Current-limit switch is digitally programmable

By Budge Ing, Maxim Integrated Products, Sunnyvale, CA -- EDN, 8/17/2000

Current-limit switches are virtually ubiquitous in system controls. They provide a safe means for regulating the current delivered to a load circuit. The switches allow the load current to increase to a programmed limit but no higher. Typically, the current limit is a function of the voltage across an external resistor, produced by the current from a fixed source internal to the switch IC. This voltage serves as the reference for an internal current-limiting amplifier. By replacing the resistor with a digital potentiometer, you can easily program the current limit (Figure 1). IC₁ is a current-limit switch with a maximum programmable limit of 1A. The limit equals 1380/R_SET, where R_SET is the resistance between pins 5 and 6 of IC₂. IC₂ is a 50-kW digital potentiometer whose resistance is programmable in 32 equal increments. With  held low, high-to-low transitions at  (Pin 1) increments IC₂'s internal counter.

These transitions increase the resistance between W and L when  is low and reduce it when  is high. IC₁ includes a thermal-shutdown capability that turns the load current completely off when the chip temperature exceeds 135°C. It restores the load current when the temperature cools by 10°C. If the short-circuit fault is still present, the switch cycles off and on, yielding a pulsed load current. An open-drain Fault output (Pin 8) switches low when the load demands current beyond the programmed limit, enabling an external system to monitor the condition of the current switch. (DI #2577)