Hall-effect hot-swap controller debuts
By Paul Rako, Technical Editor -- 8/17/2006
The new ACS760 series of ICs from Allegro Microsystems incorporates Hall-effect sensing to control inrush and protect the power buses in blade servers and other applications. This IC provides a major advantage over other approaches, which use a dropping resistor to yield a current measurement. These sensing resistors are expensive, take up precious board space, and generate heat. Using a copper-board trace as the resistor is futile because the large TCR (temperature coefficient of resistance) of copper, as well as the variation in board production make this method inaccurate and unreliable. In addition, engineers must design copper traces leading to the resistors to remove the resistor’s heat. This approach takes up even more board space and creates a hot area that customers may perceive as a design shortcoming. When you use these resistors in blade servers and other high-density systems, all the heat from the sensing resistors can increase the budget for cooling fans and other peripherals. Heat also shortens semiconductor lifetimes in direct proportion to the heat at which they operate.
In addition, any dropping-resistor scheme must galvanically connect to the bus, unless you design in complex and expensive isolation circuits. Optoelectonics all age significantly over their lifetime, making a stable, accurate, long-term isolation scheme a challenge to design. The Allegro ICs sense the magnetic field that the current flowing in the conductor creates, and they isolate the chip’s power and output from the high-current bus. This approach can allow isolation to the bus if a user designs in the simpler isolation scheme for the bus NFET.
The ACS760ELF-20B can provide 240 VA of protection and inserts only 1.5 mΩ of resistance into the bus. In addition to controlling the gate of the N-channel high-side bus FET, it provides an analog-voltage output proportional to the signal. It also contains an overcurrent comparator that you can use to provide a digital trip point. An internal charge pump provides above-the-rail voltage to control an N-channel high-side FET. The IC operates at 10.8 to 13.2V. The analog-signal bandwidth is 50 MHz, and the circuit, including 12 pins for the dc bus, fits into a thin, 24-pin QSOP.
The first part in a future family of parts, the ACS760 targets low-voltage bus control, so it does not take advantage of the isolation that Hall sensing provides. Instead, it differentially senses the voltage at the load, so that it can account for the power calculations to provide thorough bus control. Allegro expects the ACS760ELF-20B to sell for $2.48 (10,000).
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