Switching elements, such as Field Effect Transistors (FETs), are used in a variety of applications to drive various types of loads. As one example, FETs are used to drive LED loads such that when the FET conducts, current passes through the LED (or string of series-coupled LEDs or parallel such strings) to illuminate the LED load. The particular level of current through the LED can control the LED turn on/turn off and/or brightness.
Driver circuits for driving a switching element such as a FET generally provide a control signal to a control terminal of the FET (e.g., the gate terminal) which causes the FET to conduct when the control signal reaches a predetermined voltage level, as may be referred to as a conduction threshold (VT), associated with the FET. An illustrative control signal ramps up at a predetermined slew rate when the system demands that the FET turn on to provide current to the load, remains at a substantially constant, predetermined level to maintain the FET in a conductive state, and then ramps down at a predetermined slew rate to turn off the FET and cease the flow of current to the load.
The turn on and turn off time intervals and characteristics of the FET can be affected by a parasitic capacitance of the FET between the gate and source terminals (CGS). In particular, it can take longer than otherwise possible to turn on a FET with a relatively large CGS since the control signal will charge the capacitance CGS causing it to take longer for the control signal to reach the conduction threshold. Additionally, it can take longer than otherwise possible to turn on a FET with a relatively high threshold voltage VT since the control signal must be ramped up through a larger voltage range. While these considerations suggest that it is desirable to speed up the slew rate at which the FET control signal is ramped up, stability issues can arise when a FET is switched on and/or off too fast. Additionally, fast switching of the FET can cause or exacerbate electromagnetic interference. Certain compliance standards, such as certain FCC standards or automotive standards, require electromagnetic emissions to be within certain levels.