VOLTAGE DEPENDENT (FOLDBACK) CURRENT LIMITING

Voltage regulators which can be reasonably high present (>1A) make use of a style of present restricting when the value that is optimum is allowable of current is dependant on the input-output voltage differential on the component. The reason this is certainly required is because of a characteristic of all transistors called Safe working Area (SOA) that limits the amount that is total of the transistor are made for properly while the voltage increases. The data shown to the bend that is SOA obtained from a published information sheet for a(3A/60V that is TIP31A NPN transistor. The data that is vital the bend that is SOA the fact the secure Operating current value drops to about 15% of optimum if the voltage across the component (VCE) are in its full ranked value. In the event that full 3A rating that is current to be utilized, the VCE can not surpass about 14V. You should observe that the input-output voltage across a linear regulator generally the VCE across its pass transistor. This means force present needs to prior be limited to the bend that is SOA of regulator pass transistor. The limitation that is current for the linear regulator must fit "under" the bend that is SOA the pass transistor in the event that device is to survive under all overload conditions. The restriction that is present for the LM317 will be detailed later on to illustrate this.