Design of input protection
Design of input protections
The figure above shows the typical input circuitry for a linear voltage regulator. Though input filtering is not mandatory for the stability of a linear regulator, some external devices and filtering circuits are recommended in order to protect the linear voltage regulator against external disturbances and damage.
|A ceramic capacitor at the input in the range of 100nF to 470nF is recommended to filter out the high frequency disturbances imposed by the line, e.g. ISO pulses 3a/b. This capacitor must be placed very close to the input pin of the linear voltage regulator on the PCB.|
|An aluminum electrolytic capacitor in the range of 10µF to 470µF is recommended as an input buffer to smooth outhigh energy pulses, such as ISO pulse 2a. This capacitor should be placed close to the input pin of the linear voltage regulator on the PCB.|
|An overvoltage suppressor diode can be used to further suppress any high voltage beyond the maximum rating of the linear voltage regulator and protect the device against any damage due to overvoltage.|
|For linear voltage regulators with a NPN bipolar or a MOSFET power stage, a reverse polarity diode is mandatory to protect the device from damage due to reverse polarity. Though the regulators with a PNP power stage have internal reverse polarity protection, a reverse polarity diode is still recommended in order to avoid damage due to excessively high reverse voltage, e.g. the ISO pulse 1. The reverse polarity diode can be put anywhere on the module between the battery and the input pin of the regulator. It can also be shared with other elements on the module.|