved through the TPS2350 greatly enhances applications designed to support PCI Industrial Computer Manufacturers Group's (PICMG) Advanced Telecom Computing Architecture (AdvancedTCA) specification. Plus, it permits application developers to meet the 1% undervoltage lockout (UVLO) specifications of the AT&T Network Equipment Development Standards (NEDS v4.0) and the American National Standard (ANSI 315) for Telecommunications. TPS2350 also provides a power-good output to enable down-stream power converters and a fault output to indicate load programs. TPS2350 comes in a 14-pin thin shrink small outline package (TSSOP). And it is priced at $1.90 per 1,000 units.

Higher Integration

Targeting telecom and networking equipment makers, Motorola Semiconductor has released two new negative voltage hot-swap power controllers. Motorola's MC33652/3 are crafted to minimize the use of external components. Toward that goal, MC33652/3 integrate on-chip power MOSFET and other protective and programmable functions. The controllers are offered in miniature SOIC packages. The integration of FET, along with compact packaging can reduce board space by about 60%, claims the developer.

Other salient features of Motorola's controller include overcurrent limit with auto retry (default or programmable), charging current limit (default or programmable), start-up and current fault timer (programmable option on the MC33652), overvoltage and undervoltage detection (programmable option on the MC33652), active high and low-power good output signals, disable input control with active-high or -low, and thermal shutdown. The programmable undervoltage and overcurrent detection circuitry monitors the input voltage to check that it is within its operating range. To control the inrush current, the units employ a two-level current limit technique.

Maximum output current capability for MC33652 is 2.0A, while MC33653 is rated for 1.0A. Input voltage range for these controllers is -15V to -80V. The MC33652 is offered in a 16-pin SOIC, and MC33653 comes in an eight-pin version. In 10,000 pieces, the MC33652 is priced at $1.75, and MC33653 at $1.34.

Designed for -48V telecom power backplanes, Maxim's hot-swap controller, MAX5920/21 and MAX5939, offer unprecedented reliability and fault protection. While MAX5920 serves as a drop-in, second source for LT1640 and LT4250, MAX5921 and MAX5939 are pin-compatible with LT1640 and LT4250 with enhanced features. Aside from withstanding -100V input voltage transients, Maxims controllers are also immune to input voltage steps caused by battery switchover in telecom systems. During an input voltage step, these devices limit the current drawn by the load to a safe level without turning off the power to the load, says the maker.

For better fault protection, the MAX5921 and MAX5939 exponentially pull-down the gate current during severe overcurrent fault condition. In case of short circuit, these devices provide a fast, 450mA gate pull-down current. To protect the Gate of the external MOSFET from high-voltage potentials, these controllers also feature an internal voltage clamp. Other features include input undervoltage and output overvoltage protection, thermal shutdown, and power-good status. Operating over the temperature range of -40°C to +85°C, Maxim's hot-swap controllers come in 8-pin SO packages. In lots of 1000, the MAX5920 is priced at $1.47, and MAX5921/MAX5939 cost $1.80.

Unlike others, Linear Technology's controller LTC4240 targets compactPCI bus voltages. As shown in Fig.2, it controls all four CompactPCI supplies by employing two external N-channel transistors for the 3.3V and 5V supplies, while two on-chip switches regulate the ±12V supplies. Electronic circuit breakers protect all four supplies against overcurrent faults and a power-good output indicates when all of the supply voltages are within tolerance.

The LTC4240 has a built-in two-wire I2C compatible interface that permits software control and monitoring of device functions as well as power supply status. It also gives the user the ability to turn the device On or Off, set the RESETOUT, turn on a status LED driver and ignore ±12V faults. The part also provides a pre-charge bias for the I/O pins and combines the PCI_RST# signal with the HEALTHY# to make the LOCAL_PCI_RST# signal, and meet the tight PCI timing requirements. This eliminates the need for external logic.

The LTC4240 is housed in a 28-pin narrow SSOP package and is available for both commercial and industrial temperature ranges. Pricing starts at $4.25 each for 1,000-piece quantities.

Tracking And Sequencing

Heeding the call of today's complex digital circuits, Micrel Semiconductor's latest dual-channel hot-swap power controllers bring power supply tracking and sequencing capability to the boards using positive voltages. Besides standard fault protection, the MIC2584/85 also offer designers a flexible means of controlling the sequencing of supply voltages to various ICs in a system. In fact, these controllers offer true tracking of each supply relative to the other without using external clamping diodes.

Both the devices control inrush current during board insertion, and preserve system voltage regulation requirements. A single-pin enable controls both channels and permits the orderly removal of external power FET drive upon board removal. In addition to providing system protection against overcurrent fault conditions, the MIC2585 also monitors each output for overvoltage and undervoltage events. In order to support multiple power supplies, the MIC2585 can also be easily cascaded.

The supply voltage range for Maxim's hot-swap controllers is from 1.0V to 13.2V with input surge protection up to 20V. While MIC2584 comes in 16-pin TSSOP package, the MIC2585 is encased in a 24-lead TSSOP. In 10,000-piece quantities, the MIC2584 is priced at $2.95, and MIC2585 at $3.16, respectively.

Texas Instruments, Dallas, Tx; www.ti.com
CIRCLE 344 on Reader Service Card

Motorola, Phoenix, Ariz.; www.Motorola.com
CIRCLE 345 on Reader Service Card

Maxim Integrated Products, Sunnyvale, Calif.
www.maxim-ic.com; CIRCLE 346 on Reader Service Card

Linear Technology, Milpitas, Calif.; www.linear.com
CIRCLE 347 on Reader Service Card

Micrel Semiconductor, San Jose, CA; www.micrel.com
CIRCLE 348 on Reader Service Card

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