Controller with Programmable Power and Current Limits

Jan 1, 2004 12:00 PM
By Ashok Bindra, Editor, Power Electronics Technology

Although high-voltage hot-swap controllers for positive systems have been around for a few years, the product line's latest entrant brings flexibility, programmability and accuracy at a low cost. Unlike controllers that use voltage ramp-up to keep inrush current to the board within safe limits, Texas Instruments (TI) has deployed a power ramp technique to permit safe insertion or removal of boards under live conditions. TI also has exploited its newest 0.7-µm LBC silicon-on-insulator (SOI) analog process to ensure complete MOSFET safe operating area (SOA) protection. The result is a high-voltage hot-swap power manager featuring programmable power- and current-limit functions for positive 9-V to 80-V systems, with accuracy of ±10%. Maximum voltage is 100 V.

Labeled TPS2490 and TPS2491, the controllers are housed in a 10-pin, 3-mm × 5-mm MSOP package. They support and protect emerging positive high-voltage distributed power systems, such as 12-V, 24-V and 48-V server back planes, storage area networks, medical systems, plug-in modules and wireless base stations.

Built to safely drive an external low-R_DS(on) N-channel MOSFET switch inside a selected SOA under harsh operating conditions, they offer adjustable power- and current-limit function. While current limit can be set to maximum full-load current, power limit controls junction-temperature rise, and overload time controls case-temperature rise.

This monolithic device also provides other features needed for a positive hot-swap controller, including: programmable undervoltage lockout/logic to enable a source low-voltage shutdown or system-level logic control; adjustable turn-on inrush limit; adjustable overload time-out (also called an electronic circuit breaker); charge-complete indicator for downstream converter coordination; and an optional automatic restart mode. Furthermore, it offers latched operation (TPS2490) and automatic retry (TPS2491); a programmable fault timer to eliminate nuisance shutdowns; a power good open-drain output that provides downstream dc-dc coordination; and fast current limit to protect input distribution during output short.

Exploiting its advanced trench isolation technique, the 0.7-µm LBC-SOI thick copper process combines CMOS, bipolar and DMOS power components, as well as polysilicon resistors, poly-poly capacitors, Schottky diodes and three levels of metal interconnect to deliver efficient integrated single-chip solutions. The LBC-SOI process also offers many advantages, including higher packing density, smaller parasitic capacitances, inherent latch-up immunity and the ability to eliminate substrate currents. The TPS2490's and TPS2491's programmable power-limiting feature helps ensures the external FET never operates outside its SOA in applied voltage, current and time. During normal operation, the external FET works with the gate-to-source voltage maximized to provide the lowest possible channel resistance. During start-up and short-circuit events, the gate-to-source voltage is modulated to provide a defined turn-on time and to prevent damage to the external FET. A timer function limits how long the device will be in power-limit mode. The power-limit circuitry, which monitors the drain current and drain-to-source voltage of the external FET, computes the power dissipation and controls the gate-to-source voltage to prevent the FET from dissipating power greater than the user-programmed level. When the drain-to-source voltage of the external FET is low, the circuitry operates in a current-limit mode, preventing the drain current from surpassing the user-programmed level. In quantities of 1000, the controller is priced at $1.70.

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