Apex Microtechnology ~ Question and answer
What is current limit, and how do I use it?
Current limit is a must-have protection feature for any Power Amplifier in any high-reliability environment. This feature is available in almost all Apex parts, and although it comes in many flavors, the working principle is typically as follows:
All output current flows through a current-limit resistor: a low-Ohm value resistor placed inside the feedback loop of an op-amp. This placement inside the feedback loop is important - so that the voltage drop does not show up as an error on the output. This current-limit resistor parallels the base-to-emitter junction of a bipolar transistor (BJT) inside the Apex Power Amplifier. Once the voltage across the resistor reaches roughly 0.65V (the VBE saturation voltage of most transistors), the internal BJT turns on and disables the output stage of the Power Amp. Then, the Power Amp begins working as a constant current source, regardless of the input signal. As soon as the condition that caused excess current is removed, the amplifier recovers back into its normal operating mode.
When selecting a current limit resistor, use the equation or plot provided in the specific product datasheet. Each product may use a different BJT with a different VBE saturation voltage, so 0.65V is not always used in the equation. Additionally, tolerances on VBE(SAT) can be wide. Allow for a 20% deviation by multiplying the calculated RCL by 0.8. Lastly, recall that VBE(SAT) is temperature-sensitive. Most BJTs decrease their VBE(SAT) by about 2.2mV for every 1°C rise in temperature.
Equally important to the value of the current limit resistor is its power rating. Use P=I2R to calculate the power dissipation (in Watts) in the current limit resistor, where I is the current limit set point (in Amperes), and R is the current limit resistance value (in Ohms).