One-Touch-Down Circuit for Power Windows and Power Sunroofs: Application Overview

The express open feature of power windows and power sunroofs is becoming common on nearly all passenger cars and trucks. The most common technique switch manufacturers employ is to latch a relay in the energized state with a secondary activation switch. Once the relay is latched, the express, or "one touch" operation begins. Current flowing through the motor is monitored, by measuring the voltage drop across a precision value sense resistor. When the motor reaches the end of the travel, "stall condition", the current flowing will typically increase by several times the run current value. Therefore, the voltage measured across the sense resistor will also increase proportionally. An operational amplifier or a comparator circuit changes states when a predetermined threshold voltage is reached. This de-activates the latched relay. Additional electronic functions are required to reject inrush current and transient voltages, and a fail-safe time-out feature is often added as a back-up for the event the primary system fails. The electronic components employed to make up these circuits can range from standard off-the-shelf semiconductor products to custom-manufactured ASICs.